Cytotoxic T-cell activity antagonized by naturally occurring HIV-1 Gag variants

Selective mechanisms utilized by persistent and oncogenic viruses to interfere with antigen processing and presentation

Cell-mediated immunity is effective against cells harboring active virus replication, and is critical for the elimination of ongoing infections, regression of virus-associated tumors, and reducing or preventing the reactivation of persistent viruses. The capacity of persistent and oncogenic viruses to maintain a long-term relationship with their host presupposes viral mechanisms for circumventing antiviral defenses. By suppressing the expression of molecules associated with antigen processing and presentation, viruses abrogate the major immune mechanism that deals with the elimination of infected and tumor cells. This is accomplished either by transcriptional downregulation of genes encoding class I MHC antigens, peptide transporter molecules, and the proteasome-associated LMP subunits, or by interfering with transport of class I molecules to the cell surface. In some cases viruses shut off the expression of most viral proteins during latency or express mainly nonimmunogenic or antagonistic peptide epitopes. This review describes selective mechanisms utilized by viruses for interference with antigen processing and presentation, and addresses their significance for in vivo viral persistence and tumor progression.

Specific inhibition of granzyme B by parainfluenza virus type 3

T-cell-mediated cytotoxicity is an important means of defense against viral pathogens; however, several viruses possess mechanisms to disrupt cytotoxicity, thereby allowing them to avoid immune clearance. These viruses have been shown to inhibit cytotoxicity by interfering with the capacity of T lymphocytes to specifically recognize infected cells. An alternative mechanism for virally induced cytotoxic dysfunction is identified in this report. We show that parainfluenza virus type 3, a negative-stranded RNA virus, can inhibit cytotoxicity by causing a defect in the cytotoxic effector apparatus. This defect is identified as a selective inhibition of granzyme B mRNA.

Ligands for the T-cell receptor: hard times for avidity models

T-cell activation occurs when the T-cell receptor (TCR) binds to a self major histocompatibility complex (MHC) molecule carrying a specific peptide. Small changes in peptide structure can alter or inhibit this response. Although this phenomenon is normally attributed to a lowering of the avidity of the ligand for its receptor, recent data suggest that conformational changes in the receptor itself may be playing a critical role, as discussed here by Charles Janeway.

The association of an HPV16 oncogene variant with HLA-B7 has implications for vaccine design in cervical cancer

HLA-restricted cytotoxic T-lymphocyte (CTL) recognition of human papillomavirus (HPV) oncogene products may be important in the control of the HPV infections associated with the development of cervical cancer. We have identified, in HLA-B7 individuals, a consistent variation in the HPV16 E6 oncoprotein sequence, which alters an HLA-B7 peptide binding epitope in a way likely to influence immune recognition by CTLs. These results illustrate a biologically relevant mechanism for escape from immune surveillance of HPV16 in HLA-B7 individuals. Thus, both HLA type and HPV16 strain variation need to be considered in the screening of at-risk individuals and for the rational design of anti-HPV vaccines.

Anti-HIV type 1 cytotoxic T lymphocyte effector activity and disease progression in the first 8 years of HIV type 1 infection of homosexual men

Cytotoxic T lymphocytes (CTL) may play an important role in host defense against HIV-1 infection. In this study, we examined the responses of circulating effector CTL (CTLe) specific for Gag, Pol, Env, and Tat in 57 HIV-1-infected men, 49 of whom were asymptomatic and had documented time since seroconversion of < 8 years. CTLe responses to at least one of the four HIV-1 gene products were detected in 83% of the subjects. The magnitude and prevalence of the anti-Tat responses were significantly less than the responses to Gag, Pol, and Env. Cell depletion studies indicated that the lytic activity against the HIV-1 structural proteins was mediated by CD8+ T cells, although 30% of Env-specific lysis was mediated by CD16+ natural killer cells. Anti-HIV-1 CTLe responses against Gag and Pol were significantly less in subjects infected for over 6 years as compared to those infected for shorter periods of time. We found no correlation, however, between anti-HIV-1 CTLe responses and either CD4+ or CD8+ T cell counts, rates of CD4+ T cell loss, HIV-1 infectious viral load, use of antiviral medications, or subsequent progression to AIDS. Our results indicate that anti-HIV-1 CTLe activity is relatively stable in asymptomatic subjects infected < 6 years, and is not an early marker for risk of disease progression.

