164 Immunoglobulin Molecules on Rat

A central problem in cellular immunology is the elucidation of the nature of the antigen receptor molecules on the surface of thymus-dependent lymphocytes (T cells). Antigen receptors on thymus-independent lymphocytes (B cel|s) are readily detectable and have been identified as being mainly monomeric IgM molecules (1-6). However direct attempts to demonstrate immunoglobulin (Ig) molecules on the surface of T cells by immunofluorescence, cytotoxic, or autoradiographic methods have either been unsuccessful or have detected only small amounts of immunoglobulin (5-9). This is consistent with results of experiments that indicate that T cells have many fewer antigen-binding sites than do B cells (10). Indirect evidence in support of the notion that Ig molecules act as antigen-recognition sites on T cells has come from the demonstration that in some instances anti-Ig sera can interfere with antigen-binding and certain functions ascribed to T cells (11-15). In both the direct and indirect experiments only immunoglobulin light chain determinants have been detected with any consistency on T cells. This has led some investigators to postulate that the antigen receptors on T cells are immunoglobulin molecules that possess unique heavy chain determinants ("IgX") (16). 1 Still others have suggested the possibility that the antigen receptors on T cells are not immunoglobulin molecules at all, but an as yet undescribed product of the immune response (It) genes of the major histocompatibility locus (17). Recently, Marchalonis et al. (18), using an enzymatic radioiodination method to label surface proteins on living lymphocytes (19), have reported that monomeric IgM molecules serve as antigen receptors on human and mouse T cells. Surprisingly, they found that T and B cells had approximately the same number of IgM molecules on their surfaces. They also presented evidence to support the hypothesis, earlier proposed by Greaves and Hogg (20), that Ig molecules on T cells, but not on B cells, tend to be wholly or partially masked. According to this view, only a small number of