An immunodominant epitope of the human immunodeficiency virus envelope glycoprotein gp160 recognized by class I major histocompatibility complex molecule-restricted murine cytotoxic T lymphocytes

Carboxyl-terminal and central regions of human immunodeficiency virus-1 NEF recognized by cytotoxic T lymphocytes from lymphoid organs. An in vitro limiting dilution analysis

Cytotoxic T lymphocytes (CTL) specific for human immunodeficiency virus (HIV) proteins have been analyzed in lymphoid organs from seropositive patients. Indeed, an active HIV replication coexists with a major CD8+ lymphocytic infiltration in these organs. We have shown in a previous report that HIV-seropositive patients lungs were infiltrated by HIV specific CD8+ lymphocytes. In the present report, we show that HIV-specific CTL responses can also be detected in lymph nodes and spleens, and were mainly directed against the ENV, GAG, and NEF HIV-1 proteins. The primary NEF-specific CTL responses were further characterized by epitope mapping. Determination of epitope-specific CTL frequencies were performed by limiting dilution analysis. Our results indicated that, in addition to the central region of NEF (AA66-148), a new immunodominant region is recognized by CTL. This region corresponds to the carboxyl-terminal domain of NEF (amino acids 182-206). AA182-206 is recognized in association with at least two common human histocompatibility leukocyte antigen (HLA) molecules (HLA-A1 and B8), with clonal frequencies of one CTL per 10(-5) to 10(-6) splenic lymphocytes. Our data indicate that lymphoid organs may represent a major reservoir for in vivo activated HIV-specific CTL. Furthermore, the carboxyl-terminal domain of NEF was found to be conserved among several HIV strains. Therefore, our finding is of interest for further HIV vaccines development.

Detailed mapping of the antigenicity of the surface unit glycoprotein of equine infectious anemia virus by using synthetic peptide strategies

We describe here a detailed analysis of the antigenic determinants of the surface unit glycoprotein (gp90) of equine infectious anemia virus (EIAV), using a comprehensive panel of synthetic peptides in enzyme-linked immunosorbent assays with immune serum from naturally and experimentally infected horses and with a panel of gp90-specific neutralizing and nonneutralizing monoclonal antibodies. The results of these studies identify immunoreactive segments throughout the conserved and variable domains of gp90 but localize immunodominant (100% reactivity) determinants to the amino and carboxyl termini of the glycoprotein molecule. Analysis of peptide reactivities with longitudinal serum samples taken from experimentally infected ponies revealed that antibody responses to conserved B-cell determinants appeared earlier and at higher titers than do antibodies specific for determinants contained in the variable domain of gp90. These observations suggest an evolution of antibody responses in EIAV-infected ponies that may correspond to the establishment of immunological control of virus replication and disease routinely observed in EIAV infections. In addition, the mapping of monoclonal antibody epitopes to peptides of 9 to 12 amino acids demonstrated that all of the neutralizing epitopes are located in the variable domain of gp90. The arrangement of neutralizing epitopes and critical structural considerations suggest that EIAV gp90 contains a principal neutralizing domain similar to the V3 loop of human immunodeficiency virus type 1. These antigenic analyses provide an important foundation for further analyzing the protective immune response generated during persistent EIAV infections and also provide potential peptide substrates for diagnostic assays and for vaccine strategies.

Human immunodeficiency virus type 1 clones chimeric for the envelope V3 domain differ in syncytium formation and replication capacity

Chimeric human immunodeficiency virus type 1 (HIV-1) molecular clones differing only in the envelope V3 region were constructed. The V3 regions were derived from two HIV-1 isolates with a non-syncytium-inducing, non-T-cell-tropic phenotype and from four HIV-1 isolates with a syncytium-inducing, T-cell-tropic phenotype. When assayed in SupT1 cells, the two chimeric viruses with a V3 region derived from the non-syncytium-inducing isolates did not induce syncytia and showed a low level of replication. The four chimeric viruses with a V3 region derived from the syncytium-inducing isolates did induce syncytia and replicated efficiently in SupT1 cells. In A3.01 cells, which do not support syncytium formation, the V3 loop affected replication similarly. Upon prolonged culture in SupT1 cells, the phenotype of a non-syncytium-inducing, low-replicating chimeric HIV-1 converted into a syncytium-inducing, high-replicating phenotype. Mutations within the usually conserved GPGR tip of the loop, which were shown to be responsible for the conversion into the syncytium-inducing, high-replicating phenotype, had occurred. In vitro mutagenesis showed that coupled changes of amino acids at both sides of the tip of the V3 loop were able to convert the viral phenotype from non-syncytium-inducing, low replicating into syncytium inducing, high replicating. Our data show that the V3 loop is involved in both syncytium forming and replicative capacity of HIV-1.

Human immunodeficiency virus type 1 envelope gene structure and diversity in vivo and after cocultivation in vitro

Nested-primer polymerase chain reaction (PCR) has been applied to the molecular cloning of 4.6-kb half-genome fragments of human immunodeficiency virus type 1 (HIV-1) taken directly from the peripheral blood mononuclear cells (PBMC) of an individual with neurological symptoms of HIV-1 infection. In a similar manner, gp120-coding portions of the envelope gene were cloned after PBMC from the same blood sample were cocultivated with uninfected PBMC for 28 days. The complete 1.6-kb nucleotide sequence of the gp120 gene was determined from each of 35 clones examined. Two of 13 (15%) PBMC-derived gp120 genes and 3 of 22 (14%) coculture-derived gp120 genes were defective as a result of frameshifts and an in-frame stop codon(s). Mean diversity between individual gp120-coding sequences in PBMC was fivefold greater (3.24%) than after coculture (0.65%). A predominant sequence of "strain" was found after coculture that was distinct from the diverse viral genotypes detected in vivo and therefore was selectively amplified during in vitro propagation. Multiple distinct third variable (V3) regions encoding the principal neutralizing domain of the envelope protein were detected in PBMC-derived genes, suggesting the presence of immunologic diversity of HIV env genes in vivo not reflected in the cocultured virus sample. The large size of the HIV fragments generated in this study will permit analysis of the diversity of immunologic reactivity, gene function, and pathogenicity of HIV genomes present within infected individuals, including the functional significance of the loss of diversity that occurs upon coculture.

Human immunodeficiency virus

A successful AIDS vaccine must elicit an immune state that will prevent the establishment of an HIV-1 persistent infection. This is a unique and difficult goal for a vaccine. Most vaccines elicit or prime for immune responses that prevent or attenuate the expression of clinical disease following infection with the pathogen. However, current evidence suggest that, following persistent infection with HIV-1, antiviral immune responses do not prevent the long-term progression to disease. Hence, it seems that the development of the persistent infection must be prevented. The ability of the immune response to accomplish this goal depends upon the efficiency with which the virus establishes persistence in the host. This is unknown for HIV-1. As a result, early efforts at vaccine development have focused on humoral immune responses directed against the virus particle in the attempt to prevent any infection of the host's cells. Studies with chimpanzees, as a model for HIV-1 infection, suggest that virus-neutralizing antibodies directed against the third hypervariable (V3) domain of the viral gp120 envelope glycoprotein may be particularly effective in preventing this infection. Studies also are in progress, both in chimpanzees and humans, to define the immunogenicity and effectiveness of various immunogens derived from the viral envelope and core structural proteins. Efforts that have concentrated on the gp120 V3 domain (or PND) have defined the extent of this region's variability and have established elements of generally conserved structure and sequence. The construction of these elements into practical and effective immunogens is an important goal. Finally, it is essential that basic studies be performed to determine if humoral or cellular immune responses directed against virus-infected cells would aid in preventing the establishment of an HIV-1 persistent infection. Such immune responses, if effective and in conjunction with specific virus-neutralizing antibody responses, would enhance the probability that an effective HIV-1 vaccine could be developed.

