The mixed leukocyte culture reaction

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.

Human mixed leukocyte culture: identification of the proliferating lymphocyte subpopulation by sex chromosome markers

The nature of the cells which proliferate in response to allogeneic histoincompatible lymphocytes in human mixed leukocyte cultures (MLC) was investigated. Direct information on the differentiation of proliferating cells was obtained in an in vitro chimeric system with T- and B-cell sex chromosome markers. Highly purified T and B cells were prepared from the peripheral blood of three pairs of HL-A genotype identical, MLC-negative siblings of opposite sex by E-rosette sedimentation techniques. Recombined T and B cells of the siblings were stimulated with allogeneic cells, and karyotype analysis of proliferating cells performed. 96% of the proliferating cells were found to carry the sex chromosome marker of the respective T-cell population. These results indicate that proliferation in human MLCs is a feature of T cells.

Stimulated lymphocyte cultures: responder recruitment and cell cycle kinetics

Lymphocytes, stimulated with different doses of plant mitogens or allogeneic cells, incorporate varying amounts of [(3)H]thymidine. Theoretically, this may be due to different numbers of responding cells, to earlier proliferative response of these cells, and/or to their more or less rapid transit through the cell cycle. Dog peripheral blood lymphocytes were stimulated in vitro with different doses of phytohemagglutinin (PHA), or with allogeneic lymphocytes. After their synchronization by incubation with hydroxyurea (4 mM), the mean durations of the cell cycle, and of the different cell cycle phases were constant and unrelated to strength or type of stimulation. PHA-stimulated lymphocyte cultures were maintained in the presence of colchicine, to prevent clonal proliferation of responding lymphocytes. DNA uptake in this setting, attributed to first generation responders, was related to the strength of proliferative lymphocyte response in control cultures without colchicine. Furthermore, cell proliferation occurred earlier with greater stimulation. It is concluded that higher [(3)H]thymidine uptake in vitro by stimulated lymphocytes is due to greater numbers of responding cells, which are triggered into proliferative response earlier, and not to a more rapid transit of the responding cells through the cell cycle.