Phylogenetically three related CXC chemokines are classified
Phylogenetically three related CXC chemokines are classified as the possible ligands for the teleost CXCR1 and CXCR2. They are referred to as CXCL8_L1 (CXCL8/IL-8/CXCa), CXCL8_L2 (CXCc) and CXCL8_L3 (Alejo and Tafalla, 2011, Chen et al., 2013, Laing et al., 2002, Laing and Secombes, 2004, Nomiyama et al., 2008, van der Aa et al., 2010). The CXCL8_L1 is found ubiquitously among all teleost species studied to date, but the other two lineages seem to be limited to some species. All fish CXCL8 members (except for cod CXCL8_L1) lack the ELR+ motif, a characteristic sequence feature for the inflammatory CXC chemokines (Chen et al., 2013), a rule that apparently is not applicable to the fish CXC chemokines. Fish CXCL8_L1 and CXCL8_L2 have been shown to be chemoattractants for neutrophils and macrophages and can be modulated by ligands of opioid and adrenergic GPCRs, suggesting that these molecules are important players in regulation of immune response and neuroendocrine functions (Chadzinska et al., 2009, Chadzinska et al., 2012, Harun et al., 2008, Verburg-van Kemenade et al., 2013, Wang et al., 2013). Studies in zebrafish demonstrate that both CXCL8_L1 and CXCL8_L2 signal through CXCR2 to enhance neutrophil recruitment in response to wounds and infections (de Oliveira et al., 2013). In some teleost species, which contain only a single potential CXCL8 ligand but two groups of receptors (e.g. salmonids), the ligand/receptor pairing remains to be determined.
CXCR3 In mammals CXCR3 is the sole receptor shared by three closely related ELR- chemokines CXCL9–11 and also interacts with CXCL4 and CCL21 (Lacotte et al., 2009, Soto et al., 1998). CXCL9–11 can be induced by the Th1 signature cytokine IFN-γ and hence are referred to as IFN-γ inducible proteins. CXCR3 is primarily expressed in activated T cells, in particular CD4+ Th1 brucine (Groom and Luster, 2011), but also in cytotoxic CD8+ T cells (Tc), NK cells and macrophages (Loetscher et al., 1998, Zhou et al., 2010). CXCR3 is crucial for the interaction between dendritic cells and CD4+ Th1 cells, guiding migration of CD4+ T cells and hence influencing their regulatory functions (Groom et al., 2012, Qin et al., 1998, Rabin et al., 2003). Other functions of CXCR3 include the regulation of angiogenesis and cancer growth (Kawada et al., 2007, Romagnani et al., 2001) and the control of endothelial cell migration and differentiation (Yang and Richmond, 2004). In addition, it has been shown that CXCR3 is constitutively expressed by endothelial cells located in blood vessels of medium and large calibre but not in small vessels from different organs (García-López et al., 2001). CXCR3 exists as a single gene in mammals. Alternative splicing gives rise to two transcript variants, translating into a full length receptor with an extended N-terminal extracellular domain and a truncated variant (CXCR3B) lacking the 6th and 7th transmembrane domains and the entire intracellular tail (Ehlert et al., 2004, Lasagni et al., 2003, Loetscher et al., 1996). The two receptor variants, CXCR3A and 3B, display opposite functions in regulating cell growth, either promoting or inhibiting cell growth mediated by CXCL9–11 (Balan and Pal, 2014, Lasagni et al., 2003). In humans, the growth of CXCR3B+ microvascular endothelial cells is inhibited by CXCL9–11 whilst CXCR3A+ cells are not affected (Lasagni et al., 2003). Two apparent CXCR3 groups are found in poikilothermic (cold blooded) vertebrates including bony fish, amphibians and reptiles (Bhatt et al., 2014, Chang et al., 2007, Wang et al., 2008b, Xu et al., 2014a) (Fig. 1 and Supplementary file 3). In birds, the gene loci for both CXCR3 and its ligand CXCL9–11 have been lost but the evolutionary events leading to this loss have not been determined (Wang et al., 2005). Like the CXCR1/2 genes, gene synteny of the CXCR3 locus is well conserved across poikilothermic vertebrates (Xu et al., 2014a), and the genes encoding CXCR3a and CXCR3b/CXCR3rel reside next to each other in teleosts, coelacanth and frogs. A conserved CXCR3a locus can be identified in the genome of spotted gar but the presence of CXCR3a cannot be confirmed due to the poor quality of the genome sequence in this region (unpublished data). A survey of the elephant shark genome failed to identify a CXCR3 orthologue in this species (Venkatesh et al., 2014). However, our analysis here indicates that a CXCR3-like molecule is present in another cartilaginous fish species, the little skate (Leucoraja erinacea) (SkateBase contig No., LSb2-ctg96488, http://skatebase.org), and that this molecule is indeed an orthologue of CXCR3a in other vertebrate species (Fig. 1 and Supplementary file 3).