Quantitative studies of alloantigen-reactive human lymphocytes in primary and secondary MLC.

The number of alloantigen-reactive cells in human peripheral blood was estimated by a limiting dilution analysis. In MLC combinations between allogeneic unrelated donors, the frequency of alloantigen-reactive cells ranged between 1:241 to 1:486. The frequency of alloantigen-reactive cells to the specific donor was increased six- to eight-fold after priming in MLC. The results demonstrate that specific "memory" cells are enriched in long-term MLC. In limiting dilution experiments between HLA-identical siblings, the frequency of alloantigen-reactive cells ranged from 1:1160 to 1:1740. The data point to the existence of a lymphocyte-defined antigen system controlled by a genetic region that is not linked to HLA. The results suggest that the lymphocyte clones that are able to react to non-HLA antigens probably consist of a small number of lymphocytes. Finally, the response of these clones of cells to non-HLA antigens was observed only under conditions where responder cells were limiting.