MHC class I surface expression in embryo-derived cell lines inducible with peptide or interferon

It has long been recognized that the absence of expression of products of the major histocompatibility complex (MHC) during early development might allow the fetus to escape recognition by maternal lymphocytes. In addition to the MHC class I heavy chain and beta 2-microglobulin, antigenic peptide is an essential structural component of the class I molecule. Indeed, there is evidence that MHC-linked genes encoding peptide transporter molecules and possibly components of a proteolytic complex are necessary for MHC class I assembly and stability at the cell surface. Here we demonstrate that embryonic cells in general show a defect in MHC class I assembly. Surface expression was rescued in the presence of an appropriate antigenic peptide, or by treatment with interferon. Consistent with this, HAM1 messenger RNA was not constitutively expressed, but was inducible by interferon, and during differentiation in vitro. Thus, tolerance of the fetal allograft may in part be controlled at the level of peptide-dependent MHC class I assembly.

Specific lysis of human immunodeficiency virus type 1-infected cells by a HLA-A3.1-restricted CD8+ cytotoxic T-lymphocyte clone that recognizes a conserved peptide sequence within the gp41 subunit of the envelope protein

A HLA-A3.1-restricted CD8+ cytotoxic T-cell clone, E7.20, that lyses cells infected with human immunodeficiency virus type 1 was isolated from an infected individual. The epitope was localized to amino acids 768-778 (RLRDLLLIVTR, NL43 env sequence) of the cytoplasmic domain of gp41 by successive use of a panel of recombinant vaccinia viruses that express truncated env genes and synthetic peptides. The epitope is conserved on 7 (NL43, BRU, HXB2, BRVA, SC, JH3, and JFL) of 13 human immunodeficiency virus type 1 isolates from North America. Synthetic peptides of this region of strains RF and CDC4 are also recognized by E7.20 despite a nonconservative Thr----Val or Thr----Ala change at amino acid 777; however, an MN peptide, which has four amino acid substitutions, was not reactive. The epitope recognized by E7.20 has a predicted hydrophobic alpha-helical structure, with three contiguous Leu residues followed by Ile and Val at amino acids 772-776. Cytotoxicity was restricted by HLA-A3.1 using allogeneic target cells that shared HLA class I antigens with the donor and an HLA-A and -B negative human plasma cell line transfected with the HLA-A3.1 gene. The transfected cells were infectable by human immunodeficiency virus type 1 strains IIIB and MN but only the former virus sensitized them to killing by E7.20. The ability of E7.20 to specifically lyse a human lymphocyte line infected with a human immunodeficiency virus type 1 strain carrying the conserved epitope is consistent with an important role for cytotoxic T cells in controlling infection.

Identification of an immunodominant epitope within the phosphoprotein of rabies virus that is recognized by both class I- and class II-restricted T cells

Immunization of H-2k mice with live rabies virus induces cytolytic T lymphocytes to the phosphoprotein of rabies virus. The antigenic determinant responsible for stimulating this class I-restricted cytolytic response was mapped to 50 amino acids (residues 180 to 229) of the phosphoprotein by using vaccinia virus recombinants expressing either the full-length phosphoprotein or C-terminal truncations of the phosphoprotein. The epitope was more finely mapped to residues 191 to 206 by using synthetic peptides. Several CD4+, class II-restricted T-cell lines were isolated from splenocytes of H-2k mice immunized with the vaccinia virus-rabies virus phosphoprotein recombinant virus. These lines were specifically stimulated by the phosphoprotein, and in addition, each line proliferated and released lymphokines in response to the same synthetic peptide shown to stimulate phosphoprotein-specific, class I-restricted cytolytic T cells.

Priming of anti-human immunodeficiency virus (HIV) CD8+ cytotoxic T cells in vivo by carrier-free HIV synthetic peptides

The generation of antiviral cytotoxic T lymphocytes (CTLs) is a critical component of the immune response to viral infections. A safe and nontoxic vaccine for AIDS would optimally use a carrier-free synthetic peptide immunogen containing only components of HIV necessary for induction of protective immune responses. We report that hybrid synthetic peptides containing either a HIV envelope gp120 T-cell determinant (T1) or the envelope gp41 fusion domain (F) N-terminal to HIV CTL determinants are capable of priming murine CD8+, major histocompatibility complex class I-restricted anti-HIV CTLs in vivo. These data demonstrate that carrier-free, nonderivatized synthetic peptides can be used in vivo to induce anti-HIV CTL responses.

Antibody responses of chimpanzees immunized with synthetic peptides corresponding to full-length V3 hypervariable loops of HIV-1 envelope glycoproteins

Immunization of primates or humans with human immunodeficiency virus type 1 (HIV-1) glycoproteins usually elicited moderate immune responses to the principal neutralizing determinant (PND) located within the V3 hypervariable loop of gp120. Since an antibody response to the PND appears to be protective, experiments were carried out to determine the responsiveness of chimpanzees to immunization with synthetic peptides corresponding to the full-length V3 loop. Seven chimpanzees (4 preimmunized with gp160, 2 preimmunized with HIV-1 antigens unrelated to gp160, and 1 unimmunized) were vaccinated with a mixture of full-length V3 loop peptides from 21 distinct HIV-1 isolates (clones) either in unconjugated form or linked to carrier proteins from HIV-1 nef and gag P18, respectively. Six chimpanzees developed high levels of antibodies to the peptides (dilution endpoints 1: greater than 25,000), and 5 had high levels of antibodies to gp120 from HIV-1IIIB (endpoint titers 1: greater than 500,000). Chimpanzees immunized with peptide-carrier conjugates (4) had antibodies to the carrier proteins nef and gag P18, respectively (endpoint titers 1: greater than or equal to 35,000). Virus-neutralizing (VN) antibodies were detected in sera of 5 of 7 chimpanzees, but were present at titers of 1: greater than or equal to 400 only in sera of 2 chimpanzees. One of these was challenged with HIV-1 and was protected against infection, as reported elsewhere. The antibodies were primarily specific for the HIV-1 isolate used for primary immunization before boosting with peptides. The relatively low dilution endpoints of VN antibodies as compared with endpoints determined by site-specific immunoassays probably can be ascribed to imperfect mimicry of conformational epitopes by synthetic peptides. Nevertheless, sequential or simultaneous immunization with recombinant envelope glycoproteins of HIV-1 and selected synthetic peptides offers an approach for eliciting protective immunity against HIV-1.

