One of the most elegant master regulatory cascades in cell biology is the system of small G proteins controlling actin-based structures and coordinating these with cell polarity and cell cycle: Cdc42, Rac, and Rho. In this issue, Burbage et al. examine the role of one of these regulators, Cdc42, in B cell function using conditional gene targeting in mice. They find that this master regulator is everything it’s cracked up to be—selective Cdc42 deletion in developing B cells led to a series of deficits that included decreased B cell receptor (BCR) signaling and delayed egress from the bone marrow, as well as an almost complete functional collapse when Cdc42-deficient B cells were asked to make antibodies in response to viral infection.
Classical studies describe how stromal cells generate—in response to growth factors—fine, spiky protrusions called filopodia to probe the environment, ruffling protrusions to allow fluid uptake, and, finally, a network of stress fibers to exert contractile forces. The Rho family of small GTPases controls these distinct F-actin structures: Cdc42 for spikes, Rac for ruffles, and Rho for contraction. The generation of such structures is important for immune cells, as actin-based protrusions are the most sensitive part of cells for antigen recognition, and contractile force is an important ingredient in ligand recognition by T and B cell receptors.
In this study, Burbage et al. define roles for Cdc42 in development and mature B cell function. The first nonredundant role of Cdc42 is in pro-B to pre-B cell differentiation. Of potential interest, this is a transition for which CD19 is required, which might entail F-actin regulated recruitment of CD19 to pre-BCR signaling clusters. The authors also suggested that Cdc42 might be required for immature B cell responses to the egress signal, sphingosine 1 phosphate (S1P), although this was not directly investigated; such decisions could involve competition between retention and egress signals, either or both of which could contribute to the phenotype. However, an earlier study found that B cell chemotaxis to CXCL12 was intact in Cdc42-deficient B cells, which suggests that retention signals are intact. The most dramatic effects of Cdc42 deficiency uncovered in this study were found in mature B cells in response to antigen. Steady state levels of IgM and IgG were low in specific pathogen-free animals. Furthermore, mice with Cdc42-deficient B cells were unable to produce IgG antibody responses to flu infection. Vaccination revealed a 1 log defect in IgM production and 3 log defects in IgG isotypes, suggesting impaired T cell-dependent responses. BCR signaling, spreading and antigen uptake were impaired, but the partial defects could not obviously account for the defects in antibody production. Consistent with earlier reports, B cells appeared to localize normally in lymphoid tissues, but by using two-photon microscopy, the present study found that Cdc42 deficient B cells moved significantly slower in lymph nodes. Furthermore, the generation of MHC-peptide complexes was strongly impaired and there was a general failure of early T–B conjugate assembly and germinal center formation. It is known that plasmablasts display a strong, sustained polarization when exiting follicles for the medulla, where they begin maturation into plasma cells. While this aspect of plasmablast behavior was not studied, there was clearly a cell autonomous defect in the differentiation of plasma cells in response to CD40 engagement independent of the defect in T–B conjugation. Cdc42 was found to play a critical role in CD40L-induced activation of AP-1, which is required to up-regulate Blimp-1, a factor known to modulate plasma cell differentiation in B cells.
The mechanisms by which Cdc42 dependent signaling contribute to B cell development, loading of MHC-peptide complexes, and AP-1 induction in plasma cells are not known. It’s also unclear how extensively quantitative defects in cell polarity are linked to these functional abnormalities. The defects in immature B cell egress and B cell migration may be secondary to polarity defects, but further studies are needed to determine if responses to chemotactic signals, such as S1P, are responsible and why this pathway would be more affected than G protein-coupled receptor–dependent responses to chemokines.