Maedi-visna virus infection in sheep: a review Michel Pépin, Christian Vitu, Pierre Russo, Jean-François Mornex, Ernst Peterhans

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Michel Pépin, Christian Vitu, Pierre Russo, Jean-François Mornex, Ernst Peterhans. Maedi-visna virus infection in sheep: a review. Veterinary Research, BioMed Central, 1998, 29 (3-4), pp.341-367. ￿hal-00902532￿

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Maedi-visna virus infection in sheep: a review

Michel Pépina Christian Vitua Pierre Russoa Jean-François Mornexb Ernst Peterhans

a Unité pathologie des petits ruminants, Cneva Sophia-Antipolis, BP 111, 06902 Sophia-Antipolis cedex, France bInra-ENV Lyon, Marcy-l’Étoile, France c Institute of Veterinary Virology, Bern, Switzerland

(Received 29 December 1997; accepted 10 March 1998)

Abstract - The maedi-visna virus (MVV) is classified as a lentivirus of the retroviridae family. The genome of MVV includes three genes: gag, which encodes for group-specific antigens; pol, which encodes for reverse transcriptase, integrase, RNAse H, protease and dUTPase and env, the gene encoding for the surface glycoprotein responsible for receptor binding and entry of the virus into its host cell. In addition, analogous to other lentiviruses, the genome contains genes for regulatory proteins, i.e. vif, rev and tat. The coding regions of the genome are flanked by long ter- minal repeats (LTR) which play a crucial role in the replication of the viral genome and pro- vide binding sites for cellular transcription factors. The organs targeted by MVV are, in descend- ing order of importance, the lungs, mammary glands, joints and the brain. In these organs, the virus replicates in mature macrophages and induces slowly progressing inflammatory lesions con- taining B and T lymphocytes. The clinical signs of MVV infection, i.e. dyspnea, loss of weight, mastitis and arthritis, are related to the location of these lesions. Infection with MVV induces the formation of antibodies which can be detected by agar gel immunodiffusion, ELISA and the serum neutralization assay. As neither antiviral treatment nor vaccination is available, diagnos- tic tests are the backbone of most of the schemes implemented to prevent the spread of MVV. How- ever, since current serological assays are still lacking in sensitivity and specificity, molecular bio- logical methods are being developed permitting the detection of virus in peripheral blood, milk and tissue samples. Future research will have to focus on both the development of new diag- nostic tests and a better understanding of the pathogenesis of MV V infection. 0 Inra/Elsevier, Paris lentivirus / maedi-visna / sheep / slow infection / review

Résumé - L’infection par le virus Maedi-visna chez le mouton : une revue. Le virus du maedi-visna appartient au genre lentivirus de la famille des rétrovirus. Le génome de ce virus com-

* Correspondence and reprints Tel.: (33) 4 92 94 37 00; fax: (33) 4 92 94 37 Ol; e-mail: [email protected] prend trois gènes de structure codant pour l’enveloppe virale (env), la capside (gag) et les enzymes telles que la transcriptase réverse, l’intégrase et la dUTPase (po/), et plusieurs gènes accessoires : vif, rev et tat. Ces gènes accessoires interviennent dans la régulation de la réplication et de l’expression du virus, la modulation de son pouvoir pathogène et de son tropisme. Les organes cibles du virus maedi-visna sont par ordre d’importance le poumon, la mamelle, les articula- tions et le cerveau. Dans ces organes, le virus infecte les macrophages matures et entraîne des lésions de type inflammatoire contenant des lymphocytes B et T, évoluant lentement, il l’ori- gine des signes cliniques de la maladie : essoufflement, amaigrissement, mammite, arthrites, etc. L’infection par le virus maedi-visna conduit à une réponse humorale à l’origine des tests diagnostiques actuellement disponibles : immunodiffusion en gélose, Elisa, etc. Ces tests séro- logiques sont à la base de la plupart des plans de prophylaxie sanitaire mis en place dans de nombreux pays, en l’absence de traitement et de vaccination. Cependant de nombreuses difficultés liées il la sensibilité et à la spécificité des tests sérologiques ont conduit à s’orienter vers la mise en évidence du virus dans le sang. le lait ou les organes par des techniques d’amplification génique. Dans un futur proche, les besoins en matière de recherche sont évidents tant pour le déve- loppement de nouveaux outils diagnostiques fiables que pour une meilleure connaissance de la pathogénie du maedi-visna afin de trouver des moyens de lutte mieux adaptés. © Inra/Elsevier, Pariss lentivirus / maedi-visna / mouton / infection lente / revue 1. INTRODUCTION ruminant lentiviruses include primary interstitial pneumonia, encephalitis, lym- The lentivirus genus of the retroviridae phadenopathy, arthritis, mastitis and family comprises pathogens of humans, chronic weight loss (Russo et al., 1991; monkeys, horses, cattle, sheep, goats and Phelps and Smith, 1993). Lentiviral dis- cats. The infections caused by maedi-visna eases are the cause of significant economic virus (MVV) in sheep and by caprine losses incurred by the sheep and goat arthritis-encephalitis virus (CAEV) in industries and also increasingly threaten goats share a number of features with the exports of live animals. infection caused by the human immuno- deficiency virus (HIV), such as an incu- Infections with MVV and CAEV per- bation period of several months or even sist for life in sheep and goats, respec- years and a slow development of disease tively, despite a humoral and cellular symptoms (table I). The major manifes- immune response. The infection is char- tations of the diseases induced by small acterized by progressive inflammatory et visna state of lesions in various organs (Cador6 al.,., called (’fading away - pro- 1996; Harkiss et al., 1991; Mornex et al., gressive apathy’ in Icelandic), is a 1994; Anderson et al., 1994; Cador6 et al., demyelinating leukoencephalomyelitis 1993). The major, if not the sole, host cells (Palsson, 1976; Watt et al., 1994). MVV of the virus are cells of the monocyte/ can also infect other organs or tissues, par- macrophage cell lineage (Gendelman et ticularly joints in which it causes arthri- al., 1986). In contrast to human (HIV) and tis (Watt et al., 1994) and the mammary simian (SIV) immunodeficiency viruses, glands where it causes mastitis (Pekelder, the small-ruminant lentiviruses do not 1993; Watt et al., 1994). The properties infect CD4+ T lymphocytes. Therefore, of sheep retroviruses are summarized in the diseases they cause provide a valuable table IL model for both the effects of studying In Iceland, MVV is likely to have lentivirus infection on macrophage biology appeared subsequent to the importation of and the role infected played by infected asymptomatic Karakul rams from