PlCXCL12a differentially modulates SGIV and RGNNV replication during single and co-infection and regulates chemotaxis-associated immune responses.

Chemokines play important roles in coordinating innate immune defense in teleost fish. In this study, a CXCL12a homolog, designated PlCXCL12a, was cloned and characterized from leopard coral grouper (Plectropomus leopardus). PlCXCL12a encodes a conserved CXC chemokine containing a typical SCY domain and showed high sequence similarity to teleost CXCL12a homologs. PlCXCL12a was constitutively expressed in multiple tissues and was significantly induced by Singapore grouper iridovirus (SGIV), red-spotted grouper nervous necrosis virus (RGNNV), and pathogen-associated molecular pattern stimulation. Subcellular localization analysis showed that PlCXCL12a was mainly distributed in the cytoplasm and partially associated with the endoplasmic reticulum and Golgi apparatus. Functional assays demonstrated that PlCXCL12a overexpression suppressed SGIV and RGNNV replication, as evidenced by reduced viral gene expression, viral protein levels, cytopathic effects, and viral titers, while PlCXCL12a knockdown promoted viral replication. Notably, under SGIV and RGNNV co-infection, PlCXCL12a showed a stronger early inhibitory effect on SGIV but a weaker inhibitory effect on RGNNV compared with the corresponding single-infection conditions. Moreover, PlCXCL12a enhanced cell migration and increased the mRNA expression of chemokine-, inflammatory-, interferon-, and antiviral-related genes. Dual-luciferase reporter assays further showed that PlCXCL12a positively regulated NF-κB and STAT1 reporter activities. These findings suggest that PlCXCL12a functions as an inducible chemokine involved in both antiviral defense and chemotaxis-associated immune regulation in P. leopardus.