The role of viral chemokine receptor US28 in the pathogenesis of HCMV disease

This thesis is the first to report in vivo expression of the US28 gene and provides strong evidence of a potential role in HCMV disease. Expression of the gene for the US28 receptor and the chemokines, MCP-1, MIP-1α, and RANTES, was found in CMV pneumonitis. US28 gene expression was observed in alveolar lining cells, which were also positive for HCMV antigen in infected human lung. In addition, the cells identified as expressing the US28 gene were identified morphologically as harbouring typical HCMV inclusions. A prominent leucocytic infiltrate comprising predominantly of T lymphocytes and macrophages, was found in association with abundant expression of MCP-1, RANTES and MIP-1α that are known to recruit these cells. These observations would suggest the development of an effective host immune response, however examination of the lung tissue revealed that the majority of the infiltrating inflammatory cells were not located in areas where large numbers of US28 positive inclusion bodies were present, as areas surrounding the inclusion bodies were devoid of inflammatory cells. These results therefore, suggest that competitive binding of chemokine ligands to US28 might prevent the recruitment of effector cells to the precise site of infection, thereby preventing effective clearance of the virus. In vitro analyses revealed that the US28 gene is expressed in HCMV-infected fibroblasts and THP-1-derived macrophages in the late phase of in vitro infection. Fibroblasts constitutively produced MCP-1 mRNA and protein in culture, which was down-regulated in parallel with increasing expression of US28. THP-1-macrophages expressed RANTES, MIP-1α, MIP-1β and IL-8, all of which were inhibited in the late phase of infection in association with increasing US28 expression. This inhibition of chemokine production by HCMV-infected cells in vitro may signify a mechanism for inhibition of chemokine dependent trafficking in the immune response to HCMV. A number of advantages may be provided to the virus by employing this strategy, including: modulation of recruitment of specific leucocytes to sites of infection, prevention of activation of leucocytes by chemokines, hence preventing elimination of infected cells by the immune response. This study provides evidence that the viral chemokine receptor, US28 may utilise these changes in the local chemokine environment to prevent activation of leucocytes and/or recruitment of leucocytes to sites of infection to evade the immune response