CSF1R-dependent macrophages control B cell development and function in the chicken immune system.

Acquired immunity in mammals depends upon capture and presentation of antigens by specialised macrophage populations in splenic marginal zone and lymph node sinuses and follicular dendritic cells (FDC) within germinal centres. Cells referred to as FDC in chickens express CSF1R, the receptor for macrophage colony-stimulating factor (CSF1) and IL34. We utilised single cell RNA-seq on CSF1R+ cells from chicken spleen to identify monocytes and two distinct populations of macrophages. TIMD4/C1Q/MAFB+ macrophages were enriched for expression of genes involved in iron metabolism. A MARCO/VSIG4+ population expressed SPIC, a transcription factor associated with red pulp macrophages in mammals but also expressed receptors (CR2) and trophic factors (TNFSF13, CXCL13) associated with mammalian FDC. SPIC+ cells were located within follicles in spleen, caecal tonsil and bursa. We generated a CSF1R knockout in the chicken germ line. Mutant birds lack macrophages in the embryo. They were indistinguishable from wild type at hatch and behaved and fed normally but from day 5-6 post hatch they failed to thrive. Loss of CSF1R function in hatchlings led to monocytopenia and granulocytosis and the loss of macrophage subpopulations in lymphoid organs. Consistent with their expression of B cell trophic factors, the loss of follicular macrophages in the bursa was associated with involution and severe B cell deficiency in the circulation and spleen. In summary, lymphoid tissues of chickens contain specialised macrophage populations with distinct expression profiles. The details of regulation by CSF1R, specialised functions and underlying transcriptional regulation are quite different between birds and mammals.