Fine structure of a virus-encoded helper T-cell epitope expressed on FBL-3 tumor cells

Antibodies against endogenous retroviruses

The human genome harbors hundreds of thousands of integrations of ancient retroviruses, amassed over millions of years of evolution. To reduce further amplification in the genome, the host prevents transcription of these now endogenous retroviruses (ERVs) through epigenetic repression and, with evolutionary time, ERVs are incapacitated by accumulating mutations and deletions. However, several members of recently endogenized ERV groups still retain the capacity to produce viral RNA, retroviral proteins, and higher order structures, including virions. The retention of viral characteristics, combined with the reversible nature of epigenetic repression, particularly as seen in cancer, allow for immunologically unanticipated ERV expression, perceived by the adaptive immune system as a genuine retroviral infection, to which it has to respond. Accordingly, antibodies reactive with ERV antigens have been detected in diverse disorders and, occasionally, in healthy individuals. Although they are part of self, the retroviral legacy of ERV antigens, and association with and, possibly, causation of disease states may set them apart from typical self-antigens. Consequently, the pathogenic or, indeed, host-protective capacity of antibodies targeting ERV antigens is likely to be context-dependent. Here, we review the immunogenicity of typical ERV proteins, with emphasis on the antibody response and its potential disease implications.

Friend retrovirus studies reveal complex interactions between intrinsic, innate and adaptive immunity

Approximately 4.4% of the human genome is comprised of endogenous retroviral sequences, a record of an evolutionary battle between man and retroviruses. Much of what we know about viral immunity comes from studies using mouse models. Experiments using the Friend virus (FV) model have been particularly informative in defining highly complex anti-retroviral mechanisms of the intrinsic, innate and adaptive arms of immunity. FV studies have unraveled fundamental principles about how the immune system controls both acute and chronic viral infections. They led to a more complete understanding of retroviral immunity that begins with cellular sensing, production of type I interferons, and the induction of intrinsic restriction factors. Novel mechanisms have been revealed, which demonstrate that these earliest responses affect not only virus replication, but also subsequent innate and adaptive immunity. This review on FV immunity not only surveys the complex host responses to a retroviral infection from acute infection to chronicity, but also highlights the many feedback mechanisms that regulate and counter-regulate the various arms of the immune system. In addition, the discovery of molecular mechanisms of immunity in this model have led to therapeutic interventions with implications for HIV cure and vaccine development.

IL17eScan: A Tool for the Identification of Peptides Inducing IL-17 Response

IL-17 cytokines are pro-inflammatory cytokines and are crucial in host defense against various microbes. Induction of these cytokines by microbial antigens has been investigated in the case of ischemic brain injury, gingivitis, candidiasis, autoimmune myocarditis, etc. In this study, we have investigated the ability of amino acid sequence of antigens to induce IL-17 response using machine-learning approaches. A total of 338 IL-17-inducing and 984 IL-17 non-inducing peptides were retrieved from Immune Epitope Database. 80% of the data were randomly selected as training dataset and rest 20% as validation dataset. To predict the IL-17-inducing ability of peptides/protein antigens, different sequence-based machine-learning models were developed. The performance of support vector machine (SVM) and random forest (RF) was compared with different parameters to predict IL-17-inducing epitopes (IIEs). The dipeptide composition-based SVM-model displayed an accuracy of 82.4% with Matthews correlation coefficient = 0.62 at polynomial (t = 1) kernel on 10-fold cross-validation and outperformed RF. Amino acid residues Leu, Ser, Arg, Asn, and Phe and dipeptides LL, SL, LK, IL, LI, NL, LR, FK, SF, and LE are abundant in IIEs. The present tool helps in the identification of IIEs using machine-learning approaches. The induction of IL-17 plays an important role in several inflammatory diseases, and identification of such epitopes would be of great help to the immunologists. It is freely available at http://metagenomics.iiserb.ac.in/IL17eScan/ and http://metabiosys.iiserb.ac.in/IL17eScan/.

