Monoclonal antibodies to HuIFN- alpha species were previously isolated and the reactive epitopes on HuIFN- alpha2 identified. When used in excess concentration the antibodies neutralise the antiviral activity of HuIFN- alpha and inhibit binding of IFN to its receptor on human and bovine cells. However, when used in equimolar concentrations the antibodies bind to IFN bound to its receptor on human but not on bovine cells. Radiolabelled antibodies and radiolabelled ligand have been used to study in detail IFN-receptor interactions in human cells. The use of the monoclonal antibodies has identified differences in the interaction in different cell lines not obvious from ligand binding studies alone. IFN receptors do not appear to be down-regulated in Daudi cells but remain on the cell surface complexed with bound IFN whereas down-regulation does occur in the human epithelial breast carcinoma cell line BT20. Although surface receptors are not decreased in Daudi cells exposed to IFN, the time course of association of radiolabelled IFN with these cells shows that cell associated radioactivity decreases after incubation for more than 1 hr at 37-degrees-C. This result cannot be explained by degradation of exogeneous IFN (which does not occur) and suggests that endogenous IFN may be involved in the interaction of HuIFN- alpha with Daudi cells.

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