The Leukocyte-Common Antigen (L-CA) Family

Abstract

The leukocyte common antigen (L-CA, T200, Ly-5 or CD45) consists of a family of abundant membrane glycoproteins of apparent Mr 180–240k present on all lymphoid and myeloid cells but absent from other tissues (1–4). The L-CA shows cell type specific heterogeneity in apparent Mr, glucosylation and antigenicity. L-CA from rat thymocytes gives a single major band on SDS PAGE of 180k Mr, whilst T cells give four bands of 180, 190, 200 and 220k Mr and B cells one broad band at 240k (5) and similar patterns are found on mouse and human leukocytes (2,6). Differences in glycosylation in these forms have been shown with anti-carbohydrate antibodies (7) and by lectin binding (8). Antigenic heterogeneity is revealed by antibodies of the higher Mr bands of L-CA seen on SDS PAGE; e.g. the monoclonal antibody. MRC OX-33 is a marker for B cells and the MRC OX-22 monoclonal antibody recognizes B cells, T cytotoxic cells and about 2/3 of T helper cells but not the remainder of the helper cells nor thymocytes (5). The division of T helper cells using this antibody is of particular interest as these sub-populations have important functional differences (9,10). In addition the MRC OX-22 antigenic determinant is stable to trypsin digestion enabling an antigenically active peptide to be purified by antibody affinity chromatography and sequenced (1).