Frontline Science: Antagonism between regular and atypical Cxcr3 receptors regulates macrophage migration during infection and injury in zebrafish

The CXCR3-CXCL11 chemokine-signaling axis plays an essential role in infection and inflammation by orchestrating leukocyte trafficking in human and animal models, including zebrafish. Atypical chemokine receptors (ACKRs) play a fundamental regulatory function in signaling networks by shaping chemokine gradients through their ligand scavenging function, while being unable to signal in the classic G-protein-dependent manner. Two copies of the CXCR3 gene in zebrafish, cxcr3.2 and cxcr3.3, are expressed on macrophages and share a highly conserved ligand-binding site. However, Cxcr3.3 has structural characteristics of ACKRs indicative of a ligand-scavenging role. In contrast, we previously showed that Cxcr3.2 is an active CXCR3 receptor because it is required for macrophage motility and recruitment to sites of mycobacterial infection. In this study, we generated a cxcr3.3 CRISPR-mutant to functionally dissect the antagonistic interplay among the cxcr3 paralogs in the immune response. We observed that cxcr3.3 mutants are more susceptible to mycobacterial infection, whereas cxcr3.2 mutants are more resistant. Furthermore, macrophages in the cxcr3.3 mutant are more motile, show higher activation status, and are recruited more efficiently to sites of infection or injury. Our results suggest that Cxcr3.3 is an ACKR that regulates the activity of Cxcr3.2 by scavenging common ligands and that silencing the scavenging function of Cxcr3.3 results in an exacerbated Cxcr3.2 signaling. In human, splice variants of CXCR3 have antagonistic functions and CXCR3 ligands also interact with ACKRs. Therefore, in zebrafish, an analogous regulatory mechanism appears to have evolved after the cxcr3 gene duplication event, through diversification of conventional and atypical receptor variants.