Rac1 and Rac2 GTPases Play Redundant but Critical Role in T-Cell Development and Survival.

The Rac subfamily GTPases of the Rho family have been implicated in the control of actin dynamics, cell proliferation, apoptosis, adhesion and migration of many blood cell types including hematopoietic stem/progenitors, neutrophils and macrophages, but their role in T cell development remains poorly understood. T cells from the Rac2 deficient mice appear to mostly undergo normal development, whereas previous constitutively active mutant Rac2 or Rac1 overexpression studies suggest Rac GTPases are required for CD4+ and CD8+ T cell maturation. Using conditional gene targeting, we have achieved specific deletion of Rac1 or Rac1 together with Rac2 in the T cell lineage by cross-breeding the Lck-Cre transgenic mice with the Rac1^flox/flox mice that contain a pair of loxP sites sandwiching the exon 1 sequences of Rac1 or the Rac1^flox/flox;Rac2^−/− mice. We show that similar to Rac2 deficiency, inactivation of Rac1 alone had little effect on various developmental stages of T cells in the animal. However, deletion of both Rac1 and Rac2 significantly affected both the immature CD4−CD8− (2.3 fold increase) and CD4+CD8+ (13% decrease) populations in the mouse thymus and the mature CD4+ and CD8+ populations in the thymus and spleen (Table). These developmental defects are associated with proliferation defects of thymocytes and splenocytes in response to ConA or PMA/ionomycin stimulation and deficient survival of various T cell populations at different developmental stages (Table). Together, these data show that Rac1 and Rac2 play overlapping and obligatory roles in T-cell development and serve as important cell survival regulators at various stages.