Regulación de la dinámica de las células B por estímulos de tipo innato: efectos de TLR4 y su ligando LPS

Abstract

B cells are constantly migrating in follicles, where they survey for the presence of potential antigens. The recognition of specific antigen through the B cell receptor (BCR) initiates the humoral immune response. B cells also express a wide range of innate-like receptors, including several members of the Toll-like receptor (TLR) family. TLR recognition of pathogen-associated ligands has important effects in a number of B cell functions: enhances antigen presentation capacity and promotes cytokine secretion, cell proliferation and differentiation into antibody-secreting cells. TLR ligands detection also modifies B cell localization and trafficking. Nevertheless, the direct role of TLR signaling on B cell dynamics remains almost entirely unknown. Using a two-dimensional model combined with real-time microscopy, we analyzed the effects of TLR4 signaling on B cell motility. We found that 4 h of TLR4 stimulation modulates B cell migration in response to chemokines. This short-term TLR4- stimulation increases B cell adhesion, polarization, migration and directionality in response to CXCL13. These effects are dependent on the MyD88-dependent signaling pathway, with little implication of the TRIF-mediated route. ERK and p38 MAPK regulate cellular migration and inflammatory responses; 4 h-TLR4 stimulation increases CXCR5 levels at the B cell surface and enhances CXCL13-mediated ERK activation. Both effects might aid in the observed enhancement of B cell dynamics; however, they do not explain all the described effects. We explored TLR4-mediated regulation of Rho GTPase family members and ERM proteins, important for cell migration. We found that TLR4 stimulation enhances Rac GTPase activity and promotes sustained Rac activation in response to chemokines. As a result, actin polymerization is increased, probably through the higher activity of the Rac-effector PAK. We also proposed that the enhanced PAK activity regulates NM-II activation and thus, actomyosin contraction and integrin-mediated adhesion in TLR4-stimulated B cells. Finally, the level of active ERM, in particular Moesin, is also increased in LPS-treated B cells. All these molecular changes might be responsible of B cell dynamics modulation by TLR4. In summary, TLR4 signaling could improve B cell antigen-seeking efficiency by a molecular mechanism that involves a Rac/PAK/NM-II/moesin signaling complex.