Viral effects on antigen processing

Ocular exposure to infectious laryngotracheitis virus alters leukocyte subsets in the head-associated lymphoid tissues and trachea of 6-week-old White Leghorn chickens

Infectious laryngotracheitis virus (ILTV), an alphaherpesvirus, causes acute respiratory disease primarily infecting the upper respiratory tract and conjunctiva. Administration of live attenuated ILTV vaccines via eye drop, drinking water, or by coarse spray elicits protective mucosal immunity in the head-associated lymphoid tissues (HALT), of which conjunctiva-associated lymphoid tissue (CALT) and the Harderian gland (HG) are important tissue components. The trachea, a non-lymphoid tissue, also receives significant influx of inflammatory cells that dictate the outcome of ILTV infection. The objective of this study was to evaluate leukocyte cellular and phenotypic changes in the CALT, HG and trachea following ocular infection with a virulent ILTV strain. At 1, 3, 5, 7 and 9 days post-infection, CALT, HG, and trachea of 6-week-old specific pathogen free (SPF) chickens ocularly-exposed to vehicle or virulent ILTV strain 63140 were dissociated, the cells enumerated and then phenotyped using flow cytometry. The CALT had the highest viral genomic load, which peaked on day 3. In ILTV-infected birds, the CALT had a decreased percentage of leukocytes. This was reflected by decreased numbers of MHCI⁺MHCII^-, MHCI⁺MHCII^low+, and CD4⁺ cells, while IgM⁺ and MHCI⁺MHCII^High+ expressing cell populations increased. In the HG, the most notable change in cells from ILTV-infected birds was a decrease in IgM expressing cells and histologically, an increase in Mott cells. In summary, an acute, ocular exposure to ILTV strain 63140 in young birds shifts subsets of lymphocyte populations in the CALT and HG with minimal impact on the trachea.

Marek's disease in chickens: a review with focus on immunology

Marek's disease (MD), caused by Marek's disease virus (MDV), is a commercially important neoplastic disease of poultry which is only controlled by mass vaccination. Importantly, vaccines that can provide sterile immunity and inhibit virus transmission are lacking; such that vaccines are only capable of preventing neuropathy, oncogenic disease and immunosuppression, but are unable to prevent MDV transmission or infection, leading to emergence of increasingly virulent pathotypes. Hence, to address these issues, developing more efficacious vaccines that induce sterile immunity have become one of the important research goals for avian immunologists today. MDV shares very close genomic functional and structural characteristics to most mammalian herpes viruses such as herpes simplex virus (HSV). MD also provides an excellent T cell lymphoma model for gaining insights into other herpesvirus-induced oncogenesis in mammals and birds. For these reasons, we need to develop an in-depth knowledge and understanding of the host-viral interaction and host immunity against MD. Similarly, the underlying genetic variation within different chicken lines has a major impact on the outcome of infection. In this review article, we aim to investigate the pathogenesis of MDV infection, host immunity to MD and discuss areas of research that need to be further explored.

Functional roles of HIV-1 Vpu and CD74: Details and implications of the Vpu-CD74 interaction

HIV-1 Vpu has a variety of functions, including CD4 degradation and the downregulation of MHCII. Downregulation of the MHCII occurs through Vpu binding to the cytoplasmic domain of CD74, the chaperone for antigen presentation. The CD74 cytoplasmic domain also plays a vital role in cell signaling through the activation of an NF-κB signal cascade for the maturation, proliferation and survival of B cells as well as by binding the macrophage inhibitory factor. In view of these functions, it follows that the Vpu-CD74 interaction has multiple downstream consequences for the immune system as it not only impairs foreign antigen presentation but may also have an effect on signal transduction cascades. It is thought that Vpu specifically targets intracellular CD74 while other HIV-1 proteins cannot. Therefore, this protein-protein interaction would be a potential drug target in order to reduce viral persistence. We review the functional importance and specific binding site of Vpu and CD74.