macrophages in the absence of immuno- Germany in 1933, which resulted in deficiency. widespread dissemination of the infection This review aims to summarize the cur- to most flocks. Following its discovery in rent knowledge of the biology of MVV Iceland, MVV infections were detected in and its interaction with its host, the sheep. various countries, although with differing We also provide a short overview of the prevalences (Madewell et al., 1987; Camp- history of the discovery of MVV. The bell et al., 1994; Schaller et al., 1995; structure and organization of the MVV Pritchard et al., 1995; de la Concha- are Aus- genome and of the encoded polypeptides Bermejillo, 1997). Exceptions tralia and New where lentiviral are described, with particular emphasis on Zealand, infections have been observed in but auxiliary genes. The clinical consequences goats of infection, the epidemiology and diag- not in sheep (Greenwood et al., 1995). In nostic tests are considered and the mech- France, Russo et al. (1980) isolated the anisms of pathogenesis discussed. first French MVV strain in 1980. Complete nucleotide sequences of sev- eral MVV strains have been published 2. HISTORY since 1985, i.e. those of strains K 15144 (Sonigo et al., 1985) from Iceland, SA- OMVV (Qu6rat et al., 1990) from South MVV was discovered in sheep by Sig- Africa, EV 1 from the UK et al., urdsson et al. (1952, 1957) in Iceland in (Sargan as well as clones KV 1772-kv72/67 the early 1950s, although the disease 1991) for neurovirulence (Andresson symptoms had been described prior to this [selected discovery in South Africa, the USA and et al., 1993)] and LV 1-1 KS 1 (Staskus et Iceland. The concept of ’slow viruses’ al., 1991 ).). resulting from this discovery prompted the name of the lentivirus [lentus (]at.) = 3. THE VIRUS slow] genus of which MVV is a member. Two distinct pathological situations, cor- responding to the main clinical manifes- tations of MVV infection, featured in those 3.1. Viral structure early descriptions. The first, called maedi (’dyspnea’ in Icelandic), is a progressive MVV virions are spherical and have a et structure. The size pneumonia (Palsson, 1976; Mornex al.,., unique three-layered 1994; Cador6 et al., 1996) and the second, of the virions is about 100 nm. The central part of the virus is the genome-nucleo- various auxiliary genes. Analogous to protein complex, associated with the other retroviruses, the MVV proviral DNA reverse transcriptase. This structure is is flanked by long terminal repeats (LTR) enclosed within an icosahedral capsid sur- that provide the cis signals required for rounded by an envelope derived from the transcription, integration and polyadeny- plasma membrane of the host cell.l . lation of viral RNA (figure 1). The num- bers and role of the auxiliary genes vary depending on the lentivirus involved and, 3.2. Organization of the viral genome to a lesser degree, between the different strains of MVV. MV virus has a genetic organization that is typical of lentiviruses: its genome is 3.2.1. Structural genes and a dimer of RNA of positive strand polar- their products (table III) ity, 9.2 kb in size, which is reverse tran- scribed into DNA, some of which proviral 3.2.1.1. The gag gene will be integrated into the chromosomal DNA. It comprises three structural genes, The gag gene encodes for three glyco- i.e. gag (group-specific antigen), pol (poly- proteins from the precursor Pr55!a!: the merase) and env (envelope), as well as capsid (p25), the nucleocapsid (p14), and the matrix (MA) protein (p 17) which 3.2.1.2. The env gene ensures the link between the capsid and the envelope. The MA protein is respon- The env gene encodes the glycoprotein sible for the association of the gag pre- of the virus. As for other retroviruses, the cursor with the cell plasma membrane; glycoprotein is synthesized as a precursor within the virus particle, the MA protein is (gp160) and is cleaved by a host cell pro- localized between the viral membrane and tease into two subunits: the surface gly- the capsid protein of the virus. The cap- coprotein (SU; gp135) and the transmem- sid protein, the most abundant protein of brane glycoprotein (TM; gp44). The TM the virion, forms the hydrophobic core of part of the viral envelope surface glyco- the virion and elicits a strong antibody protein is anchored in the lipid bilayer. response during infection, which is valu- The SU glycoprotein is non-covalently able for diagnostic tests. Within the capsid linked to TM. The envelope glycoproteins protein the nucleocapsid protein coats the of lentiviruses have many important bio- viral RNA genome (Joag et al., 1996). logical functions and contain the epitopes responsible for both the induction of neu- 3.2.1.3.2.2. Integrase antibodies and the interaction of tralizing In each virion, numerous molecules of the virus with the of the host cell. receptor reverse transcriptase and integrase are To no surface for MVV date, receptors associated with the viral RNA (Marin et have been defined conclusively although a]., 1994). Following reverse transcrip- cellular proteins able to bind to the virus tion, provirus migrates towards the have been described (Dalziel et al., 1991; nucleus, and the double-stranded DNA Clements and this contrasts Payne, 1994); genome is integrated in the host cell DNA to HIV where the and the receptor (CD4) by a mechanism mediated by integrase for co-receptors (receptors chemokines) (Stormann et al., 1995). Integration have been determined et al., (Willett involves repeated inverted sequences pre- 1997). sent in the LTR (Marin et al., 1994). 3.2.1.3.2.3. Protease 3.2.1.3. The pol gene The viral protease cleaves the gag and and resem- 3.2.1.3.1. The reverse transcriptase gag-pol polyprotein precursors bles cellular aspartic acid proteases in its The key enzyme of retroviruses is the three-dimensional structure (Joag et a]., reverse transcriptase, a RNA-dependent 1996). DNA polymerase, encoded by the pol gene which permits the transcription of the viral 3.2.2. Auxiliary genes RNA into DNA; this protein is an het- erodimer which displays RNAse H activ- The MVV has three major auxiliary ity. genes: tat, vif’(viral infectivity factor; pre- viously called Q gene), and rev (regula- 3.2.1.3.2. Other enzymes tor of virion protein expression) (Clements and Zink, 1996) (figure 1).). 