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 proerythroblasts. 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. [Display omitted] •The majority of TNCs in the bone marrow have a hematopoietic origin•Bona fide stromal-derived TNCs are selected by CD51-positive selection•Ly6D+ CD44+ CD51– TNCs are enriched in pro-B lymphoid cells•Ly6D– CD44+ CD51– TNCs contain “pre-proerythroblasts” that expand in anemia Bone marrow triple-negative CD45– Ter119– CD31– cells are thought to contain heterogeneous stromal cell populations. Boulais et al. show these cells are mostly hematopoietic in origin and contain previously unappreciated stromal-associated erythroid and B-lymphoid progenitor populations.