Acquired nonpermissive BM microenvironment impairs HSC proliferation and maintenance, and B-cell development after...

Defects in B-cell reconstitution upon hematopoietic stem cell (HSC) transplant (HSCT) are a common observation, yet the mechanism remains unexplained. The bone marrow (BM) stroma, including mesenchymal stromal cells (MSCs), guides HSC maintenance and B-lymphopoiesis by secreting crucial cytokines. We report acquired, permanent, selective, and complete B-cell deficiency in the context of full donor chimerism in a patient with X-linked lymphoproliferative disease, and aimed to identify the contribution of the BM microenvironment in disrupted B-cell reconstitution after HSCT. We studied longitudinal BM samples from the patient and his identical twin, both of whom underwent HSCT with the same donor with opposite outcomes in B-cell reconstitution. In the BM of the index patient, we observed progressive loss of proliferation of HSCs and a selective block at the pre-BI cell stage. In vitro modeling studies showed limited survival of patient HSCs and a relative accumulation of pre-B cells. Patient-derived MSCs failed to support survival and proliferation of HSCs and B-cell development of healthy HSCs, which was correlated with reduced CXCL12 levels. Using bulk RNA-sequencing of MSCs and in vitro functional studies, we showed global changes in the patients' MSCs, and a progressive loss of CXCL12 expression. Indeed, survival of the patients' HSCs improved upon supplementing the in vitro development culture with CXCL12, suggesting a contribution of defective CXCL12 signaling to the phenotype. In summary, our data show that an acquired defect in the BM-stromal microenvironment and exhaustion of HSCs and committed progenitors may cause a permanent nonpermissive state for normal B-cell development.