Mice of the resistant H-2(b) haplotype mount broad CD4(+) T cell responses against 9 distinct Friend virus epitopes

To date, only a single Friend virus (FV) peptide recognized by CD4(+) T cells in FV-infected mice of the resistant H-2(b) haplotype has been described. To more thoroughly examine the repertoire of CD4(+) T cell responses in H-2(b) mice infected with this retrovirus, 18mer peptides spanning the FV gag, pol, and env coding regions with 11mer overlaps were synthesized. The peptides were then used to stimulate whole splenocytes and purified CD4(+) T cells from FV-infected mice in an IFNγ ELISPOT assay. Nine new CD4(+) T cell epitopes were identified, 3 encoded by gag, 1 by pol, and 5 by env. The high resistance of H-2(b) mice could be related to this very broad CD4(+) T cell response against multiple peptides during FV infection.

Distinct roles of NK cells in viral immunity during different phases of acute Friend retrovirus infection

BACKGROUND

In many virus infections natural killer (NK) cells are critical for the rapid containment of virus replication. Polymorphisms in NK cell receptors as well as viral escape from NK cell responses are associated with pathogenesis and viral loads in HIV-infected individuals, emphasizing their importance in retroviral immunity. In contrast, NK cells of LCMV-infected mice dampened virus-specific T cell responses resulting in impaired virus control. Thus, the exact role of NK cells during different phases of viral infections remains elusive. In this study we characterized the NK cell response at different time points of an acute retroviral infection by using the Friend retrovirus (FV) mouse model.

FINDINGS

Depletion of NK1.1⁺ cells during the initial phase of FV infection (3 to 4 days post infection) resulted in increased viral loads, which correlated with enhanced target cell killing and elevated NK cell effector functions. At days 7 to 15 post infection, NK and NKT cells did not contribute to anti-retroviral immunity. In the transition phase between acute and chronic infection (30 days post infection), NK and NKT cells exhibited an inhibitory role and their depletion resulted in reduced viral loads and significantly improved FV-specific CD8⁺ T cell responses.

CONCLUSIONS

Our results demonstrate an opposed activity of NK cells during retroviral infection. They were protective in the initial phase of infection, when adaptive T cell responses were not yet detectable, but were dispensable for viral immunity after T cell expansion. At later time points they exhibited regulatory functions in inhibiting virus-specific CD8⁺ T cell responses.

Interferon-alpha subtype 11 activates NK cells and enables control of retroviral infection

The innate immune response mediated by cells such as natural killer (NK) cells is critical for the rapid containment of virus replication and spread during acute infection. Here, we show that subtype 11 of the type I interferon (IFN) family greatly potentiates the antiviral activity of NK cells during retroviral infection. Treatment of mice with IFN-α11 during Friend retrovirus infection (FV) significantly reduced viral loads and resulted in long-term protection from virus-induced leukemia. The effect of IFN-α11 on NK cells was direct and signaled through the type I IFN receptor. Furthermore, IFN-α11-mediated activation of NK cells enabled cytolytic killing of FV-infected target cells via the exocytosis pathway. Depletion and adoptive transfer experiments illustrated that NK cells played a major role in successful IFN-α11 therapy. Additional experiments with Mouse Cytomegalovirus infections demonstrated that the therapeutic effect of IFN-α11 is not restricted to retroviruses. The type I IFN subtypes 2 and 5, which bind the same receptor as IFN-α11, did not elicit similar antiviral effects. These results demonstrate a unique and subtype-specific activation of NK cells by IFN-α11.

The innate immune response mediated by cells such as natural killer (NK) cells can contribute to immunity against viral infections. NK cells can kill virus-infected cells and thus inhibit virus replication and spread during acute infection. However, in infections with retroviruses, like HIV, these cells are not sufficient to prevent pathology. Here, we describe a new strategy to augment natural killer cell responses during virus infections by using a subtype of the type I interferon family as antiviral drug. This therapy strongly activated NK cells and enabled them to control retrovirus as well as herpes virus infections in mice. The new approach might have great potential for the treatment of many infectious and tumor diseases in which natural killer cells play a significant role in immunity.

