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Skin Diseases Associated with , 7, and 8 Infection

Andrew Blauvelt Branch, National Cancer Institute, Bethesda, Maryland, U.S.A.

Relatively recently, the discovery and analysis of now believed to be the long sought after etiologic three new human herpesviruses, human herpesvirus agent of Kaposi's sarcoma. The evidence for these (HHV)-6, HHV-7, and Kaposi's sarcoma-associated skin disease associations and key ®ndings from recent herpesvirus (KSHV), also known as HHV-8, has basic science investigations on viral pathogenesis are contributed greatly to our understanding of the discussed in this review. In addition, possible thera- pathogenesis of several common dermatoses. HHV-6 peutic implications of these research studies are and HHV-7 are closely related b-herpesviruses that explored. Key words: rosea/KSHV/Kaposi's have been linked with roseola (mostly HHV-6), sarcoma/pathogenesis. Journal of Investigative Dermatology severe drug eruptions (HHV-6), and pityriasis rosea Symposium Proceedings 6:197±202, 2001 (mostly HHV-7). KSHV is a g-herpesvirus that is

he family of human herpesviruses contains eight HHV-7 was discovered in 1990 by June and colleagues members and is subdivided into three subfamilies (Frenkel et al, 1990). Serendipitously, these investigators noted (Table I). These include the a-herpesviruses (herpes cytopathic effects in cultures of activated CD4+ T cells isolated simplex types 1 and 2 and varicella-zoster virus), from a healthy individual. This ®nding led to isolation of Tthe b-herpesviruses [cytomegalovirus (CMV), human HHV-7 and further ultrastructural and genetic characterization. herpesvirus (HHV)-6, and HHV-7], and the g-herpesviruses Of note, the initials of this healthy individual were R.K., and [Epstein-Barr virus (EBV) and Kaposi's sarcoma-associated herpes- designation of HHV-7 as RK virus, or an RK HHV-7 strain, is virus (KSHV), also known as HHV-8]. These subfamilies are based sometimes mentioned in the virology literature. The genomes upon similarities in viral genomes and biologic behavior. For of HHV-7 and both variants of HHV-6 are closely related, example, the a-herpesviruses all cause blistering skin diseases, with 20%±75% nucleic acid homology depending on the genes whereas the g-herpesviruses are distinguished by their potential to being compared (Braun et al, 1997; Black and Pellett, 1999). induce cellular proliferation and malignancy. In this review, I will focus on the skin diseases caused by the three most recently Epidemiology and disease associations HHV-6 and HHV- described human herpesviruses, HHV-6, HHV-7, and KSHV. 7 are both ubiquitous herpesviruses. Most commonly, HHV-6 Because of their marked similarities, HHV-6 and HHV-7 will be infects infants between the ages of 3 and 6 mo as protection discussed together. from maternal antibodies wanes (Okuno et al, 1989). By the age of 3 y, 90% of the U.S. population has been infected with HHV-6 (Braun et al, 1997). Respiratory transmission among HHV-6 AND HHV-7 persons with close contact with one another is the major route of primary infection. Historical aspects HHV-6 was discovered by Gallo and The peak age of initial infection for HHV-7 is slightly later than colleagues in 1986 (Salahuddin et al, 1986). It was isolated from that for HHV-6; the most common age range for infection is the peripheral blood lymphocytes of six individuals with 18 mo to 3 y of age (Wyatt et al, 1991). By age 5 y, 90% of the lymphoproliferative disorders. HHV-6 was named human B- U.S. population demonstrates evidence of HHV-7 infection (Black lymphotropic virus, or HBLV, because it was initially found within and Pellett, 1999). Similar to HHV-6, HHV-7 is transmitted B cells of infected individuals; however, subsequent work revealed among close contacts via the respiratory route. that CD4+ T cells were the major cell type infected by HHV-6 Primary infection with HHV-6 can either be asymptomatic, (Lusso et al, 1988; Takahashi et al, 1989). In 1991, two variants of cause roseola (Yamanishi et al, 1988), or be associated with an HHV-6 were differentiated from one another, HHV-6A and unspeci®ed febrile illness of childhood (Chiu et al, 1998). HHV-6B HHV-6B (Schirmer et al, 1991). is the particular subtype associated with roseola (Schirmer et al, 1991). Children with roseola typically present with high , Manuscript received March 20, 2001; revised June 25, 2001; accepted usually without obvious signs of upper respiratory infection. The for publication June 25, 2001. onset of the classically appears on the third day of fever, often Reprint requests to: Dr. Andrew Blauvelt, Dermatology Branch, coinciding with resolution of the fever. The rash of roseola is National Cancer Institute, Building 10/Room 12 N238, 10 Center Dr MSC 1908; Bethesda, MD 20892±1908. Email: [email protected] characterized by coin-sized erythematous macules or slightly Abbreviations: CMV, cytomegalovirus; EBV, Epstein-Barr virus; HHV, elevated on the head and neck (Fig 1A). Primary infection human herpesvirus; KS, Kaposi's sarcoma; KSHV, Kaposi's sarcoma- with HHV-7 is most often asymptomatic; however, it can also associated herpesvirus; PR, pityriasis rosea. cause a roseola-like illness in patients (Tanaka et al, 1994).

