Influenza virus hemagglutinin-specific cytotoxic T cell response induced by polypeptide produced in Escherichia coli

Combination vaccines: synergistic simultaneous induction of antibody and T-cell immunity

Vaccines have traditionally been designed to induce antibody responses and have been licensed on their capacity to induce high titers of circulating antibody to the pathogen. With our increased knowledge of host-pathogen interactions, it became apparent that induction of the cellular arm of the immune response is crucial to the efficacy of vaccines against intracellular pathogens and for providing appropriate help for antibody induction. Diverging strategies emerged that concentrate on developing candidate vaccines that solely induce either cellular or humoral responses. As most microbes reside at some point in the infectious cycle in the extracellular as well as intracellular space, and there is interplay between antibody and T cells, it is now apparent that both arms of immunity are essential to effectively control and eliminate the infection. It is, therefore, necessary to develop vaccines that can effectively induce a broad adaptive immune response. For vaccines targeted at diseases of the developing world, such as HIV, tuberculosis and malaria, it is imperative that these vaccines are simple to deliver and cost effective, that is,that optimum T-cell and antibody immunity is achieved with the minimum number of vaccinations. Combination vaccines, where an antibody-inducing subunit protein vaccine is coadministered with a T-cell-inducing poxvirus-based vaccine fulfill these requirements and induce sterile immunity to pathogen challenge.

Revival of the identification of cytotoxic T-lymphocyte epitopes for immunological diagnosis, therapy and vaccine development

Immunogenic T-cell epitopes have a central role in the cellular immunity against pathogens and tumors. However, in the early stage of cellular immunity studies, it was complicated and time-consuming to identify and characterize T-cell epitopes. Currently, the epitope screening is experiencing renewed enthusiasm due to advances in novel techniques and theories. Moreover, the application of T-cell epitope-based diagnoses for tuberculosis and new data on epitope-based vaccine development have also revived the field. There is a growing knowledge on the emphasis of epitope-stimulated T-cell immune responses in the elimination of pathogens and tumors. In this review, we outline the significance of the identification and characterization of T-cell epitopes. We also summarize the methods and strategies for epitope definition and, more importantly, address the relevance of cytotoxic T-lymphocyte epitopes to clinical diagnoses, therapy and vaccine development.

Individual mouse analysis of the cellular immune response to tumor antigens in peripheral blood by intracellular staining for cytokines

Among the experimental animal models, mice remain the most widely used for the evaluation of immunotherapeutic strategies. Vaccines against parasites and viral antigens are commonly administered to the appropriate mouse strain which also allows testing of the therapeutic effect. Similarly, in mice transgenic for human tumor associated antigens (TAA), cancer vaccines must lead to breakage of immune tolerance to elicit a significant effect on the tumor. However, one of the major drawbacks in the monitoring of cellular immune responses induced by vaccination is that functional immunological assays require suppression of the animals to collect the spleen or lymph nodes for analysis. Here, we report the application of a rapid intracellular staining (ICS) method to quantify antigen-specific T cells responses in small volumes of murine blood. Genetic vaccination with plasmid DNA followed by electroporation (DNA-EP) and the use of adenoviral vectors (Ad) encoding CEA as a model target antigen were applied to different strains of mice. Optimal blood volume, number of lymphocytes, sensitivity and reproducibility of intracellular staining for IFN-gamma were determined both in non-tolerant/wild type mice as well as in tolerant CEA transgenic mice upon restimulation of PBMCs with CEA peptides. Groups of vaccinated mice were then sacrificed and PBMCs and splenocytes from individual animals were compared for intracytoplasmic detection of IFN-gamma and TNF-alpha. A significant correlation was observed between splenic and blood immune responses. Finally, the cellular immune response was followed over time in groups of vaccinated mice. The kinetics of IFN-gamma producing effectors were measured after priming and successive boosting with adenoviral vectors. We show that intracellular staining for mouse PBMCs is a rapid and simple method to measure antigen-specific immune responses. It does not require animal euthanasia and mirrors the response observed in lymphoid organs such as the spleen.

Chloroquine enhances human CD8+ T cell responses against soluble antigens in vivo

The presentation of exogenous protein antigens in a major histocompatibility complex class I–restricted fashion to CD8⁺ T cells is called cross-presentation. We demonstrate that cross-presentation of soluble viral antigens (derived from hepatitis C virus [HCV], hepatitis B virus [HBV], or human immunodeficiency virus) to specific CD8⁺ T cell clones is dramatically improved when antigen-presenting dendritic cells (DCs) are pulsed with the antigen in the presence of chloroquine or ammonium chloride, which reduce acidification of the endocytic system. The export of soluble antigen into the cytosol is considerably higher in chloroquine-treated than in untreated DCs, as detected by confocal microscopy of cultured cells and Western blot analysis comparing endocytic and cytosolic fractions. To pursue our findings in an in vivo setting, we boosted groups of HBV vaccine responder individuals with a further dose of hepatitis B envelope protein vaccine with or without a single dose of chloroquine. Although all individuals showed a boost in antibody titers to HBV, six of nine individuals who were administered chloroquine showed a substantial CD8⁺ T cell response to HBV antigen, whereas zero of eight without chloroquine lacked a CD8 response. Our results suggest that chloroquine treatment improves CD8 immunity during vaccination.

