In vitro stimulation of lymphocytes by combinations of antigens and allogeneic cells

The mixed leucocyte reaction and the delayed hypersensitivity antigens histoplasmin and tuberculin purified protein derivative were used in combinations to test the clonal selection theory in vitro. When histoplasmin was added to mixed leucocyte cultures containing sensitized cells from one donor, there was always an additive blastogenic effect. Experiments using different proportions of histoplasmin positive and negative cells in mixed cultures indicated that the response to histoplasmin was a specific one, and depended on the number of sensitized cells present in the cultures. Preincubating positive or negative donor cells with histoplasmin prior to setting up mixed cultures led to the same conclusion. On addition of both histoplasmin and tuberculin to unmixed and mixed leucocyte cultures containing sensitized cells, it was demonstrated that, under appropriate conditions, summation occurred. This summation, on multiple antigenic stimuli, supports a clonal proliferation mechanism in in vitro lymphocyte cultures