Functional Characterization of the ELR Motif in Piscine ELR+CXC-Like Chemokine

To elucidate the functional role of piscine incomplete ELR motif, the recombinant CXC and its mutants (mELR and mLoop) were produced in Escherichia coli M15 based on the predicted mature peptide coding sequence of the black sea bream CXC (BS CXC) chemokine. Assays showed that the BS rCXC proteins displayed strong ability to induce fish blood neutrophils and head kidney (HK) macrophage migration in a dose-independent manner (10 to 200 ng), both in black sea bream and common carp. Although the ELR motif and the N-terminal loop of ELR+CXC chemokines are essential for chemotactic activity and receptor binding in mammals, the mELR and mLoop mutants showed no significant difference in their induction of chemotaxis of fish blood neutrophils compared with the full-length rCXC at the same dose. Human recombinant IL-8 (hrIL-8) can clearly induce piscine blood neutrophil migration and has no effect on macrophages, whereas the BS rCXC cannot induce chemotaxis in higher vertebrates, such as rat blood neutrophils or macrophages, even if the incomplete ELR motif in rCXC was mutated to ELR. The BS CXC and its mutants can promote the phagocytosis ability of piscine blood neutrophils and HK macrophages both in black sea bream and common carp, but have no effect on rat neutrophils or macrophages. Results showed that the piscine ELR+CXC-like chemokine represents an ancient version of a CXC chemokine; the ELR motif still does not show the higher specific polarization of function as found in mammalian.