HLA-restricted epitope identification and detection of functional T cell responses by using MHC-peptide and costimulatory microarrays

Identification of T cell epitopes is a vital but often slow and difficult step in studying the immune response to infectious agents and autoantigens. We report a spatially addressable technique for screening large numbers of T cell epitopes for both specific antigen recognition and functional activity induced. This system uses microarrays of immobilized, recombinant MHC-peptide complexes, costimulatory molecules, and cytokine-capture antibodies. The array elements act as synthetic antigen-presenting cells and specifically elicit T cell responses, including adhesion, secretion of cytokines, and modulation of surface markers. The method allows facile identification of pertinent T cell epitopes in a large number of candidates and simultaneous determination of the functional outcome of the interaction. Using this method, we have characterized the activation of human CD4(+) and CD8(+) T cells responding to vaccinia, influenza, HIV-1, and Epstein-Barr viruses.

A simple and efficient method for the monitoring of antigen-specific T cell responses using peptide pool arrays in a modified ELISpot assay

In this study, we describe a simple and efficient method for both the monitoring of antigen-specific CD4 and CD8 T cell responses as well as the identification of novel CD4 and CD8 T cell epitopes using a modified ELISpot assay and pools of 20mer peptides. We have demonstrated that pools containing as many as 64 20mer peptides may be used to screen for CD4 and CD8 T cell responses to HPV16 L1, E1, and E7 in mice. Using arrays of pools of overlapping 20mer peptides, we have identified novel CD4 and CD8 epitopes in both HPV16L1 and HPV16E1 which are presented in Balb/c mice. We have further shown that the use of 20mer peptides is equivalent to using minimal 9mer epitopes for the stimulation of CD8 T cell responses in our assay. While our experiments are conducted in mice, the use of peptide pool arrays allows for the identification of epitope-specific responses using far fewer cells than is required for testing a panel of overlapping peptides individually, making this strategy particularly useful in clinical settings where immune cells may be limiting.

Alternative pathways for processing exogenous and endogenous antigens that can generate peptides for MHC class I-restricted presentation

The concept of distinct endogenous and exogenous pathways for generating peptides for MHC-I and MHC-II-restricted presentation to CD4+ or CD8+ T cells fits well with the bulk of experimental data. Nevertheless, evidence is emerging for alternative processing pathways that generate peptides for MHC-I-restricted presentation. Using a well characterized, particulate viral antigen of prominent medical importance (the hepatitis B surface antigen), we summarize our evidence that the efficient, endolysosomal processing of exogenous antigens can lead to peptide-loaded MHC-I molecules. In addition, we describe evidence for endolysosomal processing of mutant, stress protein-bound, endogenous antigens that liberate peptides binding to (and presented by) MHC-I molecules. The putative biological role of alternative processing of antigens generating cytotoxic T-lymphocyte-stimulating epitopes is discussed.

Particulate vaccine candidate for Japanese encephalitis induces long-lasting virus-specific memory T lymphocytes in mice

We previously reported that extracellular particles (EPs) composed of premembrane (prM) and envelope (E) proteins were released from cells infected with recombinant vaccinia viruses encoding Japanese encephalitis (JE) virus prM and E genes. In the present study, EPs were evaluated for induction of JE virus-specific antibody and specific T lymphocytes in mice. Six- to 8-week-old male Balb/c mice were inoculated intraperitoneally once or twice (at a 3-week interval) with purified EPs containing 1 microgram of E without adjuvant. Neutralizing antibody was detected and spleen cells proliferated against JE viral antigen 3 weeks after the second immunization with EPs. Neutralizing antibody and JE virus-specific T lymphocytes were also detected 10 months after immunization with EPs containing 2 micrograms of E. Spleen cells obtained from EP-immunized mice and stimulated in vitro with live JE virus, expressed JE virus-specific cytotoxic activity. The cytotoxic activity was reduced by treatment with anti-CD3 antibody and complement. These results indicate that immunization with EPs induces long-lasting specific antibody and memory T cells in mice.

Analysis of T-cell responses in cattle immunized against heartwater by vaccination with killed elementary bodies of Cowdria ruminantium

Cattle were successfully immunized against heartwater with a lysate of Cowdria ruminantium formulated in Freund's adjuvant. Vaccinated animals proved fully resistant to virulent challenge 3 and 10 months after vaccination. For the first time a helper T lymphocyte response to Cowdria antigens was observed and characterized. Cowdria-specific T-cell lines generated from vaccinated animals by in vitro restimulation with Cowdria lysates are 95 to 100% CD4+, are MHC class II restricted, and produce gamma interferon. They proliferate in response to autologous monocytes infected with live Cowdria but not in response to uninfected monocytes. These T-cell lines will facilitate the search for Cowdria antigens that are immunogenic for T cells and will therefore be of relevance in the development of a subunit vaccine against the disease.

