Bone marrow stromal cell transplantation ameliorates cytopenias caused by depletion of FAP-α expressing cells

Mesenchymal stromal cell (MSC) therapies are being successfully developed for immune-mediated inflammatory diseases as phase 3 clinical trials have demonstrated. However, the en-dogenous roles of stromal cells (SSCs) are not fully elucidated. Fibroblast activation protein-α (FAP) is a marker of tissue resident SSCs removable in FAPDM2 mice using diphtheria toxin (DT) receptor-mediated conditional cell knockout (TRECK). Two DT doses confirmed FAP-α+ cell ablation at 72h associated with anaemia and altered haematopoiesis. Our hypothesis relies on the administration of a single intravenous (i.v) dose of stromal cells 24h after DT exposure alleviates the phenotypes associated with FAP-α+ cell depletion and attributed them to FAP-α+ SSCs. Peripheral blood (PB) counts and flow cytometry tests on haematopoietic and stromal cell (HSC & SSC) populations in BM were examined post ablation and MSC administration. FAP-α+ cell ablation resulted in anaemia, thrombocytopenia and neutropenia in PB and general hypo-cellularity was observed in BM, specifi-cally, megakaryocytes progenitors (MEP) but increased multipotent progenitors (MPP2). In addi-tion to FAP-α+ SSCs, other stromal cell subsets decreased; namely PDFGR-α, Leptin-R, gp38, and SDC2. Administration (i.v) of a single MSC (2.5x105) dose expressing FAP-α+ rescued anaemia, ame-liorated thrombocytopenia and neutropenia in PB as well as improved the cellularity of the mar-row: specifically progenitors (Lin-Sca-1+) and MEP, whereas the percentage of MPP2 was reversed indicating the lineage biased of the MPP2 to erytrocytosis and megakaryocytosis. FAP-α+ SSCs are necessary for marrow homeostasis and a single dose of BM-MSCs is sufficient to restore anaemia, alleviate thrombocytopenia and neutropenia and increase the haematopoietic progenitor’s absolute numbers in BM