3.2.1.3.2.1. dUTPase 3.2.2.1. Rev In lentivirus genomes, the gene encod- The of the rev is a ing the dUTPase enzyme is located in the product gene pro- pol reading frame between the RNAse H tein of 19 kDa (167 amino acids) derived and integrase coding regions. Primate from an mRNA of 1.4 kb. The MVV rev consists of four exons: a leader lentiviruses lack dUTPase. This enzyme gene seg- activity has been identified only in FIV ment (exon 1), an untranslated portion (feline immunodeficiency virus), EIAV (exon 2), and two translated exons, 3 and (equine infectious anemia virus), CAEV 4. As rev is an early gene of lentiviruses et it a and MVV (Elder et al., 1992); dUTPase (Mazarin al., 1990), plays major role in mRNA from appears to decrease the frequency of G- transporting unspliced to-A mutations. In vitro, dUTPase defi- the nucleus to the cytoplasm. The rev pro- tein contains nuclear which cient CAE viruses replicate more slowly in export signals the to the macrophages (Turelli et al., 1996). In vivo permit protein pass through nuclear membrane and to exert its they are slightly attenuated (Turelli et al., regu- role via a element 1997), albeit to a lesser degree than the latory responsive (RRE for rev located in the dUTPase-deficient EIAV (Steagall et al., responsive element) env The RRE is of 1995; Lichtenstein et al., 1997). In con- gene (figure 1). capable RNA via an RNA site. trast, dUTPase-deficient MVV appeared to binding binding be as pathogenic in vivo as the wild type The essential role of rev is illustrated virus (Petursson et al., 1998). by site-directed mutagenesis in the 4th exon (Toohey and Haase, 1994): rev-muta- be mediated by stimulation of cellular genized virus was shown to be non-infec- genes, such as cytokine genes (Philippon et tious (table lV). al., 1994). A recent paper examining the role of tat by studying tat-deleted CAE 3.2.2.2. Tat viruses has shown that this gene is not essential for virus replication (Harmache et The tat gene ( 1.7 kb mRNA) encodes al., 1995b). It may, however, contribute to for a of 10 kDa which was first protein a successful interaction between the virus described for its role in stimulating gene and its host or directed the viral by recruiting modulating expression by promoter cellular factors involved in the initiation located in the 5’ LTR. The Tat protein of the maturation pro- mediates the accumulation of viral mRNA transcription during cess of monocytes to which via the AP-1 and AP- macrophages, (activator protein-1) leads to increased viral in 4 sites in the U3 of the gene expression binding region vivo (Carruth et al., 1994). Moreover, the LTRs (Gdovin and Clements, 1992; presence or absence of AP-1 or AP-1-like Saltarelli et al., 1993) or via cellular factors sequences may at least partially explain such as c-Fos and c-Jun et (Neuveut al., the differences in tropism between vari- 1993). A leucine-rich domain in present ous MVV isolates such as EV 1 (a British Tat is to be for likely responsible target- isolate) and SA-OMVV (a South African ing the Tat at AP-1 sites in the viral protein isolate) versus K I S 14 (an Icelandic iso- LTR (Carruth et al., 1996). it has Recently, late) (Andresdottir et al., 1994; Sutton et al., been that the Tat of MVV recognized 1997). belongs to a group of Tat proteins charac- terized by a weak transactivation poten- 3.2.2.3. Vif tial, in contrast to the Tat proteins of HIV- 1 and -2, SIV or BIV, which strongly The vif gene encodes for a 29-kDa pro- transactivate their LTRs by binding to a tein (230 aa) which, in naturally infected TAR (Tat-activated region) sequence. The animals, induces a weak immune response Tat protein of MVV itself may contribute that can be detected in western blots to viral pathogenesis by inducing follicular (Audoly et al., 1992). This protein is not lymphoproliferative disorders in various homologous to cysteine protease in its organs (Hayman et al., 1993; Vellutini et entirety but contains a motif which is al., 1994). The action of the tat gene may homologous to cysteine protease and is translated during the late stages of viral may also present a problem in the diag- replication (Audoly et al., 1992; Harmache nosis of lentiviral infections and remains et al., 1995a). The importance of vif for a formidable obstacle to vaccine develop- the replication of MVV is unknown but ment. Antigenically distinct viruses have investigations using CAEV and HIV indi- been isolated from sheep persistently cate that v(fplays a crucial role in the late infected with MVV; these variants arise stages of the viral life cycle, i.e. during by point mutations in the env gene (Braun the morphogenesis of the viral nucleo- et al., 1987). protein core (Hoglund et al., 1994; Har- mache et al., 1996a; Simon et al., 1997). Genetic variation in MVV has been determined by PCR amplification of por- tions of the viral genome (Zanoni et al., 1992; Sargan et al., 1995; Rosati et al., 3.3. Variability 1995) or by analysing PCR products in denaturing gradient gel electrophoresis Genetically, lentiviruses are quite het- (Woodward et al., 1994). These studies erogeneous. This manifests itself in their have allowed the extent of variability in antigenic diversity, differences in viru- different regions of the genome (LTR, lence and growth characteristics in vitro. gag, pol, env) to be compared and the het- This genetic plasticity is believed to con- erogeneity of MVV strains to be deter- tribute to viral persistence in the host ani- mined. Moreover, these techniques per- mal by permitting evasion of the immune mitted the phylogeny of lentiviruses to be response. Moreover, antigenic diversity established (figure 2). Analysis of a 475- nt fragment in the pol gene of ovine 4.2. Vertical transmission lentiviruses from France revealed that the French isolates form a group closely Vertical transmission (including hered- related to the Cork CAEV strain and only itary and congenital infection, and infec- distantly related to a group of ovine tion at parturition) is a key feature of the lentiviruses consisting of the K1514, EV1I epidemiology of maedi visna (Russo et and SA-OMVV strains. This analysis was al., 1991 ). In an endemically infected confirmed by studying the sequence vari- flock, virus-infected cells and free virus ability in a fragment of the env gene (Ler- are passed from ewes to their lambs via oux et al., 1995). Similarly, in North colostrum and milk (Watt et al., 1994; de America, sequence analysis in the env la Concha-Bermejillo, 1997). The perme- gene of ovine field isolates showed them ability of the gut of newborn lambs greatly to be more similar to CAEV than to the favours vertical transmission (Houwers, ovine prototype strains, which suggests 1997). The duration of infection in the that ovine and caprine lentiviruses may ewe and the extent of the contamination have descended from a common caprine of the progeny appear to be correlated ancestral genotype (Chebloune et a]., (Houwers et al., 1989). Naturally, lambing 1996). Another study suggests that CAEV is a time of high lentivirus expression originated from an ancestral ovine which facilitates the spread of infection lentivirus adapted to a caprine host (Valas (Guiguen et al., 1992). Since mastitis is et al., 1997). frequent in affected animals, vertical trans- mission may be facilitated by the recruit- ment of mononuclear infected cells to the mammary glands (Zink and Johnson, 4. VIRUS TRANSMISSION 1994).

