The Majority of CD45- Ter119- CD31- Bone Marrow Cell Fraction Is of Hematopoietic Origin and Contains Erythroid and Lymphoid Progenitors

The non-hematopoietic cell fraction of the bone marrow (BM) is classically identified as CD45(-) Ter119(-) CD31(-) (herein referred to as triple-negative cells or TNCs). Although TNCs are believed to contain heterogeneous stromal cell populations, they remain poorly defined. Here we showed that the vast majority of TNCs (-85%) have a hematopoietic rather than mesenchymal origin. Single cell RNA-sequencing revealed erythroid and lymphoid progenitor signatures among CD51(-) TNCs. Ly6D(+) CD44(+) CD51(-) TNCs phenotypically and functionally resembled CD45(+) pro-B lymphoid cells, whereas Ly6D(-) CD44(+) CD51(-) TNCs were enriched in previously unappreciated stromal-dependent erythroid progenitors hierarchically situated between preCFU-E and proery-throblasts. Upon adoptive transfer, CD44(+) CD51(-) TNCs contributed to repopulate the B-lymphoid and erythroid compartments. CD44(+) CD51(-) TNCs also expanded during phenylhydrazine-induced acute hemolysis or in a model of sickle cell anemia. These findings thus uncover physiologically relevant new classes of stromal-associated functional CD45(-) hematopoietic progenitors.