Beta 2 microglobulin enhances the infectivity of cytomegalovirus and when bound to the virus enables class I HLA molecules to be used as a virus receptor.

We have previously demonstrated that human cytomegalovirus (CMV) binds the host protein beta 2 microglobulin (beta 2m) from body fluids or from cell culture media. In this report we have examined the effect of the beta 2m on viral infectivity. We have shown that the addition of human purified beta 2m, or a fraction of foetal calf serum corresponding to bovine beta 2m, 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 beta 2m by CMV increased its infectivity. We have shown that CMV and beta 2m compete for binding sites on fibroblasts. As the main binding site on cells for beta 2m 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 beta 2m-coated CMV was significantly higher to Raji cells than to Daudi cells. Furthermore, CMV could compete with beta 2m 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 beta 2m, CMV has the capacity to displace beta 2m from the class I HLA heavy chain-beta 2m dimer on the cell surface and bind to cells. The fact that beta 2m enhances infectivity suggests that such binding leads to productive infection of cells.