Recombinant CD4-Pseudomonas exotoxin hybrid protein displays HIV-specific cytotoxicity without affecting MHC class II-dependent functions

The present study describes several in vitro activities of CD4(178)-PE40, a recombinant protein containing a portion of human CD4 linked to active regions of Pseudomonas aeruginosa exotoxin A. Using assays for cell viability, we demonstrate that the hybrid toxin displays highly selective cytotoxicity for HIV-infected T lymphocytes. In a latently infected human T-cell line which is inducible for HIV expression, toxin sensitivity is observed only upon virus induction. At concentrations which readily kill HIV-infected T cells, CD4(178)-PE40 has no observable cytotoxic effects on uninfected human cell lines expressing surface major histocompatibility complex (MHC) Class II molecules, and does not interfere with cellular responses known to be dependent on functional association between CD4 and MHC Class II molecules.

Transient dominant selection of recombinant vaccinia viruses

A general method for constructing and selecting recombinant vaccinia viruses with insertions, deletions, or mutations in any gene that is similar in principle to one originally devised for Saccharomyces cerevisiae (S. Scherer and R. W. Davis, Proc. Natl. Acad. Sci. USA 76:4951-4955, 1979) is described. The selectable marker used, Escherichia coli guanine phosphoribosyltransferase, is not retained within the final recombinant virus, and hence, this procedure may be used serially to introduce several foreign genes or to make multiple site-directed mutations.

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.

Role of DNA replication in vaccinia virus gene expression: a naked template is required for transcription of three late trans-activator genes

The DNA replication requirement for vaccinia virus late gene expression was mimicked by transfecting a late promoter-controlled reporter gene into infected cells in the presence of a DNA synthesis inhibitor. This late promoter activation block was overcome by cotransfecting either naked linear vaccinia virion DNA or three cloned viral genes encoding trans-activator polypeptides of 17, 26, and 30 kd. These newly identified trans-activator genes were independently transcribed only from replicated or transfected DNA. These data suggest a regulatory cascade in which the parental viral genome serves as a template for the RNA polymerase and early promoter-specific transcription factors that are packaged in the infectious particle; the newly replicated DNA is accessible to sequentially synthesized intermediate promoter- and late promoter-specific trans-activators.

Interaction and mutual stabilization of the two subunits of vaccinia virus mRNA capping enzyme coexpressed in Escherichia coli

The genes D1 and D2, predicted to encode the 95- and 31-kDa subunits of the vaccinia virus mRNA capping enzyme, were coexpressed from the same plasmid in Escherichia coli. Induction with low concentrations of isopropyl beta-D-thiogalactoside was necessary to obtain soluble enzyme. The active heterodimer was purified by column chromatography and was shown to have both RNA guanylyltransferase and mRNA (guanine-N7-)-methyltransferase activities. Formation of the m7G(5')pppG cap structure was verified by enzyme digestion and thin-layer chromatography. Each subunit was also expressed individually in E. coli. Without the large subunit, the small one was very unstable in some bacterial strains and could only be detected by pulse labeling with radioactive amino acids. The individually expressed large subunit contained the guanylyltransferase domain, but the activity from E. coli was less than 2% of that obtained with both subunits. Two other products of the D1 open reading frame were formed: a 55-kDa subfragment with the GMP binding site and a 38-kDa C-terminal fragment that started at amino acid 498. Expression of this heterodimeric enzyme in E. coli may facilitate the analysis of its functional domains and provide a useful reagent for the specific 5' labeling of uncapped or capped RNA and for enhancing RNA translatability in eukaryotic systems.

