Immune lymph node cells from guinea pigs respond to soluble antigen in vitro by an increase in DNA synthesis. Optimal conditions for this proliferative response were studied in the present article. Under such conditions, immune cells showed increasing responses with increasing antigen concentration in vitro, the threshold dose of activation frequently being as low as 0.02 µg per culture. In contrast, normal lymph node cells (from FCA-stimulated animals) did only respond to antigen at very high doses (20 mg/culture), and immune cell dilution studies could be performed in normal cells without changing the kinetics of the antigen specific response of immune cells.

Fractionation on anti-Ig columns indicated that purified, immune T lymphocytes were quite capable of proliferating in vitro upon antigen stimulation. However, our attempts to adsorb the proliferating cells onto chemically defined immunoadsorbants failed despite the fact that immune B cells (as measured by the rosette assay) were retained almost completely by such a procedure.

Purified, immune T lymphocytes from guinea pigs immunized with different antigen concentrations in vivo and/or obtained at different times after immunization were tested for a differential sensitivity toward antigen-induced DNA synthesis in vitro. However, we were not able to demonstrate any regular increase in sensitivity to antigen in vitro, and if found, it seemed to be more dependent upon the number of antigen reactive cells in the population studied rather than upon differences in the average avidity of the receptors on the cells proliferating in vitro. The results in the present article are discussed in relation to current knowledge and hypotheses on T-lymphocyte receptors.

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