T-cell-receptor-like antibodies - generation, function and applications

Tumour and virus-infected cells are recognised by CD8+ cytotoxic T cells that, in response, are activated to eliminate these cells. In order to be activated, the clonotypic T-cell receptor (TCR) needs to encounter a specific peptide antigen presented by the membrane surface major histocompatibility complex (MHC) molecule. Cells that have undergone malignant transformation or viral infection present peptides derived from tumour-associated antigens or viral proteins on their MHC class I molecules. Therefore, disease-specific MHC-peptide complexes are desirable targets for immunotherapeutic approaches. One such approach transforms the unique fine specificity but low intrinsic affinity of TCRs to MHC-peptide complexes into high-affinity soluble antibody molecules endowed with a TCR-like specificity towards tumour or viral epitopes. These antibodies, termed TCR-like antibodies, are being developed as a new class of immunotherapeutics that can target tumour and virus-infected cells and mediate their specific killing. In addition to their therapeutic capabilities, TCR-like antibodies are being developed as diagnostic reagents for cancer and infectious diseases, and serve as valuable research tools for studying MHC class I antigen presentation.

Influenza A virus abrogates IFN-gamma response in respiratory epithelial cells by disruption of the Jak/Stat pathway

The innate immunity to viral infections induces a potent antiviral response mediated by interferons (IFN). Although IFN-gamma is detected during the acute stages of illness in the upper respiratory tract secretions and in the serum of influenza A virus-infected individuals, control of influenza A virus is not dependent upon IFN-gamma as evidenced by studies using anti-IFN-gamma Ab and IFN-gamma(-/-) mice. Thus, we hypothesized that IFN-gamma is not critical in host survival because influenza A virus has mechanisms to evade the antiviral activity of IFN-gamma. To test this, A549 cells, an epithelial cell line derived from lung adenocarcinoma, were infected with influenza virus strain A/Aichi/2/68 (H3N2) (Aichi) and/or stimulated with IFN-gamma to detect IFN-gamma-stimulated MHC class II expression. Influenza A virus infection inhibited IFN-gamma-induced up-regulation of HLA-DRalpha mRNA and the IFN-gamma induction of class II transactivator (CIITA), an obligate mediator of MHC class II expression. Nuclear translocation of Stat1alpha upon IFN-gamma stimulation was significantly inhibited in influenza A virus-infected cells and this was associated with a decrease in Tyr701 and Ser727 phosphorylation of Stat1alpha. Thus, influenza A virus subverts antiviral host defense mediated by IFN-gamma through effects on the intracellular signaling pathways.

Marek's disease virus up-regulates major histocompatibility complex class II cell surface expression in infected cells

Many herpesviruses modulate major histocompatibility complex (MHC) expression on the cell surface as an immune evasion mechanism. We report here that Marek's disease virus (MDV), a lymphotrophic avian alphaherpesvirus, up-regulates MHC class II cell surface expression in infected cells, contrary to all other herpesviruses examined to date. This MDV-induced class II up-regulation was detected both in vitro and in vivo. This effect was not solely an indirect effect of interferon, which is a highly potent natural inducer of MHC class II expression, since MHC class II up-regulation in cultured primary fibroblast cells was confined to the infected cells only. MHC class II up-regulation was also observed in infected cells of the bursa of Fabricius during the lytic phase of MDV infection in birds and upon reactivation of MDV from latency in an MDV-transformed cell line. As MDV is a strictly cell-associated virus and requires activated T cells for its life cycle, this up-regulation of MHC class II in infected cells may contribute to virus spread within the infected host by increasing the chance of contact between productively infected cells and susceptible activated T cells.