Negative selection by an endogenous retrovirus promotes a higher-avidity CD4+ T cell response to retroviral infection

Effective T cell responses can decisively influence the outcome of retroviral infection. However, what constitutes protective T cell responses or determines the ability of the host to mount such responses is incompletely understood. Here we studied the requirements for development and induction of CD4+ T cells that were essential for immunity to Friend virus (FV) infection of mice, according to their TCR avidity for an FV-derived epitope. We showed that a self peptide, encoded by an endogenous retrovirus, negatively selected a significant fraction of polyclonal FV-specific CD4+ T cells and diminished the response to FV infection. Surprisingly, however, CD4+ T cell-mediated antiviral activity was fully preserved. Detailed repertoire analysis revealed that clones with low avidity for FV-derived peptides were more cross-reactive with self peptides and were consequently preferentially deleted. Negative selection of low-avidity FV-reactive CD4+ T cells was responsible for the dominance of high-avidity clones in the response to FV infection, suggesting that protection against the primary infecting virus was mediated exclusively by high-avidity CD4+ T cells. Thus, although negative selection reduced the size and cross-reactivity of the available FV-reactive naïve CD4+ T cell repertoire, it increased the overall avidity of the repertoire that responded to infection. These findings demonstrate that self proteins expressed by replication-defective endogenous retroviruses can heavily influence the formation of the TCR repertoire reactive with exogenous retroviruses and determine the avidity of the response to retroviral infection. Given the overabundance of endogenous retroviruses in the human genome, these findings also suggest that endogenous retroviral proteins, presented by products of highly polymorphic HLA alleles, may shape the human TCR repertoire that reacts with exogenous retroviruses or other infecting pathogens, leading to interindividual heterogeneity.

Distinct roles of CD4+ T cell subpopulations in retroviral immunity: lessons from the Friend virus mouse model

It is well established that CD4⁺ T cells play an important role in immunity to infections with retroviruses such as HIV. However, in recent years CD4⁺ T cells have been subdivided into several distinct populations that are differentially regulated and perform widely varying functions. Thus, it is important to delineate the separate roles of these subsets, which range from direct antiviral activities to potent immunosuppression. In this review, we discuss contributions from the major CD4⁺ T cell subpopulations to retroviral immunity. Fundamental concepts obtained from studies on numerous viral infections are presented along with a more detailed analysis of studies on murine Friend virus. The relevance of these studies to HIV immunology and immunotherapy is reviewed.

Polyinosinic-polycytidylic acid treatment of Friend retrovirus-infected mice improves functional properties of virus-specific T cells and prevents virus-induced disease

The induction of type I IFN is the most immediate host response to viral infections. Type I IFN has a direct antiviral activity mediated by antiviral enzymes, but it also modulates the function of cells of the adaptive immune system. Many viruses can suppress type I IFN production, and in retroviral infections, the initial type I IFN is weak. Thus, one strategy of immunotherapy in viral infection is the exogenous induction of type I IFN during acute viral infection by TLR ligands. Along these lines, the TLR3/MDA5 ligand polyinosinic-polycytidylic acid [poly(I:C)] has already been used to treat viral infections. However, the immunological mechanisms underlying this successful therapy have not been defined until now. In this study, the Friend retrovirus (FV) mouse model was used to investigate the mode of action of poly(I:C) in antiretroviral immunotherapy. Postexposure, poly(I:C) treatment of FV-infected mice resulted in a significant reduction in viral loads and protection from virus-induced leukemia. This effect was IFN dependent because type I IFN receptor-deficient mice could not be protected by poly(I:C). The poly(I:C)-induced IFN response resulted in the expression of antiviral enzymes, which suppressed FV replication. Also, the virus-specific T cell response was augmented. Interestingly, it did not enhance the number of virus-specific CD4(+) and CD8(+) T cells, but rather the functional properties of these cells, such as cytokine production and cytotoxic activity. The results demonstrate a direct antiviral and immunomodulatory effect of poly(I:C) and, therefore, suggests its potential for clinical treatment of retroviral infections.

Minimal motif mapping of a known epitope on human zona pellucida protein-4 using a peptide biosynthesis strategy

An important step in the development of a human zona pellucida (huZP) peptide vaccine is to define the minimal amino acid motif for a mapped B cell epitope peptide within huZP4. Identification of this minimal motif is necessary to remove an overlapping T cell epitope that induces a pathogenic T cell response. Here we describe motif (PLTLEL(314-319)) mapping of an 18mer B cell epitope peptide(308-325) on huZP4 protein (previously known as huZP1/ZPB protein), achieved using a set of 22 biosynthetic 8mer peptides fused with truncated glutathione S-transferase (GST) or truncated streptavidin protein, and detected using rabbit anti-porcine zona pellucida (pZP) IgG. The immunogenicity of the B cell epitope peptide was evaluated in rabbits using expressed B cell epitope peptide fused with truncated streptavidin as the antigen. This construct elicited high titer antibody to the 18mer B cell epitope peptide, with reactivity to native human ZP, the biosynthetic 18mer peptide and the 18mer peptide GST fusion protein.

