The rules for T-cell-mediated control of viruses that infect via the respiratory mucosae show both common themes and differences, depending on the nature of the pathogen. Virus-specific CD8+ cytotoxic T lymphocytes (CTLs) are the key effectors of virus clearance in mice infected with both negative strand RNA viruses (influenza and Sendai) and a DNA virus, the murine gamma-herpesvirus-68 (MHV-68). Recently completed experiments establish that these activated CD8+ T cells indeed operate primarily via contact-dependent lysis. Perforin-mediated cytotoxicity seems to be the preferred mode, though a Fas-based mechanism can apparently serve as an alternative mechanism. Immune CD4+ T cells functioning in the absence of the CD8+ subset cannot eliminate MHV-68 from lung epithelial cells, are somewhat less efficient than the CD8+ CTLs at clearing the RNA viruses, and are generally ineffectual in mice that lack B lymphocytes. Though cytokine secretion by CD4+ and CD8+ T cells in the virus-infected lung may promote both T-cell extravasation and macrophage activation, such processes are not alone sufficient to deal consistently with any of these infections. However, CD4+ T help is mandatory for an effective B-cell response, and can operate to promote the clonal expansion of virus-specific CD8+ T cells in the lymph nodes and spleen. Furthermore, a concurrent CD4+ T-cell response seems to be essential for maintaining continued CD8+ T-cell surveillance and effector capacity through the persistent, latent phase of MHV-68 infection in B cells. Thus, the evidence to date supports a very traditional view; CD8+ T cells function mainly as killers and the CD4+ T cells as helpers in these respiratory virus infections.

All gammaherpesviruses encode a virion glycoprotein positionally homologous to Epstein-Barr virus gp350. These glycoproteins are thought to be involved in cell binding, but little is known of the roles they might play in the whole viral replication cycle. We have analyzed the contribution of murine gammaherpesvirus 68 (MHV-68) gp150 to viral propagation in vitro and host colonization in vivo. MHV-68 lacking gp150 was viable and showed normal binding to fibroblasts and normal single-cycle lytic replication. Its capacity to infect glycosaminoglycan (GAG)-deficient CHO-K1 cells and NS0 and RAW264.7 cells, which express only low levels of GAGs, was paradoxically increased. However, gp150-deficient MHV-68 spread poorly through fibroblast monolayers, with reduced cell-free infectivity, consistent with a deficit in virus release. Electron microscopy showed gp150-deficient virions clustered on infected-cell plasma membranes. MHV-68-infected cells showed reduced surface GAG expression, suggesting that gp150 prevented virions from rebinding to infected cells after release by making MHV-68 infection GAG dependent. Surprisingly, gp150-deficient viruses showed only a transient lag in lytic replication in vivo and established normal levels of latency. Cell-to-cell virus spread and the proliferation of latently infected cells, for which gp150 was dispensable, therefore appeared to be the major route of virus propagation in an infected host.Herpesviruses are large, complex pathogens that use a range of different glycoproteins to spread between cells and between hosts (34). A common first step in infection is virion adsorption to cell surface glycosaminoglycans (GAGs), such as heparan sulfate, which are widespread on epithelial surfaces (5). Herpesvirus-GAG interactions are thought to concentrate virions on cell surfaces and so promote specific protein receptor binding, which is then followed by membrane fusion (32). Cell binding by purified virions and recombinant viral glycoproteins has been studied extensively in vitro, but relatively little is known about how individual herpesvirus glycoproteins contribute to infection of the natural host. Here simple fluid-filled spaces are rare, extracellular matrix is abundant, and viral spread is probably limited to defined anatomical pathways. A significant role for direct cell-to-cell spread in herpesvirus infections is implied by pseudorabies virus that lacks glycoprotein D and infects cells poorly as free virus propagating efficiently in mice (28) and by herpes simplex virus that lacks glycoprotein E (required only for cell-to-cell spread of virus) disseminates inefficiently from the rat cornea (14).Our understanding of in vivo glycoprotein function is especially limited with gammaherpesviruses such as Epstein-Barr virus (EBV) and the Kaposi's sarcoma-associated herpesvirus (KSHV), whose narrow host ranges have largely precluded studies of pathogenesis. In vitro lytic replication systems have also proved difficult to establish. Nonetheless, it is important for practical purposes, suc...

The ␥-herpesviruses, in contrast to the ␣-and ␤-herpesviruses, are not known to inhibit antigen presentation to CD8 ؉ cytotoxic T lymphocytes (CTLs) during lytic cycle replication. However, murine ␥-herpesvirus 68 causes a chronic lytic infection in CD4 ؉ T celldeficient mice despite the persistence of a substantial CTL response, suggesting that CTL evasion occurs. Here we show that, distinct from host protein synthesis shutoff, ␥-herpesvirus 68 down-regulates surface MHC class I expression on lytically infected fibroblasts and inhibits their recognition by antigen-specific CTLs. The viral K3 gene, encoding a zinc-finger-containing protein, dramatically reduced the half-life of nascent class I molecules and the level of surface MHC class I expression and was by itself sufficient to block antigen presentation. The homologous K3 and K5 genes of the related Kaposi's sarcoma-associated virus also inhibited antigen presentation and decreased cell surface expression of HLA class I antigens. Thus it appears that an immune evasion strategy shared by at least two ␥-herpesviruses allows continued lytic infection in the face of strong CTL immunity.

The murine gamma-herpesvirus-68 K3 (MK3) is a PHD/LAP finger protein that downregulates major histocompatibility complex (MHC) class I expression. In transfected cell lines, MK3 was expressed in the endoplasmic reticulum (ER) membrane, where it bound the cytoplasmic tail of newly synthesized H-2D(b) glycoproteins and targeted them for degradation. Proteasome inhibitors blocked the degradation and led to an accumulation of ubiquitinated H-2D(b). Because this retained its native conformation, ubiquitination preceded any denaturation or dislocation to the cytosol. The PHD/LAP finger of MK3 was not required for H-2D(b) binding but was essential for its ubiquitination and degradation. Thus, gamma-herpesviruses have adapted the cellular PHD/LAP motif to immune evasion, apparently for the catalysis of MHC class I ubiquitination.