Infection enhancement of influenza A H1 subtype viruses in macrophage‐like P388D1 cells by cross‐reactive antibodies

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Abstract
The contribution of cross‐reactive hemagglutination inhibition (HI) antibodies to infection enhancement of influenza A H1 subtype NWS virus and two antigenic drift strains was investigated in a macrophage‐like cell line P388D1. When P388D1 cells, previously treated with neuraminidase (NA) to remove the viral receptors, were infected with NWS virus exposed to rabbit antiviral immunoglobulin (IgG) showing various levels of cross‐HI titers, virus yields were enhanced in the presence of a subneutralizing antibody, depending on their cross‐HI titers. By flow cytometric analysis using a fluorescein isothiocyanate (FITC)‐labeled NWS virus, the efficiency of attachment of virus‐rabbit IgG complexes to Fc receptors on NA‐treated cells showed close correlation with its cross‐HI titer. These data suggest that cross‐reactive HI antibodies could contribute to infection enhancement through the formation of potent infectious immune complexes with drift strains to mediate virus infection via Fc receptor uptake. Two monoclonal antibodies (mAB) in mouse IgG subclasses IgG1 and IgG2a showing strain‐specific or cross‐reactive HI activity were tested for their infection enhancement characteristics. A strain‐specific mAB enhanced infection of homologous NWS virus, but not that of two other drift strains in either antibody dilution. In contrast, a cross‐reactive mAB caused infection enhancement of all three virus strains in the presence of the subneutralizing antibody. This indicates that cross‐reactivity, but not the IgG subclass, acts as an enhancing factor to this phenomenon. The antibody, with the same specificity as cross‐reactive mAB, was detected semiquantitatively by competitive enzyme‐linked immunosorbent assay (ELISA) with results almost consistent with cross‐HI titers of polyclonal rabbit antiviral IgGs. These data suggest that the antibody detected by this assay might be one of the potent antibodies governing cross‐HI activity as a whole antibody and causing infection enhancement of drift strains.