4.2.1. In utero transmission

4.1. Horizontal transmission In utero virus transmission is a highly controversial issue. Despite strict lamb- ing controls, some unexplained cases of Free virus or virus-infected cells are seropositive lambs have been observed in horizontally transmitted by inhalation of flocks in which MVV eradication pro- respiratory secretions (Zink and Johnson, grams are carried out (Houwers et al., 1994; Watt et al., 1994). Co-infections 1983). Some authors have reported their with other viruses or bacteria may also failure to detect virus in experimentally contribute to the spread of MVV via pul- infected Texel sheep embryos (Watt et al., monary exudates (Palsson, 1976). Several 1994). Others have reported the isolation studies support the hypothesis that, under of ovine lentivirus from a 100-day-old certain circumstances, viral transmission cesarean-sectioned fetus from a naturally between adult animals may play an impor- infected ewe (de la Concha-Bermejillo, tant role in the spread of MVV (Campbell 1997). In an endemically infected flock, et al., 1994). In addition, horizontal trans- the PCR yielded positive results in periph- mission is closely associated with close eral blood mononuclear cells of about confinement in winter stabling (de la Con- 10 % of lambs tested prior to colostrum cha-Bermejillo, 1997), the duration of the ingestion (Brodie et al., 1994). Additional presence of the virus in the flock (Houw- observations suggest that co-infections, ers et al., 1989), and annual acquisition of such as Border disease, might permit an replacements (Houwers, 1997). in utero infection (Russo, pers. comm.). Embryo transfer might be a safe way of Narayan, 1986). Antiviral antibodies in ensuring virus-free flocks (Watt et al., serum of naturally infected animals are of 1994). However, in utero infection may the IgG ! subtype, with no detectable IgG2 be a rare occurrence and is not supported (Bird et al., 1995). However, this appears by epidemiological evidence (Houwers et unlikely to be related to the controversial al., 1989). role of virus neutralization in the persis- tence of virus in vivo. Although it was demonstrated that antibodies, in principle, 4.3. Venereal transmission were also capable of neutralizing virus in macrophages, the affinity of virus bind- to exceeded that of As the virus is present in all body fluids, ing macrophages its it might, theoretically, also be transmit- binding to antibody. This finding the that ted by mating. However, to date no well- prompted suggestion neutralizing antibodies be to the documented case of venereal transmission might unable prevent has been reported (de la Concha-Berme- virus from spreading between and jillo, 1997). Co-factors, such as infection macrophages (Kennedy-Stoskopf It was also that with Brucella ovis or leucocytospermia of Narayan, 1986). argued neutralization determinants of certain viral unknown origin, may increase the shed- strains not be detected antibod- ding of virus in the semen (de la Concha- might by ies and Bermejillo et al., 1996). that the emergence of antigenic variants might be yet another factor con- tributing to the antibodies’ failure to achieve control 5. PATHOGENESIS immunological (Narayan et al., 1987; Clements et at., 1988).

Maedi visna is characterized by a longg Due to the tropism for mononuclear incubation period and, typically, symp- phagocytes, lymphoid tissues are consid- toms take several months or even to years ered to be important targets for the repli- (for a review, see and develop Narayan cation of MVV. Indeed, studies Cork, 1985). Infection for life and elegant persists using lymph node cannulation infected animals are a constant reservoir of techniques have yielded important information with infection which, consequently, permits the regard to virus-host interaction. The pro- virus to persist in its host. liferative responses of efferent lymph cells In contrast to infections with HIV, SIV were shown to be depressed transiently and FIV, immunosuppression is not a fea- after experimental infection (Bird et al., ture of maedi visna (Clements and Zink, 1993, 1995). Moreover, the decrease in 1996). This explains why secondary infec- the numbers of CD4(+) and the increase in tions with opportunistic agents are infre- the numbers of CD8(+) cells caused an quent in affected flocks. Nevertheless, the inversion of the ratios of CD4(+)/CD8(+) immune response to maedi-visna virus T cell populations in bronchoalveolar exhibits certain peculiar features which lavage fluids of experimentally infected may contribute to the persistence of infec- sheep (Maslak and Schmerr, 1993). As tion. Early studies demonstrated that sheep suggested by the presence of circulating infected with maedi-visna virus develop precursors of cytotoxic T cells of the a humoral immune response that is CD8(+) phenotype, however, there also markedly slower than that directed against exists a vigorous cell-mediated immune viruses causing acute infections (Gud- response to infection (Blacklaws et al., nadottir and Kristinsdottir, 1967; Sihvo- 1994, I 995). All in all, these observations nen, 1980; Kennedy-Stoskopf and indicate that certain aspects of the immune response may lack fine tuning, even if they opmental stage of monocyte/macrophage do not suggest that the failure to eliminate host cell is a key feature of virus-cell inter- the virus may per se be due to a general action. In Maedi visna, it was demon- failure to mount an antiviral immune strated a number of years ago that viral response. For instance, as work with other gene expression increases during the lentiviruses suggests, the failure to effi- development of the monocyte to its mature ciently neutralize infection is the result of tissue form, the macrophage (Gendelman as yet undetermined mechanisms (Pan- et al., 1985). The fact that monocytes in cino et al., 1994; Bertoni et al., 1994). Par- the blood may carry the viral genome ticularly in the light of observations made without sustaining its replication was with attenuated HIV vaccines, it seems referred to as the ’Trojan horse mecha- well worth investigating whether certain nism’, which indicates that this type of immunodominant regions of Env may interaction with the host cell may permit actually serve as decoys, thus decreasing the virus to be transported to tissues with- a possible protective antibody response out being detected by the immune system (Garrity et al., 1997). Moreover, work with (Peluso et al., 1985). In this context, the the closely related CAE virus in goats sug- observation by Brodie and coworkers gests that a dominant type 2 (i.e. antibody (Brodie et al., 1995) is of interest: in con- centered) immune response may be asso- trast to entry, viral replication in ciated with disease (Perry et al., 1995).). rnacrophages may be restricted in certainn tissues. In addition, different strains of Other important factors contributing to Maedi-visna virus may, in vivo, differ in viral persistence in infected sheep are the their host cell (Clements and Zink, host-cell tropism and certain aspects of tropism lentivirus-host cell interaction. Maedi- I 996). visna virus, as well as CAE virus, were shown in vivo to have a for tropism Incidentally, the tropism for cells of the mononuclear phagocytes (Narayan et al., monocyte lineage with an attendant lack of 1982; Gendelman et al., 1985; Lairmore et viral replication in monocytes also poses Several studies that cer- al., 1987). suggest the evolu- tain other cell less interesting questions regarding types, although