NK T cells are an unusual T lymphocyte subset capable of promptly producing several cytokines after stimulation, in particular IL-4, thus suggesting their influence in Th2 lineage commitment. In this study we demonstrate that, according to the cytokines present in the microenvironment, NK T lymphocytes can preferentially produce either IL-4 or IFN-+ . In agreement with our previous reports showing that their IL-4-producing capacity is strikingly dependent on IL-7, CD4 − CD8 − TCR § g + NK T lymphocytes, obtained after expansion with IL-1 plus granulocyte-macrophage colony-stimulating factor, produced almost undetectable amounts of IL-4 or IFN-+ in response to TCR/CD3 cross-linking. However, the capacity of these T cells to produce IFN-+ is strikingly enhanced when IL-12 is added either during their expansion or the anti-CD3 stimulation, while IL-4 secretion is always absent. A similar effect of IL-12 on IFN-+ production was observed when NK T lymphocytes were obtained after expansion with IL-7. It is noteworthy that whatever cytokines are used for their expansion, IL-12 stimulation, in the absence of TCR/CD3 cross-linking, promotes consistent IFN-+ secretion by NK T cells without detectable IL-4 production. Experiments in vivo demonstrated a significant upregulation of the capacity of NK T cells to produce IFN-+ after anti-CD3 mAb injection when mice were previously treated with IL-12. In conclusion, we provide evidence that the functional capacities of NK T cells, which ultimately will determine their physiological roles, are strikingly dependent on the cytokines present in their microenvironment.

In this study, we examined the consequences of Fas deficiency on hematopoiesis in C57BL/6-lpr/lpr mice. We found a striking extramedullary increase in hematopoietic progenitor cells, comprising erythroid and nonerythroid lineages alike. These modifications preceded the lymphadenopathy, because early progenitors (colony-forming units-spleen [CFU-S] day 8) were already augmented in day-18 fetal livers of the lpr phenotype. Three weeks after birth, CFU-S increased in peripheral blood and spleen and colony-forming cells (CFU-C) began to accumulate 1 to 3 weeks later. Extramedullary myelopoiesis augmented progressively in Fas-deficient mice, reaching a maximum within 6 months. By then, mature and immature myeloid cells had infiltrated the spleen, the liver, and the peritoneal cavity. Similar changes occurred in C57BL/6-gld/gld mice, indicating that they resulted from Fas/FasL interactions. Medullary hematopoiesis was not significantly modified in adult mice of either strain. Yet, the incidence of CFU-S decreased after Fas cross-linking on normal bone marrow cells in the presence of interferon γ, consistent with a regulatory function of Fas/FasL interactions in early progenitor cell development. These data provide evidence that Fas deficiency can affect hematopoiesis both during adult and fetal life and that these modifications occur independently from other pathologies associated with the lpr phenotype.

Recently, a marked extramedullary myelopoiesis in Fas/CD95-or FasL/CD95L-deficient mice has been reported. In the present in vitro study, the mechanisms underlying Fas-induced apoptosis of normal peripheral colony-forming unit-C (CFU-C) progenitors in the spleen were analyzed. Surprisingly, it was found that clonogenic progenitors were protected from ␥IFN plus Fas-induced programmed cell death when Lin ؉ cells were removed from cultured splenocytes. The cells that rendered CFU-C sensitive to the activa-

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B lymphocyte generation in bone marrow (BM) compensates for cell loses. The Fas/Fas ligand (FasL) pathway has been implicated in apoptosis of various cell types. Abnormalities of the Fas receptor or of FasL expression are associated with excessive T cell proliferation and autoimmunity. To examine the role of the Fas/FasL system in B cell differentiation, we created double-chimeric mice by transferring both C57BL/6 (B6)-Fas + and lpr-FasL + BM cells into RAG-2-/-hosts. Equal numbers of stem cells were co-injected into sublethally irradiated recipients, and their progeny were studied by using antibodies directed against the B6-Ly5.1 + 5.2 + and lpr-Ly5.1-5.2 + populations. A longitudinal study lasting for up to 6 months revealed that cells of the lpr phenotype dominated the B6 phenotype in the BM, as a result of their active proliferation. Analysis of the B cell compartment showed more lpr than B6 cells among immature HSA hi B220 lo populations. In contrast, the lpr and B6 phenotypes were equally represented among mature B cells. BM transfer to second hosts indicated that B6-derived B cell progenitors were absent from the first host. These data suggest that activation of the Fas/FasL pathway disturbs the early steps of B cell development and might therefore contribute to the onset of autoimmune disorders.

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