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Fusion activity and inactivation of influenza virus: kinetics of low pH-induced fusion with cultured cells

Nejat Düzgüne^1,2,3

Dennis Alford^2,

¹ Department of Microbiology, University of the Pacific School of Dentistry, 2155 Webster Street, San Francisco, California 94115
² Cancer Research Institute
and the³ Department of Pharmaceutical Chemistry
and⁴ Department of Pathology, University of California, San Francisco, California 94143, U.S.A.
⁵ Center for Cell Biology and the Department of Chemistry, University of Coimbra, 3049 Coimbra, Portugal
and The⁶ Seagram Centre for Soil and Water Sciences, Faculty of Agriculture, The Hebrew University of Jerusalem, 76100 Rehovot, Israel

The kinetics of fusion of influenza virus (A/PR/8/34) with human promyelocytic leukaemia (HL-60), human T lymphocytic leukaemia (CEM) and murine lymphoma (S49) cells were investigated. Fusion was demonstrated by electron microscopy, and monitored by fluorescence dequenching of octadecylrhodamine incorporated in the virus membrane. Rapid fusion was induced upon mild acidification of the medium. At pH 5, all virus particles were capable of fusing with the cells. The initial rate and the extent of fusion were maximal between pH 4.9 and 5.2 and declined sharply below and above this range. The rate constants of adhesion of influenza virus to cells or erythrocyte ghosts were large, indicating a diffusion-controlled process. The rate constants of fusion of the virus with cells were smaller than those found previously for fusion with various liposomes. Although preincubation of the virus at acidic pH in the absence of target membranes almost completely inactivated the virus in its ability to fuse with erythrocyte ghosts, it reduced the extent of fusion with cultured cells by only 20 to 40%. Kinetic analysis of fusion revealed a mode of inactivation of the virus bound to erythrocyte ghosts or suspension cells, below pH 5.4, different from that of the virus preincubated at low pH without target membranes.

Present address: Department of Bioscience and Biotechnology, Drexel University, Philadelphia, Pennsylvania 19104, U.S.A.

Received 20 May 1991; accepted 28 September 1991.

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