POSTNATAL-DEVELOPMENT OF MURINE SPLEEN-CELL PROLIFERATIVE RESPONSES INDUCED BY INTERLEUKIN-2 OR PHORBOL ESTER AND CALCIUM IONOPHORE-A23187

To determine the kinetics of the ontogeny of polygenically controlled proliferative responses, spleen cells from 5, 10 and 20 day-old mice, previously identified as high (B10 PL-Thy 1a, AKR.M) and low (C3H-A) responders were tested. Cells (10(5)) were stimulated in vitro with human recombinant IL-2 (35 U), a mixture of either IL-2 (0.7 U) and 13-myristate 12-acetate phorbol (5 ng) or phorbol and calcium ionophore A23187 (5 ng). Two parameters were assessed; proliferation measured by H-3-thymidine incorporation and the synergistic effect expressed as a synergism coefficient (SC): the ratio of proliferation induced by a mixture to the sum of proliferations induced by each component alone. At the age of 5 days, all three strains, regardless of stimulation, produced similar proliferation and showed no synergistic effect (SC - 1.0). At the age of 10 days, proliferation of high responder cells was 3-5 times higher than that of the low responder, but only high responders displayed marginal synergistic effect (2 > SC > 1.5). At the age of 20 days, proliferative responses of the high responders increased somewhat and a definitive synergistic effect became clearly demonstrable (SC > 3.0). This effect increased further after the age of 20 days to reach the level of SC > 5.0 in adult high responders. The delay in expression of synergistic effect was due to a substantial proliferation induced by phorbol alone in 10 day-old animals. The available data allow the conclusion that phenotypic strain variation in proliferative responses is first detectable at the age of 10 days and becomes fully developed only after attaining the age of 20 days or more. Since the magnitude of proliferative responses was not associated with frequencies of 11-2R alpha(1) cells it seems that differences in responsiveness can not be attributed to the induction of IL 2R alpha chain, and they may reflect intrinsic abilities of cells to proliferate.