Cytotoxic therapies cause irreversible damage to the niches for multipotent progenitor cells, resulting in impaired long-term repopulating potential of bone marrow

Abstract

Cytotoxic therapies used for the treatment of cancer are known to dramatically affect hematopoiesis and can result in prolonged periods of pancytopenia. While cytotoxic therapies are known to directly impact on hematopoietic cells, the effects of cytotoxic therapies on different bone marrow microenvironment cell types (aka niches) are largely unknown. Using adult male mice, we have extensively analyzed the effects of: 1) lethal irradiation, 2) lethal irradiation accompanied by a bone marrow transplantation or 3) a single dose of the chemotherapy drug, 5-fluorouracil (5-FU) on different bone marrow microenvironment cells (osteoblast lineage cells, vasculature and adipocytes) at early and late time points post-cytotoxic therapy. We have assessed the changes in the numbers of HSCs and progenitor cells at all time points. Transplantation studies were performed to assess the functional potential of whole bone marrow and HSCs at 8 weeks and 16 weeks after cytotoxic therapies. Studies assessing bone parameters were also performed in human allogeneic stem cell transplant (alloSCT) patients. All therapies caused early, transient changes in various bone marrow microenvironment cell types. Irreversible loss of bone occurred in response to each therapy and was also observed in alloSCT patients. The bone loss was due to transient increases in osteoclasts and could be prevented by a single dose of the bisphosphonate, zoledronic acid, given prior to the cancer therapy. High levels of serum monocyte chemoattractant protein-1 (MCP-1) preceded the bone loss in both mice and humans, and correlated with the loss of bone observed at both the femoral neck and hip in alloSCT patients. In mice, the numbers of multipotent progenitor cells (MPPs) were significantly reduced at all time points assessed, whereas the numbers of long-term repopulating hematopoietic stem cells (LT-HSCs) normalized within 14 days post-therapy. Transplantation studies revealed that whole bone marrow cells obtained from mice at 8 or 16 weeks post-therapy had significantly impaired repopulation capacity compared to bone marrow from age-matched controls. In contrast, the numbers and competitive repopulating potential of purified HSCs obtained from the same bone marrow at these time points were similar in cancer therapy-treated and age-matched control mice. Collectively, our data show that dramatic changes occur in bone marrow microenvironment cells after cytotoxic therapies. Our findings suggest that, similar to that recently observed for steady-state conditions, MPPs are the primary source of blood cell production after cytotoxic therapies. Furthermore, the production of MPPs post-cancer therapy is significantly impaired long-term. This is likely due to impaired extrinsic (MPP niche) regulation as opposed to intrinsic changes in the output of MPPs from HSCs. The identification and characterization of the MPP niche may lead to treatments aiding hematological recovery after cancer therapies.