Identification of the vaccinia virus gene encoding an 18-kilodalton subunit of RNA polymerase and demonstration of a 5' poly(A) leader on its early transcript

The DNA-dependent RNA polymerase of vaccinia virus contains 8 to 10 virus-encoded polypeptides. We have mapped the gene encoding an 18-kilodalton RNA polymerase subunit to D7R, the seventh open reading frame of the HindIII D genomic subfragment. Localization of this gene was achieved by using antibody to the purified RNA polymerase for immunoprecipitation of the in vitro translation products of in vivo-synthesized early mRNA selected by hybridization to cloned DNA fragments. The identification was confirmed by translation of D7R transcripts made in vitro with bacteriophage T7 RNA polymerase. The phenotypes of two previously isolated conditionally lethal temperature-sensitive mutants that map to D7R (J. Seto, L. M. Celenza, R. C. Condit, and E. G. Niles, Virology 160:110-119, 1987) are consistent with an essential role of this subunit in late transcription. This polymerase gene, designated rpo18, predicts a polypeptide of 161 amino acids with a molecular mass of 17,892. The rpo18 gene is transcribed early in infection, even though the 5'-TAAATG-3' motif, which is conserved among many genes of the late class, is present near the RNA start site. Characterization of the 5' end of the early transcript by several different methods, including cDNA cloning, revealed a poly(A) leader with up to 14 adenylate residues, whereas only 3 are present in the corresponding location of the DNA template. Similar but somewhat longer poly(A) leaders have previously been observed in mRNAs of late genes. We noted a TAAATG motif near the initiation site of several other early genes, including the viral DNA polymerase, and carried out additional experiments to demonstrate that their early transcripts also have 5' poly(A) leaders. Thus, formation of the poly(A) leader is not exclusively a late function but apparently depends on sequences around the transcription initiation site.

Human immunodeficiency virus envelope glycoprotein/CD4-mediated fusion of nonprimate cells with human cells

Human immunodeficiency virus (HIV) infects human cells by binding to surface CD4 molecules and directly fusing with the cell membrane. Although mouse cells expressing human CD4 bind HIV, they do not become infected, apparently because of a block in membrane fusion. To study this problem, we constructed a recombinant vaccinia virus that can infect and promote transient expression of full-length CD4 in mammalian cells. This virus, together with another vaccinia recombinant encoding biologically active HIV envelope glycoprotein gp160, allowed us to study CD4/gp160-mediated cell-cell fusion in a wide variety of human and nonhuman cells in the absence of other HIV proteins. By using syncytium formation assays in which a single cell type expressed both CD4 and gp160, we demonstrated membrane fusion in lymphoid and nonlymphoid human cells but not in any of the 23 tested nonhuman cell types, derived from African green monkey, baboon, rabbit, hamster, rat, or mouse. However, in mixing experiments with one cell type expressing CD4 and the other cell type expressing gp160, all of these nonhuman cells could form CD4/gp160-mediated syncytia when mixed with human cells; in 20 of 23 cases, membrane fusion occurred only if the CD4 molecule was expressed on the human cells whereas in the other three cases, CD4 could be expressed on either one of the fusing partners. Interestingly, in one mouse cell line, CD4-dependent syncytia formed without a human partner, but only if a C-terminally truncated form of the HIV envelope glycoprotein was employed. Our results indicate that nonhuman cells are intrinsically capable of undergoing CD4/gp160-mediated membrane fusion, but this fusion is usually prevented by the lack of helper or the presence of inhibitory factors in the nonhuman cell membranes.

Removal of cryptic poxvirus transcription termination signals from the human immunodeficiency virus type 1 envelope gene enhances expression and immunogenicity of a recombinant vaccinia virus

The in vivo role of the proposed poxvirus early transcription termination signal TTTTTNT was confirmed by analysis of the RNA species made by recombinant vaccinia viruses. Premature transcription termination occurred following each of two TTTTTNT sequences present naturally within the coding region of the human immunodeficiency virus type 1 envelope gene. Alteration of the TTTTTNT sequences, without changing the encoded amino acids, resulted in production of full-length early mRNAs, improved protein expression, and a more consistent immune response.

