277. Characterization of the Macrophage Response in Hematopoietic Stem Cell-Mediated Tissue Repair in Cystinosis

Cystinosis is a lysosomal storage disorder resulting from genetic mutations in the ubiquitously expressed CTNS gene, which encodes the lysosomal transmembrane transporter called cystinosin. In the absence of functional cystinosin, cystine accumulates in all tissues of the body, eventually depositing as crystals. Thus, cystinosis is a multisystemic degenerative disorder, with end-stage kidney failure as the cardinal manifestation.Our previous work has shown that transplantation of syngeneic wild-type hematopoietic stem cells (HSCs) in the mouse model of cystinosis, Ctns-/- mice, was effective in treating cystinosis and led to long-term tissue preservation. Our study clearly established that transplanted HSCs integrated abundantly into all tissues and differentiated into tissue-resident macrophages, such as Kupffer cells in the liver, microglia in the brain, and dendritic cells in the kidney. Addressing the cellular mechanism, we showed that HSC-derived macrophages corrected the genetic defect by delivering functional cystinosin to adjacent host Ctns-/- cells via long intercellular bridges called tunneling nanotubes (TNTs). In the kidney, TNTs were shown to transfer cystinosin-containing lysosomes from transplanted interstitial macrophages into diseased proximal tubular cells directly across the dense, continuous, and formidable barrier of the tubular basement membrane. This is the first demonstration of correction of a genetic lysosomal defect by vesicular exchange via TNTs and suggests broader potential for HSC transplantation for other disorders due to defective vesicular proteins.Overall, correlation between differentiation of HSCs into macrophages and improved biochemical, functional, and anatomical outcomes post-transplantation suggest that macrophages are key mediators of tissue repair. To test this hypothesis and to characterize the molecular protagonists involved, Ctns-/- mice have been transplanted with HSCs isolated from mouse strains that are deficient in macrophage differentiation. This work will open new perspectives in regenerative medicine that could spur the development of novel stem cell-based therapy.