THE RAPID DETECTION, TITRATION, AND DIFFERENTIATION OF VARIOLA AND VACCINIA VIRUSES BY A FLUORESCENT ANTIBODY-COVERSLIP CELL MONOLAYER SYSTEM

Assessment of virus infectivity by the immunofluorescent and immunoperoxidase techniques

The immunofluorescent and immunoperoxidase cell-counting techniques were comparable in precision and reproducibility for the quantitative assessment of influenza virus infectivity. The dose-response function was linear with each procedure, and comparable results were obtained for estimating neutralizing antibodies in antiviral serum.

Cell attachment and penetration by influenza virus

Attachment and penetration of influenza virus into clone 1-5C-4 cells were quantitatively determined by the immunofluorescent cell-counting assay. Aided by centrifugal force, more than 95% of virus inocula of five representative influenza virus strains (A(0)/PR8, A(1)/Ann Arbor, A(2)/Japan, B/Lee, B/Great Lakes) were attached to cells at a linear rate within 10 min, in contrast to approximately 35% after stationary incubation at 35 C for 2 h. By the former procedure, a proportionality between the number of infected cells and volume of inoculum was revealed which was not evident when stationary incubation was employed. Maximal binding of virus to cells occurred at 0.2 M NaCl. The salt requirement, added to evidence of pH dependence and temperature independence, indicated that the initial virus-cell union involved electrostatic forces. Virus penetration into cells, measured by the insensitivity of virus-cell complexes to antiviral serum, was linear and complete within 15 min at 35 C for all five virus strains tested. Maximal virus penetration occurred at 0.1 to 0.2 M NaCl; the process was pH- and temperature-dependent. Both virus attachment and penetration processes were partially inhibited in the presence of diethylaminoethyl-dextran.

Comparison of the immunofluorescent-cell counting and plaque methods for the assay of vaccinia virus

The immunofluorescent-cell counting method was compared with the agarand liquid-overlay plaque techniques for the assay of vaccinia virus. In addition to being as quantitative, reproducible, and simple to use as the two plaque techniques, the immunofluorescent assay method was found to be more sensitive, rapid, and specific.

Accuracy of smallpox diagnosis by immunfluorescence with a purified conjugate

A direct fluorescent-antibody test for smallpox is described which utilizes a conjugated antivaccinia serum that was purified by diethylaminoethyl fractionation. The purity of the conjugate was analyzed by density gradient centrifugation, and specific staining of smallpox and nonsmallpox specimens was measured quantitatively by a photovoltmeter. Variola elementary bodies were identified in all of the specimens of vesicular and pustular fluid collected from 50 smallpox patients, and no false-positive diagnoses were made on specimens from 27 patients with varicella or other nonpox viral exanthems. However, 63.8% of the specimens of vesicular and pustular fluid smeared on slides in the field were unusable because of bright nonspecific fluorescence. For this reason, the fluorescent-antibody test does not compare favorably with other routine laboratory tests for smallpox.

Immunofluorescence in the study of viruses in tissue culture. I. Development of an immunofluorescent focus assay for herpes simplex virus

A rapid and sensitive assay method has been developed for herpes simplex virus in kidney cells of Cercopithecus aethiops. Using immunofluorescence, foci of infection produced by this virus can be scored within 20–22 hours after inoculation. It has been demonstrated that this procedure gives reproducible results and exhibits a direct relationship between virus concentration and the number of fluorescent foci. The technique is slightly more sensitive than the conventional plaque assay method using the same host system.

Primary virus-cell interactions in the immunofluorescence assay of Venezuelan equine encephalomyelitis virus

The conditions under which Venezuelan equine encephalomyelitis (VEE) virus attached to host cells markedly influenced the assay of virus by the fluorescent cell-counting technique. When virus inoculum was centrifuged onto McCoy cell monolayers, approximately 97% of virus was attached to cells within 10 min, in contrast to 34% after stationary incubation at 35 C for 2 hr. Maximal binding of virus occurred only in the presence of 0.1 to 0.15 m NaCl. This salt requirement, added to evidence of (p)H dependence and temperature independence of VEE virus attachment to cells, indicated that the initial union involved electrostatic forces. Virus penetration, measured by the insensitivity of virus-cell complexes to viral antiserum, was complete in 30 min at 35 C. The process was temperature-dependent and un-affected by the ionic content of medium. For assay of VEE virus by the fluorescent cell-counting technique, infected cells may be enumerated as early as 12 hr after infection of cell monolayers. The relationship between virus concentration and cell-infecting units was linear; the distribution of fluorescent cells was random. The virus assay was equivalent in sensitivity but more precise and rapid than that of intracerebral inoculation of mice.