Immunobiology and pathogenesis of hepatocellular injury in hepatitis B virus transgenic mice

The role of the immune response to hepatitis B virus (HBV)-encoded antigens in the pathogenesis of liver cell injury has not been defined because of the absence of appropriate experimental models. HBV envelope transgenic mice were used to show that HBV-encoded antigens are expressed at the hepatocyte surface in a form recognizable by major histocompatibility complex (MHC) class I-restricted, CD8+ cytotoxic T lymphocytes specific for a dominant T cell epitope within the major envelope polypeptide and by envelope-specific antibodies. Both interactions led to the death of the hepatocyte in vivo, providing direct evidence that hepatocellular injury in human HBV infection may also be immunologically mediated.

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.

Attenuation and immunogenicity in primates of vaccinia virus recombinants expressing human interleukin-2

Vector-directed lymphokine expression represents a novel approach to the attenuation of live recombinant viruses which might be used as vaccines. Expression of interleukin-2 (IL-2) by recombinant vaccinia virus has been shown to significantly attenuate virus virulence in rodent species without diminishing immunogenicity. Skin lesion formation and immunogenicity of vaccinia/IL-2 recombinants in three species of primates was examined. IL-2 expression was associated with a 15-fold reduction in the area of induration after intradermal inoculation of recombinant viruses in patas monkeys. Wild type and a control vaccinia recombinant produced large (greater than 5000 mm2) skin ulcers in this species, but the IL-2 expressing recombinant produced no ulceration. Production of antibodies to vaccinia virus and to influenza A virus haemagglutinin expressed by recombinant vectors was examined in rhesus and squirrel monkeys. IL-2 expression accelerated the resolution of skin lesions in rhesus but not squirrel monkeys. Despite this, antibody production was equivalent in the presence or absence of IL-2. IL-2 expression can greatly reduce the skin lesions formed by live recombinant vaccinia vectors in primates, indicating significant attenuation, without reducing the immunogenicity of the vaccine.

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.

A synthetic HIV-1 protease inhibitor with antiviral activity arrests HIV-like particle maturation

A synthetic peptidemimetic substrate of the human immunodeficiency virus 1 (HIV-1) protease with a nonhydrolyzable pseudodipeptidyl insert at the protease cleavage site was prepared. The peptide U-81749 inhibited recombinant HIV-1 protease in vitro (inhibition constant Ki of 70 nanomolar) and HIV-1 replication in human peripheral blood lymphocytes (inhibitory concentration IC50 of 0.1 to 1 micromolar). Moreover, 10 micromolar concentrations of U-81749 significantly inhibited proteolysis of the HIV-1 gag polyprotein (p55) to the mature viral structural proteins p24 and p17 in cells infected with a recombinant vaccinia virus expressing the HIV-1 gag-pol genes. The HIV-1 like particles released from inhibitor-treated cells contained almost exclusively p55 and other gag precursors, but not p24. Incubation of HIV-like particles recovered from drug-treated cultures in drug-free medium indicated that inhibition of p55 proteolysis was at least partially reversible, suggesting that U-81749 was present within the particles.

Oligomeric structure of the human immunodeficiency virus type 1 envelope glycoprotein

The envelope (env) glycoprotein of human immunodeficiency virus type 1 (HIV-1) consists of two noncovalently associated subunits, gp120 and gp41, that are formed gradient sedimentation, polyacrylamide gel electrophoresis, gradient sedimentation, polyacrylamide gel electrophoresis, and chemical cross-linking, we show that gp160 is synthesized as a monomer and subsequently forms stable homodimers. The molecule remains dimeric after cleavage to gp120/gp41 but is less stable to detergent solubilization and centrifugation. Analysis of wild-type and mutated env proteins indicated that interactions between the ectodomain regions of adjoining gp41 subunits are important for dimer formation and stability. A higher-order oligomeric form was also recovered, probably a tetramer consisting of two noncovalently associated dimers. The proposed subunit composition of the HIV-1 env protein is identical to that previously observed for the paramyxovirus envelope proteins F and HN.

