Thymic lymphocytes. II. Phenotypic modifications of thymocytes after concanavalin A stimulation in the presence of interleukin 2: early modifications of Lyt 1+2+ subset and later proliferation of cells with more mature phenotypes.

Cortical thymocytes are devoid of any immune function, as tested by presently available techniques. The ability of this subpopulation to respond to mitogens or antigens in the presence of interleukin 2 (IL-2) produced by activated mature T lymphocytes has been claimed but is still questioned. In an attempt to study the participation of the different thymocyte subsets and especially that of the cortical type, phenotypic modifications were examined during concanavalin A activation in the presence of IL-2. An immunofluorescent double labeling technique with anti-Lyt 1 and anti-Lyt 2 antibodies was used which led to the determination of four different phenotypes: Lyt 1+2+, Lyt 1+2-, Lyt 1-2+, and Lyt 1-2-. Careful analysis of cell viability in culture and expression of the results in absolute numbers of living cells per culture allowed us to follow modifications of small cellular subsets. Cultures of total thymocytes and PNA-agglutinated (enriched in Lyt 1+2+ cells) and non-PNA-agglutinated cells (enriched in Lyt 1+2-, Lyt 1-2+, and Lyt 1-2- cells) were studied. It was shown that thymocyte activation began by early phenotypic modifications which took place within the first 2 hr of culture but only when Con A plus IL-2 were used. These modifications imply the reduction of the Lyt 1+2+ pool and a compensatory enhancement of Lyt 1-2+ and Lyt 1-2- cells, without modification of the total cell number or [3H]thymidine incorporation. These early phenotypic changes are interpreted as the modulation of antigens on the surface of Lyt 1+2+ cells. The second phase of thymocyte activation implies cell death (essentially Lyt 1+2+ cells) and cell proliferation. The cells which specifically proliferate in the presence of Con A and IL-2 are Lyt 1+2- and Lyt 1-2+, the latter always being present in greater number. Cell survival and absolute number of Lyt 1+2- and Lyt 1-2+ cells in the activated PNA- -enriched population are always higher than in total thymocyte and PNA+ cells cultures. Thus, if Lyt 1+2+ cortical thymocytes do not proliferate by themselves, they seem to intervene by providing Lyt 1-2+ cells which proliferate secondarily.