Effects of a minor isoleucyl tRNA on heterologous protein translation in Escherichia coli

In Escherichia coli, the isoleucine codon AUA occurs at a frequency of about 0.4% and is the fifth rarest codon in E. coli mRNA. Since there is a correlation between the frequency of codon usage and the level of its cognate tRNA, translational problems might be expected when the mRNA contains high levels of AUA codons. When a hemagglutinin from the influenza virus, a 304-amino-acid protein with 12 (3.9%) AUA codons and 1 tandem codon, and a mupirocin-resistant isoleucyl tRNA synthetase, a 1,024-amino-acid protein, with 33 (3.2%) AUA codons and 2 tandem codons, were expressed in E. coli, product accumulation was highly variable and dependent to some degree on the growth medium. In rich medium, the flu antigen represented about 16% of total cell protein, whereas in minimal medium, it was only 2 to 3% of total cell protein. In the presence of the cloned ileX, which encodes the cognate tRNA for AUA, however, the antigen was 25 to 30% of total cell protein in cells grown in minimal medium. Alternatively, the isoleucyl tRNA synthetase did not accumulate to detectable levels in cells grown in Luria broth unless the ileX tRNA was coexpressed when it accounted for 7 to 9% of total cell protein. These results indicate that the rare isoleucine AUA codon, like the rare arginine codons AGG and AGA, can interfere with the efficient expression of cloned proteins.

Influenza A subtype cross-protection after immunization of outbred mice with a purified chimeric NS1/HA2 influenza virus protein

Influenza A/PR/8/34-derived chimeric (D) protein (SK&F 106160) composed of the first 81 amino acids (aa) of NS1 fused to the conserved 157 C-terminal aa of HA2 (NS1 1-81-HA2 65-222) was previously shown to induce H-2d-restricted protective cytotoxic T-lymphocyte (CTL) immunity in inbred mice. However, D protein, like other small peptides, exhibited haplotype dependence and was not immunogenic in H-2b and H-2K mice. A potential use of this antigen in humans and the role of T cells in any protection were evaluated in outbred Swiss and inbred CBF6F1 (H-2d/b) mice. Mice immunized with D protein and challenged by small-particle aerosol with a lethal dose of influenza virus were significantly protected against mortality from influenza A/H1N1 and A/H2N2 (p < 0.05-< 0.0000001), but not from A/H3N2 and influenza B viruses when compared with control mice. D protein did not induce serum virus-neutralizing antibody but caused virus to be cleared faster in immunized mice. Protection was long-lasting. In vivo depletion of either Lyt2 (CD8+) or L3T4 (CD4+) T cells with monoclonal antibodies led to abrogation of in vitro-generated CTL activity in CF6F1 mice and significant reduction in the protective efficacy of D protein against virus challenge in both Swiss and CF6F1 mice. These results suggest that protection was mediated by CD8+ and/or CD4+ cells and not antibody. Thus D protein, via a conserved sequence on the HA2 polypeptide, has the potential to induce partially cross-reactive CTL that may protect against influenza virus disease in humans.

Presentation of an exogenous antigen by major histocompatibility complex class I molecules

Cytotoxic T lymphocytes (CTL) generally recognize peptides derived from endogenously expressed proteins in association with nascent major histocompatibility complex (MHC) class I molecules. In contrast, peptides derived from exogenous proteins associate with MHC class II following endocytosis to an endosomal compartment. However, we have recently demonstrated that exogenous fusion proteins consisting of the binding and translocating domains of Pseudomonas exotoxin (PE) fused with CTL epitopes derived from either influenza matrix protein (PEMa) or nucleoprotein are internalized, processed, targeted to and presented by MHC class I (Donnelly et al. 1993, Proc. Natl. Acad. Sci. USA 1993. 90: 3530). PE is known to be internalized, processed in endosomes, and translocated to the cytosol during intoxication of cells. However, our present studies demonstrate that, unlike PE, PEMa does not require translocation to the cytosol to exert its effect. First, two inhibitors of PE toxicity that exert their effects at steps subsequent to endosomal processing had no effect on the sensitization of target cells for CTL-mediated lysis by PEMa. NH4Cl, which inhibits PE by raising endosomal pH, and brefeldin A, which inhibits PE by disrupting the Golgi complex, did not inhibit sensitization of targets cells by PEMa. Second, PEMa was capable of sensitizing for lysis T2 mutant cells, which are defective in transport of peptides from the cytosol to the lumen of the endoplasmic reticulum for presentation by MHC class I. These results suggest that PEMa is proteolytically processed in endosomes, and association with MHC class I does not require nascent MHC molecules. Such a process may involve internalized MHC class I, and subsequent expression of the peptide-MHC complexes on the cell surface would then lead to recognition by CTL.

Selective stimulation of murine cytotoxic T cell and antibody responses by particulate or monomeric hepatitis B virus surface (S) antigen

In the murine system, we tested in vivo the immunogenicity of different preparations of the yeast-derived surface antigen (S-antigen or S-protein) of hepatitis B virus (HBV). Native S-protein molecules self-assemble into stable 22-nm particles. BALB/c mice immunized with low doses of native S-particles without adjuvants efficiently generated an H-2 class I-restricted CD8+ cytotoxic T lymphocyte (CTL) response, and developed easily detectable serum antibody titers against conformational determinants of the native S-particle or linear epitopes of the denatured S-protein. Disruption of S-particles with sodium dodecyl sulfate and beta-2-mercaptoethanol generated p24 S-monomers. Injection of an equal dose of S-monomers into mice efficiently primed CTL, but did not stimulate an antibody response against conformational or linear epitopes of the native or denatured S-protein. In vivo priming of CTL by S-particles or S-monomers required "endogenous" processing of the antigen because the injection of an equimolar (or higher) dose of an antigenic, S-derived 12-mer peptide into mice did not prime CTL. Native (particulate) or denatured (monomeric) S-antigen injected with mineral oil (incomplete Freund's adjuvant) or aluminum hydroxide failed to stimulate a CTL response. Hence, different preparations can be produced from a small protein antigen which specifically stimulate selected compartments of the immune system.

