The cells involved in cell-mediated and transplantation immunity in the rabbit. XII. The establishment of the optimum conditions for the demonstration of a consistent response for the circulating white blood cells in the mixed leukocyte reaction.

Rabbit circulating leukocytes (WBC) have been shown to respond consistently in the allogeneic one-way mixed leukocyte culture reaction (MLR). Culture of the cells for 5--6 days in 5% CO2 in air in medium RPMI 1640 supplemented with normal decomplemented rabbit serum to a concentration of 2.5% provided conditions for an optimal MLR response. The WBC of the majority of the normal rabbits tested (82%) responded in the MLR, in contrast to the negative findings of other investigators. The WBC of a minority of the normal rabbits (18%) did not respond in the conventional MLR; however, the spleen cells of these rabbits consistently responded in the MLR, demonstrating that these rabbits possessed MLR responsive cells which may not have been present in the circulation at the time of analysis. Furthermore, the WBC of the 'non-responder' rabbits responded well in the MLR when the allogeneic WBC stimulator cells were replaced by allogenic spleen cells, suggesting that the WBC responder cells require a helper cell provided by the splenic stimulator cells to facilitate the blastogenic response. Whether the MLR responsive cell in the rabbit is a T cell or a 'T-like' cell with B cell properties has still not been resolved.