1087-0024/01/$15.00 ´ Copyright # 2001 by The Society for Investigative Dermatology, Inc. 197 198 BLAUVELT JID SYMPOSIUM PROCEEDINGS

Table I. Classi®cation of known human herpesviruses

Subfamily Herpesvirus type Distinguishing features a 1 Variable host range, relatively short reproductive cycle, rapid spread in culture, ef®cient destruction of infected cells, Herpes simplex 2 capacity to establish latent infections primarily (but not exclusively) in sensory ganglia Varicella-zoster virus b Cytomegalovirus Restricted host range, relatively long reproductive cycle, slow growth in culture, infected cells become enlarged, HHV-6 carrier cultures easily established HHV-7 g Epstein-Barr virus Restricted host range, replicate in lymphoblastoid cells and sometimes in epithelioid and ®broblastic cells, latent KSHV (HHV-8) virus frequently detected in l ymphoid tissue, associations with malignancies

Figure 1. Clinical examples of the skin diseases associated with HHV-6, -7, and -8 infection. (A) Roseola, most often associated with HHV-6 primary infection. (B) PR, purported to be associated with reactivation of HHV-7. (C) KS, characterized by tumor spindle cells latently infected with KSHV, also known as HHV-8.

Reactivation of HHV-6 and HHV-7 has been reported in cell-free serum, and saliva from most patients with PR.1 Plasma and immunocompromised individuals and can lead to widespread serum samples from healthy individuals were always negative. Viral multiorgan infection in these patients (Cone et al, 1993; Drobyski et localization studies are currently underway to help expand and al, 1993, 1994; Knox and Carrigan, 1994). Encephalitis, pneumo- understand the signi®cance of our PCR results. nitis, and hepatitis are most commonly reported. HHV-6A is the Recently, three patients have been reported with severe drug± subtype commonly reactivated in AIDS patients (Schirmer et al, induced hypersensitivity syndromes associated with systemic HHV- 1991); because it infects CD4+ T cells, HHV-6A has also been 6 reactivation (Suzuki et al, 1998; Tohyama et al, 1998). All three postulated as a cofactor in the progression of HIV disease (Lusso and had extensive exfoliative . Allopurinol and sulfasalazine Gallo, 1995). Interestingly, HHV-6 can clearly be activated from were identi®ed as the medications that triggered the eruptions. The latency by HHV-7 reactivation (Katsafanas et al, 1996; Tanaka- precise role of HHV-6 in these types of cases needs to be further Taya et al, 2000). The mechanism for this phenomenon is explored. unknown. Therapeutic implications and future research In 1997, Drago and colleagues reported on the detection of directions The HHV±6 disease associations with roseola in HHV-7 DNA in the tissue of patients with pityriasis rosea (PR) children and widespread organ involvement in immunosuppressed (Drago et al, 1997a, b), a common papulosquamous disease that individuals are well established. Unfortunately, acyclovir and its most often occurs in otherwise healthy young adults (Fig 1B). derivatives have little antiherpesviral activity against HHV-6 and Speci®cally, HHV-7 DNA was found in 12 of 12 fresh lesional skin HHV-7 (Takahashi et al, 1997). Foscarnet, ganciclovir, and , in 12 of 12 peripheral blood mononuclear cell (PBMC) cidofovir have some activity at high concentrations, but all of preparations, and in 12 of 12 cell-free serum samples isolated from these medications are of limited practical use because of the need to PR patients. By contrast, HHV-7 DNA was detected in no skin administer them