Strategies and mechanisms for host and pathogen survival in acute and persistent viral infections

Persistent viral infections causing serious diseases derive, primarily, from altered function of the immune system. Knowledge of the very complex composition and function of the innate and adaptive branches of the immune system is essential to understanding persistent infection. The best solution to the problem of persistent infection is by prevention using prophylactic vaccines. Hit and run viruses evade immune destruction by infecting new hosts and rarely persist. Hit and stay viruses evade immune control by sequestration, blockade of antigen presentation, cytokine escape, evasion of natural killer cell activities, escape from apoptosis, and antigenic change. Twelve prophylactic vaccines against hit and run agents exist, and there are only three vaccines against hit and stay viruses, all of which are of DNA composition. Several new vaccines against hit and stay viruses are feasible, but protective vaccines against RNA HIV and hepatitis C agents are highly unlikely, short of a major breakthrough. Therapeutic vaccines are very improbable without a magnitude of favorable new discoveries. In the meantime, antiviral chemotherapy, chemotherapy/prophylactic vaccination, and short interfering RNA silencing are worthy of intense investigation.

Interferon gamma expression and clinical features in patients with acute retinal necrosis syndrome

BACKGROUND

Interferon gamma (IFN-gamma) has been reported to play an important role during virus infections. The purpose of this study was to examine the relationship between IFN-gamma expression and the clinical course of patients with acute retinal necrosis syndrome (ARN) associated with the varicella-zoster virus (VZV).

METHODS

Six patients with ARN were studied. The aqueous and/or vitreous were examined by reverse transcriptase-polymerase chain reaction (RT-PCR) and enzyme-linked immunosorbent assay during the follow-up period. The presence of VZV genome was also determined by PCR. The results were correlated with the clinical data and features and compared with patients with other ocular diseases.

RESULTS

A statistically significant higher level of IFN-gamma was detected in the aqueous and/or vitreous in eyes with ARN than in eyes with other ocular diseases. A statistically significant positive correlation was observed between the level of IFN-gamma in the vitreous and the final visual acuity. IFN-gamma was reduced to undetectable levels within 30 days after the initial eye symptoms. Three of five patients had severe inflammation initially, and the visual acuity gradually recovered with the disappearance of VZV and higher levels of IFN-gamma. Conversely, the other 2 patients showed mild inflammation, had a slow decrease of visual acuity with persistent VZV, and lower levels of IFN-gamma expression.

CONCLUSION

Our results suggest that IFN-gamma may be one of the factors that plays an important role in the clinical course of VZV-associated ARN.

Immune evasion strategies of Kaposi's sarcoma-associated herpesvirus

To establish lifelong infection in the presence of an active host immune system, herpesviruses have acquired an impressive array of immune modulatory mechanisms that contribute to their success as long-term parasites. Kaposi's sarcoma-associated herpesvirus (KSHV) is the most recently discovered human tumor virus and is associated with the pathogenesis of Kaposi's sarcoma, primary effusion lymphoma, and multicentric Castleman's disease. KSHV has acquired a battery of genes to assist in viral survival against the host immune response. These viral gene products target a variety of host immune surveillance mechanisms, including the cytokine-mediated immune response, apoptosis, natural killer (NK) cell killing and T cell-mediated responses. This review summarizes our understanding of the role of these viral proteins in the escape from host immune surveillance, which ultimately contributes to lifelong infection and pathogenesis of KSHV.

Glycoprotein B from strain 17 of herpes simplex virus type I contains an invariant chain homologous sequence that binds to MHC class II molecules

Major histocompatibility complex class I (MHCI) molecules are major targets of virus evasion strategies because they introduce antigens from the biosynthesis pathway into the antigen-processing and presentation pathways for immune recognition by CD8+ T cells. Little is known about viral strategies that interfere with the MHC class II (MHCII) antigen presentation pathway. We identified a six amino acid sequence from type I herpes simplex virus (HSV-1) glycoprotein B (gB) that is identical to a sequence of human leucocyte antigen D (HLA-D) -associated invariant chain (Ii). In addition, this gB sequence is adjacent to a highly conserved HLA-DR1 binding motif. Both viral sequences together resemble the class II binding site of human Ii, consisting of a MHCII groove binding segment and a promiscuous binding site. We cloned gB from HSV-1 strain 17 and demonstrate association of the virus envelope protein to three HLA-DR allotypes. With chimeric Ii/gB fusion proteins we identified gB sequences that mediate promiscuous or allotype-specific binding to the HLA-DR peptide-binding domain. Mutation of two Lys residues in the viral segment of Ii/gB abolished promiscuous binding to HLA-DR heterodimers. The result indicates promiscuous binding of the virus sequence to HLA-DR molecules and suggests a potential for HSV-1 to manipulate antigen processing and presentation.

