Dendritic cells (DCs) form a remarkable cellular network that shapes adaptive immune responses according to peripheral cues. After four decades of research, we now know that DCs arise from a hematopoietic lineage distinct from other leukocytes, establishing the DC system as a unique hematopoietic branch. Recent work has also established that tissue DCs consist of developmentally and functionally distinct subsets that differentially regulate T lymphocyte function. This review discusses major advances in our understanding of the regulation of DC lineage commitment, differentiation, diversification, and function in situ.

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.

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.

CD103+ dendritic cells (DCs) in nonlymphoid tissues are specialized in the cross-presentation of cell-associated antigens. However, little is known about the mechanisms that regulate the development of these cells. We show that two populations of CD11c+MHCII+ cells separated on the basis of CD103 and CD11b expression coexist in most nonlymphoid tissues with the exception of the lamina propria. CD103+ DCs are related to lymphoid organ CD8+ DCs in that they are derived exclusively from pre-DCs under the control of fms-like tyrosine kinase 3 (Flt3) ligand, inhibitor of DNA protein 2 (Id2), and IFN regulatory protein 8 (IRF8). In contrast, lamina propria CD103+ DCs express CD11b and develop independently of Id2 and IRF8. The other population of CD11c+MHCII+ cells in tissues, which is CD103−CD11b+, is heterogenous and depends on both Flt3 and MCSF-R. Our results reveal that nonlymphoid tissue CD103+ DCs and lymphoid organ CD8+ DCs derive from the same precursor and follow a related differentiation program.

Dendritic cells (DCs) are key players in the immune system. Much of their biology has been elucidated via culture systems in which hematopoietic precursors differentiate into DCs under the aegis of cytokines. A widely used protocol involves the culture of murine bone marrow (BM) cells with granulocyte-macrophage colony-stimulating factor (GM-CSF) to generate BM-derived DCs (BMDCs). BMDCs express CD11c and MHC class II (MHCII) molecules and share with DCs isolated from tissues the ability to present exogenous antigens to T cells and to respond to microbial stimuli by undergoing maturation. We demonstrate that CD11c(+)MHCII(+) BMDCs are in fact a heterogeneous group of cells that comprises conventional DCs and monocyte-derived macrophages. DCs and macrophages in GM-CSF cultures both undergo maturation upon stimulation with lipopolysaccharide but respond differentially to the stimulus and remain separable entities. These results have important implications for the interpretation of a vast array of data obtained with DC culture systems.

Although, much progress has been made in our understanding of DC ontogeny and function, the transcriptional regulation of DC lineage commitment and functional specialization in vivo is poorly understood. We performed a comprehensive comparative analysis of CD8+, CD103+, CD11b+, and plasmacytoid DC subsets and the recently identified Macrophage DC precursors and Common DC precursors across the entire immune system. Here we characterize candidate transcriptional activators involved in myeloid progenitor commitment to the DC lineage and predicted regulators of DC functional diversity in tissues. We identify a molecular signature that distinguishes tissue DC from macrophages. We also identify a transcriptional program expressed specifically during steady-state tissue DC migration to the draining lymph nodes that may control tolerance to self-tissue antigens.

Induction of immunity and peripheral tolerance requires contacts between antigen-bearing dendritic cells (DCs) and cognate T cells. Using real-time two-photon microscopy, we have analyzed the dynamics of CD8(+) T cells in lymph nodes during the induction of antigen-specific immunity or tolerance. At 15-20 h after the induction of immunity, T cells stopped moving and established prolonged interactions with DCs. In tolerogenic conditions, despite effective initial T cell activation and proliferation, naive T cells remained motile and established serial brief contacts with multiple DCs. Thus, stable DC-T cell interactions occur during the induction of priming, whereas brief contacts may contribute to the induction of T cell tolerance.

SUMMARY GM-CSF (Csf-2) is a critical cytokine for the in vitro generation of dendritic cells (DCs) and is thought to control the development of inflammatory DCs and resident CD103+ DCs in some tissues. Here we showed that in contrast to the current understanding, Csf-2 receptor acts in the steady state to promote the survival and homeostasis of nonlymphoid tissue-resident CD103+ and CD11b+ DCs. Absence of Csf-2 receptor on lung DCs abrogated the induction of CD8+ T cell immunity after immunization with particulate antigens. In contrast, Csf-2 receptor was dispensable for the differentiation and innate function of inflammatory DCs during acute injuries. Instead, inflammatory DCs required Csf-1 receptor for their development. Thus, Csf-2 is important in vaccine-induced CD8+ T cell immunity through the regulation of nonlymphoid tissue DC homeostasis rather than control of inflammatory DCs in vivo.