Hematopoietic stem cells (HSCs) are required for lifelong blood cell production and, to prevent exhaustion, the majority of HSCs are deeply quiescent during steady-state hematopoiesis. Paracrine factors produced by specialized bone marrow niche cells maintain HSC quiescence. Yet, evolution demands a rapid hematopoietic response to stressors such as infection, bleeding or toxin exposure. These triggers set off a remarkable adaptation in hematopoiesis that sacrifices HSPC quiescence, and the protection it affords, to meet an urgent need for new blood cell production. A great deal is known about how the HSPCs are mobilized during these periods of stress but mechanisms driving the return to steady-state hematopoiesis after stress have not been identified.
Because TGFβ is one of few negative regulators of hematopoiesis, we measured the levels of active TGFβ and assessed the effects of its downstream signaling during hematopoietic recovery after chemotherapy. During late hematopoietic regeneration, as hematopoietic homeostasis is restored, the levels of active TGFβ spike in whole bone marrow and downstream signaling (as reported by Smad2 phosphorylation) increases in hematopoietic stem and progenitor cells (HSPCs). To assess how TGFβ signaling alters hematopoietic regeneration, we administered myelosuppressive chemotherapy to mice and then, during recovery, treated cohorts with either an antibody that specifically neutralizes active TGFβ (1D11), an isotype control antibody (13C4) or nothing at all. We found that TGFβ blockade accelerated blood count and bone marrow regeneration, and promoted HSCP self-renewal/proliferation by delaying the return of HSPCs to quiescence (G0). In contrast, TGFβ blockade during homeostasis elevates neither blood counts nor bone marrow cellularity and fails to induce HSPCs to emerge from quiescence.
The de facto paradigm is that homeostasis is passively reestablished as stress mediators normalize. Our data strongly suggest that this paradigm is incorrect. Rather than being a passive process, it appears that steady-state hematopoiesis is actively reimposed. TGFβ pathway activation marks regenerating HSPCs returning to quiescence and this context-depending signaling helps reestablish homeostasis during recovery from chemotherapy. Together, our data demonstrate that myelosuppression does not drive hematopoiesis using only a cytokine-fueled gas pedal but also taps an active braking mechanism once sufficient recovery has been attained. More directly, our data strongly suggests that TGFb pathway inhibitors could be used to promote multi-lineage hematopoietic reconstitution. This finding significantly extends prior work in the field by opening the door to new, potentially superior, ways to modulate the hematopoietic adaptation to stress.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.