PROGRAMMED DIFFERENTIATION OF MURINE THYMOCYTES DURING FETAL THYMUS ORGAN-CULTURE

Fetal thymus organ culture (FTOC) has become widely used to investigat e the impact of immunomodulators on T cell development. However, these studies have given variable results among different laboratories. In this study, we have found that fetal tissue age and mouse strain diffe rences can affect the development of T cell phenotypes in this system. T cell development in FTOC occurred in two 'waves', defined as peaks of cell recovery. The first wave consisted initially of CD4(-)CD8(-) d ouble negative (DN) cells and CD4(-)CD8(+) single positive (SP) T cell s expressing gamma delta T cell receptor (TCR). CD4(+)CD8(+) double po sitive (DP) cells expressing low levels of alpha beta TCR were produce d soon thereafter; and these cells dominated the cultures for the bala nce of the first wave. Prolonged FTOC resulted in the production of an other wave of T cells which were relatively enriched for CD4 or CD8 SP cells expressing high levels of alpha beta TCR, as well as DN cells a nd CD4(-)CD8(+) SP T cells expressing high levels of gamma delta TCR. As defined by cell number and differentiation of alpha beta TCR SP cel ls, development was delayed in FTOC using fetal thymus tissue from you nger fetuses relative to that observed when older fetal thymus tissue was used. The degree of development of T cells in FTOC was also strain dependent. Organ cultures derived from 14 gestation days (gd) C.B-17 scid/scid fetal thymus did not generate TCR-bearing mature SP cells, b ut they did produce TCR-negative CD4 and CD8 SP cells likely to be pre cursors of DP thymocytes. Such cultures made from 18 gd tissue did not produce SP cells. Negative selection in FTOC was also evaluated. Mtv- specific V(beta)3 cells were deleted in FTOC of C3H/HeN tissue. Deleti on occurred only in late FTOC, suggesting a late encounter between the Mtv deleting elements and susceptible T cells during ontogeny. These results show that while FTOC recapitulates normal thymic development b y a variety of criteria, results can be influenced by the length of cu lture, as well as by the age and strain of fetal thymus tissue utilize d.