Construction of peptides encompassing multideterminant clusters of human immunodeficiency virus envelope to induce in vitro T cell responses in mice and humans of multiple MHC types

To make synthetic peptide vaccines effective in a broad population of outbred humans, one would have to incorporate enough antigenic determinants to elicit recognition by T cells of most HLA types. We have previously defined multideterminant regions of the human immunodeficiency virus (HIV) envelope that include overlapping determinants seen by proliferating T cells of three or four haplotypes of mice. We have now tested the hypothesis that synthetic peptides encompassing such multideterminant regions will be recognized by T cells of multiple murine MHC types as well as by human T cells representing multiple HLA types. Six such peptides of 20-33 residues in length were prepared, and tested for their ability to stimulate T cells from mice of four distinct MHC types immunized with recombinant envelope protein rgp 160, as well as from 42 HIV-infected humans of different HLA types. Results identify several such peptides that are broadly recognized by mice of four H-2 types and by 52-73% of infected humans who still retain IL-2 productive responses to control recall antigens such as influenza A virus or tetanus toxoid. 86% of such infected donors tested against at least three peptides respond to at least one of the six peptides, and 77% of an additional group of seropositives respond to a mixture of the peptides. Moreover, the peptides can be used to immunize mice to elicit T cells reactive with the intact HIV envelope protein. These peptides therefore may be useful for both vaccine development in the broad human population, and diagnostic or prognostic use.

Defective assembly of class I major histocompatibility complex molecules in an embryonic cell line

Developmentally regulated expression of the products of the major histocompatibility complex (MHC) is thought to play a key role in maternal tolerance of the fetal allograft. Here we analyze a cell line (EE2H3), derived from early post-implantation-stage mouse embryos, that is defective for MHC class I assembly. To follow expression of a single well-defined class I product, we introduced the H-2Dd gene under control of the human beta-actin promoter. We found that the transfected EE2H3 cells expressed abundant levels of H-2Dd heavy chains and beta 2-microglobulin protein, but only small amounts of H-2Dd surface protein. Surface expression was rescued by the addition of an appropriate antigenic peptide, or by culturing the cells at low temperature. The phenotype exhibited by EE2H3 is thus remarkably similar to that described for class I-negative somatic cell variants selected using antibodies and complement. However, a striking difference was that surface expression in H-2Dd-transfected EE2H3 cells was markedly enhanced in response to treatment with interferon. Thus, we have identified a novel class I assembly-defective cell line. Considering that EE2H3 was established from primary cultures of mouse embryo cells without immunoselection, and is therefore likely to represent a cell population normally present in post-implantation-stage embryos, these findings raise the possibility that expression of class I surface antigens during early development may in part be controlled post-translationally at the level of MHC class I assembly.

Peptides mimicking selected disulfide loops in HIV-1 gp120, other than V3, do not elicit virus-neutralizing antibodies

The positions of all 9 intrachain disulfide bonds within the envelope glycoprotein gp120 of the human immunodeficiency virus (HIV-1) have been established recently. Peptides expected to mimic some of the disulfide-bonded domains [(120-133)-(203-221); (133-138)-(164-203); (224-254); (391-425) and (385-392)-(425-452)] were synthesized. All peptides, except (120-133)-(203-221), elicited in immunized rabbits relatively high levels of antibodies reacting with gp120 in enzyme-linked immunosorbent assay (ELISA) and/or Western immunoblot assays. However, these antibodies failed to neutralize the infectivity of HIV-1. Combined with earlier reports concerning other gp120 loop peptides, these results confirm the uniqueness of the V3 (303-338) loop in encompassing a principal determinant(s) involved in virus neutralization.

An HLA-C-restricted CD8+ cytotoxic T-lymphocyte clone recognizes a highly conserved epitope on human immunodeficiency virus type 1 gag

A unique epitope on the gag protein of human immunodeficiency virus type 1 (HIV-1), located at amino acid 145 to 150, has been mapped by using a CD8+ cytotoxic T-lymphocyte (CTL) clone. This epitope is highly conserved among 18 HIV-1 strains. The HIV-1 gag-specific human leukocyte antigen (HLA) class I-restricted CD8+ CTL clone was generated from fresh peripheral blood mononuclear cells of an HIV-seropositive donor by stimulation with gamma-irradiated allogeneic peripheral blood mononuclear cells in the presence of an anti-CD3 monoclonal antibody and recombinant interleukin-2. This gag-specific CTL clone killed autologous target cells infected with a recombinant vaccinia virus containing the gag gene of HIV-1 and target cells pulsed with an authentic p24gag construct expressed in Escherichia coli. Fine specificity was determined by using a panel of overlapping 30-amino-acid-long synthetic peptides and subsequently using smaller peptides to precisely map the CTL domain on p24. The epitope is on a highly conserved region, and it overlaps with a major B-cell epitope of gag. This CD8+ T-cell epitope is restricted by HLA-Cw3, which has not been previously identified as a restricting element for human CTL responses.

Induction of HIV-specific CTL and antibody responses in mice using retroviral vector-transduced cells

Recombinant retroviral vectors can efficiently transduce and express foreign genes in mammalian cells. We have examined the utility of retroviral vector-mediated gene transfer to deliver genes which encode human immunodeficiency virus type I (HIV) antigens capable of stimulating specific immune responses. Murine fibroblast cell lines were transduced with a nonreplicating murine retroviral vector carrying the gene encoding the HIV-IIIB envelope protein and were shown to express the gp160/120 protein. Mice immunized with syngeneic vector-transduced cells developed CD8+, class I major histocompatibility complex (MHC)-restricted cytotoxic T lymphocytes (CTL) specific for targets expressing the HIV envelope protein. The CTL also exhibited lytic activity on target cells coated with synthetic peptides derived from the gp120 V3 hypervariable region of both the HIV-IIIB and HIV(MN) isolates. Following adoptive transfer in a murine tumor model, these CTL were shown to be effective in vivo by their ability to eliminate established tumor cells expressing the HIV protein. Vector-transduced syngeneic cells were also capable of eliciting HIV envelope-specific antibody responses in immunized mice. Sera obtained from these mice were found to bind to the HIV-IIIB gp160 protein as well as a peptide-defined neutralizing antibody epitope contained within the V3 domain of gp120. These sera exhibited virus-neutralizing activity in that they markedly reduced the ability of HIV to infect and form syncytia of a human T-cell line. This is the first demonstration that cells transduced with a retroviral vector encoding the HIV-IIIB envelope protein are capable of inducing effective HIV-specific cellular and humoral immune responses in mice.

Identification of a major human immunodeficiency virus-1 reverse transcriptase epitope recognized by mouse CD4+ T lymphocytes

Delineation of major T helper cell recognition sites of human immunodeficiency virus (HIV-1) proteins represents one important step in the design of an efficient acquired immune deficiency syndrome (AIDS) vaccine. Towards this end, we have explored the immunogenicity of HIV-1BRU proteins in the mouse model. Preliminary experiments revealed that inbred mice primed with whole inactivated HIV-1 developed strong CD4+ T cell proliferative responses to a variety of recombinant viral proteins including reverse transcriptase (RT). To characterize further the mouse T cell responses to this protein, several Ad- or Ed-restricted T hybridoma cells (THC) were established from BALB/c or DBA/2 mice. These THC were tested for their capacity to recognize a series of 15-mer synthetic overlapping peptides spanning three segments of HIV-1 RT that had been preselected on the basis of either alpha-helicity, amphipaticity, and/or for containing rare amino acid sequence patterns. Peptides corresponding to a C-terminal region (residues 528-560) of RT were recognized by several of the THC established from RT-primed mice. Furthermore, a non-alpha-helical peptide from this region (A3, 528-543) was capable of priming mice with different H-2 haplotypes for both peptide A3 and native RT CD4+ T cell recognition. In addition to the recently identified RT determinant 203-219 capable of triggering both mouse and human CD8+ CTL, the present results identify a good candidate for an immunodominant RT epitope capable of eliciting RT-specific T helper cell responses.