promi- tion of the viral in infected ani- also sustain viral in genome nent, may replication mals. The evolution of the viral infected animals. A detailed study of the genome depends on several parameters, among central nervous system of Icelandic sheep them the error rate of the viral polymerase, produced evidence of viral replication in a evolutionary pressure exerted by func- variety of cell types, inter alia, epithelial tional constraints and by the immune sys- cells and fibroblasts of the chorioid plexus tem, and the rate and extent of viral repli- et al., 1989), and viral tran- (Georgsson cation. Viral variants have been shown to scripts were detected in cells of epithelial in infected animals but there is no the and small intestines of emerge thyroid, kidneys clear evidence that these variants arise as goats infected with CAEV (Zink et a]., a result of immune et al.,., 1990). Very recently, endothelial cells pressure (Lutley 1983; Thormar et al., 1983; et were shown to the of Narayan support replication al.. 1987). Viral RNA and have MVV in vitro, with interesting differences antigen been demonstrated in bone marrow cells, on the of the tissue used depending origin but it has remained unclear whether the to prepare the endothelial cells (Craig et cells as and a]., 1997). staining macrophages pro- ducing virus were in fact monocyte pre- In all lentiviruses, the restriction of cursors or macrophages residing in the virus replication depending on the devel- marrow (Gendelman et al., 1985). A major difference between small rumi- The mechanisms responsible for main- nant lentiviruses and the immunodefi- taining and gradually increasing the ciency-causing HIV, SIV and FIV lies in mononuclear inflammation typical of the nature of the histological lesion. Even maedi visna have remained an enigma. A though the viruses causing immunodefi- lentivirus-specific interferon (IFN) may ciency may induce inflammation initially, be one of the pro-inflammatory factors. It in the later stages there is extensive cell was described as the result of an interac- depletion (Gougeon et al., 1993). By con- tion between infected macrophages and trast, both maedi-visna and CAE viruses lymphocytes (Narayan et al., 1985). The interferon was shown cause progressive inflammation. Inflam- lentivirus-specific to restrict viral slow down mation is observed in different organs, replication, and have chemo- most prominently, in the lungs and mam- macrophage maturation, tactic for and mary glands and, less frequently, in the properties lymphocytes synovial membranes of the joints and in mononuclear phagocytes (Kennedy et al., the brain. The extent of inflammation and 1985; Zink and Narayan, 1989). The over- all actions of the inter- the spectrum of affected organs may lentivirus-specific feron would tend to aggravate and per- depend on the genetics of the infected ani- petuate inflammation, providing new host mal as well as on the strain of the infecting cells for the virus and, at the same time, virus (DeMartini et al., 1991 ). Different preventing the virus from replicating at a breeds may differ in the degree of their high rate that might stimulate a more vig- susceptibility to developing ovine pro- orous antiviral immune It should the North American response. gressive pneumonia, be noted, however, that this interpretation form of maedi visna et al., 1986). (Cutlip reflects an of in vitro data In addition, visna, the classical CNS form extrapolation to a situation in vivo, which is likely to of infection originally observed in Ice- be considerably more complex. In the light land, was only rarely seen elsewhere. The of more recent data obtained relating to mechanisms of determined genetically HIV, it would be interesting to know resistance to clinical disease have not been whether part or all of the activity by this determined in but in are sheep goats they lentivirus-specific interferon may be linked to the MHC class I and class II anti- related to the recently described and gens (Ruff et al., 1993). Field isolates chemokines that were shown to have a established strains of maedi- laboratory profound effect on the extent of replica- visna virus are highly heterogeneous tion of HIV (D’Souza and Harden, 1996). (Qu6rat et al., 1984; Sargan et al., 1991; Zanoni et al., 1992; Andresdottir et al., Investigations on the role in vivo of 1994; Carey and Dalziel, 1994; Leroux et classical type 1 and type 2 interferons and differ- al., 1996). Undoubtedly, genetic of other cytokines are scarce. Maedi-visna ences are for the differences in responsible virus-infected sheep with severe lymphoid virulence between individual strains of interstitial pneumonia had significantly virus although the underlying differences elevated levels of spontaneous interferon have not yet been determined in detail. production originating from pulmonary However, work by the group of De Martini leukocytes, as compared to both infected suggests that the extent of viral replica- animals with mild or no lesions of lym- tion and degree of cytopathicity may be phoid interstitial pneumonia and non- important markers of virulence, ’rapid- infected controls. It was thought that high’ strains being more virulent than increased local production of IFN in ’slow-low’ strains (Lairmore et al., 1987, lentivirus-infected host tissues may serve 1988b). to increase the numbers of leukocytes entering the sites of viral replication. This the virus may be an important trigger for promotes cell-mediated tissue damage and the formation of the lesion. In line with also provides larger numbers of cells for this interpretation, UV-inactivated virus virus replication (Lairmore et al., 1988a). was found to stimulate the expression of Infection of alveolar macrophages resulted GM-CSF in ex vivo alveolar macrophages in increased expression of interleukin-8 (Woodall et al., 1997), indicating that virus (IL-8) mRNA. Interestingly, mRNA of replication was not required for the virus this cytokine was also demonstrated to be to have this effect. Infection with maedi increased in the lungs of sheep infected visna was also shown to alter the capacity with maedi-visna virus. Expression of IL- of macrophages for generating reactive 8 correlated with the severity of the oxygen species (Cottin et al., 1996), which lesions. This led to the suggestion that IL- indicates that the virus alters an important 8 may play an active role in shaping the effector function implicated in the defence histological changes observed in the lungs against microorganisms as well as in host of sheep suffering from maedi visna pathology. The generation of enhanced (Legastelois et al., 1996). This situation levels in response to exogenous stimula- appears to differ from that found in the tion is of particular interest because it is synovial membranes of CAE virus- indicative of ‘priming’, i.e. the cells show infected goats. Even though CAE virus a difference in function only when caused an increased expression of IL-8 in responding to exogenous stimulation. This cultured macrophages, no such effect was particular type of functional alteration may found in the synovial membranes (Lechner also occur when the cells respond to stim- et al., 1997b). Since IL-8 expression was ulation by ’physiological’ signals origi- found to be increased only in those nating from other cells in situ. We have macrophages in which virus replicated at recently observed that macrophages a very high rate, this difference between infected with CAE virus in vitro show the status in the lungs of MV-infected altered expression