Properties of mouse leukemia viruses: IV. Hemagglutination assay and characterization of hemagglutinating surface components

A hemagglutination (HA) assay for mouse leukemia virus (MuLV) concentrates is described. The assay gives linear dose-response data between concentrations of approximately 10 11 and 8 × 10 12 particles per milliliter. One HA unit was equivalent to approximately 5 sX 10 7 virus particles in those preparations where surface projections (knobs) were readily detectable on the virions. When knobs were removed during purification or by bromelain treatment, the HA activity was reduced or eliminated. Bromelain-treated virus also lacked glycoprotein component (s), did not absorb neutralizing MuLV antibody, and was noninfectious. Thus, the HA substructure is clearly a surface component of the virion and has an apparent association with the surface knobs, glycoprotein (s), and type-specific antigens. Treatment with neuraminidase and phospholipase C, as required for demonstration of HA activity, damaged the envelope of the virion but did not release hemagglutinating subunits or surface knobs. Active hemagglutinin recovered from enzyme-treated virus by degradation with Tween 80 and ether and density gradient centrifugation was stable under various physical treatments but was heat sensitive. Active hemagglutinin could also be recovered from non-enzyme-treated virus by similar procedures. Concentrated preparations of avian myeloblastosis virus and feline leukemia virus agglutinated chicken and sheep erythrocytes, respectively, but in both cases the specific HA activities were low and the reactions could not be inhibited by specific antisera.