Antibody and cytotoxic T-cell responses to soluble hepatitis B virus (HBV) S antigen in mice: implication for the pathogenesis of HBV-induced hepatitis

Immune responses to components of hepatitis B virus (HBV) are assumed to play an essential role not only in the elimination of the virus but also in the pathogenesis of HBV-induced hepatitis. Protective humoral immunity to HBV is mediated by immune responses to HBV surface antigen (HBsAg). It is important to know which HBsAg preparations induce which type of cellular and humoral immune responses under which immunization conditions. We studied in BALB/c mice the humoral (antibody) response and the class I-restricted cytotoxic T-lymphocyte (CTL) response to different preparations of HBsAg particles: recombinant, small protein particles; plasma-derived, mixed particles formed by large, medium, and small surface proteins; and different preparations of recombinant, mixed particles formed by large and small surface proteins. Specific antibody levels appeared in the sera of immunized mice 2 to 3 weeks after immunization and were correlated with the antigen dose used for priming. HBsAg-specific antibody levels were enhanced by boost injections or by adsorbing the antigen to aluminum hydroxide. Injected in particulate form without adjuvants in the dose range of 0.1 to 10 micrograms per mouse, all HBsAg preparations tested efficiently primed specific CD8+ CTL of defined restriction and epitope specificity. Specific CTL reactivity was detectable from 5 days to more than 4 months postimmunization. In the dose range tested, it was independent of the antigen dose used for immunization and not enhanced by repeated boost injections. CTL were not elicited by HBsAg adsorbed to aluminum hydroxide. We have thus defined conditions under which HBsAg induced preferentially either a cellular immune response or a humoral immune response. These findings may be relevant for the interpretation of HBV-associated immunopathologic phenomena.

Protective immunity of Hantaan virus nucleocapsid and envelope protein studied using baculovirus-expressed proteins

Recombinant Hantaan virus nucleocapsid protein (rNP) and recombinant envelope (rEnv) proteins were prepared using a baculovirus expression system to examine the role of Hantaan virus structural proteins in protective immunity. Passive transfer of spleen cells from mice immunized with rNP conferred partial protection or prolongation of time to death from fatal Hantaan virus infection in suckling mice which were challenged with Hantaan virus at 40 LD50 (survival rate: 43%) or 4 LD50 (survival rate: 43%). The T cell-enriched fraction protected one mouse from lethal infection but the B cell-enriched fraction had no such effect on fatal HTN infection. The protective effects of the antibody against HTN challenge were examined by passive immunization. The monoclonal antibody ECO 2 directed to NP also conferred partial survival and significant difference in time to death. Although rEnv antigen failed to induce neutralizing antibody, both immune spleen cells and immune serum to rEnv conferred partial protection upon suckling mice. These results indicate that both nucleocapsid and envelope proteins of Hantaan virus were responsible for induction of cell mediated protective immunity. Vero E 6 cells infected with Hantaan virus expressed envelope protein on the surface, as determined by flow cytometry. However, there was only negligible expression of nucleocapsid protein.

Induction of CD8+ cytotoxic T cells by immunization with killed influenza virus and effect of cholera toxin B subunit

The MHC class I cytotoxic T-lymphocyte (CTL) response in mice given formalin-inactivated influenza whole-virus vaccine (WVV) with or without cholera toxin B subunit (CTB) was studied. Intraperitoneal injection of Balb/c (H-2d) mice with high doses of A/Taiwan/1/86 (H1N1) WVV stimulated influenza A virus-specific CTL response in a dose-dependent manner. A dose of 4.4 or 44 micrograms induced CTL response equal to or greater than live influenza virus infection. Coadministration of vaccine with 5 or 25 micrograms of CTB resulted in a higher level of CTL than with vaccine alone. CTL lysed A/Taiwan and A/Shanghai (H3N2) virus-infected class I-expressing P815 (H-2d) but not virus-infected EL-4 (H-2b) target cells nor B/Yamagata virus-infected target cells. Virus-infected MHC class II- and class I-expressing A20 (H-2d) targets were also lysed. Depletion of Lyt-2+ (CD8+) T cells with monoclonal antibody completely abrogated lysis of P815 target cells and resulted only in a slight reduction of lysis of A20 target cells. Depletion of L3T4+ (CD4+) T cells or NK cells had minimal effect on lysis of either P815 or A20 target cells. Using limiting dilution analysis, the precursor CTL (pCTL) frequency paralleled CTL activity. Significant CTL activity was detected 7 months after immunization. These results demonstrate that adequate doses of influenza WVV with or without CTB can induce long-lasting influenza A cross-reactive MHC class I-restricted CD8+ CTL response in mice. Thus, coadministration of influenza WVV with CTB may lead to an effective vaccine that stimulates both CTL and antibody responses.

