Antigen processing and recognition

Cell, Vol. 71, 5-7.

October

2, 1992, Copyright

0 1992 by Cell Press

Virus Proteins That Counteract Host immune Defenses

Minireview

Linda R. Gooding Department of Microbiology and Immunology Emory University School of Medicine Atlanta, Georgia 30322

virus (Bhat and Thimmappaya, 1993) and human immunodeficiency virus (Gunnery et al., 1990). The complement cascade functions both as an innate antiviral defense and, when activated by antibody, as an effector arm of the adaptive immune response. Activation of complement can lead to virus destruction. It directly mediates lysis or phagocytosis of free virus and virusinfected cells. Indirectly, activation of the complement cascade produces a variety of mediators of the inflammatory response. Strategies to control complement functions are frequently encountered among viruses. The major secreted protein of vaccinia virus, VCP, binds the C4b fragment of complement component C4; it inhibits complement fixation mediated by the classical pathway, but not by the antibody-independent alternative pathway (Kotwal et al., 1990). A protein of similar sequence has been identified in simian herpesvirus saimiri (Albrecht and Fleckenstein, 1992). Inhibition of both alternative and classical pathway complement activation is mediated by glycoprotein C-l of herpes simplex viruses (Harris et al, 1990). Glycoprotein C-l binds the C3b fragment of complement component C3 and prevents both complement-mediated virus neutralization and cytolysis of virus-infected cells (Harris et al., 1990; McNearney et al., 1987). Herpes viruses have adopted an additional barrier to antibody-dependent complement-mediated destruction by encoding a pair of proteins, gE and gl, that bind the Fc region of IgG. The gE-gl membrane heterodimer not only prevents complement-mediated lysis of infected cells and enveloped virions, it may protect against Fc-facilitated phagocytosis as well (Bell et al., 1990). Cytokines and lymphokines are the hormones of the

Latent and/or persistent infections are a part of the lifestyles of many viruses. This capacity to maintain a longterm relationship with its host presupposes viral mechanisms for circumventing antiviral defenses. Until recently, relatively little was known about these mechanismsor their role in pathogenesis of infectious viruses. Insight into this arena is coming from the identification of viral gene products with unique functions that modify both the innate and the specific, adaptive arms of the host’s antiviral response. Characterization of these proteins is rapidly providing information on the molecular basis for viral pathogenesis. In addition, these proteins are potentially powerful biotherapeutics when selective modification of a portion of the immune system is desired. This review will briefly describe the viral proteins identified to date and the recurrent molecular themes observed among them (Table 1). Antiviral defense at the cellular level is mediated by interferons. lnterferons induce synthesis of the doublestranded RNA-activated inhibitor DAI, which, in the presence of double-stranded RNA, phosphorylates initiation factor elF-2 and prevents the initiation of translation. The earliest characterized viral countermeasure was the VA RNAs of human adenoviruses. VA RNAs block the interferon-induced autophosphorylation of DAI and hence overcome the antiviral effect of interferons (reviewed in Mathews and Shenk, 1991). More recently, inhibitors of DAI function have been identified in both Epstein-Barr

Table

1. Virus

Virus

Familv

Proteins

That Modulate

Immune

Virus

Herpesviruses

Retroviruses

system,

informing

migrating

lymphoid

Product

Host

IFN (a@) CTL TNF TNF TNF

Mathews Wold and Wold and Wold and Wold and

Shope fibroma/myxoma Vaccinia Vaccinia/cowpox cowpox

T2 VCP B15R crmA

TNF complement IL-1 IL-l

Smith et al., 1991 Kotwal et al., 1990 Spriggs et al., 1992; Alcami Ray et al., 1992

Epstein-Barr virus Epstein-Barr virus Cytomegakwirus Herpes simplex I/II Herpes simplex I/II Herpes saimiri

EBER RNA BCRFl UL18 gc-1 gE-gl CCPH

IFN (a@) cytokine synthesis CTL(?) complement antibody complement

Bhat and Thimmappaya, Hsu et al., 1990 Browne et al., 1990 Harris et al., 1990 Bell et al., 1990 Albrecht and Fleckenstein.

