A comparison of the activation of thymus and lymph-node cells by concanavalin-A and phytohaemagglutinin. Effects of complement

Abstract

Cell suspensions from rat lymphoid organs were incubated for 6 hr with 3H-uridine in either bovine calf serum or homologous serum. The radioactive labelling of acid-precipitable material was then measured. Calf serum greatly inhibited the labelling of thymus cells, but had only a small effect on lymph-node cells. The inhibition was removed by decomplementing serum suggesting that cell destruction had been initiated by specific complement-fixing natural antibodies against rat thymus determinants. Activation of lymphocytes was assessed by measuring the enhancement of labelling produced by concanavalin-A (Con-A) or phytohaemagglutinin )PHA). In homologous serum and in decomplemented calf serum, thymus cells were activated by Con-A, (i) more than by PHA, and (ii) less than lymph-node cells were activated by Con-A. The latter finding indicates that the reduced transformation response of thymus cells to Con-A reported by other workers reflects the failure of cell activation, rather than of cell survival in vitro. In calf serum, lymph-node cells were activated by low concentrations of Con-A and inhibited by high concentrations of Con-A; the latter reaction required complement. In calf serum, thymus cells were (i) inhibited by Con-A concentrations low enough to stimulate lymph-node cells, and (ii) stimulated by low PHA concentrations. The results suggest that thymus cells consist of at least two populations, a major population destroyed by calf serum and activated by Con-A but not by PHA, and a minor population which is activated by PHA but not by Con-A. Attention is drawn to discrepancies between the Con-A dose-response relationships described by different workers. These may be understood in terms of the changing pattern of dose-response curves with time of culture, which is described elsewhere (Milthorp and Forsdyke, 1973).