The lytic cycle of Epstein-Barr virus is associated with decreased expression of cell surface major histocompatibility complex class I and class II molecules

Human herpesviruses utilize an impressive range of strategies to evade the immune system during their lytic replicative cycle, including reducing the expression of cell surface major histocompatibility complex (MHC) and immunostimulatory molecules required for recognition and lysis by virus-specific cytotoxic T cells. Study of possible immune evasion strategies by Epstein-Barr virus (EBV) in lytically infected cells has been hampered by the lack of an appropriate permissive culture model. Using two-color immunofluorescence staining of cell surface antigens and EBV-encoded lytic cycle antigens, we examined EBV-transformed B-cell lines in which a small subpopulation of cells had spontaneously entered the lytic cycle. Cells in the lytic cycle showed a four- to fivefold decrease in cell surface expression of MHC class I molecules relative to that in latently infected cells. Expression of MHC class II molecules, CD40, and CD54 was reduced by 40 to 50% on cells in the lytic cycle, while no decrease was observed in cell surface expression of CD19, CD80, and CD86. Downregulation of MHC class I expression was found to be an early-lytic-cycle event, since it was observed when progress through late lytic cycle was blocked by treatment with acyclovir. The immediate-early transactivator of the EBV lytic cycle, BZLF1, did not directly affect expression of MHC class I molecules. However, BZLF1 completely inhibited the upregulation of MHC class I expression mediated by the EBV cell-transforming protein, LMP1. This novel function of BZLF1 elucidates the paradox of how MHC class I expression can be downregulated when LMP1, which upregulates MHC class I expression in latent infection, remains expressed in the lytic cycle.

Human cytomegalovirus disrupts constitutive MHC class II expression

CD8(+) and CD4(+) T lymphocytes are important in controlling human CMV (HCMV) infection, but the virus has evolved protean mechanisms to inhibit MHC-based Ag presentation and escape T lymphocyte immunosurveillance. Herein, the interaction of HCMV with the MHC class II Ag presentation pathway was investigated in cells stably transfected with class II transactivator. Flow cytometry experiments demonstrate that HCMV infection decreases cell-surface MHC class II expression. HCMV down-regulates MHC class II surface expression without a significant effect on class II RNA or steady-state protein levels. SDS-stability and confocal microscopy experiments demonstrate normal levels of steady-state peptide-loaded class II molecules in infected cells and that class II molecules reach late endosomal and HLA-DM positive peptide-loading compartments. However, MHC class II positive vesicles are retained in an abnormal perinuclear distribution. Finally, experiments with a mutant HCMV strain demonstrate that this novel mechanism of decreased MHC class II expression is not mediated by one of the known HCMV immunomodulatory genes. These defects in MHC class II expression combined with previously identified CMV strategies for decreasing MHC class I expression enables infected cells to evade T lymphocyte immunosurveillance.