intravenously or the potentially worrisome side- specimens, in 44% of PBMC, and in no sera from healthy effects. Thus, patients with active HHV-6 replication and roseola individuals. These investigators have also shown increased serum should not be treated with antiherpesviral therapy. These drugs, levels of IFN-a in PR patients, indicative of acute viral infection, however, should be considered in patients with end organ and viral cytopathic changes noted on culture of PR PBMC. In dysfunction or widespread systemic disease. At this time, patients summary, these data indicated that HHV-7 reactivation was with PR, whether the association with HHV-7 is con®rmed or occuring during PR. refuted, should also not be treated with antiherpesviral therapy. Subsequent to these initial reports, however, other research Further study is needed to determine the roles of herpesviruses in teams have not been successful in establishing a de®nitive link the etiologies of both PR and severe drug-induced hypersensitivity between PR and HHV-7 (Kempf et al, 1999; Watanabe et al, 1999; reactions. Kosuge et al, 2000). Speci®cally, in one study, no HHV-7 DNA was detected in 14 formalin-®xed paraf®n-embedded tissues from lesional skin of PR patients (Kempf et al, 1999). The other two KSHV reported studies to date suggested a possible association of HHV-7 Historical aspects Moritz Kaposi, a Hungarian born in a subset of PR patients; Watanabe et al (1999) detected HHV-7 dermatologist, ®rst described idiopathic multiple pigmented DNA in 47% of plasma samples from PR patients, whereas Kosuge sarcoma of the skin in 1872 (Fig 1C) (Kaposi, 1872). et al (2000) found evidence of HHV-7 antibody increases in a few Interestingly, Kaposi was ®rst to suggest a possible infectious (but not in most) PR patients studied. We have recently reported preliminary ®ndings in abstract form 1Watanabe et al. Pityriasis rosea (PR) is associated with reactivation of on the detection of both HHV-7 and HHV-6 DNA by sensitive both human herpesvirus (HHV)-7 and -6 in blood. J Invest Dermatol nested polymerase chain reaction (PCR) in skin, cell-free plasma, 114:784, 2000 (abstr.) VOL. 6, NO. 3 DECEMBER 2001 HHV-6, HHV-7, AND KSHV 199 etiology for Kaposi's sarcoma (KS). With the epidemiologic patients was infectious (Viera et al, 1997). The discrepancy in the evidence of KS clustering in characteristic populations and mode of KSHV transmission between developed and underdevel- clinical disease settings (especially as documented in the oped nations is similar to that observed with hepatitis A virus HIV+ gay community), the search for a possible infectious cause infection, and suggests that poor hygiene may increase the risk of of KS intensi®ed (Beral et al, 1990). Many organisms were acquiring KSHV. Finally, sequence differences among KSHV in purported to be the etiologic agent of KS (reviewed in Blauvelt, different parts of the world suggest the existence of several KSHV 1999), yet evidence for causality was subsequently not strains (Poole et al, 1999; Zong et al, 1999). It is theoretically substantiated. possible that different KSHV strains may have different routes of In December 1994, the husband and wife research team of Yuan transmission and clinical relevance. Further study is needed to Chang and Patrick Moore at Columbia University reported on the determine this issue as well as other speci®c variables involved in isolation of novel herpesviral DNA sequences in KS tissue, but not the transmission of KSHV and, in particular, studies are needed that