Neutral red-labeled influenza virus loses photosensitivity during absorption to host cells but not to erythrocytes.

Neutral red (NR)-labeled influenza virus is extremely photosensitive. Unlike NR-labeled picornaviruses which lose their photosensitivity only after penetrating the host cell, NR-labeled influenza virus loses most of its photosensitivity during adsorption at 4 degrees C. We demonstrate that the underlying reaction occurs within seconds of adsorption and that it is irreversible, i.e., NR virus eluted from chick embryo cells after adsorption is hardly photosensitive anymore. In contrast to this, NR virus adsorbed to and eluted from erythrocytes retains its original photosensitivity. We suggest that the loss of photosensitivity during adsorption of NR virus to host cells reflects a conformational change in the virion which is not elicited by adsorption to red blood cells.