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Delayed-type hypersensitivity in mouse hepatitis virus infection was studied with intraperitoneally infected mice. Twenty-four hours after eliciting injection with virus-infected cell lysate on day 8 postinfection, specific footpad swelling response was observed with infiltration of mononuclear cells. The response was not enhanced by cyclophosphamide treatment made 2 days before infection. The adoptive transfer of the response from infected mice to H-2 compatible recipients was successful with spleen T cells with Lyt 2-.

The mechanisms underlying delayed-type hypersensitivity (DTH) response in mouse hepatitis virus (MHV) infection was studied using high responder B10.D2 and low responder DBA/2 mice. Bone-marrow chimeras expressed the DTH responses at the same degree as the donors. The involvement of suppressor cells in low DTH response in DBA/2 mice seems unlikely since transfer of DBA/2 spleen cells to B10.D2 mice did not suppress the response. Transfer of B10.D2 spleen cells to DBA/2 mice caused no changes in DTH response of the recipients. MHV-infected mice, which were irradiated with 500 rad of gamma-ray and then given B10.D2 bone-marrow cells 8 days p.i., showed higher DTH responses than those given DBA/2 bone-marrow cells. These results suggest that the difference in DTH responsiveness to MHV between the two mouse strains is dependent on the activity of bone-marrow cells.

The effect of transfer of spleen lymphocytes from euthymic heterozygote littermates (nu/+) or rabbit antiserum on Pneumocystis carinii (P. carinii) infection in nude (nu/nu) mice was investigated. After intravenous administration of nu/+ spleen T-cells, the number of cysts in the lung of previously infected mice was markedly decreased at day 14 and further at day 30. Intense cellular reactions were seen at day 14 and subsided lesions at day 30. Increased IgG antibody titers were observed at days 14 and 30. Multiple treatments with immune rabbit serum after infection were not effective for reducing the cyst number in the lung, neither enhanced cellular reaction, while the immune serum administered prior to infection prolonged the survival time. The results indicated an important role of T-cell mediated immunity for the resistance to P. carinii infection in mice.