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Biomedical subjects

N L Letvin

Publications and source records attributed to N L Letvin.

At least 163 records · Page 9Linked to original sources
PubMed1991

Definition of an epitope and MHC class I molecule recognized by gag-specific cytotoxic T lymphocytes in SIVmac-infected rhesus monkeys.

Infection of macaque monkeys with the simian immunodeficiency virus of macaques (SIVmac) results in disease similar to human AIDS. Therefore, the macaque monkey is proving to be an important model for testing the effectiveness of various AIDS vaccine approaches. A detailed analysis of the cellular immune responses is necessary for the evaluation of candidate vaccines. However, this has not been possible in macaques, due, in part, to the unknown nature of the MHC molecules that restrict their T lymphocytes. In our report we demonstrate that a particular MHC class I molecule involved in the rhesus monkey's effector T lymphocyte response to SIVmac is expressed at a high frequency in a colony of rhesus monkeys. SIVmac-infected monkeys that express this MHC class I molecule all develop CTL that are restricted by that molecule and recognize an identical nine amino acid epitope of the SIVmac gag protein. This MHC class I molecule has been defined as an HLA-A homolog by cDNA cloning and sequencing. It has also been expressed in an MHC class I-deficient cell line to demonstrate directly the cloned molecule's capacity to bind and present peptide Ag to CTL. These studies illustrate that AIDS virus-specific CTL can be characterized in detail in the rhesus monkey and lay the foundation for exploring novel approaches to AIDS virus vaccination in this species.

Amino Acid Sequence↗
PubMed1991

Immunization of simian immunodeficiency virus-infected rhesus monkeys with soluble human CD4 elicits an antiviral response.

Since the CD4 molecule is a high-affinity cell-surface receptor for the human immunodeficiency virus (HIV), it has been suggested that a soluble truncated form of CD4 may compete with cell-surface CD4 for HIV binding and thus be of use in the therapy of AIDS. We have utilized the simian immunodeficiency virus of macaques (SIVmac)-infected rhesus monkeys to explore another possible therapeutic application of CD4 in AIDS--the use of recombinant soluble CD4 (rsCD4) as an immunogen. SIVmac-infected rhesus monkeys immunized with human rsCD4 developed not only an anti-human CD4 but also an anti-rhesus monkey CD4 antibody response. Coincident with the generation of this antibody response, SIVmac could not be isolated easily from peripheral blood lymphocytes and bone marrow macrophages of these animals. Furthermore, the decreased number of both granulocyte/macrophage and erythrocyte colonies grown from the bone marrow of these immunized monkeys rose to normal levels. These findings suggest that a modified human CD4 molecule serving as an immunogen might elicit an antibody response in man that could induce a beneficial therapeutic response in HIV-infected individuals.

Animals↗
PubMed1991

Recombinant virus vaccine-induced SIV-specific CD8+ cytotoxic T lymphocytes.

Evidence indicates that cytotoxic T lymphocytes (CTLs) may be important in containing the spread of the human immunodeficiency virus (HIV) in the infected host. Although the use of recombinant viruses has been proposed as an approach to elicit protective immunity against HIV, the ability of recombinant viral constructs to elicit CD8+ CTL responses in higher primates has never been demonstrated. A live recombinant virus, vaccinia-simian immunodeficiency virus of macaques (SIVmac), was used to determine whether such a genetically restricted, T lymphocyte-mediated antiviral response could be generated in a primate. Vaccinia-SIVmac vaccination elicited an SIVmac Gag-specific, CD8+ CTL response in rhesus monkeys. These CTLs recognized a peptide fragment that spans residues 171 to 195 of the Gag protein. The rhesus monkey major histocompatibility complex (MHC) class I gene product restricting this CTL response was defined. Both the vaccinated and SIVmac-infected monkeys that shared this MHC class I gene product developed CTLs with the same Gag epitope specificity. These findings support the use of recombinant virus vaccines for the prevention of HIV infections in humans.

Amino Acid Sequence↗
PubMed1991

Development of a monoclonal antibody, anti-6C2, which is involved in the interaction of CD4 T helper cells and activated B cells.

