Intraperitoneal transfer of wild-type bone marrow cells in the Csf1r knockout rat repopulates resident tissue macrophages without contributing to monocytopoiesis.

Homozygous null mutation of the macrophage colony-stimulating factor receptor (Csf1r) gene in rats leads to the loss of most tissue macrophage populations and has pleiotropic impacts on postnatal growth and organ maturation leading to mortality by 8-12 weeks of age. The phenotype of the Csf1r knockout (Csf1rko) can be reversed by intraperitoneal transfer of wild-type bone marrow cells (BMT) at weaning. Here we used a Csf1r-mApple transgenic reporter, which is expressed in neutrophils and B cells as well as monocytes and macrophages, to track the fate of donor-derived cells. Following BMT into Csf1r recipients, wild-type mApple+ve cells restored IBA1+ tissue macrophage populations in every tissue donor-derived cells also completely replaced recipient macrophages in organs such as spleen, lung and liver that were only partly macrophage-deficient in the Csf1rko. However, monocytes, neutrophils and B cells in bone marrow, blood and lymphoid tissues remained of recipient (mApple-ve) origin. An mApple+ve cell population expanded in the peritoneal cavity and invaded locally in the mesentery, fat pads, omentum and diaphragm. One week after BMT, distal organs contained foci of mApple+ve, IBA1-ve immature progenitors that appeared to proliferate, migrate and differentiate locally. We conclude that rat bone marrow contains progenitor cells that are able to restore and maintain all tissue macrophage populations in a Csf1rko rat directly without contributing to the bone marrow progenitor or blood monocyte populations. ### Competing Interest Statement The authors have declared no competing interest.