Viruses and diabetes

Insulin-dependent diabetes mellitus (IDDM) is a multifactorial disease. Besides a genetic predisposition environmental factors have been implicated in the pathogenesis of beta cell destruction. Among these environmental factors viruses have been the focus of many studies. Some viruses are diabetogenic in animals, and others have been implicated as triggers in human IDDM by temporal and geographical association between IDDM and viral infections, serological evidence of infection in recently diagnosed diabetic patients, and the isolation of viruses from the pancreas of affected individuals. We discuss possible pathomechanisms of viral infections in beta cell destruction and review the studies on involvement of enteroviruses, retroviruses, rubella viruses, cytomegaloviruses, and Epstein-Barr viruses in human IDDM. We also report on studies of diabetogenic viruses in animal models as well as on viral infections protecting from IDDM. Some of the difficulties in linking viral infections to IDDM will be illustrated with data from a transgenic mouse model in which IDDM can be precipitated by infections with certain strains of lymphocytic choriomeningitis virus (LCMV). Emerging treatment concepts that do not rely on defining the initiating autoantigens but involve self-reactive regulatory lymphocytes such as oral antigen administration, as well as DNA vaccines, will be discussed briefly.

Physical association of the K3 protein of gamma-2 herpesvirus 68 with major histocompatibility complex class I molecules with impaired peptide and beta(2)-microglobulin assembly

To persist in the presence of an active immune system, viruses encode proteins that decrease expression of major histocompatibility complex class I molecules by using a variety of mechanisms. For example, murine gamma-2 herpesvirus 68 expresses the K3 protein, which causes the rapid turnover of nascent class I molecules. In this report we show that certain mouse class I alleles are more susceptible than others to K3-mediated down regulation. Prior to their rapid degradation, class I molecules in K3-expressing cells exhibit impaired assembly with beta(2)-microglobulin. Furthermore, K3 is detected predominantly in association with class I molecules lacking assembly with high-affinity peptides, including class I molecules associated with the peptide loading complex TAP/tapasin/calreticulin. The detection of K3 with class I assembly intermediates raises the possibility that molecular chaperones involved in class I assembly are involved in K3-mediated class I regulation.

Human cytomegalovirus binding to heparan sulfate proteoglycans on the cell surface and/or entry stimulates the expression of human leukocyte antigen class I

Human cytomegalovirus (HCMV) is known to down-regulate the expression of human leukocyte antigen (HLA) class I, the process of which involves a subset of virus genes. Infection of human foreskin fibroblast (HFF) cells with UV-inactivated HCMV (UV-HCMV), however, resulted in an increase in HLA class I presentation on the cell surface in the absence of HCMV gene expression. Heparin, which inhibits the interaction of virus particles with cell surface heparan sulfate proteoglycans (HSPGs), blocked the effect of UV-HCMV on HLA class I expression. Pretreatment of cells with heparinase I decreased in a dose-dependent manner the effect of UV-HCMV on HLA class I expression enhancement. Sodium chlorate, which is known to inhibit the sulfation of HSPGs, gave a similar result. Pretreatment of UV-HCMV with trypsin or monoclonal antibody reactive with the envelope glycoprotein gB reduced the increase in HLA class I expression on the HFF cell surface by UV-HCMV. RT-PCR analysis demonstrated that the increase in HLA class I presentation on the HFF cell surface was due to an increase in HLA class I transcription. Thus, binding of HCMV particles to cell surface HSPGs appears to be required for the stimulation of HLA class I expression. It is also possible that virus entry, in addition to binding to HSPGs, may be involved in the stimulation of HLA class I expression, since the UV-HCMV entered the cells and all treatments to block virus binding to HSPGs would necessarily prevent virus entry.

The recognition of HLA-B27 by human CD4(+) T lymphocytes

HLA-B27 transgenic animal models suggest a role for CD4(+) T lymphocytes in the pathogenesis of the spondyloarthropathies, and murine studies have raised the possibility that unusual forms of B27 may be involved in disease. We demonstrate that CD4(+) T cells capable of recognizing B27 can be isolated from humans by coculture with the MHC class II-negative cell line T2 transfected with B27. These CD4(+) T cells recognize a panel of B27-transfected cell lines that are defective in Ag-processing pathways, but not the nontransfected parental cell lines, in a CD4-dependent fashion. Inhibition of responses by the MHC class I-specific mAb w6/32 and the B27 binding mAb ME1 implicates the recognition of a form of B27 recognized by both of these Abs. We suggest that B27-reactive CD4(+) T cells may be pathogenic in spondyloarthropathies, particularly if factors such as infection influence expression of abnormal forms of B27.

