Perspectives Mechanisms of Viral Pathogenesis Distinct Forms of Reoviruses and Their Roles during Replication in Cells and Host Max L. Nibert,** Deirdre B. Furlong,* and Bernard N. Fields*"1 *Department ofMicrobiology and Molecular Genetics and OShipley Institute ofMedicine, Harvard Medical School, and Departments oftPathology and I1Medicine, Brigham and Women's Hospital, Boston, Massachusetts 02115 Recent years have seen a resurgent interest in mechanisms by mal viruses infect complex host organisms in the natural world, which viruses cause disease, or viral pathogenesis. Studies of however, considerations beyond the level of the cell are needed viral pathogenesis often address the capacity of a virus to cause for identifying other steps in viral replication. In this perspec- injuries to cells and tissues; yet, injuries are secondary conse- tive we wish to emphasize that viral growth in the host is a quences of many fundamental interactions that occur between complex process, involving multiple cycles of replication virus and single cells, as well as between virus and host organ- within cells as well as other uniquely host-level steps (illustrated ism, during infection. For example the tropism ofa virus, or its for reoviruses in Fig. 2). We therefore refer to the complete capacity to grow within particular cells in the host, reflects sev- process as the replication program of an animal virus. eral of these interactions. Studies of more basic aspects of viral Features specific to viral replication in the host have been infection are therefore critical for understanding viral pathogen- reviewed elsewhere (5, 6). Briefly, viruses infect their hosts in esis. We begin this perspective by introducing some general three general patterns: local, "local + systemic," and systemic concepts in regard to the replication and structure of animal infections (5). Local infection refers to growth of virus near its viruses. site of entry into the host, and systemic infection refers to Infections with mammalian reoviruses provide useful mod- growth at sites distant from the entry site. The pattern exhib- els for characterizing mechanisms of viral pathogenesis (re- ited by reoviruses depends on numerous factors, but in infec- viewed in references 1 and 2). Mammalian reoviruses (reovi- tions by the enteric route, reoviruses often exhibit local + sys- ruses for short) are medium-sized nonenveloped viruses that temic infection (7, 8). Host-level steps in viral replication in- have a genome consisting of 10 segments of double-stranded clude entry into the host, primary replication, spread to distant (ds)' RNA. In humans, reoviruses are not linked definitively sites, and secondary replication (6) (Fig. 2). Viral tropism is with any disease more severe than a mild enteric or respiratory reflected in both primary and secondary replication but is often illness, but in mice and rats they cause a number of disease discussed in regard to secondary replication because at that syndromes involving major organs like brain (3) and heart (4). step virus grows in only some ofthe many cells it encounters in Studies with reoviruses demonstrate that facts obtained at mul- the host (Fig. 2). tiple levels of inquiry can be integrated in the attempt to under- Two additional steps that can be identified as essential to stand viral pathogenesis (1, 2). In the major part ofthis perspec- viral replication in the host are release of virus from the in- tive, we describe how the structure of reoviruses relates to their fected host and survival in the environment between hosts (9) strategy for replication in cells and host organisms and discuss (Fig. 2). These steps highlight the fact that a virus must be several similarities in this regard between reoviruses and other capable of spreading between hosts to survive in the natural animal viruses. world. Survival and spread between hosts are formally similar to survival and spread between sites of replication within the Some general concepts regarding the replication and host: outside the host, virus must contend with the barrier of structure ofanimal viruses distance and physicochemical challenges like drying and radia- Defining the replication program of an animal virus. Animal tion; inside the host, there are anatomical barriers and chal- viruses proceed through an orderly series ofsteps when replicat- lenges like those imposed by the host immune response. The ing within cells (illustrated for reoviruses in Fig. 1). These steps relevance of studying viral survival in the face of specific chal- are said to define the replication cycle of a virus. Because ani- lenges is not immediately apparent when thinking only about viral replication in isolated cells. A schematic description ofthe structure ofanimal viruses. Address