of some, but not all, sheep and that in the synovial membranes cytokines investigated (Woodall et al., of CAEV-infected goats may be accounted 1997; Lechner et al., 1997a). Hence, such for by a more restricted replication of priming for an altered response may con- CAEV in the synovial membranes of goats tribute to the chronic inflammation which in contrast to a higher rate of replication of is a hallmark of infection by small rumi- MVV in the lungs of sheep. An active role nant lentiviruses. of infected macrophages in promoting inflammation is further suggested by the To date, the question of how the induction of procoagulant activity both in histopathological lesions typical of maedi cultured cells and in the lungs of infected visna may be generated is unresolved. sheep (Lena et al., 1994). Recently, Clearly, the mononuclear inflammation Woodall et al. reported elevated levels of itself is the major pathogenic factor pro- mRNA for granulocyte-macrophage moting some of the more obvious alter- colony-stimulating factor (GM-CSF), ations seen in the final stages of chronic interleukin-2 receptor (IL-2R), and inter- maedi. These include, among others, lung leukins 1 f3,4 4 and 10 (IL-1 (3, IL-4, IL-10) fibrosis and its well-known manifestations (Woodall et al., 1997). Interestingly, such as a decrease in the gas exchange in expression of TNF-a was not increased, the lungs due to a decrease in the total which suggests that, in maedi, not all pro- alveolar surface and an increase in the dis- inflammatory cytokines are upregulated. tance between the luminal and vascular The severity of inflammation was corre- sides of the alveoli. Lung fibrosis is a com- lated with the viral load, indicating that plex process, to which cytokines and other immune mediators produced during the CAEV are closely related (Gogolewski et chronic inflammation contribute. In addi- al., 1985), AGID based on this antigen has tion, reactive oxygen species generated for a long time been routinely used to by macrophages and other phagocytic cells detect small ruminant lentivirus infections may shift the fragile protease/antiprotease in both sheep and goats. Precipitating anti- balance in favour of the former which, in bodies identified by AGID are anti-p25 turn, results in enzymatic tissue destruction with a crude concentrate and large periph- (Hutchison, 1987). Associated with fibro- eral wells (macroimmunodiffusion test) sis is a decrease in the compliance of the (Cutlip et al., 1977), and anti-gp135 with lung tissue which manifests as the kind of a microhexagonal well pattern, with alter- strained breathing which is called ’maedi’ nate large and small peripheral wells by the Icelanders. Similar to other forms of (microimmunodiffusion test) (Winward lung fibrosis, bacterial infections may et al., 1979; Dawson et al., 1982). Several become more frequent and more difficult studies demonstrated that an AGID assay to control efficiently. In turn, such infec- with CAEV gp 135 is more sensitive than tions have an adverse effect on the health an AGID assay with CAEV p25 (Grewal, status, as do other lung diseases such as 1986; Adams and Gorham, 1986). adenomatosis, an infectious tumor caused Radioimmunoprecipitation assays revealed by an oncovirus (Snyder et al., 1983, Daw- that goats infected with CAEV have much son et al., 1990). higher antibody titers against gp135 than against p25, emphasizing the necessity of using the appropriate antigen in AGID to 6. DIAGNOSIS attain a high sensitivity and specificity in the detection of antibody to CAEV or MVV (Knowles et al., 1994). Hence, pre- 6.1 Antibodies and nucleic acids cipitation in an agar gel requires multiple epitope-antibody interactions, whereas Due to the persistence of circulating the radioimmunoprecipitation assay antibodies, the diagnosis of MVV infec- requires only the binding to a single epi- tion is most commonly based on serolog- tope (Knowles et a]., 1994). Thus, the ori- ical tests. In recent years, as an alterna- gin and characteristics of the viral strain tive to serology, methods have been producing the precipitating antigen appear developed that allow the detection of the to be of major importance (Klein et al., viral genome by PCR. 1985). However, the interpretation of the microimmunodiffusion test with gpl35 6.1.1. The first generation assays: antigen sometimes appears difficult or AGID, whole-virus ELISA subjective and may lead to false-positive and immunoblot or false-negative results, although, theo- retically, this assay is more sensitive than The most widely used test is agar gel the anti-p25 AGID. In unclear cases, the immunodiffusion (AGID), first described results must be confirmed in ELISA or in the late 1970s (Cutlip et al., 1977; Win- immunoblot (western blot). ward et al., 1979). The antigen, a concen- trate of medium harvested from cell cul- The development of indirect ELISAs tures infected with the WLC 1 MVV strain, has improved the diagnosis of MVV and contains both major structural proteins, CAEV. The antibody response is detected i.e. the core protein p25 and the major at an earlier timepoint and yields a semi- envelope glycoprotein gp135 (Dawson et quantitative assessment of the level of cir- al., 1996). As, antigenically, MVV and culating antibodies (Vitu et al., 1982; Houwers et al., 1982; Russo et al., 1988). or monoclonal antibody conjugates These first-generation ELISAs used par- (Bosgiraud et al., 1989; Zanoni et al., tially or highly purified preparations of 1989, 1994). Generally, a sample is con- whole-virus antigens, and polyclonal sidered positive in the western blot if anti- enzyme conjugates, thus generating false- bodies to at least two gene products are positive results, and necessitating a high detected (Brodie et al., 1992; Johnson et dilution of the sera to avoid non-specific al., 1992). Nevertheless, the western blot, reactions. although valued as a confirmation test, is not suitable for large numbers of sera, and Immunoblot techniques (western blot) can still yield false-positive results. have been used to analyse the antibody responses to each of the major viral pro- 6.1.2. The second teins, i.e. the gp135 - also referred to as generation assays: gp105 (Kajikawa et al., 1990; Brodie et assays based on monoclonal recombinant al., 1992), the transmembrane (TM) pro- antibodies, proteins tein gp44 (or gp55) and the internal pro- and peptides teins p25, p17 and p14, in sera of experi- mentally or naturally infected sheep By using monoclonal antibodies anti- (Houwers and Nauta, 1989; Johnson et p25 and a double-sandwich method, al., 1992; Torfason et al., 1992) or goats improved assays with a higher specificity (Vitu et al., 1993; Barlough et al., 1994). and reliability were developed (Houwers In some cases, in the western blot, anti- and Schaake, 1987; Reyburn et a]., 1992). bodies to p25 were detected before anti- Numerous recombinant proteins have been gpl35 in experimentally infected sheep developed since 1990, following the initial (Houwers and Nauta, 1989), and localization of immunodominant regions detectable antibody responses to p14, p 177 in the Env and Gag proteins (Kwang and and even p25 may be absent in sheep with Cutlip, 1992; Kwang et al., 1996). The MVV lesions (Houwers and Nauta, 1989). gp70 (a degradation product of either the Technical problems may well explain that precursor, gp160 or of SU, gp135) and different authors recorded differing results gp40 (equivalent to TM glycoprotein, concerning a delayed anti-gp135 response gp44) were expressed in Escherichia coli, (Houwers