in surrounding normal skin (Chang et al, 1994). The DNA was examine these factors in different parts of the world. detected in a KS genomic library using the powerful molecular In addition to KS, Chang and Moore initially detected KSHV biologic method of representational difference analysis, a technique DNA in three of 27 cases of AIDS-associated lymphomas (Chang et that combines features of subtractive hybridization and PCR al, 1994). On subsequent analysis, it was determined that these (Lisitsyn et al, 1993). Many other studies have now con®rmed the patients had primary effusion lymphoma (PEL), a rare disease presence of this virus within all clinical types of KS (reviewed in previously known as body cavity-based B cell lymphoma Blauvelt, 1999). Thus, KSHV is found within lesions of AIDS- (Cesarman et al, 1995). Patients present with collections of serous associated, classical, endemic, and iatrogenic KS, as well as in KS ¯uid, usually with no solid tumor mass, within the pleural, involving skin, lymph nodes, lungs, or the gastrointestinal tract. pericardial, or peritoneal cavities; the ¯uid contains malignant Because of this tight disease association, this new virus became B cells that are infected with KSHV and often dually infected with known as KSHV (currently the preferred term), although some EBV. The original report by Cesarman et al that de®nitively linked refer to it as HHV-8. KSHV with PEL was con®rmed by many other studies (Nador et al, 1996; Dupin et al, 1999). Importantly, several KSHV-infected B Epidemiology and disease associations The detection of cell lines have been isolated from patients with PEL and have antibodies speci®c for KSHV has yielded insight into the allowed investigators to study KSHV gene expression and the viral geographic distribution of this virus. Current assays show that life cycle in detail (Sarid et al, 1998; Jenner et al, 2001; Zoeteweij et approximately 95% of KS patients (all clinical types) are seropositive al, 2001). for KSHV (Gao et al, 1996; Kedes et al, 1996; Simpson et al, 1996). KSHV has also been linked with con®dence to the plasmablastic Interestingly, most studies have also shown relatively high levels of variant of Castleman's disease (CD), also called multicentric KSHV seropositivity in populations where KS is prevalent. For angiofollicular lymphoid hyperplasia (Soulier et al, 1995; Dupin et example, HIV+ gay men, sub-Saharan Africans, and individuals al, 2000). This variant of CD is typically (but not always) associated from southern Italy who have no clinical evidence of KS, yet all of with HIV disease. Clinically, patients with CD present with fever, whom represent ``at risk'' populations for KS, have a high lymphadenopathy, and splenomegaly, whereas histologic examin- incidence of KSHV seropositivity (approximately 30%±40%, ation of affected tissues shows polyclonal lymphoid hyperprolifera- 50%±60%, and 15%±30%, respectively) (Gao et al, 1996; Kedes et tion with vascular hyperplasia. al, 1996; Simpson et al, 1996). By contrast, speci®c individuals at Other than KS, PEL, and CD, there are a fairly large number of low risk of developing KS (e.g., HIV-infected women and children reports on the PCR detection of KSHV DNA in a wide variety of from the U.S.A.) have a low prevalence of KSHV-speci®c diseases (reviewed in Blauvelt, 1999). Some of these include antibodies (Simpson et al, 1996; Blauvelt et al, 1997; Kedes et al, angiosarcoma, a variety of lymphoproliferative disorders, basal cell 1997). Similarly, the prevalence of KSHV infection in