Interaction between complement receptor gC1qR and hepatitis C virus core protein inhibits T-lymphocyte proliferation.

Hepatitis C virus (HCV) is an important human pathogen that is remarkably efficient at establishing persistent infection. The HCV core protein is the first protein expressed during the early phase of HCV infection. Our previous work demonstrated that the HCV core protein suppresses host immune responses, including anti-viral cytotoxic T-lymphocyte responses in a murine model. To investigate the mechanism of HCV core-mediated immunosuppression, we searched for host proteins capable of associating with the core protein using a yeast two-hybrid system. Using the core protein as bait, we screened a human T cell-enriched expression library and identified a gene encoding the gC1q receptor (gC1qR). C1q is a ligand of gC1qR and is involved in the early host defense against infection. Like C1q, HCV core can inhibit T-cell proliferative responses in vitro. This core-induced anti-T-cell proliferation is reversed by addition of anti-gC1qR Ab in a T-cell proliferation assay. Furthermore, biochemical analysis of the interaction between core and gC1qR indicates that HCV core binds the region spanning amino acids 188 to 259 of gC1qR, a site distinct from the binding region of C1q. The inhibition of T-cell responsiveness by HCV core may have important implications for HCV persistence in humans.

Melanomas with concordant loss of multiple melanocytic differentiation proteins: immune escape that may be overcome by targeting unique or undefined antigens.

Melanoma-reactive HLA-A x 0201-restricted cytotoxic T lymphocyte (CTL) lines generated in vitro lyse autologous and HLA-matched allogeneic melanoma cells and recognize multiple shared peptide antigens from tyrosinase, MART-1, and Pme117/gp100. However, a subset of melanomas fail to be lysed by these T cells. In the present report, four different HLA-A x 0201+ melanoma cell lines not lysed by melanoma-reactive allogeneic CTL have been evaluated in detail. All four are deficient in expression of the melanocytic differentiation proteins (MDP) tyrosinase, Pme117/gp100, gp75/ trp-1, and MART-1/Melan-A. This concordant loss of multiple MDP explains their resistance to lysis by melanoma-reactive allogeneic CTL and confirms that a subset of melanomas may be resistant to tumor vaccines directed against multiple MDP-derived epitopes. All four melanoma lines expressed normal levels of HLAA x 0201, and all were susceptible to lysis by xenoreactive-peptide-dependent HLA-A x 0201-specific CTL clones, indicating that none had identifiable defects in antigen-processing pathways. Despite the lack of shared MDP-derived antigens, one of these MDP-negative melanomas, DM331, stimulated an effective autologous CTL response in vitro, which was restricted to autologous tumor reactivity. MHC-associated peptides isolated by immunoaffinity chromatography from HLA-A1 and HLA-A2 molecules of DM331 tumor cells included at least three peptide epitopes recognized by DM331 CTL and restricted by HLA-A1 or by HLA-A x 0201. Recognition of these CTL epitopes cannot be explained by defined, shared melanoma antigens; instead, unique or undefined antigens must be responsible for the autologous-cell-specific anti-melanoma response. These findings suggest that immunotherapy directed against shared melanoma antigens should be supplemented with immunotherapy directed against unique antigens or other undefined antigens, especially in patients whose tumors do not express MDP.

Suppression of host immune response by the core protein of hepatitis C virus: possible implications for hepatitis C virus persistence.

Hepatitis C virus (HCV) is a major human pathogen causing mild to severe liver disease worldwide. This positive strand RNA virus is remarkably efficient at establishing chronic infections. Although a high rate of genetic variability may facilitate viral escape and persistence in the face of Ag-specific immune responses, HCV may also encode proteins that facilitate evasion of immunological surveillance. To address the latter possibility, we examined the influence of specific HCV gene products on the host immune response to vaccinia virus in a murine model. Various vaccinia/HCV recombinants expressing different regions of the HCV polyprotein were used for i.p. inoculation of BALB/c mice. Surprisingly, a recombinant expressing the N-terminal half of the polyprotein (including the structural proteins, p7, NS2, and a portion of NS3; vHCV-S) led to a dose-dependent increase in mortality. Increased mortality was not observed for a recombinant expressing the majority of the nonstructural region or for a negative control virus expressing the beta-galactosidase protein. Examination of T cell responses in these mice revealed a marked suppression of vaccinia-specific CTL responses and a depressed production of IFN-gamma and IL-2. By using a series of vaccinia/HCV recombinants, we found that the HCV core protein was sufficient for immunosuppression, prolonged viremia, and increased mortality. These results suggest that the HCV core protein plays an important role in the establishment and maintenance of HCV infection by suppressing host immune responses, in particular the generation of virus-specific CTLs.