Anti-retroviral effects of type I IFN subtypes in vivo

Type I IFN play a very important role in immunity against viral infections. Murine type I IFN belongs to a multigene family including 14 IFN-alpha subtypes but the biological functions of IFN-alpha subtypes in retroviral infections are unknown. We have used the Friend retrovirus model to determine the anti-viral effects of IFN-alpha subtypes in vitro and in vivo. IFN-alpha subtypes alpha1, alpha4, alpha6 or alpha9 suppressed Friend virus (FV) replication in vitro, but differed greatly in their anti-viral efficacy in vivo. Treatment of FV-infected mice with the IFN-alpha subtypes alpha1, alpha4 or alpha9, but not alpha6 led to a significant reduction in viral loads. Decreased splenic viral load after IFN-alpha1 treatment correlated with an expansion of activated FV-specific CD8(+) T cells and NK cells into the spleen, whereas in IFN-alpha4- and -alpha9-treated mice it exclusively correlated with the activation of NK cells. The results demonstrate the distinct anti-retroviral effects of different IFN-alpha subtypes, which may be relevant for new therapeutic approaches.

Host genetic factors that control immune responses to retrovirus infections

Several host genes control retroviral replication and pathogenesis. These include genes that directly affect the replication of retroviruses in target cells and those that control the host immune responses to the viral antigens. Host genetic factors that affect retroviral replication and immune responses to the viral antigens have been best studied in mouse models of Friend leukemia virus (FV) infection. Several genes located within the major histocompatibility complex (MHC), along with a separate gene not linked to the MHC, influence the host immune responses to FV antigens. The latter, the Rfv3, regulates the production of virus-neutralizing antibodies, and thus affects the duration of viremia. T-cell responses to the viral epitopes are controlled by MHC class I and class II genotypes, and both CD8(+) and CD4(+) T-cells are required for spontaneous immune resistance to FV infection. When CD4(+) T-helper cells are efficiently primed with a viral epitope, however, CD8(+) T-cells are not required for immune protection against FV infection, while B cells are absolutely required. There are individuals who possess human immunodeficiency virus type 1 (HIV-1)-reactive IgA antibodies in their mucosal secretions and show strong T-cell responses to HIV-1 antigens, even though they are negative for HIV-1 genome and HIV-1-reactive serum IgG. These HIV-1-exposed but uninfected individuals rarely possess resistance-associated alleles at known AIDS-restricting loci such as CCR5Delta32. Recent genetic analyses have indicated that a large proportion of such exposed but uninfected individuals may share a common genetic background.

Peptide-induced immune protection of CD8+ T cell-deficient mice against Friend retrovirus-induced disease

CD8+ CTLs and virus-neutralizing antibodies have been associated with spontaneous and vaccine-induced immune control of retroviral infections. We previously showed that a single immunization with an env gene-encoded CD4+ T cell epitope protected mice against fatal Friend retrovirus infection. Here, we analyzed immune cell components required for the peptide-induced anti-retroviral protection. Mice lacking CD8+ T cells were nevertheless protected against Friend virus infection, while mice lacking B cells were not. Virus-producing cells both in the spleen and bone marrow decreased rapidly in their number and became undetectable by 4 weeks after infection in the majority of the peptide-immunized animals even in the absence of CD8+ T cells. In the vaccinated animals the production and class switching of virus-neutralizing and anti-leukemia cell antibodies were facilitated; however, virus-induced erythroid cell expansion was suppressed before neutralizing antibodies became detectable in the serum. Further, the numbers of virus-producing cells in the spleen and bone marrow in the early stage of the infection were smaller in the peptide-immunized than in unimmunized control mice in the absence of B cells. Thus, peptide immunization facilitates both early cellular and late humoral immune responses that lead to the effective control of the retrovirus-induced disease, but CD8+ T cells are not crucial for the elimination of virus-infected cells in the peptide-primed animals.

