Restrictions in the repertoire of allospecific T cells: Contribution of the α-helical sequence polymorphism of HLA-DR molecules

We have studied the molecular diversity of the allogeneic TCR repertoire specific for the HLA-DR4 variant, DRB1*0404, by analyzing the Vβ gene segment usage of CD4⁺ T cell clones from nine different donors and comparing the Vβ repertoire in allospecific and unselected T cell populations. To investigate the forces shaping the repertoire of the alloreactive T cell response we compared the diversity of TCR specificities utilized in very similar and in disparate responder/stimulator combinations. Six of the responders shared the HLA-DRB1*0401 allele, which differs from the HLA- DRB1*0404 allele by only two amino acid substitutions at positions 71 and 86 of the HLA-DRβ1 chain; three responders expressed HLA-DRB1 alleles with extensive polymorphisms compared with the stimulator HLA-DRB1 allele. In all nine responders, TCR specificities recruited to recognize HLA-DRB1*0404 were strongly biased toward the dominant usage of one to three Vβ elements. Despite some degree of heterogeneity between different responders, the overall pattern was very similar with a hierarchy of TCR Vβ elements expressed by HLA-DRB1*0404 specific T cells. A core group of Vβ elements (Vβ6, 5, 2, 13.2, 18, and 7) was preferentially used, whereas other Vβ elements including Vβ3, 4, and 8 were infrequently used or not used at all. Sequencing of the VDJβ region from selected T cell clones showed no junctional bias associated with the preferential Vβ gene segment usage. Surprisingly, the T cell diversity in focused and in complex alloreactive responses were equally biased. The finding that HLA-DR4⁺ and DR4^- donors exhibited a similar bias supports the interpretation that the α-helical HLA- DR structure of the stimulator cell has a dominant contribution in shaping the alloreactive repertoire.