Macrophages and dendritic cells

Macrophages, monocytes, and conventional dendritic cells (cDCs), together with B lymphocytes, are termed antigen-presenting cells (APCs). Monocytes and cDCs differentiate from myeloid precursor cells in the bone marrow under the influence of lineage-specific hematopoietic growth factors. After release from the hematopoietic tissue, they appear in the bloodstream as monocytes and cDC precursors. Older tissue cDCs are replaced by new cells entering the blood. Upon entry into tissues, cDC precursors undergo limited local proliferation, thus numbers of DCs in tissues are maintained by a combination of recruitment and local tissue proliferation. In contrast, tissue-resident macrophages are largely derived from embryonic precursors that seed tissues in several waves during embryonic development. In the steady state, these tissue macrophages are maintained independently from blood monocytes by local tissue proliferation, whereas during inflammatory conditions they can be replaced by recruited monocyte-derived macrophages. Tissue-resident macrophages further differentiate into diverse subsets of functionally specialized cell types which play critical roles as regulators of the adaptive immune response, as effector cells in the control of infections and in tissue homeostasis and repair. Avian macrophage biology has been studied intensively in the past, but research in avian cDC biology has only recently made progress because of the increasing availability of gene expression analysis, lineage-specific growth factors, cytokines, and cell surface markers. Here, we provide a comprehensive description of avian APC biology and discuss technologies currently available for research on these cells.