We have previously demonstrated that human cytomegalovirus (CMV) binds the host protein β microglobulin (βm) from body fluids or from cell culture media. In this report we have examined the effect of the βm on viral infectivity. We have shown that the addition of human purified βm, or a fraction of foetal calf serum corresponding to bovine βm, to culture medium increased the amount of infectious extracellular CMV, compared to that from cells grown in serum-free medium. Metabolic labelling experiments demonstrated that this effect was not due to an increase in the amount of extracellular virus but to an increase in the infectivity of the virus present in extracellular fluids. We concluded that the binding of βm by CMV increased its infectivity. We have shown that CMV and βm compete for binding sites on fibroblasts. As the main binding site on cells for βm is the class I HLA heavy chain we compared the binding of CMV to the Raji and Daudi cell lines which express or lack expression of class I HLA molecules. The binding of radiolabelled βm-coated CMV was significantly higher to Raji cells than to Daudi cells. Furthermore, CMV could compete with βm for binding to Raji cells, although the reverse was not true. These results demonstrate that CMV can use class I HLA molecules as a virus receptor. We propose that when coated with βm, CMV has the capacity to displace βm from the class I HLA heavy chain-βm dimer on the cell surface and bind to cells. The fact that βm enhances infectivity suggests that such binding leads to productive infection of cells.


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