Premature TCR alpha beta expression and signaling in early thymocytes impair thymocyte expansion and partially block their development.

In thymocyte ontogeny, Tcr-a genes rearrange after Tcr-b genes. TCR alpha beta transgenic (Tg) mice have no such delay, consequently expressing rearranged TCR alpha beta proteins early in the ontogeny. Such mice exhibit reduced thymic cellularity and accumulate mature, nonprecursor TCR(+)CD8(-)4(-) thymocytes, believed to be caused by premature Tg TCR alpha beta expression via unknown mechanism(s). Here, we show that premature expression of TCR alpha beta on early thymocytes curtails thymocyte expansion and impairs the CD8(-)4(-) --> CD8(+)4(+) transition. This effect is accomplished by two distinct mechanisms. First, the early formation of TCR alpha beta appears to impair the formation and function of pre-TCR, consistent with recently published results. Second, the premature TCR alpha beta contact with intrathymic MHC molecules further pronounces the block in proliferation and differentiation. These results suggest that the benefit of asynchronous Tcr-a and Tcr-b rearrangement is not only to minimize waste during thymopoiesis, but also to simultaneously allow proper expression/function of the pre-TCR and to shield CD8(-)4(-) thymocytes from TCR alpha beta signals that impair thymocyte proliferation and CD8(-)4(-) --> CD8(+)4(+) transition.

Characterization of the AKR thymic microenvironment and its influence on thymocyte differentiation and lymphoma development.

The thymic stroma has long been implicated in AKR thymic leukaemia. In this study an extensive panel of monoclonal antibodies was used to investigate changes in the AKR thymic microenvironment, in parallel with thymocyte differentiation of normal (2 month), preleukaemic (5-7 month) and leukaemic (> 7 month) mice. We found select alterations in the thymic stroma, including a loss of isolated medullary antigens and changes in MTS 32, a mAb detecting an antigen on both thymocytes and stroma in the thymic cortex. Stromal alterations were accompanied by shifts in thymocyte differentiation and the appearance of the leukaemogenic mink cell focus-forming (MCF) murine leukaemia virus.

Surface T cell Fas receptor/CD95 regulation, in vivo activation, and apoptosis. Activation-induced death can occur without Fas receptor.

Fas-mediated apoptosis is a form of cell death that operates through a receptor-ligand interaction. The FasR has been implicated directly in peripheral T cell tolerance and activation-induced apoptosis of T cells in vitro, although to date its expression on murine peripheral T cells has been characterized incompletely. In this study, we document substantial expression of FasR on the vast majority of recent thymic emigrants and resting peripheral T lymphocytes. FasR ligation can induce death in a minor (approximately 5%) subset of these cells. By contrast to rather slow activation-mediated FasR up-regulation in vitro, we demonstrate that in vivo T cell activation by alpha CD3 mAb or superantigen results in rapid up-regulation of the FasR. This up-regulation is paralleled by the kinetics of activation-induced apoptosis in lymph node T cells. However, we demonstrate that the FasR is not necessary for activation-induced cell death. Lymph node T cells from young, healthy, FasR expression-deficient MRL-Ipr/Ipr and animals could be activated in vivo through the TCR-CD3 complex. Most importantly, MRL-Ipr/Ipr T cells underwent massive activation-induced apoptosis in response to high and intermediate doses of alpha CD3. At a low alpha CD3 dose, however, both MRL-Ipr/Ipr and MRL +/+ T cells were activated similarly, but only the latter underwent adequate apoptosis. Taken together, these findings suggest that in vivo, the Fas pathway may not be the only regulator of activation-induced T cell death, but that this pathway may be critical in regulating responses to weak stimuli.

Expression and function of the murine CD95/FasR/APO-1 receptor in relation to B cell ontogeny.

Mice defective in Fas-mediated apoptosis (lpr phenotype) have an intrinsic B cell abnormality that predisposes them to autoantibody production. To investigate potential roles for the Fas receptor (FasR) in B cell tolerance, FasR expression and function were evaluated at different stages of B cell development. FasR expression was very low or absent on pro- and pre-B cells, but was detected in early B cell lines and was up-regulated following IFN-gamma-induced maturation of the pre-B cell line 70-Z. Whereas FasR expression was very low in resting mature sIgM+ B cells, expression was markedly increased following mitogen activation and was also elevated in two mature sIgG+ lymphoma lines. FasR expression correlated strongly with the ability of B cells to undergo Fas-mediated apoptosis. In addition, although Fas did not appear to play a direct role in apoptosis mediated by cross-linking of sIg with anti-IgM, anti-FasR and sublethal concentrations of anti-Ig were additive in the induction of apoptosis in the early B cell line WEHI 231. These findings suggest that the Fas pathway is not involved in the elimination of pro- and pre-B cells, but are compatible with an ancillary role for FasR in the elimination of early B cells and elimination of mature B cells following activation.