Dominant TCR-α Requirements for a Self Antigen Recognition in Humans

Abstract

TCR-α and -β chains are composed of somatically rearranged V, D, and J germline-encoded gene segments that confer Ag specificity. Recent crystallographic analyses revealed that TCR-α has more contacts with peptide than TCR-β, suggesting the possibility that peptide recognition predominantly relies on TCR-α. T cells specific for the self Ag Melan-A/MART-1 possess an exceptionally high precursor frequency in human histocompatibility leukocyte Ag-A2 individuals. This provided a unique situation for assessment of the structural relationship between TCR and peptide/MHC ligand at both the pre- and postimmune levels. Molecular and phenotypic analysis of many different Melan-A-specific T cell populations revealed that a structural constraint is imposed on the TCR for engagement with Melan-A peptides presented by HLA-A2, namely the highly preferential use of a particular TCRAV segment, AV2. Examination of CD8 single-positive thymocytes indicated that this preferential use in forming the Melan-A-specific TCR is mainly imposed by intrathymic positive selection. Our data demonstrate a dominant function of TCRAV2 segment in forming the TCR repertoire specific for the human self Ag Melan-A/MART-1 and support the view that Ag recognition is mediated predominantly by TCR-α.

Qualitative difference between the cytotoxic T lymphocyte responses to melanocyte antigens in melanoma and vitiligo

“…To validate the method, we mixed ten different Melan-A-specific clones previously obtained in the laboratory, which showed various affinities for the ligand, and followed their TCR down-regulation, after stimulation with T2 cells pulsed with the A27L peptide, by using either an anti-CD3 antibody or A2/Melan-A tetramers. When A2/Melan-A tetramers were employed, down-regulation was analyzed with a modified method, as described in Mantovani et al [15]. Results were found to be highly comparable in two independent experiments done (not shown).…”

“…After 3 h at 37 C, TCR internalization was measured by flow cytometry using A2/Melan-A tetramers. The percentage of surface TCR was calculated using a modified method [15]. Bars, AE SD.…”

“…These data confirm the highest affinity of T cells from vitiligo and confer a physiological significance to the difference observed between the two groups of patients. We had previously noticed an increased tumor reactivity of melanocyte-specific T cells from vitiligo: In one paper [15], cytotoxicity against a Melan-A-expressing, HLA-A2 + melanoma cell line was assessed for ex vivo sorted A2/Melan-A tetramer + cells from one melanoma and one vitiligo HLA-A2 + patient. We found a striking difference in the percentages of specific lysis obtained: 70% in the case of T cells from vitiligo versus only 30% for T cells from melanoma at a 10 : 1 effector-to-target (E/T) cell ratio.…”

“…Notably, however, cells from vitiligo patients were capable of lysing HLA-matched melanoma cell lines much more efficiently than CTL obtained from melanoma patients [13][14][15]. In this paper, we have performed for the first time a direct comparison of whole melanocyte-specific T cell populations in the two diseases.…”

“…By the use of HLA class I/epitope tetramers, a tool to measure the frequency of specific CTL precursors independently of their functional state [12], our group already contributed to the demonstration that melanocyte-specific CTL are present in the peripheral blood of both melanoma and vitiligo patients [13][14][15][16]. Notably, however, cells from vitiligo patients were capable of lysing HLA-matched melanoma cell lines much more efficiently than CTL obtained from melanoma patients [13][14][15].…”