Biomedical subjects

A M Wan

Publications and source records attributed to A M Wan.

8 recordsLinked to original sources

PubMed1988

Structural features of an antigen required for cellular interactions and for T cell activation in a MHC-restricted response.

The protein Ag, tobacco mosaic virus protein, (TMVP) and its tryptic peptide number 8 (residues 93-112 of the protein) exhibit cross-reactivity on the T cell level in some strains of mice (e.g., C3H.SW, C57BL/10); these strains are termed cross-reactive (CR). In other strains such as A/J or B10.BR, no cross-reactivity is exhibited; these strains are termed non-cross-reactive (NCR). Genetic experiments indicated that the cross-reactivity is dominant and that it is mapped to the I-A or I-E region of the MHC, with cross-reactivity exhibited by the I-Ab haplotype but not by I-Ak or I-Ek. Cell reconstitution experiments have indicated that the non-cross-reactivity is associated with the inability of the NCR APC to present Ag. Analysis of the area(s) on peptide 8 which serve(s) as epitope revealed that both strains recognize an overlapping area consisting of 11 amino acid residues in the middle of peptide 8 (residues 97-107), which by itself is nonstimulatory to TMVP- or peptide 8-immune T cells of the CR or the NCR strains. However, the addition of a few amino acid residues of the sequence of peptide 8 to this area converts it to a complete stimulatory epitope. Additivity experiments revealed that the CR strain contains two major T cell populations each recognizing this middle region of peptide 8 when elongated by a few amino acids N-terminally and C-terminally, respectively. In contrast, the NCR strain contains one major T cell population recognizing elongation only N-terminally. Because TMVP (but not peptide 8) requires processing before presentation to T cells, it is postulated that, during processing of TMVP, there occur alterations in the area of the proximal three or four N-terminal amino acids of the region consisting of peptide 8, destroying the only region containing the T cell epitope recognized by the NCR strain, hence TMVP and peptide 8 do not exhibit cross-reactivity in this strain. The same alterations of TMVP still leave intact an epitope consisting of amino acid residues C-terminal to the altered area which is recognized by the CR strain, hence the cross-reactivity exhibited by this strain. The results suggest that the difference in cross-reactivity on the T cell level between TMVP and peptide 8 exhibited by the strains may be due to differences in the orientation of presentation and the subsequent cell recognition of an epitope contained within peptide 8.(ABSTRACT TRUNCATED AT 400 WORDS)

Amino Acid Sequence↗

PubMed1988

Nature of polymorphism in HLA-A, -B, and -C molecules.

Diversity in 39 HLA-A, -B, and -C molecules is derived from 20 amino acid positions of high variability and 71 positions of low variability. Variation in the structurally homologous alpha 1 and alpha 2 domains is distinct and may correlate with partial segregation of peptide and T-cell receptor binding functions. Comparison of 15 HLA-A with 20 HLA-B molecules reveals considerable locus-specific character, due primarily to differences at polymorphic residues. The results indicate that genetic exchange between alleles of the same locus has been a more important mechanism in the generation of HLA-A, -B, and -C diversity than genetic exchange events between alleles of different loci.

Amino Acid Sequence↗

PubMed1987

Multiple genetic mechanisms have contributed to the generation of the HLA-A2/A28 family of class I MHC molecules.

The genetic events that produce diversity in class I MHC genes and proteins has been investigated by using a family of closely related HLA-A alleles. Five genes coding for HLA-A2.2Y, HLA-A2.3, and HLA-Aw68.2 have been isolated. Exon sequences are compared with the known sequences for HLA-A2.1, HLA-A2.2F, HLA-A2.4, HLA-Aw68.1, and HLA-Aw69. Pairwise comparison of the eight unique sequences shows that point mutation, reciprocal recombination, and gene conversion have all contributed significantly to the diversification of this family of alleles. These results are compared with those of other studies that have emphasized the role of gene conversion. A predominance of coding substitutions in the alpha 1 and alpha 2 domains is found, consistent with positive selection for polymorphism being a major factor in the fixation of these alleles. In the three cases examined, genes for phenotypically identical proteins gave identical nucleotide sequences, indicating that most, if not all, of the class I polymorphism is detectable by immunological methods. The apparent stability of the sequences suggests that the events generating some of the alleles occurred before the origin of modern Homo sapiens.

Alleles↗

PubMed1987

A transposable epitope of HLA-B7, B40 molecules.

The monoclonal antibody MB40.2 defines a novel subtype of HLA-B40 that is expressed by the Sweig cell line. This molecule, called HLA-B40, lacks an antigenic determinant that is common to HLA-B7 and the HLA-Bw60 subtype of HLA-B40. Genes encoding HLA-B40 and HLA-Bw60 have now been isolated and the amino acid sequences of these proteins compared with other HLA-B locus molecules. These results show that HLA-B40 is a unique protein which differs from HLA-Bw60 by eight amino acid substitutions. Comparison of the sequences for HLA-B40, -Bw60, and -B7 localizes the MB40.2 epitope to a cluster of three substitutions at positions 177, 178, and 180 at the end of the alpha 2 domain. Gene conversion or reciprocal recombination are postulated to have transferred this cluster of substitutions, and their associated epitope, during the evolution of HLA-B locus genes. The epitope may consist of an alpha helical segment which is exclusively found on MB40.2-positive molecules.

