Introduction

Cytomegaloviruses (CMVs) are species-specific betaherpesviruses that establish life-long persistence in their hosts. Their genomes, the largest among herpesviruses, are estimated to contain between 165 (Davison et al. 2003) and 252 (Murphy et al. 2003b; see the chapter by E. Murphy and T. Shenk, this volume) potential open reading frames (ORFs) encompassing up to 241,087 bp of double-stranded DNA (Davison et al. 2003). Approximately 41 human CMV (HCMV) ORFs belong to a core set of genes essential for viral replication in vitro, such as genes encoding DNA polymerase, capsid, matrix and envelope proteins (Yu et al. 2003). Approximately 88 genes were found to be nonessential for efficient CMV replication in vitro (Yu et al. 2003). Some of these nonessential CMV genes are homologous to genes of the host. Among these homologs are genes that share similarities with genes encoding proteins that are associated with the immune system, such as class I MHC proteins (Wills et al. 2005; Prod'homme et al. 2007), a TCR gamma chain (Beck and Barrell 1992), IL-10 (Wagner et al. 2003) and a TNF receptor (Poole et al. 2006). Apparently these gene homologs have been acquired from the host organism and subsequently modified during approximately 180 My of co-evolution (Davison 2002) in order to enable dissemination and maintain life-long persistence. Interestingly, CMVs possess two distinct groups of genes homologous to those of

Fig. 1 The 2D peptide structures of a CC chemokine ligand and a chemokine receptor. The N and the C denote the amino and carboxyl termini, respectively. The encircled Cs represent conserved cysteine residues. The dashed lines indicate conserved disulphide bridges. Grey boxes indicate hydrophobic transmembrane alpha helix domains

Fig. 1 The 2D peptide structures of a CC chemokine ligand and a chemokine receptor. The N and the C denote the amino and carboxyl termini, respectively. The encircled Cs represent conserved cysteine residues. The dashed lines indicate conserved disulphide bridges. Grey boxes indicate hydrophobic transmembrane alpha helix domains the host: (a) CC and CXC chemokine genes and (b) G protein-coupled receptors (GPCR) genes, the majority of which resembles chemokine receptor genes. GPCRs form a large family of 7-transmembrane receptors (Fig. 1) that include sensory receptors for sight, smell, and taste as well as receptors for many neurotransmitters, peptide hormones and chemokines. Chemokines comprise a family of immune modulatory cytokine peptides. Currently, four classes of chemokines are known. The classification is based on a conserved structure (Fig. 1) that includes either a single cysteine (C), a CC motif (Fig. 1), a CXC motif or a CX3C motif. Chemokines can be released to initiate inflammatory responses by acting as chemoattractant for infiltrating leukocytes (Glass et al. 2003). They can also stimulate differentiation, maturation and activation of many types of immune-related cells (Glass et al. 2003). Two chemokines, CXCL16 and CX3CL-1, were shown to function as adhesion molecules for leukocytes that are captured from the bloodstream onto the endothe-lial surface (Haskell et al. 2000; Nakayama et al. 2003). The purpose of this review is to summarize the (putative) functions of the CC and CXC chemokines (vCKs) as well as the chemokine-like GPCRs (vGPCRs) that are encoded by CMV.

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