Finely tuned, demand adapted hematopoiesis relies on the exchange of complex regulatory signals between hematopoietic and stromal cells of the bone marrow (BM) microenvironment. For instance, hematopoietic stem and progenitor cells (HSPCs) are maintained by mesenchymal, vascular and neural cells in defined BM niches. Viral infections and ensuing inflammatory responses are known to induce massive changes in the hematopoietic output. Nonetheless, little is known about the alterations elicited on the composition, structure and function of BM stromal cells and their regulatory crosstalk with HSCs after chronic viral challenge. We used a combination of conventional in vitro and in vivo assays, RNA-seq and 3D confocal quantitative imaging to address these questions in a model of chronic Lymphocytic Choriomeningitis Virus (LCMV) infection.
We report that beyond direct hematopoietic effects, chronic LCMV infections lead to a profound and durable alteration of the BM stromal compartment. 3D imaging revealed major damage followed by rapid remodeling of the BM vascular and ECM networks. Most importantly, we observed a strong and durable injury to CXCL12-abundant reticular (CAR) cells, which rapidly decreased in density, number and their capacity to produce HSPC-sustaining factors. Functional and structural decline of the CARc network rendered them less supportive for HSC quiescence as measured through in vivo assays. Importantly, damage to BM stromal integrity upon chronic LCMV infection is accompanied by a dramatic reduction in the number of phenotypic HSPCs and a striking and persistent loss of HSC function in competitive repopulation assays. We further demonstrate that the observed alterations in the BM are mediated by virus-specific CD8+ T cells accumulating in the BM upon infection. Blockage of IFN signaling precludes CD8 homing to the BM and prevents damage to the BM stromal microenvironment. We are currently investigating whether targeting of IFN pathways rescues HSC function. Our work demonstrates for the first time that chronic viral infections can permanently damage stromal compartments in the BM, resulting in persistent damage to HSPC function.
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