Amino Acid Sequence↗

PubMed1986

The primary structure of HLA-A32 suggests a region involved in formation of the Bw4/Bw6 epitopes.

All HLA-B locus molecules have either the Bw4 or Bw6 epitopes. In addition, the Bw4 epitope is found on HLA-Aw23, Aw24, and A32, and Bw6 is also found on HLA-Cw3. The structural basis for these determinants and the evolution of their distribution among products of the HLA-B locus has been a long standing puzzle. To identify residues that may be involved in these determinants, we have cloned a gene for A32 and sequenced the protein encoding exons. Comparison of the predicted protein sequence with other HLA-A,B,C sequences identified residues 79 through 83 of the alpha 1 domain as having a pattern of polymorphic substitution that correlates with the presence and absence of the Bw4 and Bw6 epitopes.

Amino Acid Sequence↗

PubMed1986

Antigenic requirements for T-cell activation: reconstitution of a functional antigen from an inactive peptide portion of an antigen conjugated to protein carriers.

The structural features of an antigenic peptide required for T-cell activation were examined by a novel approach: an active antigen was constructed from an inactive peptide portion of the original antigen by conjugating it to various proteins. An eicosapeptide, peptide 8, representing residues 103-112 of the tobacco mosaic virus protein (TMVP), was utilized as the model antigen for these studies. While peptide 8 was able to stimulate, in vitro, T-cells from peptide 8 primed mice, synthetic peptides representing various portions of peptide 8 were unable to activate these cells. Although the amino-terminal undecapeptide of peptide 8 (residues 93-103 of TMVP) was unable to activate T-cells from peptide 8 primed mice, conjugates which consisted of this undecapeptide coupled to certain proteins were capable of inducing antigen-specific proliferation of these T-cells. These results identify two structural antigenic features essential for T-cell activation: a T-cell-recognizable epitope within the amino-terminal undecapeptide of peptide 8 and a second region provided by the carboxy-terminal half of peptide 8 or by protein carriers. Potential roles for this second region include providing a site for antigen interaction with Ia molecules on the antigen-presenting cell or, alternatively, providing amino acids important in stabilizing the binding of the T-cell antigen receptor. The results suggest that the recognition of this second region exhibits only a limited specificity.

Animals↗

PubMed1986

Cell activation and immunogenicity.

There is presently great interest in the production of synthetic vaccines which utilize as immunogens peptides representing portions of protein antigens, either free or conjugated to protein carriers. The use of such immunogens raises questions regarding the cells which are activated an the characteristics of the resulting immune response. Using the tobacco mosaic virus protein (TMVP) as a model antigen, immune induction by the protein and by synthetic vaccines related to this protein was investigated. Specifically we have compared immune induction by the parent protein, an unconjugated eicosapeptide representing residues 93-112 of the protein, and an immunogen consisting of the C-terminal decapeptide of the above eicosapeptide conjugated to the protein carrier KLH. The comparison led to the following conclusions: Immunization with the free eicosapeptide but not with its C-terminal decapeptide leads to the activation of T and B lymphocytes. Immunization with the free eicosapeptide or with its C-terminal decapeptide-KLH conjugate induces antibodies capable of reacting with the parent protein. The isotype composition of the antibodies induced by these immunogens is similar to that induced by immunization with the whole protein. The fine specificity of the antibodies induced by all three immunogens is similar. However, the antibody populations induced by the synthetic immunogens may be devoid of one or more clonotypes depending upon constraints imposed by cellular interaction. Antigen specific T helper cells do not seem to influence the fine specificity of antibodies induced to a given epitope. Comparison of the induction of memory responses by the three immunogens led to the conclusion that immunization with the peptide hapten conjugated to the heterologous carrier KLH does not lead to an anamnestic antibody response upon encounter with the native protein. Immunization with an immunogenic peptide representing a portion of the protein recognized by T and B lymphocytes leads to an anamnestic antibody response upon encounter with the native protein.

Animals↗

PubMed1985

Immune induction by a protein antigen and by a peptide segment of the protein.

The immune induction by a protein (the tobacco mosaic virus protein-TMVP) was compared to the immune induction by the free, non-conjugated eicosa tryptic peptide fragment of the protein (tryptic peptide 8 representing residues 93-112 of the protein). The results demonstrated that like TMVP, peptide 8 was immunogenic in A/J mice. TMVP and peptide 8 do not cross react on the T cell level. However, immunization with TMVP or with peptide 8 induces antibodies which react with both TMVP and peptide 8. Characterization of the antibodies produced by both immunogens revealed that: their isotope composition is similar with IgG1 and IgG2 being the predominant isotypes; this composition indicates that both immunogens are T cell dependent antigens, the antibodies induced by TMVP and by peptide 8 are directed against the C-terminal decapeptide portion of peptide 8 (residues 103-112 of the protein), the fine specificity of these antibodies is the same. These results, and results of adoptive transfer experiments, indicate that antigen specific T cells had no effect on the expression of the fine antibody specificity. The results demonstrate the feasibility of immunizing with a portion of a protein for the purpose of inducing antibodies with the same isotype composition and specificity towards a protein epitope as those induced by immunization with the whole protein.

Amino Acid Sequence↗