Immunization with subunit human immunodeficiency virus vaccine generates stronger T helper cell immunity than natural infection

Healthy, human immunodeficiency virus seronegative (HIV-) volunteers were multiply immunized with a recombinant gp160 (rgp160) candidate acquired immunodeficiency syndrome (AIDS) vaccine. Peripheral blood lymphocytes from volunteers immunized with 40 micrograms or with 80 micrograms (two volunteers per group) of rgp160, as well as from control donors, were tested for T helper (Th) cell function either prior to immunization, 8 to 12 months after the third immunization, or 2 to 5 months after the fourth immunization. The Th cell functional tests included antigen-induced in vitro interleukin 2 (IL 2) production and proliferation in response to influenza A virus (FLU) and to four synthetic peptides of HIV gp120 and gp160, previously demonstrated to be recognized by T cells from HIV naturally infected patients. Our results demonstrate the following: (a) immunization of HIV- individuals with rgp160 results in IL 2 production and T cell proliferation in response to HIV determinants; (b) boosting with rgp160 enhances Th function; (c) HIV-specific Th function is up to 100-fold greater in the multiply immunized volunteers than that observed in asymptomatic, HIV-infected individuals; and (d) multiple immunization with rgp160 does not impair Th function to a non-HIV antigen such as influenza A virus. These results indicate that immunization of uninfected individuals with an HIV subunit vaccine results in much stronger Th cell immunity than does natural infection and suggests that vaccination against HIV may be possible.

Sequential immunizations with rgp120s from independent isolates of human immunodeficiency virus type 1 induce the preferential expansion of broadly crossreactive B cells

The gp120 envelope glycoprotein of human immunodeficiency virus type 1 (HIV-1) is a dominant target against which the host's humoral immune response is directed. Unfortunately, gp120 proteins from different isolates of HIV are antigenically distinct, complicating the use of the envelope glycoprotein in vaccines designed to prevent acquired immunodeficiency syndrome. Using an enzyme-linked immunosorbent spot assay (ELISA), BALB/c mice immunized and boosted with recombinant purified gp120 were studied at the single cell level for their humoral immune response to HIV-1 envelope proteins. Approximately 90% of responding B cells produced antibodies reactive with the immunizing form of gp120 but not with gp120s from other strains of HIV. A novel sandwich ELISA was then used to analyze the frequency with which individual in vivo activated B cells produced antibodies that crossreacted with heterologous gp120s. Repeated immunizations with a single gp120 or with a mixture of different gp120s resulted in the activation of primarily mono-specific (noncrossreactive) B cells. In contrast, the sequential immunization of mice with recombinant purified envelope proteins from different strains of HIV (IIIB, SF2, and Zr6) induced the selective expansion of B cells producing highly crossreactive antibodies.

Selection of genetic variants of simian immunodeficiency virus in persistently infected rhesus monkeys

Genetic and antigenic variation may be one means by which lentiviruses that cause AIDS avoid elimination by host immune responses. Genetic variation in the envelope gene (env) was studied by comparing the nucleotide sequences of 27 clones obtained from two rhesus monkeys infected with molecularly cloned simian immunodeficiency virus. All 27 clones differed from each other and differed from the input clone in the gp120 (SU) portion of the envelope gene. Nucleotide substitutions were shown to accumulate with time at an average rate of 8.5 per 1,000 per year in SU. Surprisingly, the majority of nucleotide substitutions (81%) resulted in amino acid changes. Variation in SU was not random but occurred predominantly in five discrete regions. Within these variable regions, a remarkable 98% of the nucleotide substitutions changed the amino acid. These results demonstrate that extensive sequence variability accumulates in vivo after infection with molecularly cloned virus and that selection occurs in vivo for changes in distinct variable regions in env.

Host range mutant of human immunodeficiency virus type 1: modification of cell tropism by a single point mutation at the neutralization epitope in the env gene

We have isolated a variant of human immunodeficiency virus type 1 (HIV-1) which is highly infectious to fibroblastlike cells (BT cells) derived from human brain as well as CD4-positive T cells. This variant HIV-1, named HIV[GUN-1V], was obtained by infecting BT cells with a prototype HIV-1 isolate, named HIV[GUN-1WT], which is highly infectious to T cells but barely infectious to BT cells. HIV[GUN-1V] infects BT cells productively and this infection appeared to be mediated by CD4. To elucidate the viral gene responsible for the host range difference between the variant and prototype HIV-1s, we cloned and analyzed the provirus genomes of the two viruses. Examination of the infectivities of BT cells by various recombinant viruses and analyses of the nucleotide sequences of HIV[GUN-1V] and HIV[GUN-1WT] showed that a single nucleotide exchange was responsible for their difference in infectivity of BT cells: HIV[GUN-1V] contains a thymine residue instead of the cytosine residue in HIV[GUN-1WT] at position 931 of the env coding sequence. Replacement of cytosine by thymine at this position of the env coding sequence of the HIV[GUN-1WT] genome induced the ability to infect BT cells. The base exchange at this position was expected to change amino acid 311 of the envelope glycoprotein, gp120, from proline to serine, which is located in a variable region containing type-specific immunodominant epitopes. Thus, HIV[GUN-1V] acquired a wider host range than HIV[GUN-1WT] by a single point mutation in the env gene.

Strain-specific neutralizing determinant in the transmembrane protein of simian immunodeficiency virus

Monoclonal antibody SF8/5E11, which recognizes the transmembrane protein (TMP) of simian immunodeficiency virus of macaque monkeys (SIVmac), displayed strict strain specificity. It reacted with cloned and uncloned SIVmac251 but not with cloned SIVmac142 and SIVmac239 on immunoblots. This monoclonal antibody neutralized infection by cloned, cell-free SIVmac251 and inhibited formation of syncytia by cloned SIVmac251-infected cells; these activities were specific to cloned SIVmac251 and did not occur with the other viruses. Site-specific mutagenesis was used to show that TMP amino acids 106 to 110 (Asp-Trp-Asn-Asn-Asp) determined the strain specificity of the monoclonal antibody. This strain-specific neutralizing determinant is located within a variable region of SIVmac and human immunodeficiency virus type 2 (HIV-2) which includes conserved, clustered sites for N-linked glycosylation. The determinant corresponds exactly to a variable, weak neutralizing epitope in HIV-1 TMP which also includes conserved, clustered sites for N-linked glycosylation. Thus, the location of at least one neutralizing epitope appears to be common to both SIVmac and HIV-1. Our results suggest a role for this determinant in the viral entry process. Genetic variation was observed in this neutralizing determinant following infection of a rhesus monkey with molecularly cloned SIVmac239; variant forms of the strain-specific, neutralizing determinant accumulated during persistent infection in vivo. Selective pressure from the host immune response in vivo may result in sequence variation in this neutralizing determinant.

Excess beta 2 microglobulin promoting functional peptide association with purified soluble class I MHC molecules

T lymphocytes expressing alpha beta receptors recognize antigenic peptide fragments bound to major histocompatibility complex class I or class II molecules present on the surface membranes of other cells. Peptide fragments are present in the two available HLA crystal structures and recent data indicate that peptide is required for the stable folding of the class I heavy chain and maintenance of its association with the class I light chain, beta 2-microglobulin (beta 2m), at physiological temperature. To explain how the exogenous peptide used to create targets for cytotoxic cells bearing CD8 antigen could associate with apparently peptide-filled extracellular class I molecules, we hypothesized that stable binding of exogenous peptide to mature class I molecules reflects either the replacement of previously bound peptide during the well documented beta 2m exchange process or the loading of 'empty' class I heavy chains dependent on the availability of excess beta 2m. In either case, free beta 2m should enhance peptide/class I binding. Using either isolated soluble class I molecules or living cells, we show here that free purified beta 2m markedly augments the generation of antigenic complexes capable of T-cell stimulation.