Fine specificity and cross-clade reactivity of HIV type 1 Gag-specific CD4+ T cells

Despite growing evidence that HIV-1-specific CD4(+) T helper (Th) cells may play a role in the control of viremia, discrete Th cell epitopes remain poorly defined. Furthermore, it is not known whether Th cell responses generated using vaccines based on clade B virus sequences will elicit immune responses that are effective in regions of the world where non-clade B viruses predominate. To address these issues we isolated CD4(+) T cell clones from individuals with vigorous HIV-1-specific Th cell responses and identified the minimum epitopes recognized. The minimum peptide length required for induction of CD4(+) T cell proliferation, IFN-gamma secretion, and cytolytic activity ranged from 9 to 16 amino acids in the five epitopes studied. Cross-clade recognition of the defined epitopes was examined for variant peptides from clades A, B, C, D, and AE. Over half the variant epitopes (17 of 32) exhibited impaired recognition, defined as less than 50% of the IFN-gamma secretion elicited by B clade consensus sequence. There was no evidence for antagonistic activity mediated by the variant peptides, and despite strong responses there was no escape of autologous virus from Th responses in the epitopes we studied. Abrogated recognition of variant CD4(+) T cell epitopes presents a potential obstacle to vaccine development.

Role of natural killer cells in resistance against friend retrovirus-induced leukemia

We have previously shown that immunization with a synthetic peptide that contains a single CD4(+) T-cell epitope protects mice against immunosuppressive Friend retrovirus infection. Cells producing infectious Friend virus were rapidly eliminated from the spleens of mice that had been immunized with the single-epitope peptide. However, actual effector mechanisms induced through T-helper-cell responses after Friend virus inoculation were unknown. When cytotoxic effector cells detected in the early phase of Friend retrovirus infection were separated based on their expression of cell surface markers, those lacking CD4 and CD8 but expressing natural killer cell markers were found to constitute the majority of effector cells that lysed Friend virus-induced leukemia cells. Depletion of natural killer cells by injecting anti-asialo-ganglio-N-tetraosylceramide antibody did not affect the number of CD4(+) or CD8(+) T cells in the spleen, virus antigen-specific proliferative responses of CD4(+) T cells, or cytotoxic activity against Friend virus-induced leukemia cells exerted by CD8(+) effector cells. However, the same treatment markedly reduced the killing activity of CD4(-) CD8(-) effector cells and completely abolished the effect of peptide immunization. Although the above enhancement of natural killer cell activity in the early stage of Friend virus infection was also observed in mice given no peptide, these results have demonstrated the importance and requirement of natural killer cells in vaccine-induced resistance against the retroviral infection.

CD4(+) T cells and gamma interferon in the long-term control of persistent friend retrovirus infection

We have used the Friend virus model to determine the basic mechanisms by which the immune system can control persistent retroviral infections. Previously we showed that CD4(+) T cells play an essential role in keeping persistent retrovirus in check. The present in vitro experiments with a Friend virus-specific CD4(+) T-cell clone revealed that these cells produce gamma interferon (IFN-gamma), which acts with two distinct mechanisms of antiviral activity. First, IFN-gamma had a direct inhibitory effect on virus production. This inhibitory effect was noncytolytic and, interestingly, was not associated with decreased cell surface expression of viral antigens. The second mechanism of IFN-gamma-mediated antiviral activity was an enhancement of CD4(+) T-cell-mediated cytolytic activity. We also found an in vivo role for IFN-gamma in the control of persistent Friend virus infections. Neutralization of IFN-gamma in persistently infected mice resulted in significantly increased levels of virus in the spleen, and a significant percentage of IFN-gamma-deficient mice were unable to maintain long-term control over Friend virus infections.

Induction of cross-reactivity in an endogenous viral peptide non-reactive to FBL-3 tumor-specific helper T-cell clones