Anti-HIV effects of CD4-Pseudomonas exotoxin on human lymphocyte and monocyte/macrophage cell lines

CD4(178)-PE40 is a recombinant protein consisting of the HIV envelope glycoprotein-binding region of human CD4 linked to active domains of Pseudomonas aeruginosa exotoxin A. The hybrid toxin selectively kills HIV-infected human T-cell lines and protects against HIV spread in mixtures of uninfected and infected cells. We now report that CD4(178)-PE40 also selectively kills chronically HIV-1-infected cells of monocyte/macrophage lineage. The results provide further support for therapeutic use of this hybrid toxin in the treatment of HIV-infected individuals.

Structure of vaccinia virus late promoters

Functional elements of vaccinia virus late promoters were characterized by mutagenesis. Synthetic oligonucleotides were inserted into a plasmid vector containing the lacZ gene of Escherichia coli flanked by sequences from the thymidine kinase (TK) gene of vaccinia virus. The lacZ gene, under control of the synthetic promoter, was introduced into the vaccinia virus genome at the TK locus by homologous recombination, and each of the 122 recombinants thus obtained was assayed for beta-galactosidase expression. The relative amounts and 5' ends of lacZ mRNAs specified by a subset of the recombinants were determined by primer extension. The analysis indicated that late promoters may be considered in terms of three regions; an upstream sequence of about 20 base-pairs, rich in T and A residues, separated by a spacer region of about six base-pairs from a highly conserved (-1)TAAAT(+4) element within which transcription initiates. All single nucleotide substitutions within the three A residues of the TAAAT, as well as the addition of a fourth A residue, caused drastic reductions in promoter strength. All substitutions of the T residues at -1 and +4 were also detrimental to promoter activity, to an extent that depended on the strength of the promoter as determined by the upstream sequence. mRNA synthesis appeared to initiate within the three A residues regardless of promoter strength. The 5'-poly(A) leader, which is a unique feature of poxvirus late mRNAs, was diminished in length when either of the T residues at -1 and +4 was mutated, was absent or limited to a few nucleotides when any of the three A residues was substituted, but was unaffected by changes outside the TAAAT sequence. The data are consistent with a model for the generation of the normal 5'-poly(A) leader by an RNA polymerase slippage mechanism requiring three consecutive A residues. Single nucleotide substitutions within the six base-pairs upstream and three base-pairs downstream from the TAAAT sequence had modest effects on promoter strength. The most and least favourable changes led to a fourfold increase and an eightfold decrease in activity, respectively. Sequences further upstream were essential for late promoter function; tracts of T or A residues enhanced expression up to 20-fold, the former conferring much greater activity. Highest expression was obtained with a tract of 18 or 20 T residues.(ABSTRACT TRUNCATED AT 400 WORDS)