We have developed a mAb anti-6C2, by immunizing mice with T cell line derived from the Callithrix jacchus (common marmoset). Anti-6C2 is reactive with approximately 50% of unfractionated T cells, 50% of CD4+ cells, and 40% of CD8+ cells. Regarding CD4+ cells, anti-6C2-reactive cells substantially overlap with the CD29+CD45RO+ Th cell population. Moreover, anti-6C2 can divide these T cells into 6C2+ and 6C2- subpopulations. The CD4+CD45RO+6C2+ cells maximally respond to soluble Ag such as tetanus toxoid and provide strong helper function for PWM-driven B cell IgG synthesis. Most interestingly, anti-6C2 was also reactive against activated B cells but not resting B cells; furthermore, this epitope was inducible through activation of resting B cells or B cell line. Biochemical characterization showed that anti-6C2 precipitated two glycoproteins with the relative molecular weights of 180,000 and 95,000 from 125I-surface labeled cell lysate. Sequential immunoprecipitation studies demonstrated that these two glycoproteins were the lymphocyte function-associated antigen (LFA-1) Ag complex (CD11a/18). Significantly, although this antibody did not inhibit cytotoxic killer T cell responses and Ag-induced T cell proliferation as did conventional anti-LFA-1, it did inhibit PWM-driven B cell IgG synthesis. Because 6C2 expression was induced after B cell activation, the above results strongly suggest that the 6C2 molecule may play a role in the interaction of CD4 helper cells and activated B lymphocytes.

Antibodies, Monoclonal↗
PubMed1991

Soluble human CD4 elicits an antibody response in rhesus monkeys that inhibits simian immunodeficiency virus replication.

Rhesus monkeys infected with the simian immunodeficiency virus of macaques (SIVmac) demonstrate significant virologic and clinical improvement as a result of treatment with human recombinant soluble CD4 (rsCD4). We show that human rsCD4 does not efficiently inhibit SIVmac replication in bone marrow macrophages of rhesus monkeys and does not significantly augment bone marrow hematopoietic colony formation in vitro. However, plasma of human rsCD4-treated rhesus monkeys does exhibit significant anti-SIVmac activity in vitro. Plasma of these animals efficiently blocks SIVmac replication in peripheral blood lymphocytes and bone marrow macrophages. It also increases granulocyte/macrophage colony formation in vitro by bone marrow cells of SIVmac-infected monkeys. This plasma and the IgG fraction of plasma from a rhesus monkey immunized with human rsCD4 in adjuvant demonstrate reactivity with a soluble form of the rhesus monkey CD4 molecule, exhibit binding to CD4+ but not CD8+ concanavalin A-activated rhesus monkey peripheral blood lymphocytes, and precipitate the CD4 molecule from surface-labeled activated rhesus monkey peripheral blood lymphocytes. Moreover, anti-viral activity is demonstrable in the IgG fraction of plasma from a human rsCD4-immunized monkey. These studies raise the possibility that a modified human CD4 molecule serving as an immunogen might elicit an antibody response that could potentially induce a beneficial therapeutic response in human immunodeficiency virus-infected individuals.

Animals↗
PubMed1991

Segmental exchange between MHC class I genes in a higher primate: recombination in the gorilla between the ancestor of a human non-functional gene and an A locus gene.

Classical human major histocompatibility complex (MHC) class I molecules are the products of highly diverse gene loci. It has been suggested that segmental exchange may play a role in the generation of diversity at the antigen recognition site of MHC class I molecules. Here we present the cloning, sequencing and expression of two gorilla A locus cDNAs. One of these cDNAs shows remarkable similarity to the non-functional HLA-AR locus gene (5.4-LBF) only in exon 2. The remainder of the cDNA, however, is most closely related to other classical higher primate A locus genes. This suggests that a segmental exchange may have occurred between the ancestor of the non-functional HLA-AR gene and a classical gorilla A locus gene. Furthermore, the recombination event resulting in Gogo-A3 has affected its antigen recognition site. These data, therefore, demonstrate that segmental exchange can generate diversity at the antigen recognition sites of primate MHC class I molecules and suggest that non-functional genes can contribute to the generation of diversity of classical MHC class I genes.

