Identification of two IL-4/13 homologues in large yellow croaker (Larimichthys crocea) revealed their similar roles in inducing alternative activation of monocytes/macrophages

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Mammalian interleukin-4 (IL-4) and −13 (IL-13), two anti-inflammatory T helper cell type 2 (Th2) cytokines, play the central roles in mediating the alternative activation of monocytes/macrophages (MO/Mφs). However, exact functions in MO/Mφs polarization of IL-4/13 homologues in teleost fish remain largely unknown. In this study, we identified two IL-4/13 homologues from large yellow croaker Larimichthys crocea, LcIL-4/13A and LcIL-4/13B, which share low amino acid sequence identities to the known fish IL-4/13 molecules. Phylogenetic analysis showed that LcIL-4/13A is evolutionarily closely related to Dicentrarchus labrax IL-4/13A, and LcIL-4/13B to Takifugu rubripes IL-4/13B. The two LcIL-4/13 genes were constitutively expressed in all examined tissues, but with different expression levels. Both LcIL-4/13A and LcIL-4/13B were up-regulated by inactivated trivalent bacterial vaccine in the head kidney, and LcIL-4/13B appeared more responsive to bacterial vaccine than LcIL-4/13A. Recombinant LcIL-4/13A and LcIL-4/13B proteins (rLcIL-4/13A and rLcIL-4/13B) produced in Escherichia coli could significantly decrease production of reactive oxygen species (ROS) and nitrogen oxide (NO) in the head kidney MO/Mφs from large yellow croaker. Furthermore, rLcIL-4/13A and rLcIL-4/13B obviously down-regulated expression of pro-inflammatory cytokine (IL-1β and TNF-α) and inducible NO synthase (iNOS) genes in MO/Mφs, while they increased mRNA expression of anti-inflammatory cytokines (TGF-β and VEGF) and arginase-2. Additionally, the phagocytic activity of MO/Mφs was also inhibited by rLcIL-4/13A or rLcIL-4/13B. All these results therefore indicated that both LcIL-4/13A and LcIL-4/13B, although exhibiting a lower degree of sequence identity of 15.6% and differential expression pattern, have the similar roles in promoting alternative activation of head kidney MO/Mφs.

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