the general carcinoma, vulgaris and foliaceus, multiple myeloma, adult population in most areas of the U.S.A. and western Europe is and sarcoidosis. Subsequently, however, many teams of independ- low (less than 5%) (Gao et al, 1996; Kedes et al, 1996; Simpson et al, ent investigators have been unable to substantiate the claims of 1996). these studies, adding strength to the conclusion that KSHV is Documented cases of primary KSHV infection are rare de®nitively linked to only three diseases: KS, PEL, and CD. This (Oksenhendler et al, 1998; Marcelin et al, 2000). These patients experience has proven that rigorous evaluation of possible future have been described as having an infectious mononucleosis-like KSHV disease associations is needed. Because of the great potential syndrome consisting of fever, lymphadenopathy, and splenomegaly. for false positive PCR results, it will be important to prove or No rash has yet been reported with primary KSHV infection. By disprove initial PCR reports by performing viral localization studies contrast to this description, Luppi et al recently reported primary within tissue (e.g., electron microscopy, in situ hybridization, KSHV infection following transplantation; two patients experi- immunostaining) and by examination of patients' sera for KSHV enced bone marrow failure within months after each receiving antibodies. transplanted kidneys from the same KSHV-infected donor (Luppi et al, 2000). These cases suggest that immunosuppression could alter Pathogenesis of KS Using the techniques of in situ PCR, in normal primary immune responses to KSHV and lead to severe situ hybridization, electron microscopy, and immunohisto- disease. chemistry, KSHV is localized to tumor spindle cells, lymphatic In regard to modes of transmission, epidemiologic studies clearly endothelial cells lining slit-like vascular spaces, and tumor- show that KSHV is a sexually transmitted disease in the gay in®ltrating leukocytes (Boshoff et al, 1995; Orenstein et al, 1997; community (Martin et al, 1998; O'Brien et al, 1999). Interestingly, Dupin et al, 1999). Ultrastructurally, the in®ltrating leukocytes KSHV has been detected within gastrointestinal mucosa of HIV- appear to be lymphocytes (Orenstein et al, 1997), although more positive individuals (Thomas et al, 1996), suggesting a possible basis studies will be required to identify the nature of these cells. The for fecal-oral or oral-fecal transmission of KSHV. There are also localization studies con®rmed the earlier PCR studies and added clear studies that suggest nonsexual routes of KSHV transmission further insight into the role that KSHV may be playing in the (e.g., via saliva), especially in African countries (Andreoni et al, pathogenesis of KS by identifying speci®c cellular targets for viral 1999; Rezza et al, 2000). In general, herpesviruses are frequently infection. detected in saliva of infected individuals. KSHV is no different, in Most studies have now established that KSHV can be found in that this virus has been readily detected in the saliva of most KS the blood of over half of all KS patients (Ambroziak et al, 1995; patients and in 15%±33% of HIV+ patients without KS (Koelle et Whitby et al, 1995). Speci®cally, virus has been localized to al, 1997; Pauk et al, 2000). Of possible relevance to transmission, circulating B cells (Ambroziak et al, 1995; Henry et al, 1999), one study demonstrated that KSHV isolated from saliva of KS although there are a few reports of KSHV within circulating 200 BLAUVELT JID SYMPOSIUM PROCEEDINGS