Can soluble antigens induce CD8+ cytotoxic T-cell responses? A paradox revisited

Cytotoxic T lymphocyte (CTL) induction usually follows processing of antigens via endogenous pathways before presentation on the cell surface in association with MHC class I molecules. Soluble antigens do not, in general, induce specific CTL responses. Here, Syamal Raychaudhuri and John Morrow suggest that a novel adjuvant formulation can elicit CTL responses to soluble antigens and discuss the implications for vaccine development.

Protective cross-reactive epitope on the nonstructural protein NS1 of influenza A virus

We reported previously that adoptive immunization with an influenza A virus NS1-specific H-2Ld-restricted, cross-reactive, CTL clone A-11 established by stimulation with A/PR/8/34 virus (H1N1) reduced lung virus titers in mice challenged with virus in vivo (Virology 178:174-179, 1990). Using a set of recombinant vaccinia virus constructs containing truncated portions of the NS gene we have localized this cross-protective CTL epitope to the N-terminal region of the NS1 protein. This region of NS1 is active in inducing CD8+ CTL in vivo because virus-stimulated BALB/c immune spleen cells in bulk cultures also recognized the N-terminal region of the NS1 protein.

Poliovirus-specific major histocompatibility complex class I-restricted cytolytic T-cell epitopes in mice localize to neutralizing antigenic regions

A major histocompatibility complex (MHC) class I-restricted cytotoxic T-lymphocyte (CTL) response is induced in BALB/c mice upon immunization with poliovirus serotype 1 (Mahoney strain). A similar class I-restricted response is also induced upon immunization with purified VP1 capsid proteins. Thus, poliovirus-specific MHC class I CTL responses can be induced independently of viral infection in murine hosts. In experiments using recombinant vaccinia virus vectors expressing different segments of the poliovirus capsid proteins and synthetic peptides, two regions of the VP1 capsid protein appear to contain epitopes recognized by this bulk CTL population. These epitope regions contain a Kd-restricted peptide-binding motif. Interestingly, each of these CTL epitopes is located near previously defined neutralizing antigenic sites.

Liposomal malaria vaccine in humans: a safe and potent adjuvant strategy

This study describes the safety and immunogenicity of a liposome-based vaccine injected into human subjects. Thirty healthy adult male volunteers were immunized with a liposome-encapsulated recombinant protein (R32NS181) containing epitopes from the repeat region of the circumsporozoite protein of Plasmodium falciparum. This antigen had previously been found to be poorly immunogenic in humans when it was adsorbed with Al(OH)3. In the present study, R32NS181 was encapsulated in liposomes containing monophosphoryl lipid A that were subsequently adsorbed to Al(OH)3. Increasing doses of liposomes containing antigen and monophosphoryl lipid A were used, but the liposomes were always adsorbed to the same dose of Al(OH)3. R32-specific serum IgG antibody responses to liposome-encapsulated R32NS181 were much higher than levels attained previously in humans with R32NS181 adsorbed to Al(OH)3. Geometric mean specific IgG levels after three doses ranged from 14 to 33 micrograms/ml. Sera from volunteers receiving the two highest doses inhibited P. falciparum sporozoite invasion of cultured hepatoma cells by an average of 92%, a result that was again superior to previously reported vaccines. Moderate but acceptable transient local reactogenicity was noted at high doses of the vaccine formulation, but little or no systemic toxicity was seen despite liposomal monophosphoryl lipid A doses up to 2200 micrograms. We conclude that encapsulation of poorly immunogenic circumsporozoite protein repeat peptides in monophosphoryl lipid A-containing liposomes is a successful adjuvant strategy in humans for inducing high levels of specific antibody production.

Induction of protective class I MHC-restricted CTL in mice by a recombinant influenza vaccine in aluminium hydroxide adjuvant

Induction of class I MHC-restricted cytotoxic T lymphocyte (CTL) responses by soluble proteins or peptides requires complex adjuvants or carrier systems which are not licensed for use with human vaccines. The data presented in this report show that vaccination with a highly purified recombinant influenza protein antigen in aluminium hydroxide adjuvant, the only adjuvant currently licensed for clinical use, elicited class I restricted CTL and protection from lethal challenge with H1N1 and H2N2 viruses. The antigen (D protein, SK&F 106160) is produced by expression of H1N1 influenza virus-derived cDNA (strain A/PR/8/34) in Escherichia coli, and is composed of the first 81 N-terminal amino acids (aa) of the non-structural protein 1 (NS1) fused via a nine nucleotide non-viral linker sequence to the 157 C-terminal aa of the haemagglutinin 2 subunit (HA2). Previous work by Kuwano et al demonstrated that in vitro stimulation of spleen cells from influenza virus-primed mice, with a partially purified preparation of the D protein, selected for CD8+ CTL clones which facilitated lung clearance of H1N1 and H2N2 viruses. In the current study, these results were extended by studying the responses of mice actively immunized with highly purified D protein in the presence or absence of adjuvants. Vaccination of CB6F1 (H-2dxb) mice with D protein in aluminum hydroxide or Freund's complete adjuvant generated H1N1 cross-reactive, H-2d-restricted, CD8+ CTL directed against an immunodominant HA2 epitope (aa 189-199). D protein without adjuvant did not elicit CTL, regardless of the route of injection. However, long-lived (greater than 6 months) splenic memory CTL were elicited by boosting mice intraperitoneally (i.p.) with the D protein in the absence of adjuvant. In mice injected subcutaneously with D protein in aluminium hydroxide at weeks 0 and 3, survival was increased relative to controls up to 16 weeks beyond the second vaccination, after which time additional boosting was required for protection. Studies in H-2b and H-2k mice vaccinated with the D protein showed that induction of CD4+ T-cell or antibody responses, in the absence of CD8+ CTL, did not correlate with protection. Passive transfer of immune sera from CB6F1 mice was also not protective. This prototype H1N1 recombinant subunit vaccine in aluminium adjuvant should directly address the feasibility of achieving a protective cell-mediated immune response in human influenza.

