Lymphocyte transformation in vitro. IV. Recruitment in antigen-stimulated cultures.

The additive effect of two or three different antigens upon the total DNA synthesis in lymphocyte transformation was investigated. Evidence was obtained that definite, but incomplete summation occurs when the response to each of the antigens alone is of moderate degree, but that there is a ceiling of total reactivity above which no further increase is possible. If the response to one of the antigens is already in this region, no additive effect is achieved by other antigens. It was demonstrated that in antigen-stimulated and mixed lymphocyte cultures a small sediment surface area in the presence of the stimulating agent is required during the first 24 hr for optimum stimulation. In further experiments the effect was studied of the addition of non-sensitized lymphocytes from an HL-A identical sib to sensitized cells from the other sib. It was shown that 1 × 10 sensitized cells mixed with 2 × 10 lymphocytes from the non-sensitized HL-A identical sib gave a response more or less similar to that obtained using 3 × 10 sensitized lymphocytes alone. These three series of experiments seem to indicate that recuitment of non-sensitized lymphocytes for participation in specifically induced transformation may play an essential role in total reactivity, and that maximum reactivity is limited by the size of the pool of recruitable cells.