RhoH-Deficient Mice Show Altered B Cell Populations In Vivo.

RhoH is a GTPase-deficient, hematopoietic-specific member of the family of Rho GTPases (Li et al, 2002). RhoH has been described as regulating proliferation and engraftment of hematopoietic progenitor cells (Gu et al, 2005) and integrin-mediated adhesion in T cells (Cherry et al, 2004). Additionally, RhoH plays a critical role in T-cell development and T-cell receptor signaling (Gu et al, 2006; Dorn et al, 2007). However, the potential role of RhoH in the differentiation and biological functions of B cells are unknown. To answer these questions, we analyzed the B-cell phenotype of RhoH−/− mice and the in vitro properties of RhoH-deficient splenic B cells compared to their wild-type counterparts. RhoH−/− mice showed increased B-cell numbers in the bone marrow, mainly due to an increase in the number of pro-B, pre-B and immature B cells. In the spleen, lymph nodes and peripheral blood, RhoH−/− mice showed a significant decrease in the number of follicular (B-2) cells (B220+ CD93– IgD^high CD21^low). The number of splenic marginal zone B cells (B220+ CD93– IgD^low CD21^high), plasma cells (CD93– CD38+ CD138+) in bone marrow and spleen, and B-1 cells (IgM+ CD5+) in peritoneal cavity were not significantly different from those in wild-type animals. These alterations have functional significance, since the serum concentrations of IgM and IgG1 were significantly lower in RhoH−/− mice. However, splenic B cells isolated from RhoH−/− mice did not show any significant differences in their in vitro activation by anti-IgM, CD40 ligation or IL-4 stimulation, nor did they differ in their proliferative response to lipopolysaccharide. In vitro migration of RhoH-deficient B cells in response to CXCL12 or CXCL13 was similar to that of wild-type B cells. Given the important role of RhoH in signal transduction downstream the T cell receptor, we investigated the possible role of RhoH in B cell receptor signaling. Although total splenic B cells from RhoH−/− mice showed markedly increased phosphorylation of SYK and ERK after anti-IgM stimulation compared to wild-type B cells, sorted populations of splenic B-2 and marginal zone B cells from RhoH−/− and wild-type animals did not differ in the activation of these kinases, suggesting that the observed difference can be attributed to the different cellular composition of the B cell compartment (i.e. B-2 vs marginal zone B cells) in RhoH−/− mice. These data imply that the phenotype observed in RhoH−/− mice may not reflect an intrinsic defect in B cells but may be attributed to crosstalk between B cells and other hematopoietic cell populations.