Structure of vaccinia virus early promoters

Functional elements of a vaccinia virus early promoter were characterized by making a complete set of single nucleotide substitutions, as well as more complex mutations, and assaying their effects on gene expression. Synthetic oligonucleotides, based primarily on the sequence of the 7.5-kD early promoter, were inserted into a plasmid vector containing the lacZ gene of Escherichia coli flanked by sequences from the thymidine kinase (TK) gene of vaccinia virus. The lacZ gene, under control of the synthetic promoter, was introduced into the vaccinia virus genome at the TK locus by homologous recombination, and each of the 331 different recombinant viruses thus obtained was assayed for beta-galactosidase expression. The relative amounts and precise 5' ends of lacZ mRNAs specified by a subset of the recombinants were determined by primer extension. Many promoters were tested for their ability to direct specific transcription in vitro. A generally good correlation was noted between measurements of promoter strength estimated by beta-galactosidase expression, primer extension of in vivo mRNA and transcription in vitro. A relatively simple picture emerged from the analysis. The early promoter consists of a 16 base-pair critical region, in which most single nucleotide substitutions have a major effect on expression, separated by 11 base-pairs of a less critical T-rich sequence from a seven base-pair region within which initiation with a purine usually occurs. For the critical region of the 7.5-kD promoter, AAAAgTaGAAAataTA, any substitution of an upper-case nucleotide reduced expression, usually drastically, whereas certain substitutions of lower-case nucleotides maintained or significantly enhanced expression. On the basis of this analysis, the wide range of activities of natural promoters could be attributed to the presence of one or more non-optimal nucleotides in the critical region. Moreover, single nucleotide substitutions in such promoters had the predicted enhancing effects. Most mutations in the critical region of the 7.5-kD promoter behaved independently, but some nucleotide substitutions compensated for potentially detrimental nucleotides at other positions. Promoters substantially stronger than any natural ones examined were constructed by combining several up-mutations within the critical region of the 7.5-kD promoter and by repeating the critical region sequence. Like the TATA box of eukaryotic RNA polymerase II promoters, the critical region specifies the site of transcriptional initiation.

Bromouridine triphosphate inhibits transcription termination and mRNA release by vaccinia virions

Termination of transcription in vitro by purified vaccinia virus RNA polymerase occurs downstream of a cis-acting signal UUUUUNU in the nascent RNA strand and requires a trans-acting termination factor, VTF, that is associated with the viral mRNA capping enzyme. Factor-dependent termination can be inhibited specifically by incorporation of BrUMP (from BrUTP) into nascent RNA in place of UMP. The relevance of VTF action to early vaccinia mRNA biogenesis was demonstrated in the present study of the effects of BrUTP on mRNA synthesis and release by permeabilized vaccinia virions. BrUMP incorporation inhibited the release of newly made transcripts from the virus particle, resulting in the accumulation of transcripts within virus cores. This effect was observed also with IUMP, but not with BrCMP or IMP incorporation. Transcripts synthesized in the presence of BrUTP were heterogeneous in size and severalfold larger than transcripts made in the presence of UTP. The progressive increase in the size of the core-associated, BrUMP-containing transcripts indicated that they were still engaged by elongating RNA polymerase. These results are consistent with a predominant pathway of mRNA 3'-end formation by virions that involves VTF-dependent transcription termination. These data do not support an alternative model of 3'-end formation by endonucleolytic cleavage of larger RNA precursors.

CD4-Pseudomonas exotoxin hybrid protein blocks the spread of human immunodeficiency virus infection in vitro and is active against cells expressing the envelope glycoproteins from diverse primate immunodeficiency retroviruses

We previously described an unusual recombinant protein, designated CD4(178)-PE40, containing the gp120 binding region of human CD4 linked to active regions of Pseudomonas exotoxin A. The ability of this molecule to selectively inhibit protein synthesis in cells expressing the surface envelope glycoprotein of human immunodeficiency virus (HIV) suggested this molecule may be useful in treating infected individuals. To further evaluate its therapeutic potential, several in vitro properties of this hybrid toxin were examined. CD4(178)-PE40 was found to be an extremely potent cytotoxic agent, selectively killing HIV-infected cells with IC50 values around 100 pM. In a coculture system employing mixtures of HIV-infected and -uninfected cells, the hybrid toxin inhibited spread of the infection, as judged by a delay in HIV-induced cell killing and a dramatic suppression of free virus production. Experiments with control recombinant proteins indicated that this protective effect was primarily due to selective killing of the HIV-infected cells, rather than to a simple blocking effect of the CD4 moiety of the hybrid toxin. Using recombinant vaccinia viruses as expression vectors, we found the hybrid toxin to be active against cells expressing the envelope glycoproteins of divergent isolates of HIV-1, as well as HIV-2 and simian immunodeficiency virus. These results provide further support for the therapeutic potential of CD4(178)-PE40 in the treatment of HIV-infected individuals.

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.