Amongst the many types of regulatory cells that have been described during the past few years, the spontaneously occurring population that is characterized by co-expression of CD4 and CD25 appears to play a key role in the prevention of autoimmunity and the maintenance of transplantation tolerance. In this study we have examined the ability of CD4 + CD25 + T cells to regulate human CD8 + T cells, and the behavior of CD8 + T cells following activation in the presence of regulatory CD4 + CD25+ T cells. The experiments described here demonstrate that human CD4 + CD25+ T cells cause pronounced and sustained inhibition of CD8 + T cell proliferation in response to polyclonal and allogeneic stimulation. The regulation of CD8 + T cell activation was cell contact-dependent and included inhibition of perforin, granzyme B and IFN-+ cytokine production at the transcriptional level and impaired cytotoxicity. The regulated CD8+ T cell population showed sustained hyporesponsiveness and refractoriness to exogenous IL-2. These data provide insights into the short-and long-term effects of CD4 + CD25 + T cells on CD8 + T cells that could be of considerable value in optimizing vaccination against tumor and viral antigens.

Chronic rejection represents a major cause of long-term kidney graft loss. T cells that are predominant in long-term rejected kidney allografts (35 ± 10% of area infiltrate) may thus be instrumental in this phenomenom, which is likely to be dependant on the indirect pathway of allorecognition only. We have analyzed the variations in T cell repertoire usage of the Vβ chain at the complementary determining region 3 (CDR3) level in 18 human kidney grafts lost due to chronic rejection. We observed a strongly biased intragraft TCR Vβ usage for the majority of Vβ families and also a very high percentage (55%) of Vβ families exhibiting common and oligoclonal Vβ-Cβ rearrangements in the grafts of patients with chronic rejection associated with superimposed histologically acute lesions. Furthermore, Vβ8 and Vβ23 families exhibited common and oligoclonal Vβ-Jβ rearrangements in 4 of 18 patients (22%). Several CDR3 amino acid sequences were found for the common and oligoclonal Vβ8-Jβ1.4 rearrangement. Quantitative PCR showed that biased Vβ transcripts were also overexpressed in chronically rejected kidneys with superimposed acute lesions. In contrast, T lymphocytes infiltrating rejected allografts with chronic rejection only showed an unaltered Gaussian-type CDR3 length distribution. This pattern suggests that late graft failure associated with histological lesions restricted to Banff-defined chronic rejection does not involve T cell-mediated injury. Thus, our observation suggests that a limited number of determinants stimulates the recipient immune system in long-term allograft failure. The possibility of a local response against viral or parenchymatous cell-derived determinants is discussed.

The capacity of T cells to interact with nonself-APC, also referred to as direct allorecognition, is an essential feature of the cellular response involved in graft rejection. However, there is no study on TCR repertoire biases associated with direct restricted T cell activation. In this paper, we have addressed the impact of direct recognition on the whole naive T cell repertoire, using a new approach that provides, for the first time, an integrated depiction of the quantitative and qualitative alterations in the TCR Vβ transcriptome. This method can differentiate resting patterns from polyclonally activated ones, as evidenced by superantigen usage. According to this new readout, we show that direct recognition of nonself-MHC molecules triggers mRNA accumulation of several TCR Vβ families, specific to the combination studied. Moreover, in marked contrast to the situation that prevails in indirect allorecognition, T cell activation through the direct presentation pathway was not associated with skewing of the complementarity determining region (CDR) 3 length distribution. Altogether, these data argue for the significance of TCR contacts with the MHC framework in direct allorecognition. In addition, the TCR diversity mobilized by this interaction and the massive TCRβ mRNA accumulation observed after a few days of culture suggest that a significant proportion of naive T cells receive a signal leading to TCRβ transcriptional activation even though only a few of them engage in mitosis.