CXCR4 Drives Lympho-Myeloid Fate of Hematopoietic Progenitors Via mTOR and Mitochondrial Metabolic Pathways

Blood production is a tightly regulated process that starts with hematopoietic stem cells (HSCs). In adults, HSCs are unique in their capacity to self-renew and replenish the entire blood system through production of a series of increasingly committed progenitor cells within the bone marrow (BM) microenvironment. HSCs form a rare, quiescent population that displays a metabolism skewed towards anaerobic glycolysis at the expense of mitochondrial oxidative phosphorylation (OXPHOS) to preserve its quiescent state and long-term reconstitution capacity. However, when HSCs differentiate, they undergo a metabolic switch from anaerobic glycolysis to mitochondrial OXPHOS, a process that is in part mediated by the metabolic sensor mTOR. It is well-established that HSCs in the BM adapt the production of myeloid and lymphoid cells depending on the needs of the body and that metabolic plasticity is a critical driver of HSC fate decisions. This has never been assessed for multipotent progenitors (MPPs) which constitute the stage at which the major divergence of lymphoid and myeloid lineages occurs. In mice, common lymphoid progenitors (CLPs) and common myeloid progenitors (CMPs) are generated from phenotypically and functionally distinct subpopulations of lineage-biased MPPs, i.e. MPP2 and MPP3 are reported as distinct myeloid-biased MPP subsets that operate together with lymphoid-primed MPP4 to control blood leukocyte production. This question is thus of paramount importance to understand how the lympho-myeloid specification process is regulated.

Signaling by the G protein-coupled receptor CXCR4 on MPPs in response to stimulation by its natural ligand, the chemokine CXCL12, produced by BM perivascular stromal cells constitutes a key pathway through which the niches and MPPs communicate. However, the mechanisms whereby CXCR4 signaling regulates MPP specification are still unknown. We addressed this point using BM samples of patients with WHIM Syndrome (WS), a rare immunodeficiency caused by inherited heterozygous autosomal gain-of-CXCR4-function mutations affecting desensitization of CXCR4 and characterized by chronic lympho-neutropenia, as well as a unique WS mouse model which phenocopies severe pan-leukopenia.

We unraveled myeloid skewing of the hematopoietic stem and progenitor cell (HSPC) compartment in BM of patients with WS and of WS mice. This relied on CXCR4 signaling strength that controls the output of the lymphoid and myeloid lineages by coordinating the composition and molecular identity of the MPP compartment. The fate of the lymphoid-biased MPP4 subset was central in such a process. Indeed, CXCR4 signaling termination was required for efficient generation and maintenance of the MPP4 pool, while regulating intrinsically their cell cycle status and lymphoid-myeloid gene landscape. In fact, we demonstrated for the first time that enhanced mTOR signaling, accumulation of damaged mitochondria and overactive OXPHOS-driven metabolism promoted cell-autonomous molecular changes that reprogram mutant MPP4 away from lymphoid differentiation. Consistent with this, in vivo chronic treatment with the CXCR4 antagonist AMD3100/Plerixafor or the mTOR inhibitor Rapamycin normalized mitochondrial metabolism and MPP4 differentiation.

Thus, our study shows that CXCR4 signaling acts through the mTOR pathway as an essential gatekeeper for integrity of the mitochondrial machinery, which in turn controls lymphoid potential of MPP4.