C3H Male Mice with Severe Combined Immunodeficiency Cannot Clear a Urethral Infection with a Human Serovar of Chlamydia trachomatis

ABSTRACT The pathogenesis of an infection of the male genitourinary tract of mice with a human serovar of Chlamydia trachomatis has not been characterized. To establish a new model, we inoculated C3H/HeN (H-2 k ) mice in the meatus urethra with C. trachomatis serovar D. To determine the 50% infectious dose (ID50), male mice were inoculated with doses ranging from 102 to 106 inclusion-forming units (IFU). The mice were euthanized 10 days post infection (p.i.), and the urethra, bladder, epididimydes, and testes were cultured for Chlamydia. Positive cultures were obtained from the urethra, urinary bladder, and epididimydes, and the ID50 was determined to be 5 × 104 IFU/mouse. Subsequently, to characterize the course of the infection, wild-type (WT) and C3H animals with severe combined immunodeficiency (SCID animals) were inoculated with 106 IFU/mouse (20 times the ID50). In the WT mice, the infection peaked in the second week, and by 42 days p.i., it was cleared. In contrast, most of the SCID mice continued to have positive cultures at 60 days p.i. C. trachomatis-specific antibodies were first detected in WT animals' sera at 21 days p.i. and increased until 42 days p.i. The immunoglobulin G2a (IgG2a) titers were 32-fold higher than those of IgG1, indicative of a Th1-biased immune response. A lymphoproliferative assay using splenocytes showed a significant cell-mediated immune response in the WT mice. As expected, no humoral or cell-mediated immune responses were observed in the SCID animals. In conclusion, inoculation of WT male mice in the meatus urethra with a human serovar of C. trachomatis resulted in a limited infection mainly localized to the lower genitourinary tract. On the other hand, SCID animals could not clear the infection, suggesting that in male mice, the adaptive immune response is necessary to control an infection with a C. trachomatis human serovar.