TGFbeta-mediated immunoregulation.

T cell homeostasis is required for normal immune responses and prevention of pathological responses. Transforming growth factor beta (TGFbeta) plays an essential role in that regulation. Owing to its broad expression and inhibitory effects on multiple immune cell types, TGFbeta regulation is complex. Through recent advances in cell-specific targeting of TGFbeta signaling in vivo, the role of TGFbeta in T cell regulation is emerging. We demonstrated here a critical role for TGFbeta in regulating effector vs regulatory T cell homeostasis.

Functional flexibility in T cells: independent regulation of CD4+ T cell proliferation and effector function in vivo.

Proliferation and differentiation of CD4+ T cells are often correlated, but it is not clear whether they are mechanistically linked. When antigen-specific T cells are present at high frequency in vivo, they all respond to antigenic peptide stimulation by expressing activation markers, but only a subset begins to proliferate. However, noncycling cells may synthesize the effector cytokine IFNgamma even though their cell cycle is blocked in G1. These data show that proliferation and effector function are not rigidly linked in T cells. Instead, CD4+ T cells have the flexibility to engage in or bypass clonal expansion based on the integration of multiple signals, including the frequency of other responding T cells.

Selective involvement of the Fas (CD95)/Fas ligand pathway in bone marrow B cell progenitors.

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.

Involvement of the Fas (CD95) system in peripheral cell death and lymphoid organ development.

Fas-mediated apoptosis is a form of cell death that operates through a Fas-Fas ligand (FasL) interaction. In this study we investigated the role of the Fas system during development of normal and Fas-mutated lymphocytes. Irradiated RAG2-/-recipients were reconstituted with bone marrow cells from B6 and lpr mice (Fas defective) or from B6 and gld mice (FasL defective), and analyzed for long-term development. The results showed a primary role of the Fas system in peripheral cell death and thymic colonization. In the periphery, the interaction in vivo between Fas+ and Fas-T cell populations indicated that cellular homeostasis was defective. Indeed, we observed a FasL-mediated cytotoxic effect on normal-derived T cells, explaining the dominance of lpr T cells in the mixed chimeras. The Fas mutation affected neither cell activation nor cell proliferation, as the effector (Fas-) and target (Fas+) cells behaved similarly with regard to activation marker expression and cell cycle status. However, Fas-T cells failed to seed the periphery and the thymus in the long term. We suggest that this could be due to the fact that FasL is involved in the structural organization of the lymphoid compartment.

In vivo CD4+ lymph node T cells from lpr mice generate CD4-CD8-B220+TCR-beta low cells.

Double-negative CD4-CD8-T cells (DNT) have been shown to be the major population of T cells responsible for the massive lymphadenopathy associated with the early onset of the lupus-like syndrome in mice bearing the lpr gene. Previously, we demonstrated that these cells do not proliferate in the peripheral lymphoid organs that they invade; furthermore, we showed that a wide range of CD4 Ag expression was observed on lymph node CD4+ T cells. In this study, we used an in vivo transfer system to analyze the progeny of CD4+ T cells from B6-lpr/lpr mice. Purified CD4+ T cells injected into B6 nude mice are able to generate DNT cells; furthermore, phenotypic and functional characterizations of the DNT cells generated in vivo show that they share the same properties as DNT cells from B6-lpr/lpr mice. We also show that, after in vitro bromodeoxyuridine incorporation, only CD4+ cells cycle. From these studies, we conclude that the lymphoproliferation occurs at the CD4+ stage and that down-regulation of this Ag probably is followed by arrest of the cell cycle.