Marion Espéli

Inflammatory form of Waldenström macroglobulinemia (iWM) predicts outcomes after immuno-chemotherapy and Bruton tyrosine kinase inhibitors, but its origin is unknown. Here, we unravel increased clonal hematopoiesis in patients with iWM (61% vs 23% in noninflammatory WM), suggesting a contribution of environmental cells to iWM.

Background: WHIM (Warts, Hypogammaglobulinemia, Infections, and Myelokathexis) syndrome is an ultra-rare, combined primary immunodeficiency and chronic neutropenic disorder characterized by a range of clinical presentations, including peripheral neutropenia, lymphopenia, and recurrent infections. WHIM syndrome is most often caused by gain-of-function mutations in the gene encoding C-X-C chemokine receptor 4 (CXCR4). As such, inhibition of CXCR4 with XOLREMDI^® (mavorixafor), an orally bioavailable CXCR4 antagonist, demonstrated clinically meaningful increases in absolute neutrophil and lymphocyte counts and concomitant reduction in infections in patients with WHIM syndrome, resulting in its recent U.S. Food and Drug Administration approval. The impact of CXCR4 antagonism on other aspects of the pathobiology in WHIM syndrome, such as lymphopoiesis and leukocyte trafficking between primary and secondary lymphoid organs, is less understood.Methods: In the current study, the effects of CXCR4 antagonism on leukocyte trafficking and distribution in primary and secondary lymphoid organs were investigated in a mouse model of WHIM syndrome carrying the heterozygous Cxcr4¹⁰¹³ mutation. Cxcr4^+/1013 and Cxcr4 wild-type mice received the orally bioavailable CXCR4 antagonist X4-185. Blood, spleen and bone marrow samples were collected for numeration, flow cytometry, and functional studies.Results: Cxcr4^+/1013 mice exhibited profound peripheral blood leukopenia as seen in patients with WHIM syndrome. CXCR4 antagonism corrected circulating leukopenia and mobilized functional neutrophils without disrupting granulopoiesis in the bone marrow of Cxcr4^+/1013 mice. Furthermore, Cxcr4^+/1013 displayed aberrant splenic T and B-cell counts and frequency. Treatment with X4-185 normalized splenic T-cell abnormalities, correcting the reduced CD8⁺ T-cell numbers, restoring the CD4/CD8 T-cell ratio, and ameliorating peripheral blood T-cell lymphopenia. In addition, CXCR4 antagonism was able to correct the abnormal frequencies and numbers of splenic marginal zone and follicular B cells in Cxcr4^+/1013 mice, and ultimately normalize B-cell lymphopenia in the peripheral circulation.Conclusions: Our study provides comprehensive evidence that oral dosing with a CXCR4 antagonist can effectively correct WHIM-associated neutrophil and lymphocyte abnormalities in a mouse model of WHIM syndrome. These findings extend our understanding of how targeting the dysregulated CXCR4 signaling pathway can ameliorate the pathogenesis of WHIM syndrome.

Plasma cells (PCs) are highly specialized cells representing the end stage of B-cell differentiation. We have shown that PC differentiation can be reproduced in vitro using elaborate culture systems. The molecular changes occurring during PC differentiation are recapitulated in this in vitro differentiation model. However, a major challenge exists to decipher the spatiotemporal epigenetic and transcriptional programs that drive the early stages of PC differentiation. We combined single cell (sc) RNA sequencing (RNA-seq) and assay for transposase-accessible chromatin with high throughput sequencing (scATAC-seq) to decipher the trajectories involved in PC differentiation. ScRNA-seq experiments revealed a strong heterogeneity of the preplasmablastic and plasmablastic stages. Among genes that were commonly identified using scATAC-seq and scRNA-seq, we identified several transcription factors with significant stage specific potential importance in PC differentiation. Interestingly, differentially accessible peaks characterizing the preplasmablastic stage were enriched in motifs of BATF3, FOS and BATF, belonging to activating protein 1 (AP-1) transcription factor family that may represent key transcriptional nodes involved in PC differentiation. Integration of transcriptomic and epigenetic data at the single cell level revealed that a population of preplasmablasts had already undergone epigenetic remodeling related to PC profile together with unfolded protein response activation and are committed to differentiate in PC. These results and the supporting data generated with our in vitro PC differentiation model provide a unique resource for the identification of molecular circuits that are crucial for early and mature PC maturation and biological functions. These data thus provide critical insights into epigenetic- and transcription–mediated reprogramming events that sustain PC differentiation.