Table II. KSHV-encoded genes that may be involved in growth dysregulation or evasion of immune surveillance

ORFa Name Presumed function

K2b vIL-6 pro-in¯ammatory, block apoptosis K4 vMIP-II pro-in¯ammatory, angiogenic K6 vMIP-I pro-in¯ammatory, angiogenic K9 vIRF growth promotion, block interferon signaling K12 kaposinc growth promotion 16 vBcl-2 block apoptosis 71 vFLIPc block apoptosis 72 v-cyclinc growth promotion 73 LANAc tethers viral DNA to chromosome, growth promotion 74 GPCR growth promotion, angiogenic

aOpen reading frame, or gene. bKSHV genes that are not homologous with other herpesvirus genes are prece- ded with the letter K. c ``Latent genes'' expressed in the majority of tumor spindle cells.

Figure 2. Hypothetical scheme of KS pathogenesis.

with apoptotic pathways through Bcl-2 binding (Ojala et al, 2000). CD34+ cells and CD8+ T cells. Like EBV, B cells appear to be the Kaposin has been shown to have transforming capabilities, but its major latent reservoir cell for KSHV in blood. Interestingly, the mechanism of action is unknown (Muralidhar et al, 1998; ability to detect KSHV DNA in peripheral blood leukocytes from Muralidhar et al, 2000). Finally, vFLIP may be involved in individuals without KS is predictive for future development of KS preventing apoptosis within KS tumors (Thome et al, 1997). (within 2±4 y) (Whitby et al, 1995; Moore et al, 1996). Taken Identifying the precise functions of these potential viral oncopro- together, these data suggest that reactivation from latency is teins, and determining how they interact with one another, should occuring in peripheral blood B cells, perhaps leading to ``seeding'' lead to a more complete understanding of how KSHV stimulates of virus within dermal lymphatics (Fig 2). Multiple factors are cell growth. likely to be involved in regulating latent-to-lytic switching of Therapeutic implications and future research KSHV and eventual KS development in a given individual, some of directions Knowledge of the KSHV life cycle has important which include age, sex hormones, concomitant HIV disease, pro- therapeutic implications for KSHV-infected individuals. For in¯ammatory and angiogenic cytokines and growth factors, example, all known antiherpesviral medications, including immunosuppressive drugs, and perhaps other processes that impair acyclovir and its derivatives, foscarnet, cidofovir, and ganciclovir, cell-mediated immunity (Fig 2). Interestingly, we have recently act by blocking active viral replication. Because of this, they are provided evidence that tissue hypoxia (Davis et al, 2001) and genetic predisposition (Foster et al, 2000; Lehrnbecher et al, 2000) likely to have no signi®cant impact on existing KS lesions, where may also contribute to the onset of KS. Importantly, a complete the majority of tumor spindle cells are latently infected with KSHV. understanding of this variety of factors may lead to speci®c By contrast, these drugs may be expected to prevent onset of new prophylactic interventions in KSHV-infected individuals at risk for lesions, or perhaps prevent the onset of KS in those who are developing KS. infected with KSHV. Indeed, several retrospective analyzes suggest Like EBV, KSHV belongs to the g-herpesvirus subfamily. These that HIV+ patients who have taken foscarnet or ganciclovir have a herpesviruses share sequence homology and the ability to infect and lower incidence of KS when compared with those who have not transform lymphocytes (Table I). Although the EBV genes taken these drugs (Costagliola and Mary-Krause, 1995; Jones et al, involved in cellular transformation are well known, there are no 1995). Past use of acyclovir was not protective for future homologous genes within the KSHV genome (Russo et al, 1996). development of KS. Interestingly, but not surprisingly, these Instead, KSHV, which contains over 80 open reading frames, has epidemiologic observations have been bolstered by laboratory the striking feature of having many genes that encode for proteins reports showing that foscarnet, ganciclovir, and cidofovir, but not with interesting cellular homologs (Table II) (Russo et al, 1996); acyclovir, block lytic induction of KSHV in phorbol ester- however, most of these genes have expression patterns limited to stimulated PEL cell lines (Kedes and Ganem, 1997; Zoeteweij et active viral replication, or the lytic life cycle (Sarid et al, 1998; al, 1999). It is hoped that new treatments for KS will be designed to Jenner et al, 2001). The vast majority of KS tumor spindle cells are speci®cally target the antigens expressed