Altered HSC Programming Drives Aberrant Blood Output Underlying RA Pathogenesis

Rheumatoid arthritis (RA) is a debilitating autoimmune disorder characterized by autoantibody production towards collagen type-II in the synovial joints of the skeletal system, leading to progressive joint damage. RA is accompanied by chronic systemic production of pro-inflammatory cytokines including IL-6, TNF, and IL-1. Importantly, RA patients often suffer from significant hematological complications including production of tissue-damaging macrophages, chronic anemia, impaired lymphopoiesis and suppressed bone marrow function. As blood-forming hematopoietic stem cells (HSC) constitute the root of the immune system, identifying mechanisms by which altered HSC function may contribute to disease could provide key insights into RA pathogenesis and therapeutic intervention.

In this study, we used a mouse model of collagen-induced arthritis (CIA), which faithfully recapitulates many features of human autoimmune arthritis, to identify the impact of disease on the hematopoietic compartment, and identify mechanisms underlying alteration in blood lineage output and function. Strikingly, bone marrow hematopoietic populations were significantly altered in CIA mice. This was typified by decreased numbers of peripheral red blood cells (RBC) and bone marrow erythroid progenitors, alongside decreases in bone marrow B cells and common lymphoid progenitors (CLP). On the other hand, we observed increased numbers of platelets, bone marrow granulocytes, and myeloid-biased multipotent progenitors (MPP3). Strikingly, we also observed reductions in the stromal cells comprising the bone marrow niche, with decreases in epithelial, mesenchymal, and osteoblastic cell types, suggesting that the inflammatory effects of RA influence the bone marrow compartment at multiple levels. We functionally validated our phenotypic analyses using colony forming unit (CFU) assays, and observed increased myeloid CFU, with concomitant decreases in erythroid and lymphoid colony forming activity in unfractionated bone marrow cells from CIA mice. Despite these changes, transplantation assays using purified HSCs from CIA and control mice showed that long-term reconstitution potential of HSCs was largely unaltered, suggesting that HSC self-renewal capacity is not significantly impaired. Taken together, these observations suggest that autoimmune arthritis induces substantial remodeling of the bone marrow, resulting in overproduction of myeloid cells at the expense of other lineages.

In line with our phenotypic and functional analyses, RNA-seq analysis of HSCs from CIA versus control mice uncovered precocious activation of myeloid-specific gene programs that likely underlie shifts in lineage output. Moreover, in line with our transplantation data, we did not observe significant shifts in the expression of core genes governing HSC self-renewal. Collectively, these data show that the inflammatory environment of RA causes changes to hematopoietic output, leading to an increase in damaging myeloid cell types, further fueling the inflammatory environment and disease progression.

We are currently identifying the mechanism underlying myeloid overproduction in the bone marrow, with focus on systemic inflammatory factors and/or shifts in the gastrointestinal microbiota, as potential underwriters of deregulated hematopoiesis and disease development. We are also assessing the functionality of myeloid cells generated by HSCs during RA to determine whether they exhibit a stronger pro-inflammatory phenotype. Ultimately, these studies will increase our understanding of the crosstalk between RA and the hematopoietic system, and potentially opens new avenues for therapeutic intervention to re-establish the hematopoietic system and even rescue RA progression.