Plasma cells play an essential role in humoral immunity, but many questions remain regarding the heterogeneity of this population, both in terms of ontogeny and involvement in the immune response. In this work, we have identified 5 subsets of plasma cells in human and mouse lymphoid tissues. These subpopulations were distinguished by differential expression of CD62L, CXCR4, FcγRIIb and CD93. The antigenic context as well as the B cell of origin directed plasma cell differentiation towards specific subtypes that display distinct migratory and survival abilities in vivo . Altogether, ours results unveil that plasma cell phenotypic and functional heterogeneity relies on intrinsic imprinting during B cell activation.

Bone marrow (BM) long‐lived plasma cells (PCs) are essential for long‐term protection against infection, and their persistence within this organ relies on interactions with Cxcl12‐expressing stromal cells that are still not clearly identified. Here, using single cell RNAseq and in silico transinteractome analyses, we identified Leptin receptor positive (LepR⁺) mesenchymal cells as the stromal cell subset most likely to interact with PCs within the BM. Moreover, we demonstrated that depending on the isotype they express, PCs may use different sets of integrins and adhesion molecules to interact with these stromal cells. Altogether, our results constitute an unprecedented characterization of PC subset stromal niches and open new avenues for the specific targeting of BM PCs based on their isotype.

WHIM Syndrome is a rare immunodeficiency caused by gain-of-function CXCR4 mutations. Here we report a decrease in bone mineral density in 25% of WHIM patients and bone defects leading to osteoporosis in a WHIM mouse model. Imbalanced bone tissue is observed in mutant mice combining reduced osteoprogenitor cells and increased osteoclast numbers. Mechanistically, impaired CXCR4 desensitization disrupts cell cycle progression and osteogenic commitment of skeletal stromal/stem cells, while increasing their pro-osteoclastogenic capacities. Impaired osteogenic differentiation is evidenced in primary bone marrow stromal cells from WHIM patients. In mice, chronic treatment with the CXCR4 antagonist AMD3100 normalizes in vitro osteogenic fate of mutant skeletal stromal/stem cells and reverses in vivo the loss of skeletal cells, demonstrating that proper CXCR4 desensitization is required for the osteogenic specification of skeletal stromal/stem cells. Our study provides mechanistic insights into how CXCR4 signaling regulates the osteogenic fate of skeletal cells and the balance between bone formation and resorption.

Myelodysplastic syndromes (MDS) are clonal hematopoietic disorders, representing high risk of progression to acute myeloid leukaemia, and frequently associated to somatic mutations, notably in the epigenetic regulator TET2. Natural Killer (NK) cells play a role in the anti-leukemic immune response via their cytolytic activity. Here we show that patients with MDS clones harbouring mutations in the TET2 gene are characterised by phenotypic defects in their circulating NK cells. Remarkably, NK cells and MDS clones from the same patient share the TET2 genotype, and the NK cells are characterised by increased methylation of genomic DNA and reduced expression of Killer Immunoglobulin-like receptors (KIR), perforin, and TNF-α. In vitro inhibition of TET2 in NK cells of healthy donors reduces their cytotoxicity, supporting its critical role in NK cell function. Conversely, NK cells from patients treated with azacytidine (#NCT02985190; https://clinicaltrials.gov/) show increased KIR and cytolytic protein expression, and IFN-γ production. Altogether, our findings show that, in addition to their oncogenic consequences in the myeloid cell subsets, TET2 mutations contribute to repressing NK-cell function in MDS patients.

CXCR4 est un récepteur de chimiokine qui joue un rôle central dans la migration cellulaire mais également dans d’autres mécanismes essentiels, tels que le développement du système immunitaire. De concert avec son ligand naturel, la chimiokine CXCL12, cet axe de signalisation joue un rôle important dans la biologie des lymphocytes B, des stades précoces de différenciation dans la moelle osseuse à leur activation et différenciation en cellules sécrétrices d’anticorps, aussi appelées plasmocytes. Des mutations gain de fonction de CXCR4 sont retrouvées dans une immunodéficience rare, le Syndrome WHIM. Ces mutations affectent le mécanisme de désensibilisation du récepteur et entraînent un gain de fonction en réponse à CXCL12. Cette revue résume le rôle de CXCR4 dans la réponse immune humorale et, à travers l’étude du Syndrome WHIM, souligne le rôle régulateur essentiel de la désensibilisation de CXCR4 dans ces processus. Des travaux récents rapportent en effet qu’une signalisation correcte de CXCR4 est essentielle pour limiter la réponse immune dite « extra-folliculaire » et pour permettre une protection au long terme assurée par les anticorps.