Naturally occurring variants of human T-cell leukemia virus type I Tax protein impair its recognition by cytotoxic T lymphocytes and the transactivation function of Tax

There is a high degree of intraisolate sequence heterogeneity in the tax gene of human T-cell leukemia virus type I (HTLV-I), although the sequence variation between patients is small compared with that of human immunodeficiency virus type 1. In the present study, we investigated whether naturally occurring amino acid substitutions changed the properties of the Tax protein in two respects: first, recognition of the protein by cytotoxic T lymphocytes (CTL), and second, the ability of the Tax protein to transactivate various promoters. We found that (i) all of the observed amino acid substitutions that occur in known CTL epitopes abolished the recognition of the synthetic peptide representing the respective epitope; (ii) these substitutions occurred significantly more frequently in subjects carrying HLA-A2; and (iii) most of the amino acid substitutions severely reduced the ability of Tax protein to transactivate three promoters: the HTLV-I long terminal repeat, the c-fos promoter, and the interleukin-2 receptor alpha chain promoter.

Immunobiology of cytotoxic T-cell escape mutants of lymphocytic choriomeningitis virus

Infection with virus variants exhibiting changes in the peptide sequences defining immunodominant determinants that abolish recognition by antiviral cytotoxic T cells (CTL) presents a considerable challenge to the antiviral T-cell immune system and may enable some viruses to persist in hosts. The potential importance of such variants with respect to mechanisms of viral persistence and disease pathogenesis was assessed by infecting adult mice with variants of lymphocytic choriomeningitis virus (LCMV) strain WE. These variants were selected in vivo or in vitro for resistance to lysis by CD8+ H-2b-restricted antiviral CTL. The majority of anti-LCMV CTL in infected H-2b mice recognize epitopes defined by residues 32 to 42 and 275 to 289 (epitopes 32-42 and 275-289) of the LCMV glycoprotein or 397 to 407 of the viral nucleoprotein. The 8.7 variant exhibits a change in the epitope 32-42 (Val-35-->Leu), and variant CL1.2 exhibits a change in the epitope 275-289 (Asn-280-->Asp) of the wild-type LCMV-WE. The double-mutated 8.7-B23 variant had the variation of 8.7 and an additional change located in the epitope 275-289 (Asn-280-->Ser). The 8.7 variant peptide with unchanged anchor positions bound efficiently to H-2Db and H-2Kb molecules but induced only a very weak CTL response. CTL epitope 275-289 of CL1.2 and 8.7-B23 altered at predicted anchor residues were unable to bind Db molecules and were also not recognized by antiviral CTL. Infection of C57BL/6 mice (H-2b) with the variants exhibiting mutations of one of the CTL epitopes, i.e., 8.7 or CL1.2, induced CTL responses specific for the unmutated epitopes comparable with those induced by infection with WE, and these responses were sufficient to eliminate virus from the host. In contrast, infection with the double-mutated variant 8.7-B23 induced CTL activity that was reduced by a factor of about 50-fold compared with wild-type LCMV. Consequently, high doses (10(7) PFU intravenously) of this virus were eliminated slowly and only by about day 100 after infection. 8.7-B23 failed to cause lethal lymphocytic choriomeningitis after intracerebral infection with a dose of > 10(4) PFU in C57BL/6 mice (but not in mice of nonselecting H-2d haplotype); with the other variants or wild-type LCMV, doses greater than 10(6) to 10(7) PFU were necessary to avoid lethal choriomeningitis.(ABSTRACT TRUNCATED AT 400 WORDS)

Modulation of cytokine patterns of human autoreactive T cell clones by a single amino acid substitution of their peptide ligand

We demonstrate that cognate peptide ligands altered at T cell receptor (TCR) contact residues and bound to class II major histocompatability complex can change the cytokine pattern of mature T cell clones. Myelin basic protein peptide 85-99-reactive Th0 T cell clones were stimulated with altered peptide ligands, which acted both as TCR antagonist and induced new mRNA synthesis and protein secretion of TGF-beta 1, while no longer inducing mRNA synthesis or protein secretion of IL-2, IL-4, IL-10, and IFN gamma. The modified peptides failed to induce a detectable calcium flux, p56lck activation, or thymidine incorporation, yet triggered nearly equal amounts of IL-4 secretion in the presence of ionomycin. Antigen-induced modulation of T cell cytokine secretion may be important in regulating the immune response.

Persistent hepatitis C virus infection in a chimpanzee is associated with emergence of a cytotoxic T lymphocyte escape variant

Hepatitis C virus (HCV) establishes a persistent infection in humans and chimpanzees despite the presence of virus-specific, class I major histocompatibility complex-restricted CD8+ cytotoxic T lymphocytes (CTLs) in the liver. The data presented here demonstrate that CTLs directed against a conserved epitope in the HCV nonstructural 3 protein persist in the liver of a chronically infected chimpanzee for at least 2 years after infection. However, these CTLs did not recognize the HCV quasi-species present in the plasma of this animal at week 16 postinfection or at later time points. Escape from the CTL response was facilitated by an aspartic acid to glutamic acid (D-->E) substitution at amino acid position 1449 in all HCV genomes that were sequenced. The results of this study strongly support the concept that CTL responses can select for variant viruses with an enhanced ability to persist in a host and have important implications for the design of vaccines against HCV.

