We assessed tissue macrophage gene expression in different mouse organs. Diversity in gene expression among different populations of macrophages was remarkable. Only a few hundred mRNA transcripts stood out as selectively expressed by macrophages over DCs and many of these were not present in all macrophages. Nonetheless, well-characterized surface markers, including MerTK and FcγR1 (CD64), along with a cluster of novel transcripts were distinctly and universally associated with mature tissue macrophages. TCEF3, C/EBPα, BACH1, and CREG-1 were among the top transcriptional regulators predicted to regulate these core macrophage-associated genes. Other transcription factor mRNAs were strongly associated with single macrophage populations. We further illustrate how these transcripts and the proteins they encode facilitate distinguishing macrophage versus DC identity of less characterized populations of mononuclear phagocytes.

The mononuclear phagocyte system (MPS) has historically been categorized into monocytes, dendritic cells and macrophages on the basis of functional and phenotypical characteristics. However, considering that these characteristics are often overlapping, the distinction between and classification of these cell types has been challenging. In this Opinion article, we propose a unified nomenclature for the MPS. We suggest that these cells can be classified primarily by their ontogeny and secondarily by their location, function and phenotype. We believe that this system permits a more robust classification during both steady-state and inflammatory conditions, with the benefit of spanning different tissues and across species.

Monocytes participate critically in atherosclerosis. There are 2 major subsets expressing different chemokine receptor patterns: CCR2 + CX3CR1 + Ly-6C hi and CCR2 -CX3CR1 ++ Ly-6C lo monocytes. Both C-C motif chemokine receptor 2 (CCR2) and C-X 3 -C motif chemokine receptor 1 (CX3CR1) are linked to progression of atherosclerotic plaques. Here, we analyzed mouse monocyte subsets in apoE-deficient mice and traced their differentiation and chemokine receptor usage as they accumulated within atherosclerotic plaques. Blood monocyte counts were elevated in apoE -/-mice and skewed toward an increased frequency of CCR2 + Ly-6C hi monocytes in apoE -/-mice fed a high-fat diet. CCR2 + Ly-6C hi monocytes efficiently accumulated in plaques, whereas CCR2 -Ly-6C lo monocytes entered less frequently but were more prone to developing into plaque cells expressing the dendritic cell-associated marker CD11c, indicating that phagocyte heterogeneity in plaques is linked to distinct types of entering monocytes. CCR2 -monocytes did not rely on CX3CR1 to enter plaques. Instead, they were partially dependent upon CCR5, which they selectively upregulated in apoE -/-mice. By comparison, CCR2 + Ly-6C hi monocytes unexpectedly required CX3CR1 in addition to CCR2 and CCR5 to accumulate within plaques. In many other inflammatory settings, these monocytes utilize CCR2, but not CX3CR1, for trafficking. Thus, antagonizing CX3CR1 may be effective therapeutically in ameliorating CCR2 + monocyte recruitment to plaques without impairing their CCR2-dependent responses to inflammation overall.

CX3CR1+ and CD103+ dendritic cells (DCs) in intestinal lamina propria (LP) play a key role in mucosal immunity. However, the origin and the developmental pathways that regulate their differentiation in the LP remain unclear. Our results reveal that monocytes give rise exclusively to CX3CR1+CD103− LP DCs under the control of M-CSFR and Flt3 ligands. In contrast, common DC progenitors (CDP) and pre-DCs, which give rise to lymphoid organ DCs but not to monocytes, differentiate exclusively into CD103+CX3CR1− LP DCs under the control of Flt3 ligand and GM-CSF. CD103+CX3CR1− DCs but not CX3CR1+CD103− DCs in the LP constitutively express CCR7 and are the first DCs to transport pathogenic Salmonella from the intestinal tract to the mesenteric lymph nodes. Altogether, these results underline the diverse origin of the LP DC network and identify mucosal DCs that arise from pre-DC as key sentinels of the gut immune system.

Summary It is thought that monocytes rapidly differentiate to macrophages or dendritic cells (DCs) upon leaving blood. Here we have shown that Ly-6C+ monocytes constitutively trafficked into skin, lung, and lymph nodes (LNs). Entry was unaffected in gnotobiotic mice. Monocytes in resting lung and LN had similar gene expression profiles to blood monocytes, but elevated transcripts of a limited number of genes including cyclo-oxygenase-2 (COX-2) and major histocompatibility complex class II (MHC II), induced by monocyte interaction with endothelium. Parabiosis, bromodoxyuridine (BrdU) pulse-chase analysis, and intranasal instillation of tracers indicated that instead of contributing to resident macrophages in the lung, recruited endogenous monocytes acquired antigen for carriage to draining LNs, a function redundant with DCs though differentiation to DCs did not occur. Thus, monocytes can enter steady state non-lymphoid organs and recirculate to LNs without differentiation to macrophages or DCs, revising a long-held view that monocytes become tissue-resident macrophages by default.