Ila A. Davis and BarryT. Rouse.

Department of Microbiology, University of Tennessee, Knoxville, Tennessee, 37996

Received 11/25/ 97 Accepted 11/28/97

3. EVASION OF INNATE DEFENSE MECHANISMS

Most invading viruses have a window of opportunity to establish infection before the adaptive immune response becomes effective. In the case of HV, this is ample time for them to secure permanent residence in a state often refractory to subsequent immune recognition (3). To counteract initial infection, the host possesses an array of innate defenses which could minimize replication. Among the leading innate defenses are natural killer (NK) cells, macrophages, and several humoral molecules, particularly the cytokines and the complement cascade. Evading the action of one or more of these activities may ensure the success of an invading virus. Specifically, animals with genetic defects of NK cells or which have been artificially depleted of NK cells are more susceptible to murine cytomegalovirus (MCMV) and HSV (reviewed in 4). Recently cytomegalovirus (CMV), an infection also affected by NK cell function (5), was shown to encode a gene product which counteracted NK cell destruction of virus infected cells in vitro. The gene product, UL18 in the case of human CMV and m144 for murine CMV, encodes a protein which acts as a molecular mimic of MHC Class I (6,7). The presence of this 'decoy' MHC I molecule essentially instructs NK cells to not kill thus protecting the virally infected cell from the consequences of viral induced MHC Class I down regulation. Infection of immune intact and NK cell depleted mice with MCMV m144 negative mutants gives convincing evidence that this viral decoy protein indeed plays a role in virulence since the mutant virus showed substantially increased replication in NK cell depleted mice (6). However, it is not yet certain whether UL18 defends effectively against NK cell killing during initial infection in vivo or whether this gene product is the reason patent infection can be established and maintained in humans.

NK cells effect the innate immune response in two ways; by killing infected cells directly or by generating humoral factors such as interferons (8). The interferons protect uninfected cells and modulate the protective actions of other components of immune defense. Blocking the activity of interferon represents a promising evasion strategy since it should both increase the pool of susceptible target cells as well as disarm potential antiviral effectors. Some viruses do possess homologues of interferon receptors providing them the ability to tie up and impede the function of interferons. Many orthopoxviruses (myxoma virus, vaccinia virus, cow- and rabbitpox viruses) have receptor mimics not only for interferons alpha, beta and gamma but also for other cytokines such as interleukin (IL)-1 and tumor necrosis factor (TNF)-alpha (9). Additionally, the vaccinia virus E3L gene product, through its binding of double stranded RNA, effectively inhibits the interferon (IFN)- induced intracellular signaling cascade which follows receptor triggering (10). Studies on poxvirus mutants in vivo make a strong case that the possession of these receptor mimics accord higher virulence. In particular, mice infected intranasally with a B18R (type 1 IFN receptor mimic) deleted vaccinia virus showed no signs of illness whereas mice infected with either wild-type or B18R rescued virus almost uniformly died (11, 12, 13). Indeed, the poxvirus cytokine receptor story provides the best evidence in vivo supporting the immune evasion hypothesis. Curiously, it appears that poxviruses engage almost in overkill since they have multiple potential strategies (table 1) seemingly designed to overcome host resistance. It is peculiar, therefor, that poxviruses have not adopted a lifestyle of in vivo persistence but survive in the environment outside their vertebrate hosts.

Viral interference of host chemokine function may prove to be another useful viral evasion strategy. Kaposi's sarcoma-associated herpesvirus (KSHV or human herpesvirus 8, HHV-8) encodes two viral proteins from open reading frames (ORFs) K4 and K6. These have sequential homology to the human beta-chemokines MIP-1 alpha, MIP-1 beta and RANTES (14). The beta- chemokines prevent HIV-1 entry to susceptible CD4+ cells by binding to the surface CCR5 chemokine receptor which is also the HIV-1 coreceptor. The viral MIP (vMIP) displays in vitro functional similarity to the human chemokines by reducing HIV entry to HHV-8 infected cells. While it is interesting to speculate that HHV-8 may protect against infection with HIV, the significance of this interaction in vivo is currently merely speculative.

One of the most potent protective elements of the innate defense system is the complement cascade. Although perhaps most useful for protection against bacteria, the complement system also generates components which serve to destroy invading viruses. Complement activation either succeeds in disrupting viruses or virus infected cells directly or by stimulating the antiviral cells and molecules of the inflammatory response to achieve the same purpose. Some viruses such as HSV and herpesvirus saimiri (HVS) appear to have adopted mechanisms of evading complement mediated activities. HVS encodes a protein homologous to the complement regulatory protein, decay-accelerating factor (DAF). This viral protein is called complement control protein homologue (CCPH). It may be either membrane bound on or secreted from infected cells and potentially binds host complement components C3b and C4b thus minimizing the membrane destructive effects to infected cells (15). HVS encodes a second gene, HVS-A15, with significant homology to the terminal complement control glycoprotein, CD59. In vitro studies have shown the HVS CD59 homologue prevents deposition or action of the membrane attack complex on the surface of infected cells (16). However, whether these complement evasion strategies of HVS play any role in vivo remains to be determined.

HSV binds via its protein gC-1 to one of the pivotal components of complement, C3b, which protects cell-free virus from complement lysis (17). Evidence for this comes from observations that gC deleted viral strains are more susceptible than are wild-type viruses to complement mediated neutralization in vitro and, similar to all HSV mutants, less virulent in vivo. Whether the necessity to use gC in vivo to bypass complement activation explains the fact that gC minus mutants are rarely found in nature (18) requires further investigation.