Human melanoma patients recognize an HLA-A1-restricted CTL epitope from tyrosinase containing two cysteine residues: implications for tumor vaccine development.

To identify shared epitopes for melanoma-reactive CTL restricted by MHC molecules other than HLA-A*0201, six human melanoma patient CTL lines expressing HLA-A1 were screened for reactivity against the melanocyte differentiation proteins Pmel-17/gp100, MART-1/Melan-A, and tyrosinase, expressed via recombinant vaccinia virus vectors. CTL from five of the six patients recognized epitopes from tyrosinase, and recognition of HLA-A1+ target cells was strongly correlated with tyrosinase expression. Restriction by HLA-A1 was further demonstrated for two of those tyrosinase-reactive CTL lines. Screening of 119 synthetic tyrosinase peptides with the HLA-A1 binding motif demonstrated that nonamer, decamer, and dodecamer peptides containing the sequence KCDICTDEY (residues 243-251) all reconstituted the CTL epitope in vitro. Epitope reconstitution in vitro required high concentrations of these peptides, which was hypothesized to be a result of spontaneous modification of cysteine residues, interfering with MHC binding. Substitution of serine or alanine for the more N-terminal cysteine prevented modification at that residue and permitted target cell sensitization at peptide concentrations 2 to 3 orders of magnitude lower than that required for the wild-type peptide. Because spontaneous modification of sulfhydryl groups may also occur in vivo, tumor vaccines using this or other cysteine-containing peptides may be improved by amino acid substitutions at cysteine residues.

The class I antigen-processing pathway for the membrane protein tyrosinase involves translation in the endoplasmic reticulum and processing in the cytosol.

Formation of major histocompatibility complex class I-associated peptides from membrane proteins has not been thoroughly investigated. We examined the processing of an HLA-A*0201-associated epitope, YMDGTMSQV, that is derived from the membrane protein tyrosinase by posttranslational conversion of the sequence YMNGTMSQV. Only YMDGTMSQV and not YMNGTMSQV was presented by HLA-A*0201 on cells expressing full-length tyrosinase, although both peptides have similar affinities for HLA-A*0201 and are transported by TAP. In contrast, translation of YMNGTMSQV in the cytosol, as a minigene or a larger fragment of tyrosinase, led to the presentation of the unconverted YMNGTMSQV. This was not due to overexpression leading to saturation of the processing/conversion machinery, since presentation of the converted peptide, YMDGTMSQV, was low or undetectable. Thus, presentation of unconverted peptide was associated with translation in the cytosol, suggesting that processing of the full-length tyrosinase occurs after translation in the endoplasmic reticulum. Nevertheless, presentation of YMDGTMSQV in cells expressing full-length tyrosinase was TAP (transporter associated with antigen processing) and proteasome dependent. After inhibition of proteasome activity, tyrosinase species could be detected in the cytosol. We propose that processing of tyrosinase involves translation in the endoplasmic reticulum, export of full-length tyrosinase to the cytosol, and retransport of converted peptides by TAP for association with HLA-A*0201.

Shared epitopes for HLA-A3-restricted melanoma-reactive human CTL include a naturally processed epitope from Pmel-17/gp100.

