THE TIME-COURSE, LOCALIZATION AND QUANTITATION OF T-CELL AND B-CELL MITOGEN-DRIVEN APOPTOSIS IN-VIVO

Apoptosis has very recently been visualized in situ in a mammalian thy mus and spleen [1]. We report here the first in situ visualization, lo calization and quantitation of the time course of mitogen-altered basa l levels of apoptosis within the thymus and spleen of a vertebrate. Ad ult Xenopus leavis, South African clawed toads, were injected intraper itoneally with either the T-cell mitogen, Concanavalin (Con) A, or the B-cell mitogen, lipopolysaccharide (LPS). Controls, reflecting the ba sal level of apoptosis of both organs, were injected with isotonic pho sphate-buffered saline for amphibians (APBS). ConA and LPS failed to e nhance the nearly 2% apoptotic rate in the thymic cortex, which is mad e up largely of immature lymphocytes, but it did double the base level of 2% apoptosis in the mature lymphocytes of the medulla, particularl y along the corticomedullary boundary. In the lymphoid splenic white p ulp, the 2% basal level was exceeded slightly after ConA treatment, wh ile the 6% basal lymphoid apoptotic rate in the red pulp was enhanced 7-fold in 12 h. LPS induced lymphocytic apoptosis in the partly erythr opoietic red pulp of the spleen after 12 h but did not effect the whit e pulp. Extensive macrophage engulfment of apoptotic cells was apparen t in both the thymus and the spleen.