Immunization with baculovirus-expressed recombinant rotavirus proteins VP1, VP4, VP6, and VP7 induces CD8+ T lymphocytes that mediate clearance of chronic rotavirus infection in SCID mice

Clearance of chronic murine rotavirus infection in SCID mice can be demonstrated by adoptive transfer of immune CD8+ T lymphocytes from histocompatible donor mice immunized with a murine homotypic rotavirus (T. Dharakul, L. Rott, and H.B. Greenberg, J. Virol 64:4375-4382, 1990). The present study focuses on the protein specificity and heterotypic nature of cell-mediated clearance of chronic murine rotavirus infection in SCID mice. Heterotypic cell-mediated clearance was demonstrated in SCID mice infected with EDIM (murine) rotavirus after adoptive transfer of CD8+ T lymphocytes from BALB/c mice that were immunized with a variety of heterologous (nonmurine) rotaviruses including Wa (human, serotype 1), SA11 and RRV (simian, serotype 3), and NCDV and RF (bovine, serotype 6). This finding indicates the serotypic independence of T-cell-mediated rotavirus clearance. To further identify the rotavirus proteins that are capable of generating CD8+ T cells that mediate virus clearance, donor mice were immunized with SF-9 cells infected with a baculovirus recombinant expressing one of the following rotavirus proteins: VP1, VP2, NS53 (from RF), VP4, VP7, NS35 (from RRV), VP6, and NS28 (from SA11). SCID mice stopped shedding rotavirus after receiving CD8+ T cells from mice immunized with VP1, VP4, VP6, and VP7 but not with VP2, NS53, NS35, NS28, or wild-type baculovirus. These results suggest that heterotypic cell-mediated clearance of rotavirus in SCID mice is mediated by three of the major rotavirus structural proteins and by a putative polymerase protein.

Recognition of disparate HA and NS1 peptides by an H-2Kd-restricted, influenza specific CTL clone

CTLs (CD8+) are known to recognize exogenous peptide in the context of class I MHC molecules. We observed that an influenza subtype H1 and H2 cross-reactive CTL clone B7, which was stimulated by a fusion protein containing a portion of HA2 subunit of A/PR/8 virus HA, recognized a synthetic peptide (residues 515-526) of the HA2 subunit of A/PR/8 virus strain. This CTL clone also recognized a structurally disparate NS1 peptide 50-68 of the same A/PR/8 virus. We examined the recognition of the NS1 peptide 50-68 and the HA peptide 515-526 by the subcloned CTL clone, B7-B7. Cold target inhibition experiments showed that the recognition of the HA peptide by the CTL clone B7-B7 could be competed by NS1 peptide-treated target cells and vice versa. The recognition of both NS1 peptide and HA peptide by the CTL clone B7-B7 was restricted by the same allele, H2Kd. In addition, this NS1 peptide requires approximately a 600-fold higher concn for optimal CTL recognition than did the HA peptide. We conclude that the TCR on clone B7-B7 recognizes the HA peptide or the NS1 peptide as comparable complexes with the same class I MHC molecules, although there is no obvious homology in the primary sequences of HA 515-526 and NS1 50-68 peptides. CTLs elicited with certain antigens appear to recognize distinctively different antigens complexed to the same presenting MHC molecule.

Cross-reactive protection against influenza A virus infections by an NS1-specific CTL clone

An influenza A subtype cross-reactive CTL clone (A-11) was established following stimulation of A/PR/8 virus-immune spleen cells of Balb/C (H-2d) mice. This T cell clone lysed target cells infected with influenza viruses of the H1, H2, or H3 subtypes, and recognizes a conserved epitope on the NS1 protein. The clone is restricted by the H-2Ld allele. Adoptive transfer of A-11 significantly reduced virus titers in the lungs of mice infected with influenza A viruses of the H1, H2, or H3 subtypes. These results suggest that the conserved epitope on NS1 which is recognized by A-11 may be a useful component to consider for inclusion in experimental cross-reactive influenza vaccines.

Presentation of exogenous antigen with class I major histocompatibility complex molecules

Soluble antigens (Ags) in the extracellular fluids are excluded from the class I major histocompatibility complex (MHC)-restricted pathway of Ag presentation in most cells. However, an exogenous Ag can be internalized, processed, and presented in association with class I MHC molecules on specialized Ag-presenting cells (APCs). These APCs express class II molecules and can simultaneously present exogenous Ags to both class I and class II MHC-restricted T cells. These APCs may be important participants in the regulation of host immune responses. This APC activity may explain several phenomena of cytotoxic T lymphocyte (CTL) priming in vivo and might be exploited for eliciting CTL responses to protein vaccines.

