Fatal attraction: cytomegalovirus-encoded chemokine homologs

Members of the cytomegalovirus (CMV) subfamily of betaherpesviruses infecting primates and rodents encode divergent proteins with sequence characteristics and activities of chemokines, a class of small, secreted proteins that control leukocyte migration and trafficking behavior. Human CMV genes UL146 and UL147 encode proteins with sequence characteristics of CXC chemokines, whereas, murine CMV encodes a CC chemokine homolog (MCK-2). Human CMV UL146 encodes a neutrophil-attracting chemokine denoted viral CXC chemokine-1 (vCXCL1) that is as potent as host IL-8 and functions via the CXCR2 receptor, one of two human IL-8 receptors. Murine CMV MCK-2 is composed of a chemokine domain derived from open reading frame (ORF) m131 (and denoted MCK-1) as well as a domain derived from m129 that does not have sequence similarity to any known class of proteins. A synthetic version of murine CMV m131 (MCK-1) protein carries out many of the activities of a positive-acting chemokine, including transient release of intracellular calcium stores and cell adhesion of peritoneal macrophage populations. In the context of the viral genome and infection of the mouse host, the m131-m129 (MCK-2) gene product confers increased inflammation, higher levels of viremia, and higher titers of virus in salivary glands, consistent with a role in promoting dissemination by attracting an important mononuclear leukocyte population. Other characterized primate CMVs, but not other primate betaherpesviruses, encode gene products similar to human UL146 and UL147. Other characterized rodent CMVs encode a gene product similar to the murine CMV chemokine homolog, although not as a spliced gene product. Thus chemokines, like viral proteins that downmodulate MHC class I expression or have sequence homology to host MHC class I proteins, have evolved in primate and rodent CMVs to carry out an analogous set of immunomodulatory functions during infection of the host even though they arise from distinct origins.