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 specialized macrophage populations in splenic marginal zone and lymph node sinuses and follicular dendritic cells within germinal centers. Cells referred to as follicular dendritic cells in chickens express CSF1R, the receptor for CSF1 and IL34. We used single-cell RNA sequencing on CSF1R+ cells from chicken spleen to identify monocytes and 2 distinct populations of macrophages. TIMD4/C1Q/MAFB+ macrophages were enriched for expression of genes involved in iron metabolism and antigen capture and presentation, suggesting a functional relationship with both red pulp and marginal zone macrophages in mammals. Conversely, a MARCO/VSIG4+ population expressed SPIC, a transcription factor associated with red pulp macrophages in mammals, but also expressed receptors and trophic factors associated with mammalian follicular dendritic cells. SPIC+ cells were located within follicles in spleen, cecal tonsil, and bursa. We conclude that a specialized population of CSF1R+, SPIC+ macrophages in birds differentiates to perform the function of follicular dendritic cells. To determine the function of CSF1R+ macrophages, 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; from days 5 to 6 after hatch, however, they failed to thrive. Loss of CSF1R function in hatchlings led to monocytopenia, granulocytosis, and loss of macrophage subpopulations in lymphoid organs. The loss of follicular macrophages in the bursa was associated with involution and severe B cell deficiency in the circulation and spleen.