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 occ urs by co-operation with syngeneic human antigen-presenting cells (APC ). Limiting dilution assays were used to quantify human anti-pig precu rsor cytotoxic T-cell (CTLp) frequencies and to analyse the 'kinetics' of the interaction between human lymphoid cells and porcine splenic c ells. Single-hit kinetics are demonstrative of direct recognition, as only one cell type, the CTLp, is diluted out, whereas multi-hit kineti cs indicate that more than one cell is limiting and provide evidence f or 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 t he frequency of CTLp reactive with porcine splenic cells (1/ 12,082; n = 3). However, not all of the assays performed using the xenogeneic c ombination produced single-hit kinetics. Subsequent assays were perfor med by mixing limiting numbers of human peripheral blood mononuclear c ells (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 signif icant difference in frequency between the two cell preparations after correction for the proportion of CD3(+) cells (P=0.13). There was, how ever, a significant departure from single-hit kinetics in the PBMC gro up (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 po rcine xenoantigens directly. However, the direct recognition mechanism can be altered in the presence of human APC.