Demonstration of direct xenorecognition of porcine cells by human cytotoxic T lymphocytes.

It is not known whether human cytotoxic T cells can recognize porcine major histocompatibility antigens directly, or whether recognition occurs by co-operation with syngeneic human antigen-presenting cells (APC). Limiting dilution assays were used to quantify human anti-pig precursor cytotoxic T-cell (CTLp) frequencies and to analyse the 'kinetics' of the interaction between human lymphoid cells and porcine splenic cells. Single-hit kinetics are demonstrative of direct recognition, as only one cell type, the CTLp, is diluted out, whereas multi-hit kinetics indicate that more than one cell is limiting and provide evidence for co-operative recognition of xenoantigens. Initial assays indicated that the frequency of CTLp reactive with alloantigens on human splenic targets (mean 1/1845; n = 3) was approximately sixfold greater than the frequency of CTLp reactive with porcine splenic cells (1/12,082; n = 3). However, not all of the assays performed using the xenogeneic combination produced single-hit kinetics. Subsequent assays were performed by mixing limiting numbers of human peripheral blood mononuclear cells (PBMC) or APC-depleted PBMC preparations with porcine splenocytes. There was a significant difference in the frequency of xenospecific CTLp between PBMC and APC-depleted preparations (P = 0.034). The overall frequency increased in the APC-depleted group. Variation between the seven human donors was also significant (P = 0.006). There was no significant difference in frequency between the two cell preparations after correction for the proportion of CD3+ cells (P = 0.13). There was, however, a significant departure from single-hit kinetics in the PBMC group (P = 0.004) which was not observed in the APC-depleted group (P = 0.052). It is concluded that human cytotoxic T cells can be activated by porcine xenoantigens directly. However, the direct recognition mechanism can be altered in the presence of human APC.