FeLV, HIV

p15E TAR

PK-C dep. responses IFN (alp)

Kadota et al., 1991 Gunnery et al., 1990

MuLV,

HTLV

cells

where

Responses

VA RNA E3-gp19K E3-14.7K E310.4/14.5K ElB-19K

Adenoviruses

Poxviruses

and Inflammatory

immune

Response

Reference

IFN, interferon% CTL, cytotoxic T lymphocytes; TNF, tumor necrosis factor; IL, interleukin; FeLV, virus: HTLV, human T cell leukemia virus; PK-C, protein kinase C; HIV, human immunodeficiency

and Shenk, Gooding, Gooding, Gooding, Gooding,

feline leukemia virus.

1991 1991 1981 1991 1991

virus;

and Smith,

1992

1983

1992

MuLV,

murine

leukemia

Cdl 6

to go, when to divide, and how to respond. Viruses have devised an array of strategies for interfering with cytokine functions. Human adenoviruses encode three products, E3-14.7K, E3-10.4W14.5K, and ElB-19K, that protect virus-infected cells from cytolysis by tumor necrosis factor (TNF) (Wold and Gooding, 1991). TNF is a multifunctional proinflammatory cytokine produced by activated macrophages in response to a wide variety of stimuli, including infectious viruses. Little is known about the mechanism of action of the adenovirus proteins, except that all three act at steps subsequent to TNF binding to its receptor. Similarly, poxviruses (notably Shope fibroma virus) encode a soluble TNF-binding protein T2 that competitively inhibits TNF binding to its cell surface receptors (Smith et al., 1991). Epstein-Barr virus takes an entirely different approach to the threat of cytokines by synthesizing a homolog of the cytokine synthesis inhibitory factor interleukin-10 (IL-lo) (Hsu et al., 1990). Both IL-10 and the Epstein-Barr virus protein BCRFl inhibit the synthesis of cytokines, such as TNF and interferon y, by activated macrophages and Thl helper T cell clones, thereby potentially serving to inhibit macrophage activation and the development of a delayed type hypersensitivity response in vivo (Vieira et al., 1991). Poxviruses produce an abundant, secreted IL-l-binding protein B15R that inhibits IL-1 functions in vitro (Alcami and Smith, 1992; Spriggs et al., 1992). IL-l, like TNF, is a pleiotropic cytokine that participates in a variety of both inflammatory and immune responses. In addition, cowpox virus encodes a serpin-like protease inhibitor crmA that prevents cleavage of IL-18 to its active form (Ray et al., 1992). Cytolytic T lymphocytes are critical to recovery from many virus infections. These cells appear to have evolved specifically to deal with intracellular parasites, such as viruses, by recognizing viral peptides carried to the cell surface by class I antigens of the major histocompatibility complex. Recognition by antiviral cytotoxic T lymphocytes leads to lysis of the virus-infected cell early in the replication cycle and prior to the release of infectious virus. Not surprisingly, there are an increasing number of examples of virus proteins that specifically interfere with cytotoxic T lymphocyte recognition. Human adenoviruses synthesize an integral membrane protein, E3-gp19K, that contains a C-terminal sequence that anchors it in the endoplasmic reticulum. E3-gp19K binds strongly to class I major histocompatibility antigens and prevents their translocation to the cell surface. As expected, E3-gpl9K prevents recognition and lysis of virus-infected cells by antiadenovirus cytotoxic T lymphocytes (Wold and Gooding, 1991). Human cytomegalovirus produces a class I homolog, UL18, that binds Ps-microglobulin, a subunit of class I molecules necessary for transport to the cell surface. Expression of UL18 prevents expression of cellular class I molecules on the cell surface (Browne et al., 1990); this is predicted to interfere with cytotoxic T lymphocyte recognition, although such interference,has not yet been demonstrated. There have been several reports of a decrease in class I expression following infection with other members of the herpesviruses and the poxvirus family, sug-

