Essential role of immobilized chemokine CXCL12 in the regulation of the humoral immune response

Abstract

Chemokines control the migration of a large array of cells by binding to specific receptors on cell surfaces. The biological function of chemokines also depends on interactions between nonreceptor binding domains and proteoglycans, which mediate chemokine immobilization on cellular or extracellular surfaces and formation of fixed gradients. Chemokine gradients regulate synchronous cell motility and integrin-dependent cell adhesion. Of the various chemokines, CXCL12 has a unique structure because its receptorbinding domain is distinct and does not overlap with the immobilization domains. Although CXCL12 is known to be essential for the germinal center (GC) response, the role of its immobilization in biological functions has never been addressed. In this work, we investigated the unexplored paradigm of CXCL12 immobilization during the germinal center reaction, a fundamental process where cellular traffic is crucial for the quality of humoral immune responses. We show that the structure of murine germinal centers and the localization of GC B cells are impaired when CXCL12 is unable to bind to cellular or extracellular surfaces. In such mice, B cells carry fewer somatic mutations in Ig genes and are impaired in affinity maturation. Therefore, immobilization of CXCL12 is necessary for proper trafficking of B cells during GC reaction and for optimal humoral immune responses.CXCL12 | humoral immune responses | germinal center reaction C hemokines control the migration of a large array of cells and, as a consequence, regulate cell function and homeostasis in many tissues (1). In particular, they regulate the migration and positioning of lymphocytes in secondary lymphoid organs (2). Besides specific signaling delivered by engagement of specific receptors on cell surfaces, the function of chemokines also depends on interactions between nonreceptor binding domains and the glycanicglycosaminoglycan (GAG) moiety of proteoglycan, particularly heparan sulfate (HS), of the extracellular matrix and cell surfaces (3). This interaction results in immobilization of chemokines and allows the formation of fixed local gradients that, in in vitro models, regulate the synchronous coordination of cell motility (haptotaxis) and integrin-dependent cell adhesion (2). An immobilized, but not free, chemokine is a hallmark of cell signaling (4).The importance of chemokine immobilization for their function has not been fully addressed, and its relevance has been difficult to evaluate in vivo, given the lack of information on the structure-function relationship of chemokine/HS interactions.Of the various chemokines, C-X-C motif chemokine 12 (CXCL12) [also known as stromal-cell-derived factor 1 (SDF-1)] has unique structural characteristics because its binding domains, to the receptor C-X-C chemokine receptor type 4 (CXCR4) and to HS, are distinct and nonoverlapping, permitting the separation of their respective functions (5, 6). The interaction with proteoglycans is believed to contribute to CXCL12 activity by enabling the formation of local gra...

Restriction of memory B cell differentiation at the germinal center B cell positive selection stage

“…Increased MBC differentiation, together with impaired GC B cell expansion, was also found when LZ B cells cannot sense IL21 and when LZ to DZ migration is abrogated by impaired CXCR4 signaling (Bannard et al, 2013;Barinov et al, 2017;Linterman et al, 2010;Zotos et al, 2010). In both scenarios, antigen affinity was reduced, possibly favoring MBC differentiation (Shinnakasu et al, 2016;Weisel et al, 2016).…”

A guide to chemokines and their receptors

“…Their ability to bind chemokines influences chemokine/receptor interactions; chemokine half‐life in a tissue or tissue compartment; how, and where, a chemokine operates in vivo ; and the type of cell movement or adhesion it stimulates . They are essential for maintaining interstitial chemokine functions and gradients and for the presentation of chemokines on endothelial surfaces, preventing them being washed away by the blood and so allowing them to drive leukocyte arrest and extravasation . Some chemokines, such as CCL21, are very ‘sticky’ and become rapidly immobilized, while others, such as CCL19, which activates the same receptor as CCL21, likely diffuse more readily through tissues .…”

Cell cycle modulates CXCR4 expression in germinal center B cells

“…Similar to the CXCR4 knockout, it was shown that for CXCL12 gagtm mice, in which CXCL12 is unable to bind cellular or extracellular surfaces, magnitude of the germinal center reactions is normal but affinity maturation is less effective [67]. Two observations reported in this study are particularly relevant here.…”

“…Together with CXCR4, CD86 is currently the method of choice to gate LZ and DZ GC B cells by flow cytometry (e.g. [18,54,[64][65][66][67]). Nevertheless, some questions regarding CXCR4 expression heterogeneity remain unanswered.…”

“…How could this relate to cell cycle modulation of CXCR4 levels? Perhaps higher CXCR4 expression levels in G2M ensure that crucial processes like BCR signaling in G2 [71], cytokinesis and AID activity in early G1 phase [72] happen preferentially in proximity to CXCL12 , either on the expressing reticular cell network [8] or CXCL12 immobilized on other surfaces [67]. On the other hand, high CXCR4 expression could also enhance BCR signalling in G2 phase, relying on the recently discovered tight link between CXCR4 and BCR ID in mature B cells [73].…”