Class I H-2d-restricted cytotoxic T lymphocytes recognize the neuraminidase glycoprotein of influenza virus subtype N1

Class I major histocompatibility complex-restricted cytotoxic T lymphocytes (CTL) that recognize the neuraminidase (NA) glycoprotein of subtype N1 influenza A viruses have been demonstrated in BALB/c mice. Responses to NA were obtained only in protocols that use two in vivo inoculations of virus, including a recombinant vaccinia virus containing the NA of subtype N1 influenza virus (NA-VAC) to prime or boost. Restimulation in vitro was also required for CTL recognition of NA and strongly depended on the specific N1 virus used. Influenza viruses A/Puerto Rico/8/34 (H1N1), A/CAM/46 (H1N1), J1 (H3N1), and JAP/BEL (H2N1), but not A/Bellamy (H1N1) or MEM/BEL (H3N1) virus, were able to stimulate NA-specific memory T cells in vitro. Single or double in vivo inoculation of any of the N1 viruses or a single injection of NA-VAC failed to elicit restimulatable NA-specific CTL. Lysis of NA-VAC-infected cells at low effector/target ratios was comparable to that observed toward other influenza virus proteins known to be major targets of CTL in BALB/c mice, indicating that antigenic determinants of the subtype N1 NA molecule can be efficiently presented in the context of major histocompatibility complex class I.

Active immunization against virus infections due to antigenic drift by induction of crossreactive cytotoxic T lymphocytes

We have examined whether active immunization with c13 protein, a hybrid protein of the first 81 amino acids of the viral NS1 nonstructural protein and the HA2 subunit of A/PR/8 (H1N1) hemagglutinin, could protect BALB/c mice from challenge with A/PR/8 H1 subtype virus. Mice immunized with the c13 protein had a significant reduction of pulmonary virus titers with A/PR/8 (H1) virus, but failed to limit the replication of A/PC (H3) virus, which reflects the in vitro CTL activity of c13 immune spleen cells. We observed that the epitope recognized by HA2 specific CTL, which are induced by a derivative of c13 protein, is highly conserved among H1 and H2 subtype virus strains. This led us to test whether active immunization with c13 protein would also limit pulmonary virus replication in mice infected with the A/TW virus, a virus of the H1 subtype, which was isolated in 1986, and with a virus of the H2 subtype, A/Japan/305/57. Immunized mice had significantly lower lung virus titers than did control mice, and did not possess any neutralizing antibodies to the challenger viruses. These results indicate that active immunization with a fusion protein containing the cross-reactive CTL epitope protects mice from influenza infection by inducing CTL against influenza A H1 and H2 subtype virus strains, which markedly vary in their antibody binding sites on the HA1. The ability to induce active cross-reactive immunization with a fusion protein which contains a highly conserved CTL epitope offers a model for vaccine approaches against viruses which undergo significant variations in their antibody binding sites.

Induction of ovalbumin-specific cytotoxic T cells by in vivo peptide immunization

CTL recognize peptide forms of processed, foreign antigens in association with class I molecules encoded by the MHC and are usually directed against endogenously synthesized "cellular antigens," such as those expressed by virus-infected cells. In vitro studies have shown that small exogenous peptides can directly associate with class I molecules on the cell surface and mimic the target complex derived by intracellular processing and presentation. We have recently generated OVA-specific, H-2Kb-restricted CTL by immunizing C57BL/6 mice with a syngeneic tumor line transfected with the OVA cDNA. The CTL recognize the OVA transfectant E.G7-OVA and the synthetic peptide OVA258-276, but fail to recognize the native protein. We reasoned that given the potential for direct peptide/class I association observed in vitro, OVA258-276 may induce CTL after in vivo priming. However, we found that this is not the case. OVA258-276 and peptides of increasing lengths up to OVA242-276 and OVA242-285, which are all able to form the target complex in vitro, are inefficient at priming E.G7-OVA-specific CTL responses after intravenous injection. This is also true for both native and denatured OVA. In contrast to these results the synthetic peptide OVA229-276 corresponding to a peptide in a partial tryptic digestion of OVA can efficiently prime C57BL/6 mice in vivo after intravenous injection. This peptide elicits CTL that appear identical to those derived from animals immunized with syngeneic cells producing OVA endogenously. These results are discussed in terms of separate class I and class II antigen presentation pathways and the ability of only certain, exogenous antigens to enter the cytoplasmic, class I pathway.

Cytotoxic T lymphocytes recognize a cross-reactive epitope on the transmembrane region of influenza H1 and H2 hemagglutinins

A cross-reactive cytotoxic T lymphocyte clone was produced by stimulation with a hybrid protein that contained a portion of the NS1 and the HA2 subunit of A/PR/8/34 (H1N1) virus. Transfer of this clone clears virus from the lungs of mice challenged with H1 or H2 viruses. In these experiments we define the location of the protective CTL epitope to the transmembrane portion of the influenza A virus hemagglutinin which is well-conserved on H1 and H2 subtype viruses. The H1 and H2 cross-reactive CTL clone recognized a synthetic peptide of 23 amino acids (anchor peptide) corresponding to the transmembrane domain of the A/PR/8 (H1) HA as well as the comparable anchor peptide of the A/JAP (H2) HA. The anchor peptide of the A/PR/8 HA competed against the anchor peptide of A/JAP HA in cold target inhibition tests. These results indicate that the epitope recognized by the cross-reactive CTL is located on the transmembrane region of both A/PR/8 HA and A/JAP HA. We prepared synthetic peptides to define the epitope within the transmembrane region of A/PR/8 HA which is recognized by a cross reactive CTL clone. The results indicate that residues 518-528 in the transmembrane region of A/PR/8 HA contain the cross-reactive CTL epitope.

