Peanut lectin (PNL) binds to a majority of mouse thymocytes (Thc) in suspension. By using cell affinity chromatography on a column of anti-PNL antibody, Thc populations at least 96 percent pure in PNL + or - cells, as judged by immunofluorescence, were obtained. PNL(+) cells are rich in Thy 1 and poor in H(2) antigens, cortisone sensitive, unresponsive to phytohemagglutinin (PHA), and immunologically incompetent, as judged by mixed lymphocyte reaction, popliteal lymph node graft-versus-host assay, and by testing helper activity in a primary in vitro antibody response to sheep erythrocytes; the converse is true of PNL(-) cells. Thus, PNL(+) and (-) cells appear to correspond to cortical and medullary Thc, respectively, as previously suggested. In culture, PNL(+) Thc show poor viability and a weak proliferative response to concanavalin A (Con A), except when supernate (SUP) of 24 h Con A stimulated lymph node lymphocyte cultures, or irradiated lymph node cells, are added, in which cases a strong proliferative response to the mitogen is observed. A variety of control experiments showed that the proliferating cells did not result from preferential stimulation of a few contaminating PNL(-) Thc present in the PNL(+) Thc cultures. The blasts resulting from PNL(+) Thc proliferation display mitogen-induced cytotoxicity, and give rise to a population of medium-sized lymphocytes, mostly PNL(-), poor in Thy 1 and rich in H(2) antigens, PHA responsive, and immunologically competent in the above-mentioned assays.

Fresh PNL(+) Thc responded in mixed lymphocyte reaction in the presence of SUP (lectin depleted) and since incubation in SUP alone did not confer reactivity on PNL(+) Thc, it appears therefore that (a) immature Thc possess alloantigen and mitogen-specific surface receptors but lack the capacity to respond by proliferation to receptor triggering without the help of extracellular factor(s) released by mature lymphoid cells stimulated by mitogens (b) cell division is associated with the acquisition of immunological responsiveness, characteristic of mature T lymphocytes. The implications of these findings for the ontogenesis of thymus-derived lymphocytes, and for the possible traffic of Thc within and from the thymus, are discussed.

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