Biological and immunological properties of human immunodeficiency virus type 1 envelope glycoprotein: analysis of proteins with truncations and deletions expressed by recombinant vaccinia viruses

The effects of C-terminal and internal deletions on the synthesis, transport, biological properties, and antigenicity of the human immunodeficiency virus type 1 envelope protein were determined. A family of recombinant vaccinia viruses that express N-terminal overlapping env proteins of 204, 287, 393, 502 (full-length gp120), 635, 747, and 851 (full-length gp160) amino acids was constructed. All of the proteins were detected in intra- and extracellular forms which differed in the extent of glycosylation. The 747- and 851-amino-acid proteins were cleaved, were expressed on the surface of infected cells, and bound CD4. The 635-amino-acid env protein was cleaved inefficiently, and both the precursor and product were secreted, indicating absence of the transmembrane sequence. The 635- as well as the 502-amino-acid protein, which was also largely secreted, could still bind CD4. Unexpectedly, the 393-amino-acid protein was anchored in the plasma membrane, but neither it nor smaller proteins bound to soluble CD4. When amino acids at the gp120-gp41 junction were deleted, proteolytic cleavage of gp160 did not occur. Nevertheless, gp160 was inserted into the plasma membrane and bound soluble CD4. The predominant conserved B-cell epitopes were mapped to gp41 and the C terminus of gp120, whereas cytotoxic T-cell epitopes were distributed throughout the length of the glycoproteins.

Evolution of sequences encoding the principal neutralization epitope of human immunodeficiency virus 1 is host dependent, rapid, and continuous

The principal neutralization epitope of human immunodeficiency virus 1 is localized in the third variable (V3) domain of the external envelope and has been shown to bind isolate-specific antibodies. Therefore, the extent of variation within the nucleic acid sequence encoding this epitope was studied in DNA directly obtained from peripheral blood mononuclear cells of six children and their plasma donor. This revealed that the quasi-species distribution of sequences obtained after cloning varied from recipient to recipient and that the distance from the donor sequences increased over time. V3 nucleotide evolution rates averaged 9.5 x 10(-3) per site per year for silent sites and 11.4 x 10(-3) per site per year for nonsilent sites (vs. 9.7 and 9.8 x 10(-3) per site per year for a control region 5' adjacent to the V3 region) and, although individual differences were observed, did not correlate with the serum antigen levels or disease progression. Sequences of both the epitope coding region itself (V3) and the control region upstream diverted more from the donor sequence among children not progressing to AIDS than among children progressing to AIDS. The evolution of V3 sequences is apparently host dependent, rapid, and independent of the level of antigen expression.

HGP-30, a synthetic analogue of human immunodeficiency virus (HIV) p17, is a target for cytotoxic lymphocytes in HIV-infected individuals

Evaluation of the immune response of individuals exposed to human immunodeficiency virus (HIV) is an important component of any plan designed to lead toward the development of an AIDS vaccine. Since the levels of antibodies to HIV p17 and the synthetic p17 peptide HGP-30 correlate with stages of progression to AIDS, studies were initiated to determine whether cytotoxic lymphocytes directed toward target cells pulsed with HGP-30 and radioactive chromium were present in seropositive individuals. The significance of such cells in controlling HIV viral infection has recently been enhanced by reports that HIV p17 is on the surface of infected cells and that an inactivated virus vaccine depleted of viral envelope appears to be effective in controlling expression. The selection of HGP-30 as the p17 peptide to be evaluated in early studies is based on the presence of both T-cell and B-cell epitopes as predicted by computer modeling and mouse studies and the demonstration of in vitro neutralization activity by antibodies to the epitope. By using B-lymphoblastoid cells pulsed with HGP-30 and radioactive chromium as autologous targets and mixed leukocyte culture-expanded peripheral blood lymphocytes as effectors, CD8+ cytotoxic T lymphocytes against HGP-30-coated targets were identified in seropositive individuals. In this report we demonstrate that a synthetic p17 epitope can be a target for major histocompatibility complex-restricted cytotoxic T lymphocytes in HIV-infected individuals.

HIV-1 neutralizing monoclonal antibodies induced by a synthetic peptide

We have developed a series of murine monoclonal antibodies to a region of the 120 kD envelope glycoprotein (gp120) of human immunodeficiency virus type 1 (HIV-1). This region has previously been implicated as a site for virus neutralization by antisera raised to recombinant proteins and by antibodies made to full-length gp120 purified from virus. The antigen employed was a synthetic peptide containing 15 amino acids, representing amino acid residues 308-322, RIQRGPGRAFVTIGK, of env gp120 (HTLV-IIIB isolate). Five of the monoclonal antibodies raised to this antigen have reactivity with gp120 from divergent strains of HIV-1 in Western blot assays. The two of these five which were tested with live cells infected with the divergent HIV-1 isolates IIIB, MN, and RF were specifically reactive by fluorescence analyses with cells infected with the MN and IIIB isolates. Four of the five monoclonal antibodies blocked the fusion of IIIB-infected cells with uninfected MOLT-4 target cells. The monoclonal antibody most reactive with MN-infected cells by fluorescence, #5025A, blocked the fusion of MN-infected cells with uninfected MOLT-4 cells. Four of the five monoclonal antibodies neutralized the IIIB isolate of HIV-1 in vitro, but none neutralized the MN or RF isolates at the levels of antibody tested (less than or equal to 50 micrograms/ml). Taken together these data indicate that monoclonal antibodies to the immunodominant neutralizing domain of HIV-1 gp120 display different levels of group reactivity depending on the assay system being examined.

Induction of CD4+ human cytolytic T cells specific for HIV-infected cells by a gp160 subunit vaccine

Cytolytic T lymphocyte (CTL) responses were evaluated in humans immunized with recombinant human immunodeficiency virus type 1 (HIV) envelope glycoprotein gp160. Some vaccinees had gp160-specific CTLs that were shown by cloning to be CD4+. Although induced by exogenous antigen, most gp160-specific CTL clones also recognized gp160 synthesized endogenously in target cells. These clones lysed autologous CD4+ T lymphoblasts infected with HIV. Of particular interest were certain vaccine-induced clones that lysed HIV-infected cells, recognized gp160 from diverse HIV isolates, and did not participate in "innocent bystander" killing of noninfected CD4+ T cells that had bound gp120.

