SYNERGY DURING IN VITRO CYTOTOXIC ALLOGRAFT RESPONSES : I. EVIDENCE FOR CELL INTERACTION BETWEEN THYMOCYTES AND PERIPHERAL T CELLS

A mouse in vitro allograft system was used to evaluate the concept of T-T interaction in T cell-mediated cellular immunity. In analyzing the responsiveness of thymus-processed lymphocytes as obtained from different tissues, a heretogeneity within T cells was found in regard to their capacity to be immunized in vitro against transplantation antigens. Recirculating T cells were 10–20-fold superior to thymocytes, splenic T cells being intermediate. When few (1.5 x 10⁶) peripheral T cells, in numbers too small to yield good cytotoxic responses, were mixed with 14 x 10⁶ thymocytes and the cell mixture immunized in vitro against cell-bound alloantigens, cytotoxic activity was generated exceeding about 10–20-fold the values that could be explained by a pure additive effect. Synergy occurred also in a mixture of responder T cells derived from CBA (H-2^k) and AKR (H-2^k) mice. Thus AKR anti-ϕ C3H serum could be used for discriminating between thymus-derived and peripheral T cell-derived cytotoxic lymphocytes (CL). Cytotoxic activity produced during the synergistic interaction between thymocytes and peripheral T cells was about 70% T cell derived, the remainder being thymus derived. The synoptic interpretation of this finding and "limiting dilution" experiments of the responder cells suggested strongly that peripheral T cells provide the major source for precursor cells of CL, thymocytes acting mainly as helper (amplifier) cells.