False HLA--D assignments may be caused by cytotoxic responder lymphocytes.

Lymphocytes from a healthy female repeatedly giving rise to MLC-typing response against HLA--D homozygous typing cells of three different specificities were investigated for cytotoxic capacity by the direct CML technique. Testing against a panel indicated the presence of circulating cytotoxic lymphocytes with specificity towards HLA--A2. When tested against selected HLA--D homozygous typing cells, the pattern of CML reactivity closely resembled the pattern of MLC-typing responses, i.e. typing responses were mostly restricted by the presence of HLA--A2 on the stimulator cells. This pattern was also found when time course studies of Cr--51 release were performed using experimental conditions identical to ordinary MLC typing, but involving chromium-51 labeled, irradiated homozygous typing cells as targets. These studies indicate that the presence of in vivo educated cytotoxic lymphocytes among the responder lymphocytes may in some instances mimic typing responses. Such lymphocytes are thought to lyse relevant stimulator lymphocytes prior to initiation of the proliferative response.