Regulation of B cell motility and adhesion in health and disease

Abstract

Migration and adhesion of lymphocytes are crucial to induce an efficient immune response. These activities rely on cytoskeletal dynamics, mediated by assembly of actin and microtubuli monomers leading to changes in cell shape and signaling. Different stimuli, including chemokines, cytokines and adhesive receptor triggering, induce cytoskeletal changes of lymphocytes. The Wiskott-Aldrich syndrome protein (WASP) is mutated in the severe immunodeficiency disease Wiskott-Aldrich syndrome (WAS). Exclusively expressed in hematopoietic cells, WASP coordinates upstream signaling from the Rho GTPases and other proteins to Arp2/3-induced actin assembly. We aimed at identifying the intracellular pathways involved in the cytoskeletal regulation of B cells. Likely candidates were WASP and its interacting partners Cdc42 and Rae 1.

We have investigated cytoskeletal changes of B cells activated with the T cell derived stimuli, interleukin-4 and ligation of CD40. These stimuli induce B cell motility, spreading on antibody-coated layers, homotypic B cell adhesion and increased number and length of microvilli. We found that all these processes were dependent on WASP, since WASP-deficient B cells had reduced responses. Furthermore, WASP-deficient B cells migrated poorly to chemokines. We also identified the contribution of the active forms of Cdc42 and Rac1 in formation of filopodia and lamellipodia in B cells.

In recent years it has became clear that WASP plays a crucial role in antigen-dependent activation of T cells. Dendritic cells and activated B cells present antigenic peptides on MHC class II molecules to T cells and thereby activate the T cells. T cells form a synapse, encompassing adhesive receptors, lipid rafts, polymerized actin as well as WASP, with the antigen-presenting cell. We wanted to examine whether expression of WASP in B cells and dendritic cells is important for efficient antigen presentation. We found that WASP-deficient B cells and dendritic cells presented soluble antigen efficiently to T cells, both in vitro and in vivo.

Notably, WASP-deficient dendritic cells exhibited reduced capacity to present bacterial-derived peptides to T cells. We have investigated the characteristics of IL-4-induced microvilli on B cells in more detail. We found that different membrane molecules as well as lipid rafts, preferentially localized on either microvilli or flat surface. Although we identified preferential localization of MHC class II on microvilli, we could not detect a more efficient antigen presentation under conditions where B cells express microvilli.

Finally, we found that WASP deficiency led to an impaired primary humoral immune response characterized by reduced B cell homing, germinal center formation and antibody secretion. In summary, we have defined the contribution of WASP, Cdc42 and Rae I for cytoskelatal changes in B cells. In addition, we have found that WASP plays a pivotal role in the primary humoral immune response.