and Nauta, 1989; Bosgiraud et and the entire gp70 was expressed in insect al., 1989; Kajikawa et al., 1990; Torfason cells by a recombinant baculovirus et al., 1992). They also explain the greater (Kwang et al., 1995). Fragments of the sensitivity of the radioimmunoprecipita- gp135 were expressed as fusion proteins tion assay in the early detection of anti- and used to analyse the antibody response bodies to viral glycoproteins in naturally to gpl 35 in MVV-infected sheep (Carey et and experimentally infected animals al., 1993). Different segments of Gag and (Gogolewski et al., 1985; Mazarin et al., of the 44 kDa TM glycoprotein were 1990; Knowles et al., 1994; Chebloune et expressed as glutathione S-transferase al., 1996). Nevertheless, as the radioim- (GST) fusion proteins (Power et al., 1995). munoprecipitation assay is expensive and These recombinant serve as time-consuming, it is rarely used in routine proteins antigens in new and promising assays diagnosis, but rather represents a ’last (Reyburn et al., 1992; Kwang and Cutlip, resort’ serological test for cases that can- Zanoni et Rosati et not be resolved in western blot. 1992; al., 1994; al., 1994; Power et al., 1995; Keen et al., The specificity of ELISA or western 1995; 1996; Boshoff et al., 1997), which blot can be improved by replacing the sec- are generally more sensitive and specific ond antibody by conjugates of protein G than whole virus tests. However, even if the results obtained with recombinant pro- for the direct detection of MVV in clinical teins turn out to be more sensitive, some specimens, either prior to culture or after non-specific reactions nevertheless remain, cocultivation with susceptible cells. The most notably due to antibodies binding to latter is more sensitive as only a few the GST fusion partner (Boshoff et al., PBMCs are infected by small ruminant 1997). In addition, insufficiently purified lentiviruses in vivo. In experimentally antigens may lead to false positive results, infected animals, PCR seems to be more making it necessary to check the speci- sensitive than serology, presumably ficity by using double-well ELISA kits because primers can be used that are per- which in turn increases the cost of serol- fectly complementary to the nucleotide ogy. sequences of the virus selected for infect- ing the animals (Vitu et al., 1997). Con- Synthetic oligopeptide assays, also results of cases of natu- called and versely, reported third-generation assays (Kwang infected animals indicated that direct Torres, 1994), have been developed, with rally PCR on when with immunodominant epitopes of the TM PBMCs, compared serological tests, might fail in a number gp44, or using a recombinant p25 com- of seropositive animals (Rimstad et al., bined with a peptide from the envelope 1993; Zanoni et al., 1996; Vitu et al., protein. In view of the cost of peptide tech- 1997). Moreover, direct PCR yields a pos- nology, the future use of these tests in itive result only in animals exhibiting a MVV eradication programmes will depend high virus load (Brodie et al., 1992). on their sensitivity and specificity. Attempts at PCR diagnosis of milk, 6.1.3. New developments: based on DNA and RNA extraction, detection of nucleic acids showed mixed results (Rimstad et a]., 1993; Barlough et al., 1994; Zanoni et al., Seroconversion may take a long time, 1996; Vitu et al., 1997; Leroux et al., and some infected animals may indeed 1997). In addition to the presence of an intermittent fail to develop a detectable antibody inhibitory contaminants, in milk is response. Moreover, as serologically pos- shedding of virus-infected cells itive animals may transiently become neg- suggested, which would limit the appli- ative, novel diagnostic methods are called cation of PCR in field conditions (Vitu et for to detect the presence of viral compo- al., 1997). nents in cells or tissues. PCR diagnosis has been improved by In recent years, the PCR, known for its an array of degenerate primer sets and by high sensitivity, has been applied to the nested, semi-nested and double-nested diagnosis of lentivirus infection in sheep procedures (Leroux et al., 1995; Barlough and goats to detect DNA and RNA in et al., 1994; Vitu et al., 1997; Leroux et peripheral blood and tissues. Preliminary al., 1997). Southern-blot hybridization reports have demonstrated proviral DNA with suitable probes is more sensitive than in cultured cells within 24 h post infec- ethidium bromide staining (Rimstad et al., tion (Zanoni et al., 1990b). These reports 1993; Russo et al., 1997), and significantly have also stressed the importance of select- increases the sensitivity and specificity, ing primers that take into account that dif- especially when samples are analysed ferent regions of the viral genome differ in without prior in vitro culture of cells. the degree of heterogeneity. In most stud- Results obtained with PCR techniques ies, conserved regions of LTR, and of gag indicated that seroconversion can be or pol genes were targeted for PCR delayed for several months following nat- (Zanoni et al., 1990a, b). PCR can be used ural infection with CAEV (Rimstad et al., 1993). However, the PCR test remains goat synovial membrane cells which also expensive and labour-intensive, and its support the replication of MVV (Johnson performance will have to be evaluated in et al., 1992). The cocultivation can also a greater number of field samples before it be performed with PBMCs or milk leu- can become the ’gold standard’ for detect- cocytes. Explant cultures of affected tissue ing MVV infection, which, theoretically, are carried out after necropsy. A cyto- should be possible because it can demon- pathic effect with formation of giant multi- strate the presence of a low number of tar- nucleated cells (syncytia) is expected after get sequences. 2-3 weeks, but some ovine lentivirus strains not be detected due to their The in situ PCR has been adapted to may failure to induce a clear effect amplify viral DNA in fixed cells, thus cytopathic or because the virus remains latent facilitating the identification of latent et al., In doubtful cases, infection of cells (Haase et al., 1990). In (Chebloune 1996). cell elec- situ hybridization, which detects MVV staining, immunocytochemistry, tron or RNA in cultured cells, has been reported to microscopy, reverse-transcriptase tests are (Mazarin et al., 1990). be as sensitive as PCR, and cocultivation performed studies using cells of latently infected seronegative sheep suggest that the infec- tion frequently remains undetected by 7. PREVENTION serological tests (Johnson et al., 1992). Combined with immunocytochemistry, in situ hybridization permits the simultaneous 7.1. Present methods detection of MVV antigens and RNA within the same cell (Roy et al., 1992). In In most countries, both MVV and situ is a hybridization complementary CAEV infections in sheep and goats are of classical but technique histopathology, currently controlled by an array of com- is more useful in experimental studies than plementary methods (Dion, 1991 ). Peri- as a tool. diagnostic odic serological tests using agar gel immunodiffusion (AGID) or ELISA rep- resent the standard method of detecting 6.2. Other diagnostic methods MVV-infected animals. For the eradica- tion of infection in the flocks, two differ- In cases of clinical disease, gross and ent strategies are adopted. Because, as microscopic post mortem pathology indi- described above, colostrum and milk are of cates the presence of histological alter- prime importance in the infection of