Mechanism of escape of endogenous murine leukemia virus emv-14 from recognition by anti-AKR/Gross virus cytolytic T lymphocytes

It was previously shown that spleen cells from endogenous ecotropic murine leukemia virus emv-14+ AKXL-5 mice fail to stimulate an anti-AKR/Gross virus cytolytic T-lymphocyte (CTL) response in a mixed lymphocyte culture with primed C57BL/6 responder spleen cells, whereas spleen cells from AKXL strains carrying the very similar emv-11 provirus do stimulate a response (Green and Graziano, Immunogenetics 23:106-110, 1986). We wished to determine whether the lack of response with AKXL-5 spleen cells was at the level of recognition between effector cell and target cell and whether the relevant mutation was within the emv-14 provirus. It is shown here that EMV-negative SC-1 fibroblast cells transfected with the major histocompatibility complex class I Kb gene and infected with virus isolated from the AKXL-5 strain (SC.Kb/5 cells) were not lysed by H-2b-restricted anti-AKR/Gross virus CTL. SC.Kb cells infected with virus isolated from emv-11+ strains, however, were efficiently lysed by anti-AKR/Gross virus CTL, indicating that there is nothing intrinsic to EMV-infected SC.Kb cells that would prevent them from being recognized and lysed efficiently by anti-AKR/Gross virus CTL. Analysis of virus expression for the infected SC.Kb cells by XC plaque assay and by flow cytometry indicated that emv-14 virus expression for SC.Kb/5 cells was not significantly different from that for emv-11-containing SC.Kb/9 or SC.Kb/21 cells. These data show that the mutation responsible for the lack of CTL recognition and lysis is at the level of recognition between target cell and effector cell. Furthermore, these data strongly suggest that the mutation is within the emv-14 genome. Flow cytometry experiments with monoclonal antibodies against a number of viral determinants indicated that there was no gross mutation detectable in the viral determinants analyzed. The data suggest that the relevant mutation may be a point mutation or a small insertion or deletion within a coding sequence that is critical for CTL recognition.

CD8+ T lymphocytes of patients with AIDS maintain normal broad cytolytic function despite the loss of human immunodeficiency virus-specific cytotoxicity

In this study, we have investigated the potential mechanisms responsible for the loss of human immunodeficiency virus type 1 (HIV-1)-specific cytolytic activity in the advanced stages of HIV-1 infection. We have demonstrated that HIV-1-specific cytotoxic T lymphocytes are predominantly contained within the CD8+DR+ subset. Furthermore, we have shown by a redirected killing assay that there is a dichotomy between HIV-1-specific cytolytic activity and broad cytolytic potential since the cytolytic machinery of CD8+DR+ cells is still functioning even in patients with AIDS who have lost their HIV-1-specific cytolytic activity. In addition, by comparative analysis of these two types of cytolytic activity over time we have demonstrated a progressive loss of HIV-1-specific cytolytic activity in the advanced stages of the disease, whereas the cytolytic potential remained unchanged regardless of the clinical stage. As previously shown in patients with AIDS, even in asymptomatic HIV-1-seropositive patients, CD8+DR+ cells from the same patient, compared to CD8+DR- lymphocytes, showed a substantial reduction in their ability to proliferate in vitro in response to different stimuli, such as mitogens (phytohemagglutinin and phorbol 12-myristate 13-acetate) and monoclonal antibodies directed against CD3, CD2, and CD28 molecules, and displayed a defective clonogenic potential. Thus, on the basis of these results we propose that the loss of HIV-1-specific cytolytic activity in HIV-1-infected individuals may result at least in part from a progressive decrease in the pool of HIV-1-specific cytotoxic T lymphocytes belonging to the CD8+DR+ subset whose ability to expand has been impaired.

Induction of CD8+ cytotoxic T cells by immunization with purified HIV-1 envelope protein in ISCOMs

To reduce the risks of immunization with killed or live attenuated virus vaccines, it may be advantageous to use a pure, defined antigen that contains determinants for both humoral and cellular immunity. However, although most non-living intact protein preparations induce antibodies and CD4+ major histocompatibility complex (MHC) class II-restricted helper and/or cytotoxic T lymphocytes (CTL), they do not elicit CD8+ MHC class I restricted CTL. Indeed, with a few exceptions, it has not so far been possible to induce CD8+ CTL by immunizing with intact soluble proteins. We show here that a single subcutaneous immunization in mice with immunostimulating complexes containing either purified intact gp160 envelope glycoprotein of the human immunodeficiency virus (HIV)-1 or influenza haemagglutinin results in reproducible and long-lasting priming of HIV specific or influenza-specific CD8+, MHC class I restricted CTL.

Computer predictions of antigenic domains in human immunodeficiency virus-1 envelope glycoprotein: comparison with reported experimental data

Computer analyses of amino acid sequences in the human immunodeficiency virus-1 (HIV-1) envelope glycoprotein revealed that antigenic domains in the viral protein can be predicted on the basis of the physical properties of amino acids in the polypeptide chain. Relatively high values of surface probability, flexibility, and hydrophilicity were used as markers for domains of putative antigenicity. Comparison of the computer-predicted antigenic domains in the HIV-1 envelope with those reported experimentally indicate that computer analyses are able to predict antigenic domains. This study shows the usefulness of computer programs for the prediction of the antigenic domains in the HIV-1 envelope protein.

Vaccinia virus vectors: new strategies for producing recombinant vaccines

The development and continued refinement of techniques for the efficient insertion and expression of heterologous DNA sequences from within the genomic context of infectious vaccinia virus recombinants are among the most promising current approaches towards effective immunoprophylaxis against a variety of protozoan, viral, and bacterial human pathogens. Because of its medical relevance, this area is the subject of intense research interest and has evolved rapidly during the past several years. This review (i) provides an updated overview of the technology that exists for assembling recombinant vaccinia virus strains, (ii) discusses the advantages and disadvantages of these approaches, (iii) outlines the areas of outgoing research directed towards overcoming the limitations of current techniques, and (iv) provides some insight (i.e., speculation) about probable future refinements in the use of vaccinia virus as a vector.

Anchor sequence-dependent endogenous processing of human immunodeficiency virus 1 envelope glycoprotein gp160 for CD4+ T cell recognition

Human CD4+ T cell clones and cell lines were shown to lyse recombinant vaccinia virus-infected cells that synthesize the HIV-1 envelope glycoprotein gp160. The processing of endogenously synthesized gp160 for recognition by CD4+ T cells required that the protein, after synthesis on the rough endoplasmic reticulum and during subsequent cellular transport, remain attached to the luminal/extracellular membrane face by a hydrophobic anchor sequence.

An epitope in human immunodeficiency virus 1 reverse transcriptase recognized by both mouse and human cytotoxic T lymphocytes

T-cell-mediated cytotoxicity may play an important role in control of infection by the human immunodeficiency virus (HIV). In this study, we have identified and characterized a relatively conserved epitope in the HIV-1 reverse transcriptase recognized by murine and human cytotoxic T cells. This epitope was identified using a murine antigen-specific CD8+ class I major histocompatibility complex-restricted cytotoxic T-cell (CTL) line, a transfected fibroblast cell line expressing the HIV-1 pol gene, recombinant vaccinia viruses containing different truncated versions of the pol gene, and overlapping synthetic peptides. The optimal antigenic site was identified as residues 203-219 by synthesizing extended or truncated peptide analogs of the antigenic fragment. The optimal peptide was then tested for sensitization of autologous Epstein-Barr virus-transformed B-cell targets for killing by fresh human peripheral blood mononuclear cells. It was recognized by CTLs from several HIV-seropositive patients but not from any seronegative donor. Therefore, this peptide is a good candidate for inclusion in an AIDS vaccine. This study demonstrates that the same CTL epitope can be seen by murine and human CD8+ CTLs, as previously demonstrated for epitopes recognized by CD4+ helper T cells, and suggests the utility of screening for immunodominant CTL epitopes in mice prior to carrying out studies in humans.

