CXCR4 plays a central role in B cell immune response, notably by promoting plasma cell (PC) migration and maintenance in the bone marrow (BM). Gain-of-function mutations in CXCR4 affecting receptor desensitization have been reported in the rare immunodeficiency called WHIM syndrome (WS). Despite lymphopenia, patients mount an immune response but fail to maintain it over time. Using a knockin mouse model phenocopying WS, we showed that, counter-intuitively, a gain of Cxcr4 function inhibited the maintenance of antibody titers after immunization. Although the Cxcr4 mutation intrinsically and locally promoted germinal center response and PC differentiation, antigen-specific PCs were barely detected in the BM, a defect mirrored by early accumulation of immature plasmablasts potentially occupying the survival niches for long-lived PCs. Therefore, fine-tuning of Cxcr4 desensitization is critically required for efficient PC differentiation and maintenance, and absence of such a regulatory process may account for the defective humoral immunity observed in WS patients.

The mechanisms controlling affinity maturation have been extensively studied over the last 20 years and the central role of T follicular helper cells (Tfh) in this process has now been clearly established. In order to analyze how Tfh impact on affinity maturation several models have been developed. This chapter aims to present three different techniques to evaluate antigen-specific B cell response and affinity maturation using the NP system: Flow cytometric single cell sorting and sequence analysis, ELISA and ELISpot. They have the advantages of being applicable on all types of mice independently of the presence of a transgenic BCR and to give multiple readout of the antigen-specific immune response and affinity maturation. Although first developed more than 20 years ago, these techniques are still considered to be the gold standard for the analysis of affinity maturation in vivo.