Hematopoietic stem cell loss and hematopoietic failure in severe aplastic anemia is driven by macrophages and aberrant podoplanin expression

Abstract

Severe aplastic anemia (SAA) results from profound hematopoietic stem cell loss. T cells and interferon gamma (IFNγ) have long been associated with SAA, yet the underlying mechanisms driving hematopoietic stem cell loss remain unknown. Using a mouse model of SAA, we demonstrate that IFNγ-dependent hematopoietic stem cell loss required macrophages. IFNγ was necessary for bone marrow macrophage persistence, despite loss of other myeloid cells and hematopoietic stem cells. Depleting macrophages or abrogating IFNγ signaling specifically in macrophages did not impair T-cell activation or IFNγ production in the bone marrow but rescued hematopoietic stem cells and reduced mortality. Thus, macrophages are not required for induction of IFNγ in SAA and rather act as sensors of IFNγ. Macrophage depletion rescued thrombocytopenia, increased bone marrow megakaryocytes, preserved platelet-primed stem cells, and increased the platelet-repopulating capacity of transplanted hematopoietic stem cells. In addition to the hematopoietic effects, SAA induced loss of non-hematopoietic stromal populations, including podoplanin-positive stromal cells. However, a subset of podoplanin-positive macrophages was increased during disease, and blockade of podoplanin in mice was sufficient to rescue disease. Our data further our understanding of disease pathogenesis, demonstrating a novel role for macrophages as sensors of IFNγ, thus illustrating an important role for the microenvironment in the pathogenesis of SAA.

Aged marrow macrophages expand platelet-biased hematopoietic stem cells via interleukin-1B

“…Consequently, senescent neutrophils as well as nonphagocytic Mφs, higher in IL-1B production, would remain to cause skewing even of young HSCs in mice treated with liposomal clodronate. This is consistent with our observations using clodronate-loaded liposomes to deplete Mφs in young mice and further supported by the recent demonstration that Mφ depletion in a model of severe aplastic anemia elicits a population of CD41 hi HSCs that exhibit robust platelet output (39).…”

“…In vivo treatment with clodronate-loaded liposomes (34,38) expanded CD41 + LT-HSCs (Figure 8, G-J), suggesting that loss of phagocytic Mφs is sufficient to initiate BMME changes that induce HSC megakaryocytic bias in vivo. This is consistent with observations in a mouse model of aplastic anemia in which clodronate-mediated depletion of Mφs induced increased CD41 + LT-HSCs that exhibited robust platelet output in transplantation (39).…”

Inhibition of 15-PGDH Protects Mice from Immune-mediated Bone Marrow Failure

“…Interferon gamma (IFNγ) and TNFα are cytokines classically associated with AA pathogenesis and are known to exhaust and deplete HSCs (1, 18, 19). To determine if PGDHi limits AA severity by attenuating inflammation, we measured circulating levels of the disease-driving cytokines IFNγ and TNFα, as well as IL-6 and IL-1β.…”

Myeloid Cells Restrict MCMV and Drive Stress-Induced Extramedullary Hematopoiesis through STAT1

“…IFNγ is associated with the loss of bone marrow HSCs during severe anemia (de Bruin et al, 2014), which was recently attributed to IFNγ signaling in macrophages during aplastic anemia induced by sublethal irradiation and bulk splenocyte transfer (McCabe et al, 2018). We show that MCMV infection results in reduced bone marrow cellularity and erythroblast abundance in the absence of myeloid STAT1.…”