Animals↗
PubMed1991

Unusually limited nucleotide sequence variation of the expressed major histocompatibility complex class I genes of a New World primate species (Saguinus oedipus).

Although major histocompatibility complex (MHC) class I molecules are, as a rule, highly polymorphic in mammalian species, those of the New World primate Saguinus oedipus (cotton-top tamarin) exhibit limited polymorphism. We have cloned and sequenced twelve MHC class I cDNAs from this species. Since cloned cotton-top tamarin cell lines express three to six MHC class I molecules, this species must have at least three functional MHC class I loci. There was, however, no evidence of locus-specific substitutions in the tamarin cDNAs. Unlike all other species studied, tamarin MHC class I cDNAs displayed limited nucleotide sequence variation. The sequence similarity between the two most divergent tamarin cDNAs was 95%. To ensure that the polymerase chain reaction (PCR) primers employed in these studies had amplified all of the tamarins' expressed MHC class I genes, we used another set of primers to amplify only exons 2 and 3 from RNA and DNA. PCR of genomic DNA resulted in the amplification of six distinct clones, of which only three were well expressed. Two of these nonexpressed genes were pseudogenes and the other was a nonclassical gene. Southern blot analysis demonstrated that the tamarin has 8-11 MHC class I genes, suggesting we had indeed cloned the majority of these genes. Cotton-top tamarins are, therefore, unique among mammalian species studied to date in that they express MHC class I molecules with limited nucleotide sequence variation.

Animals↗
PubMed1991

A chimpanzee-passaged human immunodeficiency virus isolate is cytopathic for chimpanzee cells but does not induce disease.

The human immunodeficiency virus type 1 (HIV-1) readily infects both humans and chimpanzees, but the pathologic outcomes of infection in these two species differ greatly. In attempts to identify virus-cell interactions that might account for this differential pathogenicity, chimpanzee peripheral blood lymphocytes and bone marrow macrophages were assessed in vitro for their ability to support the replication of several HIV-1 isolates. Although the IIIb, RF, and MN isolates did not readily infect chimpanzee peripheral blood lymphocytes, an isolate of HIV-1 passaged in vivo in chimpanzees not only replicated well in both chimpanzee peripheral blood lymphocytes and bone marrow macrophages but also was cytopathic for chimpanzee CD4+ lymphocytes. Because no evidence of HIV-induced disease has been observed in chimpanzees infected with this isolate, in vitro replication to high titers with concomitant loss of CD4+ cells is not, in this instance, a correlate of pathogenicity. These observations, therefore, indicate that caution must be used when making extrapolations from in vitro data to in vivo pathogenesis.

Animals↗
PubMed1991

An activated CD8+ lymphocyte appears in lymph nodes of rhesus monkeys early after infection with simian immunodeficiency virus.

Although alterations in T lymphocyte subset distribution and function in the peripheral blood of HIV-infected humans are well defined, the extent to which these reflect changes in other lymphoid compartments is unclear. We have characterized the coincident changes in PBL and lymph nodes (LN)1 after simian immunodeficiency virus of macaques (SIVmac) infection of rhesus monkeys. Whereas no consistent change in CD8+ PBL was noted during the first 60 d after infection, CD8+ lymphocytes increased significantly in number in LN. These CD8+ LN lymphocytes exhibited an increased expression of MHC class II and a decreased expression of leukocyte adhesion molecule-1, suggesting that they were activated, but interestingly did not express CD25 (IL-2 receptor). Moreover, there was no evidence that these CD8+ LN cells were proliferating, suggesting that they had migrated to the LN. These changes in the LN CD8+ lymphocyte population preceded any detectable change in the light microscopic appearance of the LN. When SIVmac-specific effector T cell responses were assessed, the magnitude of virus-specific effector activity was nearly identical in the PBL and LN of each monkey studied. However, the presence of SIVmac-specific effector cells in the LN did not correlate with the presence of CD8+, MHC class II+ cells. These findings suggest that this numerically important CD8+ lymphocyte subpopulation may serve a regulatory function.