Dissection of H-2Db-restricted cytotoxic T-lymphocyte epitopes on simian virus 40 T antigen by the use of synthetic peptides and H-2Dbm mutants

Five distinct cytotoxic T-lymphocyte (CTL) recognition sites were identified in the simian virus 40 (SV40) T antigen by using H-2b cells that express the truncated T antigen or antigens carrying internal deletions of various sizes. Four of the CTL recognition determinants, designated sites I, II, III, and V, are H-2Db restricted, while site IV is H-2Kb restricted. The boundaries of CTL recognition sites I, II, and III, clustered in the amino-terminal half of the T antigen, were further defined by use of overlapping synthetic peptides containing amino acid sequences previously determined to be required for recognition by T-antigen site-specific CTL clones by using SV40 deletion mutants. CTL clone Y-1, which recognizes epitope I and whose reactivity is affected by deletion of residues 193 to 211 of the T antigen, responded positively to B6/PY cells preincubated with a synthetic peptide corresponding to T-antigen amino acids 205 to 219. CTL clones Y-2 and Y-3 lysed B6/PY cells preincubated with large-T peptide LT220-233. To distinguish further between epitopes II and III, Y-2 and Y-3 CTL clones were reacted with SV40-transformed cells bearing mutations in the major histocompatibility complex class I antigen. Y-2 CTL clones lysed SV40-transformed H-2Dbm13 cells (bm13SV) which carry several amino acid substitutions in the putative antigen-binding site in the alpha 2 domain of the H-2Db antigen but not bm14SV cells, which contain a single amino acid substitution in the alpha 1 domain. Y-3 CTL clones lysed both mutant transformants. Y-1 and Y-5 CTL clones failed to lyse bm13SV and bm14SV cells; however, these cells could present synthetic peptide LT205-219 to CTL clone Y-1 and peptide SV26(489-503) to CTL clone Y-5, suggesting that the endogenously processed T antigen yields fragments of sizes or sequences different from those of synthetic peptides LT205-219 and SV26(489-503).

Cytotoxic T cells recognize a peptide from the circumsporozoite protein on malaria-infected hepatocytes

Irradiated malaria sporozoites can induce CD8+ T cells that are required for protection against infection. However, the parasite antigens targeted by this immune response are unknown. We have discovered a 16-amino acid epitope from the Plasmodium yoelii circumsporozoite (CS) protein that is recognized by cytotoxic T cells from immune mice. Lymphocytes stimulated with this peptide can kill P. yoelii liver stage parasites in vitro in an MHC-restricted, antigen-specific manner. Thus, epitopes from the CS protein are presented on the surface of infected hepatocytes and can be targets for T cells, even though intact CS protein has not been detected on the surface of the infected hepatocyte. A vaccine that induced CTL to parasite antigens might protect humans against malaria by eliminating liver stage parasites.

An immunodominant class I-restricted cytotoxic T lymphocyte determinant of human immunodeficiency virus type 1 induces CD4 class II-restricted help for itself

We have observed that a peptide corresponding to an immunodominant epitope of the HIV-1 envelope protein recognized by class I MHC-restricted CD8+ CTL can also induce T cell help for itself. The help is necessary for restimulation of CTL precursors in vitro with peptide alone in the absence of exogenous lymphokines, can be removed by depletion of CD4+ T cells, and can be replaced by exogenous IL-2. Whereas the CTL in BALB/c or B10. D2 mice are restricted by the class I molecule Dd, the Th cells are restricted by the class II molecule Ad, and the help can be blocked by anti-Ad mAb. To examine the genetic regulation of the induction of help, we studied B10.A mice that share the class I Dd molecule, but have different class II molecules, Ak and Ek. Spleen cells of immune B10.A mice behave like CD4-depleted BALB/c spleen cells in that they cannot be restimulated in vitro by the peptide alone, but can with peptide plus IL-2. Therefore, in the absence of exogenous lymphokines, peptide-specific help is necessary for restimulation with this immunodominant CTL epitope peptide, and in H-2d mice, this peptide stimulates help for itself as well as CTL. We speculate on the implications of these findings for the immunodominance of this peptide in H-2d mice, and for the selective advantage of pairing certain class I and class II molecules in an MHC haplotype.

Synthetic peptides define the fine specificity of the human immunodeficiency virus (HIV) gp160 humoral immune response in HIV type 1-infected chimpanzees

The fine specificities of antibodies produced against human immunodeficiency virus type 1 (HIV-1) gp160 were examined in sera from 23 HIV-1-infected chimpanzees. These animals had been infected with one of six isolates of HIV-1. Sera were screened by enzyme-linked immunosorbent assay for reactivity against seven synthetic peptides corresponding to regions of gp160. Chimpanzees appear to remain healthy after infection with HIV-1, suggesting that these animals may prevent extensive spread of the virus in vivo through immunologic mechanisms. Antibody specificity to gp160 epitopes may play a key role in the defense against HIV-1-related disease. Approximately one-half of all chimpanzee sera contained antibodies reactive with peptide 846-860, which corresponds to the carboxyl terminus of gp41. Less than 10% of sera from HIV-1-infected humans that were examined contained antibodies reactive with peptide 846-860, suggesting that this region is not highly immunogenic in humans. Of the human sera containing antibodies reactive with this peptide, all were from individuals classified as Walter Reed stages 1 to 3. No sera from humans with advanced stages of the disease contained antibodies reactive with peptide 846-860. Peptide 600-611, which reportedly reacts with nearly all sera from HIV-infected humans, was reactive with less than one-half of sera from HIV-1-infected chimpanzees. The observed differences in antibody reactivity to gp160 peptides in sera from HIV-1-infected chimpanzees and humans suggest that each may generate antibodies against differing sets of HIV-1 epitopes. These differences may contribute to the lack of disease progression in chimpanzees after infection with HIV-1.

Peptide component vaccine engineering: targeting the AIDS virus

Most of the successful vaccines developed to date induce protective immunity resembling that produced by natural infection. HIV infection does not induce protective immunity. Thus, previously successful approaches based on live- or killed-virus preparations may not yield an effective and safe AIDS vaccine and many feel that a more highly engineered vaccine will be required. Synthetic peptides represent extremely powerful tools for vaccine research and construct optimization. The theory and practice of vaccine engineering using synthetic peptide components is reviewed with special emphasis on progress towards development of a vaccine for AIDS.

Structural requirements for class I MHC molecule-mediated antigen presentation and cytotoxic T cell recognition of an immunodominant determinant of the human immunodeficiency virus envelope protein

In H-2d mice, the immunodominant determinant of the HIV-1-IIIB gp160 envelope glycoprotein recognized by CD8+ CTL is represented by a 15-residue synthetic peptide (315-329: RIQRGPGRAFVTIGK). This peptide is seen in association with the Dd class I MHC molecule expressed on H-2k L cell fibroblast targets. We explored the structural requirements for CTL recognition of this peptide at the levels of both the peptide molecule and the class I MHC molecule. Using several transfectants expressing recombinant Dd/Ld molecules, we found that presentation of this epitope required both the alpha 1 and alpha 2 domains of the Dd molecule, in contrast to certain instances of allorecognition for which alpha 1 of Dd was sufficient in association with alpha 2 of Ld. Because this peptide derives from a hypervariable segment of the HIV envelope, substituted peptides could be used to define not only the structures affecting interaction of peptide with class I MHC molecule and with the TCR, but also the structural basis for the effect of naturally occurring viral variation on CTL recognition. The CTL-LINE specific for this HIV-1-IIIB-derived sequence could not recognize the HIV-1-RF variant-derived sequence from exactly the same site (315-329:--HIGPGRVIYATGQ). Peptides with single amino acid substitutions from the HIV-1-IIIB sequence toward the HIV-1-RF sequence were made to test the effect of each residue significantly affected recognition, and only one, 324(F), was obligatory. Moreover, both 322(R) and 324(F) substituted peptides failed to inhibit the binding of the wild type peptide to the MHC molecule. Therefore, the amino-acids 322(R) and 324(F) seem to be involved in regulating peptide interaction with the Dd class I MHC molecule. In contrast, 325(V) appeared to affect interaction with the TCR. We suggest that sequence variations among known HIV-1 isolates that affect peptide binding to MHC such as those described here, if occurring during the course of infection of an individual, could result in failure of the MHC molecules of that individual to present the peptide. If the number of dominant HIV CTL epitopes is indeed very limited, such a blind spot could allow the virus to escape immune control, proliferate rapidly, and cause AIDS.

