Chicken cathelicidin-3 protected mice from avian pathogenic Escherichia coli infection mediated by triggering chemokine CCL3 pathway.

Host defense peptides (HDPs) possess both direct bacteriocidal and immunomodulatory activities. In this study, chicken cathelicidin-1 (cCATH-1) and chicken cathelicidin-3 (cCATH-3) were selected based on their markedly different minimum inhibitory concentrations (MICs) against avian pathogenic Escherichia coli (APEC) strains, with MICs of cCATH-1 being 4- to 16-fold lower than those of cCATH-3. However, following intraperitoneal challenge with APEC strain E058 (4 × 10⁶ CFU/mouse), 80% of cCATH-3 treated mice survived while 20% of cCATH-1 treated mice survived. Flow cytometry (FC) revealed significantly lower peripheral blood Ly6Chigh monocytes but higher peritoneal Ly6Chigh macrophages in cCATH-3-treated mice at 4 and 8 h post-treatment. Peritoneal CCL3 secretion was significantly upregulated in cCATH-3 treated mice at 8 h post-treatment. Upon knockout of macrophages, the protection of cCATH-3 in mice against APEC E058 challenge was markedly abolished. In vitro, peritoneal macrophages treated with cCATH-3 showed stronger bacteriocidal activity than those treated with cCATH-1. Mouse CCL-3 (mCCL3)-specific antibody neutralize the effect of mCCL3 on macrophages chemotaxis in vitro. mCCL3-specific antibody administration resulted in 0% survival in cCATH-3-treated mice versus 60% survival in cCATH-3-treated mice administered with mock antibody, while cCATH-1-treated mice showed 0% survival administered with either mCCL3-specific antibody or mock antibody after APEC E058 strain challenge. In summary, cCATH-3 provides robust protection against APEC infection compared with cCATH-1 in mice, partially through CCL3-mediated macrophage chemotaxis and enhanced bacteriocidal activity in activated macrophages, revealing a novel mechanism of avian cathelicidins and supporting HDPs as alternatives to antibiotics for controlling avian colibacillosis.