Lymphocyte activation. 3. Binding sites for phytomitogens on lymphocyte subpopulations.

The presence of binding sites for concanavalin A, phytohaemagglutinin and lentil mitogen on the surface of mouse B and T lymphocytes has been investigated in relation to the previously described mitogenic selectivity of these stimulants for T cells. Purified T or B or mixed T/B suspensions of cells have been analysed both for proportions of cells with affinity for phytomitogens and for the relative density of binding sites. In addition, the specificity of mitogens for simple saccharides has been investigated in relation to binding of the former to T and B cells. Microscopic agglutination and immunofluorescent studies show that virtually 100% of T and B lymphocytes have binding sites for the three phytomitogens. Quantitative absorption experiments have demonstrated that there is virtually no difference in the relative average density of binding sites on T and B cells. Immunofluorescent studies with Con-A suggested that binding sites are uniformly present over the entire cell surface. However, when lymphocytes are metabolically active an interesting altered localization of bound Con-A molecules to one pole of the cell takes place. It is suggested that binding sites for the various phytomitogens are qualitatively and topographically distinct on the cell surface but that with respect to any one mitogen T and B cells have qualitatively and quantitatively similar binding sites. The relevance of these observations to lymphocyte activation is considered.