Detection by Indirect Immunofluorescence of Fc Receptors in Cells Acutely Infected with Herpes simplex Virus

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In attempting to use the indirect fluorescent antibody test (IFA) to measure antibodies to herpes simplex virus (HSV), we found that all human sera gave a positive reaction with Chang liver cells infected with type 1 (HSV). All sera gave equivalent titers of 320-640 for acetone-fixed cells and about 40 for live cells (membrane fluorescence) in the presence of fluorescein-labeled antisera to human Ig; none of the sera reacted with uninfected cells. The fluorescence seen in fixed cells was primarily cytoplasmic; some cells showed a diffuse fluorescence, obscuring the demarcation between the nucleus and cytoplasm. Purified IgG from antibody-negative human sera and a purified Fc fragment of IgG were positive both for cytoplasmic and membrane fluorescence, whereas F(ab')2, IgM and IgA were unreactive. The reaction was also seen when an antiserum conjugate specific for the Fc fragment of IgG was used. The reactive IgG was present in freshly prepared plasma and serum; it could not be removed from serum either by ultracentrifugation or by serial absorption with HSV-infected cells. These findings suggest that the nonspecificity of the IFA results from the formation of low-avidity bonds between the large mass of native serum IgG and an Fc receptor on the plasma membrane and in the cytoplasm of cells infected with HSV. The results also suggest that extreme caution be exercised in attempting to use the IFA in the serodiagnosis of infections with HSV and perhaps the other human herpes-viruses.