Delayed-type hypersensitivity against Semliki Forest virus in mice.

After intracutaneous immunization with purified inactivated Semliki Forest virus, a delayed-type hypersensitivity without detectable antibodies in serum was obtained in BALB/c mice. Low doses of antigen given intraperitoneally induced antibodies. Intracutaneous immunization with much higher doses induced no specific antibodies, but a footpad swelling was observed after challenge with homolgous antigen. Pretreatment with cyclophosphamide before immunization enhanced footpad swelling. Microscopic examination of footpads from sensitized mice at 24 h after challenge showed a mononuclear infiltrate. The delayed-type reaction could be transferred to syngenic mice with lymph node cells, but not with spleen cells or serum. The biphasic character of the delayed-type hypersensitivity is discussed.