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A murine model was utilized to study immune responses occurring during the period of acquisition of immunity to chlamydial infection. C3H (H-2k) mice were immunized by intramuscular injection of 5 x 10(3) viable Chlamydia psittaci elementary bodies (EBs) by a protocol which permits animals to survive an otherwise lethal intraperitoneal challenge 10 days later with the homologous chlamydial strain. Spleen cells assayed during the 10-day period of development to immunity showed depressed proliferative responses in vitro to the T-cell mitogen, concanavalin A, and also exhibited suppressor cell activity. Spleen cell mitogen responses returned to normal levels by 30 days postimmunization, concomitant with the detectable development in vitro of responses to chlamydia-specific antigen. In marked contrast to the reduced proliferative responses, mitogen-stimulated production of the T-cell-derived lymphokines interleukin-2 and gamma interferon by spleen cells from immunized animals was within the normal range at 10 days postimmunization, and supernatant fluids containing these products had both microbicidal and microbistatic effects on chlamydial organisms in vitro. These results demonstrate that independent regulation of T-cell proliferation and lymphokine production occurs in vivo as part of the development of an antigen-specific protective immune response. These results also suggest that such differential modulation of T-cell responses may contribute to the development of protective immunity to chlamydiae in mice, perhaps through limited T-cell clonal expansion coupled with early or preferential maturation of cytokine-secreting helper T cells.

Differential modulation of lymphocyte proliferative responses and lymphokine secretion in mice during development of immunity to Chlamydia psittaci