Yet, this

Yet, this Lenvatinib mw has been documented in rare cases [14]. The only instance in which priming of these responses has been shown to occur in a consistent manner is in HLA-A2+ individuals with advanced metastatic melanoma [5, 12].

On the other hand, a peculiar structural feature also contributes to the abundance of Melan-A tetramer+ CD8+ T cells independently of the expression of the HLA-A2+ presenting allele. The TRAV-12-2 (formerly known as Vα2.1) TCR segment is over-represented in CD8+ T cells of this specificity so that over 90% of specific T cells express this particular segment, compared to an overall frequency in bulk CD8+ T cells of 6–8% [15, 23, 24]. At this juncture, there were still major questions left open. What is the exact reason for the preferential selection of the TRAV12-2 segment-containing TCR alpha chains? What supports a robust thymic output of Melan-A/MART-1-specific TCRs in the face of detectable expression of the Melan-A gene in mTECs [25]. Should thymic expression of Melan-A/MART-1 not lead to the negative

selection of high-affinity, specific CD8+ T cells? It was assumed that this was probably NVP-BGJ398 mouse the case and that the repertoire was devoid of the latter cells. The measurement of specific TCR binding parameters, however, suggested the existence of a large range of avidities [26] (and our unpublished data). In this issue, Sheena Pinto et al. [27] elegantly provide definitive answers for these two questions. First, the authors introduced G protein-coupled receptor kinase a single codon mutation, changing glutamine at position 31 of the CDR1 domain encoded in the TRAV12-2 gene segment, and could practically abrogate tetramer binding by T cells made to express the mutant TCR. This experiment nicely confirms and extends the structural data provided earlier by another group on the three dimensional structure of a HLA-A2/Melan-A/TCR pentamolecular complex [28]. Thus, this CDR1α, encoded in the germ line, exhibits selective affinity for the complex HLA-A2/ELAGIGILTV, whereby multiple electrostatic interactions formed between

Gln on the CDR1 domain and several amino acid residues, including Glu at P1, on the antigenic peptide provide most of the binding energy. This is also the likely explanation for the high frequency of “allorestricted” tetramer binding CD8+ T cells found in most HLA-A2– individuals. Second, the apparent paradox of productive thymic output of self/Melan-A-specific TCRs with a wide range of avidities despite the expression of Melan-A transcripts in the mTECs is now resolved in an unexpected and interesting fashion. Pinto et al. report that the predominant Melan-A transcript that can be found in mTECs is a truncated one, the product of misinitiation of transcription [27]. Consequently, the protein product lacks the immunodominant epitope as the first three exons are not transcribed. Thus, the epitope spanning residues 26–35 is not expressed in mTECs and central tolerance is simply not operating in this particular instance (Fig. 1).

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