new- ations in target organs, i.e. interstitial pneu- born lambs or kids, the lambs are removed monia with a predominant mononuclear from their infected mothers at birth and cellular infiltration (Mornex et al., 1994; raised in separate flocks. Colostrum fed Cador6 et al., 1996), presence of lymphoid to these lambs is heat-treated (56 °C for follicles in the udder parenchyma and adja- 60 min) and milk is pasteurized (Houw- cent to secretory ducts (Lujan et al., 1991) ers et al., 1983). Preferably, however, the associated with an increased number of T animals are fed colostrum and milk from lymphocytes and macrophages in milk cell certified MV-free ewes. As an alternative counts (Guiguen et al., 1992). to separating newborn lambs from their mothers, animals be Virus isolation is a delicate seropositive may technique: removed from the flock. tissue samples taken from an infected ani- mal must contain living cells for coculti- Although these methods have met with vation on sheep chorioid plexus cells or some success in several countries, these control and eradication programmes have The protective role these mutants play can a number of limitations, such as insuffi- be determined after either conventional or cient sensitivity and specificity of sero- DNA vaccination (Perk et al., 1996; Har- logical tests, relative importance of the mache et al., 1996b). Bacterial or viral other modes of transmission or resistance vectors to produce recombinant vaccines to culling seropositive animals particu- have turned out to be another promising larly in flocks raised for commercial milk approach (Cardenas and Clements, 1992; production. Bourgogne et al., 1996; Xu et al., 1997). The risk of resistant MVV strains and the cost involved have pre- emerging 7.3. Other control methods vented the development of chemotherapy for treating small ruminant lentivirus infec- The selection of with a natural tions. The only use of antiviral drugs in sheep resistance to MVV offer another maedi-visna infection is to provide a may model for in vivo testing of candidate anti- approach to controlling MVV. A few stud- ies have a natural resis- HIV drugs (Thormar et al., 1995). reported possible tance to the disease in some breeds (Perk et al., 1996). A high seroprevalence has been demonstrated in the Border 7.2. Vaccination Texel, Leicester and Finnish Landrace breeds (Vorster et al., 1996; Zink and Johnson, Not vaccines for MVV surprisingly, 1994), as opposed to the Ile-de-France are not available due to the currently breed (Houwers et al., 1989). It seems that formidable difficulties the presented by there exists a genetic susceptibility to the of the interaction between the biology disease rather than a susceptibility to the lentivirus and its host, varia- e.g. genetic infection per se (Zink and Johnson, 1994). tion of the the mechanisms virus, complex The most promising strategy would be to of and viral immunoprotection persistance learn more about the immune mechanisms et Montelaro et (Pearson al., 1989; al., involved in natural resistance to the dis- These obstacles to vaccine devel- 1989). ease at the level of the breed as a whole the of opment highlight importance pre- and at that of the individual animal. Breed- vention. ing transgenically resistant sheep, even if Despite the prospects of developing an theoretically possible (Clements et al., effective vaccine against MVV and CAEV 1994), appears, in practice, not to be an in sheep and goats remain poor (Phelps achievable goal. and Smith, 1993), a number of research groups continue to defend the feasability of a search for such a vaccine (Pearson et 8. CONCLUSION al., 1989; Cheevers et al., 1994; Perk et al., 1996; Harmache et al., 1996b), advo- In view of the impact of MVV on the cating different strategies. The use of intact farming sector of most countries, the dis- inactivated virions does not seem com- ease is classified in list B of diseases by the mendable due to inefficacy and a debat- Office International des Epizooties (Daw- able risk of such a product inducing even son et al., 1996). A continuation of more severe symptoms and lesions research is therefore called for in order to (McGuire et al., 1986; Russo et al., 1993). increase our knowledge. Inter alia, the In contrast, the use of attenuated viruses genetic variability of ovine and caprine obtained by deletion of selected genes, i.e. lentiviruses in both natural and experi- vif, tat and dUTPase, looks promising. mental infections must be analysed with a view to developing tools permitting more Andresson O.S., Elser J.E., Tobin G.J., Greenwood refined methods of lentiviral J.D., Gonda M.A., Georgsson G., Andresdottir diagnosing V., Benediktsdottir E., Carlsdottir H.M.. infections. Farmers and veterinary author- Mantyla E.O., Rafnar B., Palsson P.A., Casey ities anxious to improve their MVV erad- J.W., Petursson G., Nucleotide sequence and of ication programmes would welcome such biological properties a pathogenic proviral molecular clone of neurovirulcnt visna virus, tools. Virology 193 (1993) 89-105. In addition, the viral determinants of Audoly G., Sauze N., Harkiss G.D., Vitu C., Russo virulence and the pathogenesis of ovine P., Qu6rat G., Suzan M., Vigne R., Identifica- tion and subcellular localization of the Q gene lentivirus in its natural host must be elu- product of visna virus, Virology 189 (1992) cidated, not least to improve our under- 734-739. standing of the disease mechanisms under- Barlough J., East N.E., Rowe J.D., Vanhoosear K., lying HIV, particularly macrophage-tropic DeRock E., Bigornia L., Rimstad E., Double- nested chain-reaction for detection HIV. In this context, the determination of polymerase of caprine arthritis-encephalitis virus proviral cell receptors for MVV infection as has DNA in blood, milk, and tissues of infected recently been achieved in the case of HIV, goats, J. Virol. Methods 50 (1994) 101-1 l3. SIV and FIV (Willet et al., 1997), would Bertoni G., Zahno M.L., Zanoni R.G., Vogt H.R., constitute a major scientific advance. Peterhans E., Ruff G., Cheevers W.P., Sonigo P., Pancino G., Antibody reactivity to the Ultimately, vaccination strategies need immunodominant epitopes of the caprine arthri- virus transmembrane to be in order to extend the tis-encephalitis gp38 pro- developed tein associates with the development of arthri- armamentarium available to control maedi tis, J. Virol. 68 (1994) 7139-7147. visna in sheep. Bird P., Blacklaws B.A., Reyburn H.T., Allen D., Hopkins J., Sargan D.R., McConnell I., Early events in immune evasion by the lentivirus Maedi-Visna occurring within infected lym- ACKNOWLEDGEMENTS phoid tissue, J. Virol. 67 (1993) 5187-5197. Bird P., Reyburn H.T., Blacklaws B.A., Allen D., This review article is part of the COST Nettleton P., Yirrell D.L., Watt N.J., Sargan action no. 834 on ’Lentiviruses of sheep and D.R., McConnell I., The restricted IgGI anti- goats: pathogenesis, diagnosis and prevention’. body response to maedi-visna virus is seen fol- infection but not immuniza- This work (MP, CV, PR, JFM) was in lowing following part sup- tion with recombinant GAG Clin. from the ANRS. EP was protein, Exp. ported by grants sup- Immunol. 102 ( 1995) 274-280. ported by Swiss National Science Foundation 3100-09733.93/ and 3139-04l 859.94. Blacklaws B.A., Bird P., Allen D., McConnell L, grants T The authors wish to thank Ms R. Parham for Circulating cytotoxic lymphocyte precursors in maedi-visna virus-infected sheep, J. Gen. linguistic improvements. Virol.75 (1994) 1589-1596. 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