HLA-dependent variations in human immunodeficiency virus Nef protein alter peptide/HLA binding

In human immunodeficiency virus (HIV) infection, sequence variations within defined cytotoxic T lymphocyte (CTL) epitopes may lead to escape from CTL recognition. In a previous report, we have shown that the variable central region of HIV Nef protein (amino acids 73-144) that contains potential CTL epitopes, can escape the CTL response. We suggested that this non recognition occurs through a variety of mechanisms. In particular, we provided evidence that HIV Nef sequences recovered from HLA-A11-expressing individuals have alterations in the major anchor residues essential for binding of the two Nef epitopes (amino acids 73-82 and 84-92) to the HLA-A11 molecule. Here, we investigate in more detail whether variations in autologous Nef sequences affect HLA binding, leading to CTL escape. Potential epitopes were sought by testing Nef peptides containing the HLA-A11-specific motif or related motifs. We confirmed that only the two previously described epitopes identified in cytolysis tests have optimal reactivity with the HLA-A11 molecule. We then sequenced several viral variants from donors that do not express the HLA-A11 molecule and compared the variability of these epitopes with those obtained from HLA-A11-expressing individuals. One substitution (Leu85) found in the sequences isolated from both populations increase the reactivity of the HLA-A11-restricted epitope 84-92, and might explain the difference in immunogenicity observed between the two HLA-A11-restricted epitopes from HLA-A11+ individuals. In addition, selective variations were only detected in virus isolated from HLA-A11-expressing individuals. Furthermore, examination of the association of variant peptides with the HLA-A11 molecule demonstrated that a single substitution within the minimal epitope could not always completely abrogate HLA binding, suggesting that multiple alterations within a particular epitope may accumulate during disease progression, allowing the virus to escape CTL recognition.

Significance of T-cell stimulation by altered peptide ligands in T cell biology

Investigations of T-cell responses to altered peptide ligands have provided functional evidence that a T-cell receptor can interpret subtle structural changes in its ligand, highlighting the complexity of this antigen receptor signaling system. Over the past year, observations from many studies have suggested several roles for such analog peptides in various aspects of immune responses. Collectively, these data strongly suggest the existence of naturally occurring altered peptide ligands in the endogenous peptide repertoire, that can actively participate in the development and shaping of T-cell immunity.

Pathogen-based models favoring MHC genetic diversity

We present six models that are currently the most likely ways that pathogens might favor the evolution of MHC genetic diversity. Although each model makes one or more unique predictions, the current lack of crucial data prevents distinguishing the relative importance of each model. However, this first-time organization of these models should contribute to the design of critical experiments. This synthetic review yields at least three essentially new ideas. First, MHC-dependent immune recognition may be sufficiently redundant to render it essentially escape-proof by pathogens. Second, the four models based on pathogen escape do not work (or work weakly) for diversifying class II genes, unless class II-restricted cytotoxic T-cells are important, an idea that is controversial. Third, pathogen-escape events have traditionally been thought to result in only frequency-dependent selection but here we show that heterozygote advantage is an inevitable consequence of such pathogen evasion. Therefore, the controversy over the relative importance of these two forms of balancing selection is largely a false dichotomy.

Zeta phosphorylation without ZAP-70 activation induced by TCR antagonists or partial agonists

Small changes in the peptide-major histocompatibility complex (MHC) molecule ligands recognized by antigen-specific T cell receptors (TCRs) can convert fully activating complexes into partially activating or even inhibitory ones. This study examined early TCR-dependent signals induced by such partial agonists or antagonists. In contrast to typical agonist ligands, both an antagonist and several partial agonists stimulated a distinct pattern of zeta chain phosphorylation and failed to activate associated ZAP-70 kinase. These results identify a specific step in the early tyrosine phosphorylation cascade that is altered after TCR engagement with modified peptide-MHC molecule complexes. This finding may explain the different biological responses to TCR occupancy by these variant ligands.

Both high and low avidity antibodies to the T cell receptor can have agonist or antagonist activity

Anti-TCR antibodies can activate or block the activation of T cells. In the present experiments, we have shown that different monoclonal antibodies to the same TCR can have either agonist or antagonist activity, and we have examined the relationship between these functional effects and the avidity of the antibody for the TCR. We show here that it is not the avidity of an anti-TCR antibody that determines whether it acts as an agonist or an antagonist. Moreover, we show that monovalent Fab fragments of agonist antibodies produce detectable changes in T cell behavior. These data suggest that T cell activation may involve not just aggregation of the TCR but also some induced change in individual ligated receptors, and that agonists may produce this change while antagonists do not. We argue that similar effects may apply to peptide-MHC ligands as well.