Desensitization controls G protein-dependent signaling of chemokine receptors. We investigate the physiologic implication of this process for CXCR4 in a mouse model harboring a heterozygous mutation of the Cxcr4 gene, which engenders a desensitization-resistant receptor. Such anomaly is linked to the warts, hypogammaglobulinemia, infections, myelokathexis (WHIM) syndrome, a human rare combined immunodeficiency. Cxcr4(+/mutant(1013)) mice display leukocytes with enhanced responses to Cxcl12 and exhibit leukopenia as reported in patients. Treatment with CXCL12/CXCR4 antagonists transiently reverses blood anomalies, further demonstrating the causal role of the mutant receptor in the leukopenia. Strikingly, neutropenia occurs in a context of normal bone marrow architecture and granulocyte lineage maturation, indicating a minor role for Cxcr4-dependent signaling in those processes. In contrast, Cxcr4(+/1013) mice show defective thymopoiesis and B-cell development, accounting for circulating lymphopenia. Concomitantly, mature T and B cells are abnormally compartmentalized in the periphery, with a reduction of primary follicles in the spleen and their absence in lymph nodes mirrored by an unfurling of the T-cell zone. These mice provide a model to decipher the role of CXCR4 desensitization in the homeostasis of B and T cells and to investigate which manifestations of patients with WHIM syndrome may be overcome by dampening the gain of CXCR4 function.

During development, natural killer (NK) cells exit the BM to reach the blood. CXCR4 retains NK cells in the BM, whereas the sphingosine-1 phosphate receptor 5 (S1P5) promotes their exit from this organ. However, how the action of these receptors is coordinated to preserve NK-cell development in the BM parenchyma while providing mature NK cells at the periphery is unclear. The role of CXCR4 and S1P5 in NK-cell recirculation at the periphery is also unknown. In the present study, we show that, during NK-cell differentiation, CXCR4 expression decreases whereas S1P5 expression increases, thus favoring the exit of mature NK cells via BM sinusoids. Using S1P5 ؊/؊ mice and a new knockin mouse model in which CXCR4 cannot be desensitized (a mouse model of warts, hypogammaglobulinemia, infections, and myelokathexis [WHIM] syndrome), we demonstrate that NK-cell exit from the BM requires both CXCR4 desensitization and S1P5 engagement. These 2 signals occur independently of each other: CXCR4 desensitization is not induced by S1P5 engagement and vice versa. Once in the blood, the S1P concentration increases and S1P5 responsiveness decreases. This responsiveness is recovered in the lymph nodes to allow NK-cell exit via lymphatics in a CXCR4-independent manner. Therefore, coordinated changes in CXCR4 and S1P5 responsiveness govern NK-cell trafficking. (Blood. 2011;118(18):4863-4871) IntroductionNatural killer (NK) cells are lymphocytes of the innate immune system that are involved in the early control of infections by viruses and intracellular bacteria or parasites. 1 They can kill cells recognized as targets through a battery of surface receptors 2 and also produce large amounts of cytokines such as IFN-␥ upon activation. 1 Several NK-cell subsets have been described on the basis of surface expression of the TNF superfamily member CD27 and the integrin CD11b: CD11b low CD27 low (thereafter called double negative or "DN"), CD11b low CD27 high NK cells ("CD11b low "), CD11b high CD27 high (double positive or "DP"), and CD11b high CD27 low ("CD27 low "). 3,4 We previously found that NK-cell maturation is a 4-stage process that starts at the DN stage and follows the pathway DN 3 CD11b low 3 DP 3 CD27 low . 3 DN NK cells are very rare and immature and could arguably correspond to precursors. All 3 other subsets are quite abundant in lymphoid organs, but their distribution is very different: CD11b low NK cells are mostly located within the BM and lymph nodes (LNs); CD27 low NK cells are mostly found in the blood, the spleen, and nonlymphoid organs such as liver and lung; and DP NK cells are more evenly distributed in these organs. 5 NK cells develop mainly in the BM. Like other lymphocytes, they are thought to reach the blood circulation via venous sinusoids. 6 Once in the periphery, they can also enter the LNs through a CD62L-dependent process. 7 Exit from the LNs presumably occurs via medullar sinusoids that connect to efferent lymphatics. 6 The chemotactic receptors that control the differential distribution of NK-cell subsets a...

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.