We previously reported a helper T-cell (Th) epitope (peptide i) which corresponded to the sequence ranging from positions 462 to 479 from the N-terminus of the Friend-murine leukemia virus (F-MuLV) envelope protein (env462-479). Homologous sequences exist in both Moloney-murine leukemia (M-MuLV env452-469) and endogenous AKV (AKV env453-470) viruses, which differ from F-MuLV env462-479 in 5 and 7 amino acids, respectively. However, peptide i-specific Th clones did not respond to either of the corresponding exogenous or endogenous peptides. One amino acid substitution in M-MuLV env452-469 (Asn to Tyr at position 465: N465Y) and three amino acids in AKV env453-470 (H460S, A466Y and Y468H) endowed both peptides with the reactivity to one of the Th clones, F5-5, almost to the same degree as peptide i. However, the other Th clones responded differently to each of the modified endogenous peptides substituted by one to three amino acids. The cells responsive to the cross-reactive peptides occupied only a minor portion, if any, of the bulk cultured lymph node cells from peptide i-immune mice, and in particular, no significant response to the modified endogenous peptides was observed in repeated experiments. The exchange of at least 3 residues was necessary for the endogenous peptide to acquire sufficient cross-reactivity to two of the three Th clones. However, it was noticeable that a single substitution of alanine by tyrosine at the dominant T-cell receptor (TCR) contact position of the peptide i(e) generated a weak but significant cross-reactivity to one of the three Th clones in this study. Thus, peptides of endogenous retroviral origin that would be modified by mutational events might become 'non-self' and prime Th cells leading to auto-antibody production and resulting in autoimmune disease.

Immunoprotective determinants in friend murine leukemia virus envelope protein

Several immunological epitopes are known to be located within the Friend murine leukemia virus (F-MuLV) envelope protein, but their relative contributions to protection from Friend virus-induced disease are not known. To determine how expression of various immunological determinants affected protection, mice were immunized with recombinant vaccinia viruses expressing different portions of the F-MuLV envelope protein, and they were then challenged with a lethal dose of Friend virus complex. The disease parameters that were followed in the mice were early viremia, early splenomegaly, and late splenomegaly. Both the N-terminal and C-terminal portions of the F-MuLV gp70 were found to protect against late splenomegaly, the primary clinical sign associated with virus-induced erythroleukemia. However, neither region alone protected against early splenomegaly and early viremia, indicating poor immunological control over early virus replication and spread through the spleen and blood. In contrast, mice immunized with a vaccine expressing the entire F-MuLV envelope protein were protected against all three disease parameters. The results indicated that expression of multiple immunological determinants including both T-helper and B cell epitopes was necessary for full protection.

Specific T helper cell requirement for optimal induction of cytotoxic T lymphocytes against major histocompatibility complex class II negative tumors

This study shows that induction of tumor-specific CD4+ T cells by vaccination with a specific viral T helper epitope, contained within a synthetic peptide, results in protective immunity against major histocompatibility complex (MHC) class II negative, virus-induced tumor cells. Protection was also induced against sarcoma induction by acutely transforming retrovirus. In contrast, no protective immunity was induced by vaccination with an unrelated T helper epitope. By cytokine pattern analysis, the induced CD4+ T cells were of the T helper cell 1 type. The peptide-specific CD4+ T cells did not directly recognize the tumor cells, indicating involvement of cross-priming by tumor-associated antigen-presenting cells. The main effector cells responsible for tumor eradication were identified as CD8+ cytotoxic T cells that were found to recognize a recently described immunodominant viral gag-encoded cytotoxic T lymphocyte (CTL) epitope, which is unrelated to the viral env-encoded T helper peptide sequence. Simultaneous vaccination with the tumor-specific T helper and CTL epitopes resulted in strong synergistic protection. These results indicate the crucial role of T helper cells for optimal induction of protective immunity against MHC class II negative tumor cells. Protection is dependent on tumor-specific CTLs in this model system and requires cross-priming of tumor antigens by specialized antigen-presenting cells. Thus, tumor-specific T helper epitopes have to be included in the design of epitope-based vaccines.

Immunity to retroviral infection: the Friend virus model

Friend virus infection of adult immunocompetent mice is a well established model for studying genetic resistance to infection by an immunosuppressive retrovirus. This paper reviews both the genetics of immune resistance and the types of immune responses required for recovery from infection. Specific major histocompatibility complex (MHC) class I and II alleles are necessary for recovery, as is a non-MHC gene, Rfv-3, which controls virus-specific antibody responses. In concordance with these genetic requirements are immunological requirements for cytotoxic T lymphocyte, T helper, and antibody responses, each of which provides essential nonoverlapping functions. The complexity of responses necessary for recovery from Friend virus infection has implications for both immunotherapies and vaccines. For example, it is shown that successful passive antibody therapy is dependent on MHC type because of the requirement for T cell responses. For vaccines, successful immunization requires priming of both T cell and B cell responses. In vivo depletion experiments demonstrate different requirements for CD8(+) T cells depending on the vaccine used. The implications of these studies for human retroviral diseases are discussed.

