Glucocorticoid and B cell development: role of Glucocorticoid-induced leucine zipper

Glucocorticoids (GC) are widely used as immunosuppressive drugs and antitumor agents in some acute leukemia and multiple myeloma. Therapeutic doses of GC induce growth suppressive and cytotoxic effects on various leukocyte types including B cells. Molecular mechanisms of GC action include induction of GC target genes. Glucocorticoid-induced leucine zipper (GILZ) is a gene rapidly, potently and invariably up-regulated by GC treatment. It mediates a number of GCs effects, such as control of cell proliferation, apoptosis and differentiation. GILZ suppresses Ras/MAPK/Erk and NFkB pathways and promotes TGF-b signaling in T cells. It belongs to TSC22d family, members of which were recently found mutated in diffuse large B cell lymphoma patients. Here we address the physiologic role of GILZ in normal hematopoiesis, and evaluate its role in mediation of GC effects on various blood cells, using genetic approach. Mice deleted for gilz gene were recently generated. We have monitored white blood cell counts in wild type (wt) and in gilz knock-out (KO) mice overtime. Development of lymphoid and myeloid lineages was evaluated both by peripheral blood (PB) cell counts (Hematocrit), and by flow cytometry analysis of bone marrow (BM), spleen and PB using Mac-1, B220, CD43, IgM and IgD staining. Young gilz KO mice showed normal body and lymphoid tissues weights and cell counts in PB, thymus, spleen, peripheral lymph nodes and BM. However, overtime gilz KO mice showed a 1.5-2 fold increase in white blood cell counts in PB. Increase in lymphocyte counts was due to accumulation of B220+ cells, while the number of Mac-1+ cells did not differ between wt and gilz KO mice. Flow cytometry analyis of B220+ cell compartment in BM revealed an increase in the frequency and number of pre-B cells (IgM loIgDlo), immature (IgMhiIgDlo) and recirculating B cells (IgMloIgDhi) already in 8-week old mice. Preliminary data suggest that the defect starts as early as at common-lymphoid progenitor (CLP) stage. Treatment of purified B220+ cell with GC in vitro resulted in different degree of apoptosis in wt and gilz KO cells, suggesting that the increase in B cells in vivo may results from decreased sensitivity to the death induced by endogenous GC. Our results show that lack of GILZ results in specific defect in B cell development, leading to the expansion of B220+ cells compartment, associated with the expansion of early B cell progenitor cells and suggest that degerulation of GILZ expression may contribute to cell survival or differentiation of early B cells and pathologies of B cell lineage.