Specific lysis by CD8+ T cells of Schwann cells expressing Mycobacterium leprae antigens

In vitro cultured murine Schwann cells which were devoid of class I and class II gene products of the major histocompatibility complex expressed class I, though not class II, antigens after stimulation with recombinant interferon-gamma. Recombinant IFN-gamma-stimulated Schwann cells after priming with M. leprae were lysed by antigen-specific CD8+ T lymphocytes in vitro. These findings suggest that specific lysis of M. leprae-infected Schwann cells by CD8+ CTL plays a role in leprosy.

Cells process exogenous proteins for recognition by cytotoxic T lymphocytes

Cells exposed to intact, noninfectious influenza virus were shown to be recognized by class I-restricted anti-influenza cytotoxic T lymphocytes (CTLs). Both internal and external proteins derived from virions were processed by cells for CTL recognition. Sensitization required the inactivation of viral neuraminidase activity and could be inhibited by preventing fusion of viral and cellular membranes. These findings are important in designing vaccines to elicit CTL responses, since they demonstrate that cells can process intact, exogenous proteins for recognition by CTLs and suggest that such processing depends on introduction of exogenous proteins into the cytoplasm.

CD8+ T cells (cytotoxic/suppressors) are required for protection in mice immunized with malaria sporozoites

In recent malaria sporozoite vaccine trials in humans and mice, antibodies to the sporozoite coat protein have given only modest protection against sporozoite challenge. In contrast, irradiated sporozoites can protect mice against massive sporozoite infections. Evidence suggests that immunity in these mice is mediated by T cells. To identify the mechanism of immunity, we used monoclonal antibodies specific for either the CD4 or CD8 molecule to selectively deplete sporozoite-immunized mice of T-cell subsets. Though in vivo depletion of CD4+ T cells did not reduce immunity, depletion of CD8+ T cells abolished protection. Monoclonal antibody treatment did not affect anti-sporozoite antibody levels. Our data indicate that cytotoxic T cells are critical for immunity to large numbers of sporozoites and suggest that vaccine development should be reoriented toward stimulating cellular as well as humoral immunity.

Mycobacteria-reactive Lyt-2+ T cell lines

The biological activities of mycobacteria-reactive Lyt-2+ T cells were characterized in vitro. T cells from mice immunized with killed Mycobacterium tuberculosis or viable M. bovis were restimulated in vitro and cloned under limiting dilution conditions. Several L3T4-Lyt-2+ T cell lines, some of them KJ16+, were established. These T cell lines were capable of lysing mycobacteria-primed macrophages in an antigen-specific way. The cytolytic activity of some T cell lines was found to be class I restricted, whereas others showed antigen-specific killing in the absence of apparent H-2 restriction. Several T cell lines produced interferon-gamma after appropriate stimulation. Furthermore, these T cell lines could induce tuberculostatic macrophage capacities by apparently two different mechanisms, namely by secretion of lymphokines (most probably interferon-gamma) and by direct cell contact. We conclude that CD8 T cells with antigen-specific cytolytic potential are generated during tuberculosis and that these T cells are involved in the immune response to tubercle bacilli.

Mouse H-2k-restricted cytotoxic T cells recognize antigenic determinants in both the HA1 and HA2 subunits of the influenza A/PR/8/34 hemagglutinin

We have constructed two chimeric influenza hemagglutinin (HA) genes in which the HA1 and HA2 subunits of the HA molecule have been interchanged between influenza A/PR/8/34 (H1 subtype) and A/NT/60/68 (H3 subtype). These genes were used to construct recombinant vaccinia viruses that expressed intact chimeric HA. These recombinant viruses were used to test whether murine CTL recognize antigenic determinants in either the HA1, HA2, or both subunits. We found that both subunits of the HA molecule contain determinants for CTL. This implies that CTL have, at least in part, separate antigenic determinants from B lymphocytes, which recognize mainly epitopes within the HA1 subunit.

Influenza virus subtype-specific cytotoxic T lymphocytes lyse target cells coated with a protein produced in E. coli

We have tested the ability of the c13 protein, which is a hybrid protein of the first 81 amino acids of the viral nonstructural protein (NS1) and the HA2 subunit of viral hemagglutination produced in E. coli, to render target cells susceptible to the lytic activity of influenza virus-specific cytotoxic T lymphocytes (CTL). The results showed that P815 cells coated with c13 protein were lysed by PR8 virus-induced secondary CTL derived from BALB/c mice. Cold-target inhibition tests clearly demonstrated that c13 protein-coated P815 cells were recognized by an H1 subtype-specific CTL population. Furthermore, PR8 virus-induced CTL derived from C3H mice did not lyse c13 protein-coated P815 cells, suggesting that c13 protein was recognized by CTL in conjunction with H-2d products. These findings suggest that this protein interacts with the cellular plasma membrane and makes target cells recognizable by H-2-restricted, influenza virus subtype-specific CTL.