Human CD8+ CTL recognize peptides bound to class I MHC molecules on the surface of melanoma cells. Several peptides derived from melanocyte lineage-specific proteins have been identified as epitopes for HLA-A2 restricted melanoma-reactive CTL. Because less than half of melanoma patients express HLA-A2, it is important to identify CTL epitopes restricted by other common MHC molecules including HLA-A1 and -A3. We have generated HLA-A3-restricted human CTL that recognize one or more shared melanoma Ags. All of the melanomas recognized by one of these CTL lines express Pmel-17/gp100, and those that fail to express this Ag are not lysed. This CTL line also specifically recognizes the lymphoblastoid line C1R-A3 following infection with a recombinant vaccinia encoding the melanocyte lineage-specific protein Pmel-17/gp100. Thus, at least one Pmel-17/ gp100 peptide is an epitope for this CTL line. We have identified ALLAVGATK (Pmel-17/gp100 residues 17-25) as an epitope for this CTL line and have shown that it is naturally processed and presented by HLA-A3 on melanoma cells. A second HLA-A3-restricted melanoma-reactive CTL line recognizes at least one additional shared epitope. These findings suggest that cellular immune responses directed against multiple shared melanoma epitopes exist in the 20 to 25% of melanoma patients who express HLA-A3. In addition, immunotherapy directed against Pmel-17/gp100 and other shared melanoma Ags may be useful in a large subset of these patients.

Presentation of newly synthesized glycoproteins to CD4+ T lymphocytes. An analysis using influenza hemagglutinin transport mutants.

Human lymphoblastoid cells transiently expressing the hemagglutinin (HA) glycoprotein of influenza virus are rapidly and efficiently recognized by CD4+ HA-specific T lymphocytes. This endogenous presentation pathway is sensitive to chloroquine and is therefore likely related to the classical class II major histocompatibility complex (MHC) exogenous pathway of antigen presentation. In this study we have examined a series of transport-defective HA mutants. We correlate the intracellular fate of the native antigen with its presentation characteristics. We have found that the native antigen must enter the secretory pathway since a cytosolic form is not presented. However, surface expression and normal trafficking through the Golgi apparatus are not required for efficient presentation. Instead, escape of native antigen from the endoplasmic reticulum appears to be both necessary and sufficient for gaining access to a compartment where antigen is processed and binds class II MHC molecules.

Class I major histocompatibility complex-restricted cytolytic T lymphocytes recognize a limited number of sites on the influenza hemagglutinin.

Two distinct regions of the influenza A/JAP/305/57 hemagglutinin molecule are identifiable as sites recognized by murine class I major histocompatibility complex (MHC) (H-2d)-restricted cytolytic T lymphocytes (CTL) generated in response to immunization with infectious type A influenza virus. Each of these sites can be mimicked by a synthetic oligopeptide of approximately 20 amino acids. Data presented herein indicate that these two sites define the dominant immunogenic epitopes on the hemagglutinin recognized by H-2Kd-restricted CTL. These same sites are not efficiently recognized by hemagglutinin-specific class I MHC-restricted CTL of several unrelated MHC haplotypes. These observations show that even for a large complex glycoprotein molecule like the influenza hemagglutinin, only a limited number of class I CTL recognition sites are generated in the infected cell and that the subset of immunogenic epitopes is dependent on the MHC haplotype of the responding individual. These parameters need to be considered in the design of synthetic and recombinant vaccines.

Complete sequence of the HLA DQ alpha and DQ beta cDNA from a DR5/DQw3 cell line.

The HLA-D region is composed of three subregions termed DR, DQ, and DP. We previously reported the sequence of a DR5 beta I and two DR5 beta III cDNA from the DR5 cell line Swei. We now report on the nucleotide and deduced amino acid sequence of the DQ alpha and DQ beta cDNA from the same DR5 cell line, which also types as DQw3. Comparison with other available DQ sequences indicates that DQ alpha has one region of major variability, whereas DQ beta appears to have four regions of variability. In addition, these comparisons indicate that DQw3 alpha from DR5 is different from DQw3 alpha from DR4, but identical to DQw2 alpha from DR3. In contrast, DQw3 beta from DR5 is very similar to DQw3 beta from DR4. These data indicate that at least for DQw2 and DQw3 it is the DQ beta chain that is responsible for DQ typing. Most sequence differences in DQ alleles can be attributed to point mutations; however, codon additions/deletions in the DQ alpha chain may contribute to variability. In addition, regions of possible gene conversion in the DQ alpha and DQ beta chains is suggested by the presence of a chi-like sequence in each chain. Finally, comparison of available haplotypes suggest recombination events may take place between DQ beta and DQ alpha, between DQ alpha and DR beta I, and between DR beta I and DR beta III.