A20 deficiency in hematopoietic stem cells causes lymphopenia and myeloproliferation due to elevated Interferon-γ signals.

Inflammation and inflammatory cytokines have been shown to exert both positive and negative effects on hematopoietic stem cells (HSCs) and hematopoiesis. While the significance of inflammation driven hematopoiesis has begun to unfold, molecular players that regulate this phenomenon remain largely unknown. In the present study, we identified A20 as a critical regulator of inflammation controlled hematopoietic cell fate decisions of HSCs. A20 deficiency in HSCs leads to increased differentiation of myeloid cells and myeloproliferation. Analysis of erythroid lineage cells of A20 deficient mice indicated a striking reduction of erythrocytes in the bone marrow (BM), but elevated numbers in the spleen. Loss of A20 in HSCs causes a severe blockade of B cell differentiation in the BM and absence of peripheral B cells in the spleen, liver and blood. T cell differentiation studies revealed a reduction of both T cell progenitors and differentiated T cells in the thymus and altered T cell numbers in the spleens of A20 mutant mice. Analysis of lineage committed progenitors of the myeloid, erythroid and lymphoid lineages specified an altered composition in the A20 deficient BM. Genetic studies identified that specific loss of A20 in the myeloid lineage cells results in myeloproliferation. Bone marrow transplantation studies and mixed bone marrow chimera studies suggested an involvement of inflammatory cytokines, particularly interferon (IFN)- γ, in the onset of myeloproliferation and lymphopenia of A20 deficient mice. Finally, ablation of IFNγ signals in A20 deficient mice rescued the hematopoietic defects. In essence, these studies highlight a previously unknown role for A20 in the restriction of inflammation driven pathologic hematopoiesis. We believe that our studies based on A20 mutant mice will be helpful in understanding the pathophysiology and in the treatment of patients with A20 (TNFAIP3) mutations.