Molecular evolution of CXC chemokines: extant CXC chemokines originate from the CNS Mark O. Huising 1,2 , Rene ´ J.M. Stet 1 , Corine P. Kruiswijk 1 , Huub F.J. Savelkoul 1 and B.M. Lidy Verburg-van Kemenade 1 1 Cell Biology and Immunology Group, Wageningen University, PO Box 338, 6700 AH, The Netherlands 2 Animal Physiology Group, University of Nijmegen, Toernooiveld 1, 6525 ED, The Netherlands The mammalian CXC chemokine system comprises 16 ligands and six receptors, and its actions stretch well beyond the immune system. Recent elucidation of the pufferfish genome, a representative of an evolutionary ancient vertebrate class, has enabled analysis of the mammalian CXC chemokine system in a phylogenetic context. Comparison of the phylogenies of vertebrate CXC chemokines revealed that fish and mammals have found different solutions to similar problems, grafted on the same basic structural motif. Phylogenetic analyses showed that the large, highly redundant CXC chemokine family is a very recent phenomenon that is exclusive to higher vertebrates. Moreover, its ancestral role is found within the central nervous system and not within the immune system. CXC chemokines, named after a conserved pattern of cysteine residues, were initially identified as potent mediators of neutrophil chemotaxis [1,2]. Subsequent work has revealed a much broader spectrum of functions mediated by members of the mammalian CXC chemokine family, such as chemotaxis of monocytes and lymphocytes and angiogenesis [3,4]. Rather than providing an exhaus- tive review of the plethora of CXC chemokine functions (which have been covered recently by several excellent reviews [5–9]), we trace back through the evolutionary history of the CXC chemokine family. In doing so, we deduce remarkable features concerning the putative function of the ancestral CXC chemokine and the recent evolutionary history of the majority of mammalian CXC chemokines. It takes two to make a comparison. Molecular data of several fish species, supplemented with the recent elucidation of the Takifugu rubripes (pufferfish) genome [10], have enabled identification of the CXC chemokine system, including their receptors, in this evolutionary ancient class of vertebrates (Box 1). Where appropriate, we adhere to the standard nomenclature for chemokines [11,12] and chemokine receptors [13,14]; Table 1 summarizes the most common synonyms. Most CXC chemokines are evolutionary recent and exclusive to mammals The most striking observation derived from the phylo- genetic analyses of CXC chemokines is that only CXCL12 and CXCL14 have unambiguous orthologues in fish (Fig. 1). None of the remaining mammalian CXC chemo- kines has clear orthologues in any other vertebrate class, including birds. This suggests that CXCL12 and CXCL14 are phylogenetically the modern representatives of the ancestral CXC chemokine. The recently discovered membrane-associated CXCL16 [15] displays aberrant characteristics, complicating its assignment to a particu- lar group of chemokines. Its pattern of conserved cysteine residues resembles that of CXC chemokines, whereas the glycosylated mucin stalk by which it is attached to the cell surface is a characteristic shared with the CX 3 C class of chemokines. Human CXCL16 is located on chromosome 17 (like many CC chemokines) but on the opposite side of the centromere (17p13 vs 17q12) [5]. As this chemokine is provisionally ranked under the CXC chemokines, we have included it in our phylogenetic analyses of the multigene CXC chemokine family. CXCL16 is represented by only two mammalian sequences. The presence of the mucin stalk and transmembrane region in combination with the aberrant CXC signature of CXCL16 made it impossible to establish unequivocally the presence or absence of fish orthologues. However, phylogenetic analyses of CXC chemokine receptors (CXCRs; Fig. 2) provide a clue. CXCR6, the receptor to CXCL16, is only found in mammals, suggesting that orthologues of CXCL16 and CXCR6 are not present in fish. So far, 16 different chemokines have been discovered in mammals, although the numbers vary between individual species. This makes the CXC chemokine system in mammals much more extensive than that of fish; exhaustive sequencing and systematic database-mining yielded only five different phylogenetic CXC chemokine clusters in fish (CXCa, CXCb, CXCc, CXCL12 and CXCL14; Box 1). CXC chemokines from several mammalian species cluster largely according to nomenclature, with the notable exceptions of CXCL1 – CXCL3, and the cluster of CXCL5 and CXCL6, indicating a relatively recent common origin. Corresponding author: B.M. Lidy Verburg-van Kemenade (lidy.vankemenade@ wur.nl). Opinion TRENDS in Immunology Vol.24 No.6 June 2003 306 http://treimm.trends.com 1471-4906/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S1471-4906(03)00120-0