Identification of a Tissue-Specific, C/EBPb-Dependent Pathway of Differentiation for Murine Peritoneal Macrophages

Macrophages and dendritic cells (DC) are distributed throughout the body and play important roles in pathogen detection and tissue homeostasis. In tissues, resident macrophages exhibit distinct phenotypes and activities, yet the transcriptional pathways that specify tissue-specific macrophages are largely unknown. We investigated the functions and origins of two peritoneal macrophage populations in mice: small and large peritoneal macrophages (SPM and LPM, respectively). SPM and LPM differ in their ability to phagocytose apoptotic cells, as well as in the production of cytokines in response to LPS. In steady-state conditions, SPM are sustained by circulating precursors, whereas LPM are maintained independently of hematopoiesis; however, both populations are replenished by bone marrow precursors following radiation injury. Transcription factor analysis revealed that SPM and LPM express abundant CCAAT/enhancer binding protein (C/EBP)-b. Cebpb 2/2 mice exhibit elevated numbers of SPM-like cells but lack functional LPM. Alveolar macrophages are also missing in Cebpb 2/2 mice, although macrophage populations in the spleen, kidney, skin, mesenteric lymph nodes, and liver are normal. Adoptive transfer of SPM into Cebpb 2/2 mice results in SPM differentiation into LPM, yet donor SPM do not generate LPM after transfer into C/EBPb-sufficient mice, suggesting that endogenous LPM inhibit differentiation by SPM. We conclude that C/EBPb plays an intrinsic, tissue-restricted role in the generation of resident macrophages. A nimal tissues contain macrophages and dendritic cells (DC) that constitute a network of cells dubbed the mononuclear phagocyte system (1). Among tissues, resident phagocytes exhibit substantial phenotypic heterogeneity, likely reflecting tissue-specific function (2). Macrophage diversity is thought to result from environmental cues in tissues that induce distinct differentiation programs in macrophages or their precursors. However, the differentiation pathways that generate tissue-specific macrophages are largely unknown (3). Moreover, recent studies have challenged the long-standing hypothesis that tissue-resident macrophages arise from circulating monocytes (1), instead indicating that some mac-rophage compartments are established by fetal precursors and maintained independently of hematopoiesis (4, 5). Macrophages of the murine peritoneal cavity are among the best-studied tissue macrophage compartments. Peritoneal macrophages play important roles in clearing apoptotic cells (6) and coordinating inflammatory responses (7–9). A recent study (10) demonstrated that mouse peritoneal macrophages can be divided into two subsets: large peritoneal macrophages and small peritoneal macro-phages (LPM and SPM, respectively). LPM make up the majority of peritoneal macrophages and express high levels of F4/80, CD11b, and CD93 but lack MHC class II (MHCII); SPM, in contrast, express lower levels of F4/80, CD11b, …