gesting that this is a common strategy among infectious viruses. The immunosuppression that accompanies some retroviral infections has been attributed, at least in part, to the transmembrane viral envelope protein p15E (Snyderman and Cianciolo, 1984). Determination of the mechanism of in vivo immunosuppression by p15E has been complicated by the wide variety of in vitro functions inhibited by it (e.g., IL-e-driven T cell proliferation, monocyte chemotaxis, and natural killer and B cell activation). A unifying thread is provided by the finding that a synthetic peptide derived from the pl5E sequence inhibits signal transduction via protein kinase C (Kadota et al., 1991) which is required for all these functions. It should be noted, however, that the mechanism underlying the profound immunosuppression seen in human immunodeficiency virusinfected individuals remains complex (see Meyaard et al., 1992) suggesting that human immunodeficiency virus may have evolved multiple tools for inhibiting immune responses. What is the value of these proteins to the virus? It is assumed here that there is indeed substantial value, because most viral genomes are restricted in size by the need to fit into a capsid, and hence viruses tend to be quite thrifty with their genetic material. The most obvious answer is that viruses require these defenses to replicate in a hostile environment created by the host immune system. If so, then one would predict that removing the viral gene of interest would result in a virus that replicates less well in an immunocompetent host. So far this has been investigated in only two systems, and the results are significantly and interestingly different. Considering the poxviruses as a group, mutant viruses lacking genes predicted to counter host defenses tend to be attenuated in vivo, although all replicate at wild-type levels in vitro. Deletion of either T2 from myxoma virus or VCP from vaccinia virus severely decreases replication in infected rabbits (Upton et al., 1991; lsaacs et al., 1992). Similarly, removal of the crmA gene from cowpox markedly increases the local inflammatory response and inhibits viral replication in the chick embryo chorioallantoic membrane (Ray et al., 1992). Spriggs et al. (1992) have found a moderate attenuation of vaccinia virus pathogenesis following intracranial injection of a mutant lacking B15R. Henceamong poxviruses, inhibition of host antiviral defenses by the virus leads to an increase in viral spread and increased virulence. The pathogenesis (e.g., tissue damage) induced by poxviruses is caused primarily by virus destruction of infected cells. In addition, poxviruses appear to produce only active, acute infections. In contrast, human adenoviruses form persistent and perhaps latent infections in which asymptomatic virus shedding can be observed years after primary infection. In this case, the consequences of deleting viral immunomodulating genes are quite different from those seen with poxviruses. Deletion of most of the early region 3 (E3) transcription unit, which specifies E3-gpl9K, E3-14.7K, and E3-10.4W14.5K, has no effect on virus replication in vivo in a rodent model of adenovirus pneumonia (Ginsberg et al., 1989). In contrast to the finding with poxvi-

Minireview 7

ruses, infection with the mutant adenovirus results in a marked increase in pathogenesis. In this model, and presumably in human adenoviral pneumonias, tissue destruction is caused primarily by the host inflammatory responses that the virus counters, perhaps as a strategy for establishment of persistence. For adenoviruses, the reservoir of virus in the population is probably the persistently infected individual. In sum, the impact of removing these antihost response genes on viral pathogenesis depends entirely on the pathogenic mechanism of the virus and its infection strategies, much like the outcome of a given response in the child’s game “paper, scissor, and stone” depends on what it’s up against. Therefore, study of these virus countermeasures will provide much information about the molecular mechanisms underlying viral pathogenesis and provide potential avenues for its control. Viewed from another perspective, the penchant of infectious viruses to regulate their environment may provide an unexpectedly valuable resource in other arenas as well. Viruses are experts on the immune system and the intracellular environment, and they have evolved unique mechanisms for their management. These mechanisms can be harnessed to provide reagents for unraveling complex biological processes. For example, virtually every known second messenger system is triggered by TNF binding to its receptors. Dozens to hundreds of cellular genes are activated, but the relative importance of any or all of these responses to the TNF-induced cytolytic process is almost completely unknown. The three adenovirus proteins that inhibit TNF-mediated lysis all have different subcellular locations and probably inhibit different steps in the cascade leading to cell death. Molecular analysis of their function will establish which of the many TNF-induced intracellular events are obligate for cell killing. In addition, it is likely that many more examples of virus countermeasures of the type listed in the table will be appearing in the near future. Many viruses have large numbers of what virologists term “nonessential” genes (i.e., genes that are not required for viral replication in vitro) that have not yet been characterized and are therefore candidates for immunomodulating functions. Thus, we can anticipate an expanding wealth of resources with which to study the immune system and the biology of the cell. References Albrecht. Alcami,

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