Animals↗
PubMed1990

Two distinct lymphocyte populations mediate simian immunodeficiency virus envelope-specific target cell lysis.

The relative contributions of the effector lymphocyte responses to the AIDS virus envelope glycoprotein (env) were explored in simian immunodeficiency virus of macaques (SIVmac)-infected rhesus monkeys. CD8+, MHC class I-restricted, env-specific CTL were cloned from PBL of SIVmac-infected monkeys, indicating that such cells constitute a component of the env-specific effector lymphocyte response. A limiting dilution 51Cr release assay was then established for quantitating the frequency of SIVmac-specific effector lymphocytes in PBL of rhesus monkeys. Using this assay we demonstrate that SIVmac env-specific effector lymphocytes are comprised of both CD16-, MHC class I-restricted and CD16+, MHC class I-unrestricted cells. We also demonstrate that the env-specific response is the predominant SIVmac-specific effector lymphocyte response in rhesus monkeys. These studies document the complexity of the effector lymphocyte response to the AIDS virus envelope glycoprotein and establish the role played by two distinct effector cell populations in this response.

Animals↗
PubMed1990

Syrian hamsters express diverse MHC class I gene products.

MHC class I glycoproteins are highly diverse in most species. The Syrian hamster has long been thought to express monomorphic MHC class I molecules and thus be an exception to this rule. Here we show that Syrian hamsters express diverse MHC class I gene products. The nucleotide sequences of the alpha 1 and alpha-2 domains of classical Syrian hamster MHC class I molecules are highly variable and show evidence of having been under selective pressures at their Ag recognition sites. Interestingly, none of the Syrian hamster class I genes was closely related to their counterparts in the mouse. These observations suggest that Syrian hamsters in the wild may express diverse MHC class I molecules.

Animals↗
PubMed1990

Evolution of the MHC class I genes of a New World primate from ancestral homologues of human non-classical genes.

The products of the classical human major histocompatibility complex (MHC) class I genes (HLA-A, -B, -C) are highly polymorphic molecules that bind peptides and present them to T lymphocytes. The non-polymorphic, non-classical MHC class I gene products (HLA-E, -F, -G) are not restricting elements for the majority of T lymphocytes. The evolutionary relationship of the non-classical and classical MHC class I genes is unclear. Here we present the cloning and sequencing of the MHC class I genes of a New World primate, the cotton-top tamarin (Saguinus oedipus). The expressed MHC class I genes of this species are more closely related to the human non-classical HLA-G gene than they are to genes of the human classical HLA-A, -B, and -C loci. These observations imply that classical and non-classical genes do not necessarily constitute mutually exclusive groups over evolutionary time.

Amino Acid Sequence↗
PubMed1990

Genetically distinct cell populations in naturally occurring bone marrow-chimeric primates express similar MHC class I gene products.

The cotton-top tamarin (Saguinus oedipus) is a naturally occurring "A" + "B"----"A" bone marrow-chimeric species. These primates usually are born as dizygotic twins and, due to placental vascular anastomoses, develop sharing each others' bone marrow elements. Strikingly, almost 50% of the PBL of a member of a twin pair are derived from the hematopoietic stem cells of its cotwin. To clarify the mechanisms underlying the maintenance of tolerance in these stable chimeras, MHC gene products have been biochemically characterized in cloned, genetically distinct, male and female lymphocytes from two male/female cotton-top tamarin twin pairs. Extensive MHC class II sharing between the genetically distinct cell populations was not seen in the two twin pairs. This was consistent with the MHC class II polymorphism seen in the species. However, the MHC class I gene products expressed by one member of a twin pair were almost identical to those expressed by its cotwin. A human minisatellite probe demonstrated restriction fragment length polymorphism in DNA from these animals, indicating extensive polymorphism. Thus, MHC class I sharing did not occur due to inbreeding in these animals. Additionally, another bone marrow-chimeric primate species, the common marmoset (Callithrix jacchus), expresses MHC class I molecules with low levels of variation. These studies suggest that the stable chimerism of bone marrow-chimeric primates may be facilitated by MHC class I similarity between the genetically distinct bone marrow derived-cell populations in their circulation.

Animals↗