Long-term culture and fine specificity of human cytotoxic T-lymphocyte clones reactive with human immunodeficiency virus type 1

The definition of human immunodeficiency virus type 1 (HIV-1) immunogenic epitopes is central to the rational design of AIDS vaccine strategies. In this study, we have generated seven HIV-1 reverse transcriptase-specific cytotoxic T-lymphocyte (CTL) clones from the peripheral blood of two seropositive subjects. Epitopes recognized by these CTL clones were identified by using target cells infected with recombinant HIV-1-vaccinia virus vectors expressing truncated reverse transcriptase proteins and further defined by using target cells incubated with overlapping 25-amino acid synthetic reverse transcriptase peptides. Five different CTL epitopes were identified, and in each case recognition was restricted by class I human leukocyte antigens (HLA). Clones maintained specific cytolytic function in continuous culture for up to 11 months, requiring only periodic restimulation with a CD3-specific monoclonal antibody. These results indicate that HIV-1-specific, major histocompatibility class I-restricted CTL recognize multiple epitopes of a single viral gene product in conjunction with different host HLA antigens. In addition, they demonstrate that human virus-specific CTL can be grown in long-term culture without the need for reexposure to viral antigen.

A single amino acid interchange yields reciprocal CTL specificities for HIV-1 gp160

For the IIIB isolate of human immunodeficiency virus type-1 (HIV-1), the immunodominant determinant of the envelope protein gp160 for cytotoxic T lymphocytes (CTLs) of H-2d mice is in a region of high sequence variability among HIV-1 isolates. The general requirements for CTL recognition of peptide antigens and the relation of recognition requirements to the natural variation in sequence of the HIV were investigated. For this purpose, a CTL line specific for the homologous segment of the envelope from the MN isolate of HIV-1 and restricted by the same class I major histocompatibility (MHC) molecule (Dd) as the IIIB-specific CTLs was raised from mice immunized with MN-env-recombinant vaccinia virus. The IIIB-specific and MN-specific CTLs were completely non-cross-reactive. Reciprocal exchange of a single amino acid between the two peptide sequences, which differed in 6 of 15 residues, led to a complete reversal of the specificity of the peptides in sensitizing targets, such that the IIIB-specific CTLs lysed targets exposed to the singly substituted MN peptide and vice versa. These data indicate the importance of single residues in defining peptide epitopic specificity and have implications for both the effect of immune pressure on selection of viral mutants and the design of effective vaccines.

Molecularly engineered vaccine which expresses an immunodominant T-cell epitope induces cytotoxic T lymphocytes that confer protection from lethal virus infection

Identification of a single viral T-cell epitope, associated with greater than 95% of the virus-specific cytotoxic T-lymphocyte (CTL) activity in BALB/c (H-2d) mice (J. L. Whitton, A. Tishon, H. Lewicki, J. Gebhard, T. Cook, M. Salvato, E. Joly, and M. B. A. Oldstone, J. Virol. 63:4303-4310, 1989), permitted us to design a CTL vaccine and test its ability to protect against a lethal virus challenge. Here we show that a single immunization with a recombinant vaccinia virus-lymphocytic choriomeningitis virus (LCMV) vaccine (VVNPaa1-201) expressing the immunodominant epitope completely protected H-2d mice from lethal infection with LCMV but did not protect H-2b mice. Furthermore, we show that the success or failure of immunization was determined entirely by the host class I major histocompatibility glycoproteins. The difference in outcome between mice of these two haplotypes was consistent with the presence or absence in the immunizing sequences of an epitope for CTL recognition and is correlated with the induction of LCMV-specific H-2-restricted CTL in H-2d mice. Protection is not conferred by a humoral immune response, since LCMV-specific antibodies were not detectable in sera from VVNPaa1-201-immunized mice. In addition, passive transfer of sera from vaccinated mice did not confer protection upon naive recipients challenged with LCMV. Hence, the molecular dissection of viral proteins can uncover immunodominant CTL epitope(s) that can be engineered into vaccines that elicit CTL. A single CTL epitope can protect against a lethal virus infection, but the efficacy of the vaccine varies in a major histocompatibility complex-dependent manner.

Cloned cytotoxic T cells recognize an epitope in the circumsporozoite protein and protect against malaria

Protective immunity against malaria is induced by vaccination of hosts with irradiation-attenuated sporozoites. This immunity is mediated in part by neutralizing antibodies that are directed mainly against the repeat domain of the circumsporozoite protein. Early experiments showed, however, that B-cell-depleted mice that are immunized with sporozoites can resist challenge, indicating that T-cell effector mechanisms may also have a role in protection. This idea was supported by the recent observation that protective immunity also requires T-cells expressing the CD8 antigen (CD8+ T cells) whose target is probably the developing liver-stage parasites. Moreover, an oral Salmonella vaccine that expresses the circumsporozoite protein is able to protect against murine malaria in the absence of antibodies. Here we report the identification of an epitope contained within amino acids 249-260 of the Plasmodium berghei circumsporozoite protein that is recognized by H-2Kd-restricted cytotoxic T cells. Passive transfer into mice of cytotoxic-T-cell clones that recognize this epitope conferred a high degree of protection against challenge. These results provide the first direct evidence that CD8+ T cells that are specific for a defined epitope can confer protection against a parasitic infection.

Multiple subsets of HIV-specific cytotoxic T lymphocytes in humans and in mice

The human immunodeficiency virus type 1 (HIV-1) induces a strong cytotoxic T lymphocyte (CTL) response in humans following infection. HIV-specific CTL can be detected directly in the blood and lungs of infected patients, and can be expanded in vitro by stimulation with autologous HIV-infected lymphoblasts. Furthermore, CTL specific for HIV envelope glycoprotein gp160 have been obtained in mice by immunization with recombinant vaccinia virus (VV) that carry the HIV env gene. In this study, we show that mice also produce strong CTL responses to gag and nef proteins following immunization with VV recombinants, thus providing a convenient model system to study T lymphocyte immunity to defined HIV antigens. To determine the specificity of circulating HIV-immune CTL in humans, a panel of doubly transfected mouse P815 tumor cells was produced which express the human HLA-A2 or HLA-A3 transplantation antigen gene and one HIV-1 gene (env, gag or nef). Using these cells as targets to CTL, we show that HIV-infected humans carry co-existing CTL sub-populations of different specificities. Each subpopulation appears to vary in intensity among different individuals. Surprisingly, CTL specific for regulatory, non-structural nef protein appear to be a major constituent of the human immune response to HIV.