House dust mite exposure upregulates C3a receptor expression in pulmonary innate immune cells during the sensitization phase of allergic asthma by an IL-33-dependent mechanism

The anaphylatoxin C3a exerts its biologic functions through the activation of its cognate G-protein-coupled receptor C3a receptor (C3aR). C3aR mRNA expression has been described both in human and mouse lungs. However, no systematic analysis of C3aR expression at steady state and under pro-inflammatory conditions in the different pulmonary cell subsets has been done so far. Using a newly developed Tandem-dye (td)-Tomato-C3aR reporter knock-in mouse, we found C3aR expression in pulmonary eosinophils, resident CD11b + conventional dendritic cells (cDC), and monocyte-derived DCs (moDC) at steady state. In contrast, pulmonary CD103 + cDCs and plasmacytoid DCs stained negative. Surprisingly, lung and bronchoalveolar alveolar macrophages (AMs), as well as neutrophils, lacked td-Tomato-C3aR. C3aR antibody staining revealed that only moDCs express the receptor at the cell surface. House dust mite (HDM) is a potent allergen that drives the development of allergic asthma in humans. Using the td-Tomato-C3aR reporter mouse, we found that HDM exposure resulted in the upregulation of C3aR in AMs, eosinophils, CD11b + cDCs, and moDCs during the early phase of HDM-induced allergic asthma. Further, C3aR translocated to the cell surface in eosinophils and AMs following HDM exposure. Importantly, intra-tracheal administration of IL-33, an alarmin released by epithelial cells upon allergen contact, recapitulated these results. In summary, our findings demonstrate that allergen contact triggers the upregulation of C3aR and its surface expression in previously negative innate immune cells from the lung by an IL-33 dependent mechanism. C3aR-mediated activation of innate immune cells may play a critical role for early asthma development.