Abstract

Patients with aplastic anemia have elevated T-bet, a Th1 transcription factor, in peripheral blood CD4 and CD8 T cells, suggesting that T-bet over-expression and dysregulated Th1 immune response contributes to pathophysiology of marrow failure (

Solomou EE et al.,
Blood
.
2006
;
107
:
3983
). In the present study, we studied the role of T-bet in inducing bone marrow failure in a mouse model of immune-mediated BM failure, employing mice engineered with a germline T-bet deletion as lymphocyte donors. Compared with T-bet +/+ wild-type controls, T-bet−/− mice have similar cellular composition in various lymphohematopietic tissues including peripheral blood, spleen, thymus, lymph nodes (LN), and BM. Incubation of effector T-bet−/− LN cells with MHC-mismatched target CByB6F1 (F1) BM cells in an in vitro cytotoxicity assay resulted in a significantly lower proportion of apoptotic target cells than did wild-type T-bet+/+ LN effector cells, suggesting that T-bet−/− effector LN cells are functionally defective. While infusion of 5×106 wild-type T-bet+/+ LN cells into sublethally-irradiated F1 mice led to severe pancytopenia and aplastic bone marrow in recipient mice, infusion of the same number of T-bet−/− LN cells failed to result in marrow failure, and recipients had relatively normal blood counts and bone marrow cellularity. By flow cytometry, both expansion of CD4+ and CD8+ T cells and elevation in intracellular Th1 cytokine gamma interferon (IFN-γ), which are characteristic of marrow cells in recipients received B6 LN cells, were absent in recipients receiving T-bet −/− LN cells. Serum IFN-γ concentration in F1 mice infused with T-bet −/− LN cells was similar to the level in F1 control mice received TBI alone, and both were significantly lower than serum IFN-γ in recipients of wild-type B6 LN cells. In contrast, serum TGF-γ concentration was higher in F1 mice that received TBI alone or TBI plus T-bet −/− LN cell infusion, compared with mice that received TBI plus B6 LN cells. An increase of T-bet −/− LN cell infusion to 10×106 cells per recipient led to very mild BM failure. Contrary to the markedly increased number of CD4+ and CD8+ T cells and elevated IFN-γ level in the BM of F1 mice which have received wild type B6 LN cells, F1 mice infused with T-bet −/− LN have low CD4+ and CD8+ cells and low IFN-γ level in the BM similar to F1 mice received TBI alone, but they show increased IL4 and IL17 levels within bone marrow T cells, indicating that the diminished Th1 immune response due to T-bet deficiency was partially compensated by up-regulated Th2 and Th17 responses. Our data demonstrated that T-bet plays a critical role in immune mediated bone marrow failure. Approaches targeting to T-bet signal pathway may lead to novel treatment for bone marrow failure and other autoimmune diseases.

Disclosures: No relevant conflicts of interest to declare.

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