correspondence to Dr. Bernard N. Fields, Dept. of Microbiol- The design of a viral particle is simple, but it permits the virus ogy and Molecular Genetics, Harvard Medical School, 260 Longwood to participate in a series of complex interactions with cells and Ave., Boston, MA 02115. hosts. Although it is easy to recognize that viral structure has Receivedfor publication 12 March 1991. been molded by requirements for growth in cells, it is likely that distinct requirements for growth in the host have also contrib- 1. Abbreviations used in this paper: ds, double-stranded; ISVP, inter- uted selective viral The mediate (or infections subviral particle; M cell, microfold (or mem- pressures determining structures. latter branous) cell. pressures include, but are not limited to, those posed by the host response. J. Clin. Invest. We prefer to consider that the structural components with © The American Society for Clinical Investigation, Inc. which mature viral particles are built fall into two functional 0021-9738/91/09/0727/08 $2.00 categories: the delivery system and thepayload(Fig. 3). Compo- Volume 88, September 1991, 727-734 nents of the delivery system initiate the replication cycle by Structure and Replication ofReoviruses 727 STEP FORM PROTEIN STEP FUNCTION FORM PROTEIN (GENE) (GENE) -_. Attachment Virions or ISVPs al (Si) n F- Environment Stability Virions cr3 a 'I (S4) . Penetration ISVPs Iz1 (M2) i and Uncoating Entry into host Activation Virions l. al and Targetting (L2,(2 M2, Sl) e in GI tract lSVPs Transcription Cores )2. M3 on -0 I (L2. Li) Primary Multiplication ISVPs Multiple E c0 replication (Gl) t e< 0 Cores Translation, Assembly Multiple 0 Assembly, and Intermediates 01 Genome replication Spread Pathway Virions? al s and Stability ISVPs? (S1) 0 -Rlase Virions Tropism Targettlng to Virions? ol distant sites ISVPs? (Si) 0 Figure 1. Replication of reoviruses in cells (replication cycle). Indi- I cated are identifiable steps in the replication of reoviruses in cells, Secondary Multiplication ISVPs Multiple Is replication (targets) t 0 the particular viral forms (virions, ISVPs, and cores) that are asso- Cores ciated with these steps, and individual viral proteins that have specific 0. functions during these steps. The al protein is the cell-attachment Figure 2. Replication of reoviruses in the host (replication pro- protein that binds viral particles to specific receptors on the cell sur- gram). This figure parallels Fig. 1. Indicated are steps in the rep- face. The gsl protein is thought to mediate the penetration of cell membranes by viral particles, and its removal from particles permits lication of reoviruses in the host after infection by the enteric route and a description of viral functions associated with these activation of the viral transcriptase. The X2 and X3 proteins are steps. Also indicated are the particular viral forms associated known components of the viral transcriptase apparatus: X2 is the with the steps and individual viral proteins that have been indi- guanylyltransferase and X3 is a catalytic component of the RNA-de- cated by genetic and biochemical studies to play specific roles pendent RNA polymerase. Attachment and penetration are effected during replication in the host (described in the text). Replication by the delivery system contained in both virions and ISVPs as indi- in the host is schematized as consisting of two rounds of repli- cated at right. Cores are proposed to represent the payload of reovir- cation: primary replication associated with the portal of entry uses, whose immediate function after penetration consists of tran- into the host tract in this case) and secondary scribing the viral mRNAs. (gastrointestinal replication at distant sites. Both rounds of replication have functions of the viral delivery system and viral payload asso- ciated with them as indicated at right. An important aspect of delivering the payload to an intracellular site (cytoplasm or replication in the host are delivery functions that precede at- nucleus) within cells appropriate for replication; components tachment to the cells in which viral multiplication occurs. ofthe payload then participate in the intracellular steps of repli- cation. As might be predicted from the order of their functions, the delivery system is located more externally in viral particles though payload components act intracellularly, considerations and the payload more internally. Thus, the basic organization of replication in the host are important for understanding their of a viral particle has a simple logic (Fig. 3). functions. For example, nonspecific host-response proteins The two best known delivery functions are attachment of (e.g., interferon) can stimulate cellular activities that affect in- virus to