Dear editor and reviewers,
We would like to submit our manuscript entitled “Bactericidal ability of target acidic phospholipids and phagocytosis of CDC42 GTPase-mediated cytoskeletal rearrangement define functional conservation of CXCL12 in vertebrates” to Science China Life Sciences for publication. No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part.
The bactericidal and promoting phagocytic activities induced by CXCL12 emerged as a unique event in mammals or exists widely in vertebrates that is still clear. Present study established in grass carp (Ctenopharyngodon idella) models indicated that CiCXCL12a exerts broad-spectrum direct antibacterial activity by attracting bacterial acidic phospholipids, resulting in bacterial membrane perforation, bacterial cytoplasm leakage and death. Moreover, CiCXCL12a induces monocytes/macrophages chemotaxis toward inflammatory or infectious sites to exert phagocytosis. CiCXCL12a promotes the internalization of interacting A. hydrophila with F-actin into monocytes/macrophages by regulating CDC42 GTPase-mediated cytoskeletal polymerization and rearrangement and increasing the oligomerization of free G-actin to insoluble F-actin. Then, Rab5 and Rab7 are upregulated to mediate colocalization between A. hydrophila and early endosome, late endosome or lysosome, finally degraded in monocyte/macrophage phagolysosomes. Moreover, CiCXCL12a activates the PI3K-AKT signaling pathway to conservatively inhibits LPS-induced monocyte/macrophage apoptosis rather than using JAK-STAT5 and MAPK-ERK signaling pathways. Altogether, CiCXCL12a prevents grass carp from A. hydrophila infection that reflected by grass carp survival increase, tissue bacterial loads reduce, systemic inflammation weakens and organ injury attenuates. Our results also revealed that the direct bactericidal activity, phagocytosis, anti-apoptosis or chemotaxis mediated by CXCL12 are highly conserved across diverse vertebrate, such as Cyclostomata (Lampetra japonica), Chondrichthyes (Cetorhinus maximus), Osteichthyes (Ctenopharyngodon idella), Amphibian (Xenopus tropicalis), Reptilia (Anolis carolinensis), Aves (Gallus gallus) to Mammal (Homo sapiens). 
From an evolutionary viewpoint, these results imply CXCL12 potential roles in supporting the functional and mechanism conservation of CXCL12 in vertebrates. Furthermore, our findings also illustrate the underlying mechanism of CXCL12a-mediated immunity in teleost, and provide novel perspectives to understand the evolution of cytokines immune system in vertebrates and offers potential avenues for understanding species evolution. We believed this manuscript will be of value to the readers who are interested in antimicrobial immunity and comparative immunity, so we consider this research is suitable for publication in Science China Life Sciences. 
We appreciate your consideration of our manuscript, and we look forward to receiving comments from the reviewers.
With kind personal regards,
Respectfully yours,
这是一篇cover letter，帮我修改一下，提高水平