By Francisco Sanchez - Madrid , Janice a . Nagy , Elizabeth Robbins , Paul Simon , and Timothy

Two human molecules with surprisingly similar molecular structures, LFA-1 and Mac-1/OKM1, have recently been found to be important in cytolytic T lymphocyte-mediated killing, and in complement receptor function, respectively. Human LFA-1 contains two subunits of Mr 177,000 and 95,000 (1), and OKM 1/ Mac-1 (2) has two polypeptides of strikingly similar size. Monoclonal antibodies (MAb) to human LFA-1 block antigen-specific CTL-mediated killing and T helper cell responses, as well as natural killing (1, 3). LFA-1 participates in the Mg+2-dependent adhesion step of CTL-mediated killing (4). Human LFA-1 is a widely expressed leukocyte antigen present in lymphocytes, thymocytes, monocytes, granulocytes, and 37% of bone marrow cells (3). The participation of LFA1 in natural as well as antigen-dependent killing, and its distribution on myeloid (3) as well as lymphoid cells, suggest that LFA-1 is not an antigen receptor but has a different and perhaps more general function in cell adhesion reactions. MAb to human Mac-l(5) block adhesion by myeloid cells, mediated by the complement receptor type three (CRy), 1 to C3bi-coated particles (6). Mac-1 may thus be identical to the CRy. Human Mac-1 (5) and its homologue murine Mac1 (7-9) appear identical to the human OKM1 and Mol antigens (2, 10-12) in cell distribution and structural characteristics. Mac-l, OKM1, and Mol are expressed on monocytes, granulocytes, and natural killer cells, and in contrast to LFA-1, are absent from lymphocytes. A structural relationship between molecules implies similarities in the molecular mechanisms underlying their functions. Here, relationships between LFA1, OKM 1 (Mac-1), and further leukocyte cell surface antigens have been examined. The results show that human LFA-1, OKM1/Mol , and a third novel