Differential processing and presentation of the H-2D(b)-restricted epitope from two different strains of influenza virus nucleoprotein

The influenza virus strains A/NT/60/68 and A/PR/8/34 both have an immunodominant D(b)-restricted epitope in their nucleoprotein (NP) at amino acid residues 366-374, with two amino acid differences between the epitopes. Cross-reactive cytotoxic T lymphocytes (CTLs) were generated by priming mice with the influenza virus A/NT/60/68 NP and restimulating in vitro with influenza virus A/PR/8/34. CTLs that gave high levels of specific lysis recognized target cells infected with either strain of influenza virus with similar efficiency. Surprisingly, when target cells were infected with recombinant vaccinia viruses (VV) expressing the two different NPs, presentation of the D(b)-restricted epitope from the A/NT/60/68 NP was extremely poor, whereas presentation of the equivalent epitope from the A/PR/8/34 NP was as efficient as in influenza virus-infected cells. This difference was observed in spite of the fact that the two NP sequences show 94% identity at the amino acid sequence level. Experiments with additional cross-reactive CTL cell lines which recognized target cells less efficiently revealed a similar difference in presentation between the two NP epitopes in influenza virus-infected cells and showed a difference in the efficiency of presentation of the D(b)-restricted epitope from the two NP molecules independent of VV infection. The results show that two equivalent epitopes in highly similar proteins are processed with very different efficiency, even though they are both immunodominant epitopes. They also suggest that the previously described inhibition of antigen presentation by VV is a general, non-specific effect, which is more apparent for epitopes that are processed and presented less efficiently.

Immune evasion as a pathogenic mechanism of varicella zoster virus

Varicella zoster virus (VZV) is a human herpesvirus that causes varicella (chickenpox) during primary infection, establishes latency in dorsal root ganglia and may reactivate years later, producing herpes zoster. VZV must evade antiviral immunity during three important stages of viral pathogenesis, including the cell-associated viremia characteristic of primary infection, persistence in dorsal root ganglia during latency and the initial period of VZV reactivation. Our observations about the immunomodulatory effects of VZV document its capacity to interfere with adaptive immunity mediated by CD4 as well as CD8 T cells, ensuring the survival of the virus in the human population from generation to generation.

Factors controlling the trafficking and processing of a leader-derived peptide presented by Qa-1

Many leader-derived peptides require TAP for presentation by class I molecules. This TAP dependence can either be ascribed to the inability of proteases resident in the endoplasmic reticulum (ER) to trim leader peptide precursors into the appropriate epitope or the failure of a portion of the leader segment to gain access to the lumen of the ER. Using the Qa-1 binding epitope, Qdm derived from a class Ia leader as a model, we show that many cell types lack ER protease activity to trim this peptide at its C terminus. However, both T1 and T2 cells contain appropriate protease activity to process the full length D(d) leader (DL) when introduced into the ER lumen. Nevertheless, both T1 cells treated with the TAP inhibitor ICP47 and TAP(-) T2 cells fail to present this epitope from either the intact D(d) molecule or a minigene encoding the DL. This indicates that the portion of the leader containing Qdm does not gain access to the ER. However, changing the Arg at P7 of the DL to a Cys can alter its trafficking and allows for TAP-independent presentation of the Qdm epitope.

