Cellular distribution of a natural killer cell tumour recognition‐related surface antigen in purified human lymphocytes

Natural killer (NK) cells are large granular lymphocytes capable of human leucocyte antigen (HLA) unrestricted killing of tumour cells. A putative NK cell tumour-recognition molecule (NK-TR) was previously isolated and cloned. The predicted primary structure of the NK-TR revealed that the amino terminus of the protein shared high homology with cyclophilin proteins. In this study, we used rabbit antibodies directed against synthetic peptides corresponding to amino acids 476-497 of the NK-TR protein, to examine the expression of the NK-TR antigen in freshly purified human lymphocytes. Cell-surface staining experiments using these peptide antibodies indicated the presence of the NK-TR protein on the surface of human CD3+ T-cell populations purified from peripheral blood. There were individual donor differences in the levels of cell-surface expression of this antigen ranging from 35 to 90% in T lymphocytes and, NK cells purified from different healthy volunteers. The immunoreactivity of our peptide antibodies in immunoprecipitation showed that the NK-TR-related protein expressed in purified T cells is similar to that expressed in NK cells in terms of its electrophoretic mobility. Cell-surface staining experiments using the peptide antibodies revealed that the NK-TR-related protein is more abundantly expressed on the surface of purified T cells compared with NK cells. Northern blot analysis of the mRNA species transcribed in human lymphocytes revealed abundant expression of NK-TR-specific mRNA species in purified T cells. Furthermore, another mRNA species smaller than 7 kb was detected in both NK and T-cell populations of lymphocytes freshly isolated from peripheral blood. Expression at the cell surface of a cyclophilin-homologous protein in purified human T lymphocytes may indicate another function for the reported NK-TR protein, that is, distinct from tumour-cell recognition and cytosis.