RhoH is a small GTPase expressed only in the hematopoietic system. With the use of mice with targeted disruption of the RhoH gene, we demonstrated that RhoH is crucial for thymocyte maturation during DN3 to DN4 transition and during positive selection. Furthermore, the differentiation and expansion of DN3 and DN4 thymocytes in vitro were severely impaired. These defects corresponded to defective TCR signaling. Although RhoH is not required for TCR-induced activation of ZAP70 and ZAP70-mediated activation of p38, it is crucial for the tyrosine phosphorylation of LAT, PLC␥1, and Vav1 and for the activation of Erk and calcium influx. These data suggest that RhoH is important for pre-TCR and TCR signaling because it allows the efficient interaction of ZAP70 with the LAT signalosome, thus regulating thymocyte development.

Several experimental evidences suggested that beta1 integrin-mediated adhesion of hematopoietic stem cells (HSC) is important for their function in the bone marrow (BM). Using induced deletion of the beta1 integrin gene restricted to the hematopoietic system, we show that beta1 integrin is not essential for HSC retention in the BM, hematopoiesis, and trafficking of lymphocytes. However, immunization with a T cell-dependent antigen resulted in virtually no IgM production and an increased secretion of IgG in mutant mice, while the response to a T cell-independent type 2 antigen showed decreases in both IgM and IgG. These data suggest that beta1 integrins are necessary for the primary IgM antibody response.

The function of ␣4␤1 and ␣4␤7 integrins in hematopoiesis is controversial. While some experimental evidence suggests a crucial role for these integrins in retention and expansion of progenitor cells and lymphopoiesis, others report a less important role in hematopoiesis. Using mice with a deletion of the ␤1 and the ␤7 integrin genes restricted to the hematopoietic system we show here that ␣4␤1 and ␣4␤7 integrins are not essential for differentiation of lymphocytes or myelocytes. However, ␤1␤7 mutant mice displayed a transient increase of colonyforming unit (CFU-C) progenitors in the bone marrow and, after phenylhydrazineinduced anemia, a decreased number of splenic erythroid colony-forming units in culture (CFUe's). Array gene expression analysis of CD4 ؉ CD8 ؉ double-positive (DP) and CD4 ؊ CD8 ؊ double-negative (DN) thymocytes and CD19 ؉ and CD4 ؉ splenocytes did not provide any evidence for a compensatory mechanism explaining the mild phenotype. These data show that ␣4␤1 and ␣4␤7 are not required for blood cell differentiation, although in their absence alterations in numbers and distribution of progenitor cells were observed.

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