Early presence of ribonucleoprotein antigen on surface of influenza virus-infected cells

Abstract

HOST defence against influenza A virus in experimental animals seems to be mediated mainly by antibody^1,2. The mechanism by which antibody protects the host, however, is unknown. Antibody may either neutralise viral particles or recognise viral antigens at the surface of infected cells and then act at an early stage of infection, before viral replication is completed. The latter mechanism is theoretically more efficient, but might cause immunopathological destruction of the infected cells with harmful consequences. Viral envelope antigens (haemagglutinin and neuraminidase) have been shown to be present at the surface of influenza virus-infected cells, by haemadsorption³, electron microscopy⁴ and immunofluorescence⁵. Two main type-specific internal antigens have been described in influenza A virus—the matrix protein (MP) antigen⁶ and the ribonucleoprotein (NP) antigen⁷, a non-glycosylated polypeptide with a molecular weight of 58,000 (ref. 8). Immunofluorescence studies on fixed cells have shown that NP antigen is first demonstrable in the cell nucleus and then appears as granular masses within the cytoplasm, which become prominent at the, cell borders later in infection, suggesting a close association with the cell membrane⁹. We report here that NP antigen can be detected by immunofluorescence as early as 2 h after the infection on the surface of unfixed mouse cells infected with influenza A virus.