Evidence of T cell clonality in the infectious tolerance pathway: Implications toward identification of regulatory T cells

We have shown that a rare population of regulatory CD4+ T cells plays a key role in the acquisition of infectious tolerance in rat sensitized recipients of cardiac allografts pretreated with nondepleting anti-CD4 mAb. This study was designed to analyze the TCR Vβ expression patterns in this transplantation model. First, we used Vβ-specific RT-PCR to show that there was no differential usage of TCR Vβ genes by T cells mediating rejection or tolerance. Indeed, graft-infiltrating lymphocytes expressed most of the 22 known rat TCR Vβ genes in both recipient groups, suggesting unrestricted TCR Vβ repertoire in alloreactive T cells. Then, we applied CDR3 spectrotyping of TCR β-chain to assess the clonality of T cells at different anatomic sites. CDR3 size restriction, indicative of the presence of T cell clones, was observed in graft-infiltrating lymphocytes but not in draining lymph nodes or spleen of tolerant hosts. Consisent with the clonal expansion, T cells in tolerated grafts exhibited the memory phenotype at a much higher percentage as compared with peripheral lymphoid organs. Moreover, in tolerated graft-infiltrating lymphocytes, the CD3 size restriction occurred in limited Vβ gene families, with V β8.1 and V β18 most frequently detected. Hence, T cells at the graft site of tolerant recipients contain T cell clones expressing selective Vβ genes. This phenotypic characteristics of the tolerogenic GILs may potentially be used as a novel marker to identify operational regulatory T cells in organ allograft recipients.