Monitoring C3aR Expression Using a Floxed tdTomato-C3aR Reporter Knock-in Mouse.

C3a exerts multiple biologic functions through activation of its cognate C3a receptor. C3-/- and C3aR-/- mice have been instrumental in defining important roles of the C3a/C3aR axis in the regulation of acute and chronic inflammatory diseases, including ischemia/reperfusion injury, allergic asthma, autoimmune nephritis, and rheumatoid arthritis. Surprisingly little is known about C3aR expression and function in immune and stromal cells. To close this gap, we generated a floxed tandem-dye Tomato (tdTomato)-C3aR reporter knock-in mouse, which we used to monitor C3aR expression in cells residing in the lung, airways, lamina propria (LP) of the small intestine, brain, visceral adipose tissue, bone marrow (BM), spleen, and the circulation. We found a strong expression of tdTomato-C3aR in the brain, lung, LP, and visceral adipose tissue, whereas it was minor in the spleen, blood, BM, and the airways. Most macrophage and eosinophil populations were tdTomato-C3aR+ Interestingly, most tissue eosinophils and some macrophage populations expressed C3aR intracellularly. BM-derived dendritic cells (DCs), lung-resident cluster of differentiation (CD) 11b+ conventional DCs (cDCs) and monocyte-derived DCs, LP CD103+, and CD11b+ cDCs but not pulmonary CD103+ cDCs and splenic DCs were tdTomato-C3aR+ Surprisingly, neither BM, blood, lung neutrophils, nor mast cells expressed C3aR. Similarly, all lymphoid-derived cells were tdTomato-C3aR-, except some LP-derived type 3 innate lymphoid cells. Pulmonary and LP-derived epithelial cells expressed at best minor levels of C3aR. In summary, we provide novel insights into the expression pattern of C3aR in mice. The floxed C3aR knock-in mouse will help to reliably track and conditionally delete C3aR expression in experimental models of inflammation.