Increased reactivity of responding cells in the mixed lymphocyte reaction by a thymic humoral factor.

A thymic humoral factor (THF) induces competence in a certain sector of the lymphoid cell population. The target cell for THF activity was studied using the one-way mixed lymphocyte culture (MLC). Cells from various lymphoid tissues (responding cells) were preincubated in THF for 1 hour and then mixed with mitomycin-C-treated cells (stimulating cells). The results obtained indicate that THF increases MLC competence of lymphocytes from spleen and thymus origin of intact mice and that of spleen of thymus-deprived mice. In contrast, no enhancing effect in the MLC response by THF could be observed when bone marrow-derived spleen cells, cortisone-resistant thymocytes or lymph node cells were used as responding cells. These results confirm our hypothesis that THF leads to differentiation of young thymus-derived (T) cells, promoting them to maturation and thus to the acquisition of full immune competence.