The role of antibody in host defense against the agent of mouse pneumonitis.

Athymic nude (nu/nu) mice, which are more susceptible to the agent of mouse pneumonitis (MoPn) (murine Chlamydia trachomatis) than are their heterozygous littermates (nu/+), do not develop significant IgG or IgA antibody to the MoPn agent. Nu/+ mice develop significant titers of both specific IgG or IgA antibodies after either intranasal or intrauterine challenge. Prior intranasal or intrauterine immunization protects nu/+ but not nu/nu mice against subsequent intranasal challenge with the MoPn agent. Resistance to pneumonia due to the MoPn agent can be transferred to nu/nu mice by nu/+ mouse immune serum (with high titers of IgG and IgA antibodies to the MoPn agent) but not by nu/+ mouse normal serum (with undetectable titers of IgG and IgA antibodies to the MoPn agent). Serum components, probably specific antibodies, are important in host defense against the MoPn agent.