by latently infected cells. latently (and not lytically) infected by KSHV (Orenstein et al, Cellular immune responses directed against KSHV have not been 1997). Thus far, LANA, v-cyclin, vFLIP, and kaposin are the only adequately studied as of yet (Osman et al, 1999; Brander et al, 2000). KSHV-encoded genes shown to be expressed in latently infected The fascinating observation that KS can develop during immuno- KS tumor spindle cells (Table II) (Boshoff et al, 1995; Davis et al, suppressive therapy, and regress following withdrawal of immuno- 1997; Dupin et al, 1999; Sturzl et al, 1999). These are the likely suppressive agents, strongly suggests an important role for cell- candidate KSHV oncogenes. Among several other functions mediated immunity in the control of KSHV-induced disease. This (Schwam et al, 2000; Renne et al, 2001), LANA has been shown is true for EBV, where loss of EBV-speci®c immunity is linked to to tether KSHV DNA to human chromosomes (Ballestas et al, the onset of EBV-associated disease (Rickinson and Moss, 1997). In 1999; Cotter and Robertson, 1999) and to interfere with HIV-infected individuals, the common occurrence of KS improve- functioning of both Rb (Radkov et al, 2000) and p53 (Friborg et ment in patients receiving highly active antiretroviral therapy is al, 1999), major tumor suppressor proteins. v-Cyclin has been likely due to improved cellular immunity, since these drugs have no shown to stimulate resting cells through the cell cycle by direct effect on KSHV replication (Kedes and Ganem, 1997; overcoming normal Rb-mediated suppression of the G1, or Zoeteweij et al, 1999). Delineating the determinants of KSHV- resting, phase of the cell cycle (Godden-Kent et al, 1997; speci®c cellular immunity should prove useful for both therapeutic Swanton et al, 1997); v-cyclin has also been shown to interfere and preventive purposes. For example, advances in this area may VOL. 6, NO. 3 DECEMBER 2001 HHV-6, HHV-7, AND KSHV 201 lead to strategies designed to boost KSHV-speci®c immunity in KS Costagliola D, Mary-Krause M: Can antiviral agents decrease the occurrence of patients, or be used to design vaccines to prevent KSHV infection. Kaposi's sarcoma? Lancet 346:578, 1995 Cotter MA, 2nd, Robertson ES: The latency-associated nuclear antigen tethers the Although much has been learned in a relatively short period of Kaposi's sarcoma-associated herpesvirus genome to host chromosomes in body time, much work remains to be done in order to delineate the cavity-based lymphoma cells. Virology 264:254±264, 1999 precise role of KSHV in the pathogenesis of KS and other diseases. Davis MA, Sturzl M, Blasig C, et al: Expression of human herpesvirus 8-encoded In vitro infection of normal cells with KSHV has proven dif®cult cyclin D in Kaposi's sarcoma spindle cells. J Natl Cancer Inst 89:1868±1874, 1997 (Renne et al, 1998). There is, as of yet, no robust system for Davis DA, Rinderknecht AS, Zoeteweij JP, et al: Hypoxia induces lytic replication of studying de novo KSHV infection and the functional effects Kaposi's sarcoma-associated herpesvirus. Blood 97:3244±3250, 2001 induced by infection, although several investigators have reported Desrosiers RC, Sasseville VG, Czajak SC, et al: A herpesvirus of rhesus monkeys success in infecting endothelial cells, either in the absence (Flore et related to the human Kaposi's sarcoma-associated herpesvirus. J Virol 71:9764± 9769, 1997 al, 1998) or in the presence of other (Cannon et al, 2000) or Drago F, Ranieri E, Malaguti F, Battifoglio ML, Losi E, Rebora A: Human viral proteins (Moses et al, 1999). No good animal model for KS herpesvirus 7 in patients with pityriasis rosea. Electron microscopy exists, i.e., one that incorporates KSHV infection. Injection of PEL investigations and polymerase chain reaction in mononuclear cells, plasma cells into immunode®cient mice leads to lymphoma-like tumor and skin [see comments]. Dermatology 195:374±378, 1997a Drago F, Ranieri E, Malaguti F, Losi E, Rebora A: Human herpesvirus 7 in pityriasis formation; however, no transfer of KSHV infection to murine cells rosea [letter]. Lancet 349:1367±1368, 1997b occurs (Picchio et al, 1997; Boshoff et al, 1998). Interestingly, Drobyski WR, Dunne WM, Burd EM, et al: Human herpesvirus-6 (HHV-6) KSHV-related viruses have recently been identi®ed in nonhuman infection in allogeneic bone marrow transplant recipients: evidence of a primates, with one report linking viral infection with the KS-like marrow suppressive role for HHV-6 in vivo. 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