Physicochemical and serological characteristics of respiratory virus fluorescein-isothiocyanate conjugates for fluorescent-antibody diagnosis.

Fluorescein-isothiocyanate (FITC) conjugates were prepared by improved methods from standard reference antisera to influenza A and B, mumps, parainfluenza 1, 2, 3, and 4, herpesvirus, respiratory syncytial virus, and adenovirus hexon. The antisera, prepared in a variety of animals, were fractionated three times with selected optimal concentrations of ammonium sulfate and yielded gamma globulins of adequate purity for conjugation with FITC. Conjugates containing optimal fluorescein-to-protein ratios of between 5 and 10 were produced in 2 h by dialysis labeling. Serological titers of each antiserum and conjugate were determined by complement fixation, hemagglutination-inhibition, serum neutralization, and indirect hemagglutination tests where appropriate. When corrected for dilution, the serological titers of the FITC conjugates were identical to those of the starting antisera. The fluorescent-antibody staining titers correlated well with one of the serological parameters of the original serum. The conjugates stained homologous antigens specifically and were free of nonspecific staining at the working dilution. Undesired staining of host cells which was a problem with some of the conjugates produced from sera containing cellular antibodies was removed by absorption with packed cells. These physicochemical and serological findings were then used as a guide in preparing high quality reagents for fluorescent-antibody identification of respiratory viruses.