Inhibition of natural killer cell-mediated cytotoxicity by Kaposi's sarcoma-associated herpesvirus K5 protein

Kaposi's sarcoma-associated herpesvirus (KSHV) K3 and K5 proteins dramatically downregulate MHC class I molecules. However, although MHC class I downregulation may protect KSHV-infected cells from cytotoxic T lymphocyte recognition, these cells become potential targets for natural killer (NK) cell-mediated lysis. We now show that K5 also downregulates ICAM-1 and B7-2, which are ligands for NK cell-mediated cytotoxicity receptors. As a consequence, K5 expression drastically inhibits NK cell-mediated cytotoxicity. Conversely, de novo expression of B7-2 and ICAM-1 resensitizes the K5-expressing cells to NK cell-mediated cytotoxicity. This is a novel viral immune evasion strategy where KSHV K5 achieves immune avoidance by downregulation of cellular ligands for NK cell-mediated cytotoxicity receptors.

Obstacles to identifying viruses that cause autoimmune disease

In addition to a clear genetic disposition, environmental factors and viral infections are thought to play a role in the pathogenesis of autoimmune diseases such as type I diabetes or multiple sclerosis (MS). This article will explore, by use of paradigms developed in a transgenic mouse model, why it has been so difficult to prove a causative association between viral infections and autoimmunity. Potential pathogenetic mechanisms and their impact on devising the best strategy to prove such an association will be discussed. These thoughts might also be important for applying new immune interventions in type I diabetes or MS.

Interaction of human immunodeficiency virus type 2 Vpx and invariant chain

Vpx is a virion-associated protein of human immunodeficiency virus type 2 (HIV-2) and simian immunodeficiency viruses. The yeast two-hybrid system was used to identify invariant chain (Ii) as a cellular protein that interacts with HIV-2 Vpx. Vpx-Ii interaction was confirmed in cell-free reactions using bacterially expressed glutathione S-transferase fusion proteins and by coimmunoprecipitation in transfected and infected cells. In chronically infected cells expressing Vpx, Ii levels were markedly decreased, presumably due to enhanced degradation. These findings suggest that Vpx may disrupt major histocompatibility complex class II antigen presentation.

Downregulation of major histocompatibility complex class I molecules by Kaposi's sarcoma-associated herpesvirus K3 and K5 proteins

The T-cell-mediated immune response plays a central role in the defense against intracellular pathogens. To avoid this immune response, viruses have evolved elaborate mechanisms that target and modulate many different aspects of the host's immune system. A target common to many of these viruses is the major histocompatibility complex (MHC) class I molecules. Kaposi's sarcoma-associated herpesvirus (KSHV) encodes K3 and K5 zinc finger membrane proteins which remove MHC class I molecules from the cell surface. K3 and K5 exhibit 40% amino acid identity to each other and localize primarily near the plasma membrane. While K3 and K5 dramatically downregulated class I molecules, they displayed different specificities in downregulation of HLA allotypes. K5 significantly downregulated HLA-A and -B and downregulated HLA-C only weakly, but not HLA-E, whereas K3 downregulated all four HLA allotypes. This selective downregulation of HLA allotypes by K5 was partly due to differences in amino acid sequences in their transmembrane regions. Biochemical analyses demonstrated that while K3 and K5 did not affect expression and intracellular transport of class I molecules, their expression induced rapid endocytosis of the molecules. These results demonstrate that KSHV has evolved a novel immune evasion mechanism by harboring similar but distinct genes, K3 and K5, which target MHC class I molecules in different ways.

Posttranscriptional inhibition of class I major histocompatibility complex presentation on hepatocytes and lymphoid cells in chronic woodchuck hepatitis virus infection

