THYMIC SELECTION AND CELL-DIVISION

Cell division during thymic selection was studied with a system in whi ch purified populations of T cell antigen receptor (TCR)(-) CD4(+)8(+) (double-positive [DP]) cells and fetal thymic epithelial cells (TEC) were reaggregated in tissue culture. In this system, immature DP cells differentiate into mature single-positive (SP) CD4(+)8(-) and CD4(-)8 (+) TCR(hi) cells within 3-4 d, indicative of positive selection. By a dding the DNA precursor, bromodeoxyuridine, to the cultures and staini ng cells for bromodeoxyuridine incorporation, T cell division in reagg regation cultures was found to be high on day 1, low on day 2, and hig h on days 4-5. Cell separation studies established that cell division on day 1 was restricted to DP blast cells. In the absence of blast cel ls, small DP cells failed to proliferate and differentiated into SP ce lls without cell division, thus indicating that proliferation is not a n essential component of positive selection. This applied to SP cells generated within the first 2-3 d. Surprisingly, the SP cells generated later in culture showed a high rate of cell division; the proliferati ng SP cells were TCR(hi) and included both CD4(+)8(-) and CD4(-)8(+) c ells. Turnover of TCR(hi) SP cells was also prominent in the normal ne onatal thymus and in TEC reaggregation cultures prepared with adult ly mph node T cells. We speculate that division of mature SP cells in the perinatal thymic microenvironment is driven by stimulatory cytokines released from TEC. Such proliferation could be a device to expand the mature T cell repertoire before export to the periphery.