Factors concerning the immunity to poxvirus infection.

Two kinds of early antigens were shown in vaccinia-infected cells, one heat (56 degrees, 30 min)-stable (ES antigen) and the other heat-labile (EL antigen). Both antigens do not induce circulating neutralizing antibody in rabbits, but resistance to intradermal inoculation of active virus was observed when animals were immunized with either antigen in combination with Freund's complete adjuvant. Vaccinia-specific haemagglutinin (VHA) associated with infected cell membrane was purified and its antigenicity was also studied. Rabbits showed a good antibody response to VHA and some animals also showed neutralizing antibody, though in a small amount, to either intracellular (ICV) or extracellular (EVC) virus. Rabbits immunized with membrane antigens obtained from either VHA-positive or negative virus in combination with Freund's adjuvant showed skin restance to challenge with active virus. It was suggested that antibody against ECV might be responsible for the resistance.