Woodchuck hepatitis virus (WHV), similar to human hepatitis B virus, causes acute liver inflammation that can progress to chronic hepatitis and hepatocellular carcinoma. WHV also invades cells of the host lymphatic system, where it persists for life. We report here that acute and chronic hepadnavirus hepatitis is characterized by a profound difference in the expression of class I major histocompatibility complex (MHC) molecules on the surface of infected hepatocytes and, notably, lymphoid cells. While acute WHV infection is accompanied by the enhanced hepatocyte surface presentation of class I MHC antigen and upregulated transcription of the relevant hepatic genes, inhibition of class I antigen display on liver cells is a uniform hallmark of chronic WHV infection. This inhibition in chronic hepatitis occurs despite augmented (as in acute infection) expression of hepatic genes for class I MHC heavy chain, beta(2)-microglobulin, and transporters associated with antigen processing (TAP1 and TAP2). Further, the class I antigen inhibition is not related to the histological severity of hepatocellular injury, the extent of lymphocytic infiltrations, the level of intrahepatic gamma interferon induction, or the hepatic WHV load. Importantly, the antigen expression is also inhibited on organ lymphoid cells of chronically infected hosts. The results obtained in this study demonstrate that the defective presentation of class I MHC molecules on cells supporting persistent WHV replication is due to viral posttranscriptional interference. This event may diminish the susceptibility of infected hepatocytes to virus-specific T-cell-mediated elimination, hinder virus clearance, and deregulate the class I MHC-dependent functions of the host immune system. This multifarious effect could be critical for perpetuation of liver damage and evasion of the antiviral immunological surveillance in chronic infection and therefore could be supportive of hepadnavirus persistence.

NK cell recognition of non-classical HLA class I molecules

NK cells recognize several HLA class Ib molecules employing both immunoglobulin-like (Ig-like) and C-type lectin receptors. The CD94/NKG2 and NKG2D lectin-like molecules, respectively, interact with HLA-E and MICA; CD94/NKG2A functions as an inhibitory receptor, while CD94/NKG2C and NKG2D trigger NK cell activity. HLA-E predominantly presents nonamers from the leader sequences of other class I molecules; a peptide derived from HLA-G1 constitutes the highest affinity ligand for both CD94/NKG2 receptors. Members of the Ig-like transcript (ILT) or leucocyte Ig-like receptor (LIR) family (ILT2 or LIR-1 and ILT4 or LIR-2), expressed by other leucocyte lineages, interact with a broad spectrum of HLA class Ia molecules and HLA-G1. Among Ig-like KIRs, the KIR2DL4 (p49) receptor has been shown to specifically recognize HLA-G1; this molecule displays an unusual hybrid structure, sharing features with inhibitory and triggering KIRs.

Modulation of major histocompatibility class II protein expression by varicella-zoster virus

We sought to investigate the effects of varicella-zoster virus (VZV) infection on gamma interferon (IFN-gamma)-stimulated expression of cell surface major histocompatibility complex (MHC) class II molecules on human fibroblasts. IFN-gamma treatment induced cell surface MHC class II expression on 60 to 86% of uninfected cells, compared to 20 to 30% of cells which had been infected with VZV prior to the addition of IFN-gamma. In contrast, cells that were treated with IFN-gamma before VZV infection had profiles of MHC class II expression similar to those of uninfected cell populations. Neither IFN-gamma treatment nor VZV infection affected the expression of transferrin receptor (CD71). In situ and Northern blot hybridization of MHC II (MHC class II DR-alpha) RNA expression in response to IFN-gamma stimulation revealed that MHC class II DR-alpha mRNA accumulated in uninfected cells but not in cells infected with VZV. When skin biopsies of varicella lesions were analyzed by in situ hybridization, MHC class II transcripts were detected in areas around lesions but not in cells that were infected with VZV. VZV infection inhibited the expression of Stat 1alpha and Jak2 proteins but had little effect on Jak1. Analysis of regulatory events in the IFN-gamma signaling pathway showed that VZV infection inhibited transcription of interferon regulatory factor 1 and the MHC class II transactivator. This is the first report that VZV encodes an immunomodulatory function which directly interferes with the IFN-gamma signal transduction via the Jak/Stat pathway and enables the virus to inhibit IFN-gamma induction of cell surface MHC class II expression. This inhibition of MHC class II expression on VZV-infected cells in vivo may transiently protect cells from CD4(+) T-cell immune surveillance, facilitating local virus replication and transmission during the first few days of cutaneous lesion formation.