Functions and development of red pulp macrophages.

Macrophages are extremely heterogeneous mononuclear phagocytes widely distributed throughout the body. They play unique roles in each organ where they reside. Among macrophage subsets, red pulp macrophages (RPMs) that localize in the splenic red pulp, are critical for maintenance of blood homeostasis by actively phagocytosing injured and senescent erythrocytes and blood-borne particulates. Recent evidence indicates that RPMs are mainly generated during embryogenesis and are maintained during adult life. Furthermore, the cell-intrinsic and -extrinsic factors (namely, Spi-C, IRF8/4, heme oxygenase-1, and M-CSF) that regulate the development and survival of RPMs have been identified. Although the immunological properties of RPMs have yet to be elucidated fully, pioneering studies have demonstrated that these cells are capable of inducing differentiation of regulatory T cells via expression of transforming growth factor-β and secrete a large amount of type I interferons during parasitic infections. In this review, we describe recent advances in understanding of the functions and development of RPMs.