Identification of a gag-encoded cytotoxic T-lymphocyte epitope from FBL-3 leukemia shared by Friend, Moloney, and Rauscher murine leukemia virus-induced tumors

FBL-3 is a highly immunogenic murine leukemia of C57BL/6 origin induced by Friend murine leukemia virus (MuLV). Immunization of C57BL/6 mice with FBL-3 readily elicits CD8+ cytotoxic T lymphocytes (CTL) capable of lysing FBL-3 as well as syngeneic leukemias induced by Moloney and Rauscher MuLV. The aim of this current study was to identify the immunogenic epitope(s) recognized by the FBL-3-specific CD8+ CTL. A series of FBL-3-specific CD8+ CTL clones were generated from C57BL/6 mice immunized to FBL-3. The majority of CTL clones (32 of 38) were specific for F-MuLV gag-encoded antigen. By using a series of recombinant vaccinia viruses expressing full-length and truncated F-MuLV gag genes, the antigenic epitope recognized by the FBL-3 gag-specific CTL clones, as well as by bulk-cultured CTL from spleens of mice immune to FBL-3, was localized to the leader sequence of gPr80gag protein. The precise amino acid sequence of the CTL epitope in the leader sequence was identified as CCLCLTVFL (positions 85-93) by examining lysis of targets incubated with a series of synthetic leader sequence peptides. No evidence of other CTL epitopes in the gPr80gag or Pr65gag core virion structural polyproteins was found. The identity of CCLCLTVFL as the target peptide was validated by showing that immunization with the peptide elicited CTL that lysed FBL-3. The CTL elicited by the Gag peptide also specifically lysed syngeneic leukemia cells induced by Moloney and Rauscher MuLV (MBL-2 and RBL-5). The transmembrane peptide was shown to be the major gag-encoded antigenic epitope recognized by bulk-cultured CTL derived from C57BL/6 mice immunized to MBL-2 or RBL-5. Thus, the CTL epitope of FBL-3 is localized to the transmembrane anchor domain of the nonstructural Gag polyprotein and is shared by leukemia/lymphoma cell lines induced by Friend, Moloney, and Rauscher MuLV.

Differing T-cell requirements for recombinant retrovirus vaccines

Friend murine leukemia virus is a retrovirus complex that induces rapid erythroleukemia and immunosuppression in susceptible strains of adult mice. Using this model, we directly examined the T-cell subsets required for a protective retrovirus vaccine. Paradoxically, recovery in mice immunized with a chimeric envelope containing only T-helper (TH) and B-cell epitopes was dependent on CD8+ T cells as well as CD4+ T cells despite the fact that the vaccine contained no CD8+ cytolytic T-lymphocyte (CTL) epitopes. However, the requirement for CD8+ T cells was overcome by inclusion of additional TH and B-cell epitopes in the immunizing protein. These additional epitopes primed for more rapid production of virus-neutralizing antibody which appeared to limit virus spread sufficiently to protect even in the absence of CD8+ T cells. Inclusion of an immunodominant CTL epitope in the vaccine was not sufficient to overcome dependence on CD4+ T cells. These data suggest that TH priming is more critical for retrovirus immunity than CTL priming.

A single retroviral gag precursor signal peptide recognized by FBL-3 tumor-specific cytotoxic T lymphocytes

Several dominant T-cell receptors of cytotoxic T-lymphocyte (CTL) clones specific for FBL-3 tumor antigen were clonally amplified in mixed lymphocyte tumor cell cultures derived from an individual immune mouse. Every CTL clone analyzed had a common specificity for a single epitope in the precursor to cell membrane-associated nonstructural gag-encoded protein, Pr75gag, which can be minimally identified by nine amino acid residues, SIVLCCLCL. This epitope is located within the hydrophobic signal sequence motif that mediates translocation of the protein into the endoplasmic reticulum. These novel observations suggest that expression of Pr75gag in FBL-3 tumor cells led to the amplification of CTLs which recognize the signal sequence of the nonstructural gag-encoded glycoprotein precursor.