Immunization with a multisubunit vaccine induces immunological memory and cross-protective immunity against heterologous Chlamydia serovars.

We previously reported that intramuscular (IM) and intranasal (IN) immunization with a recombinant Vibrio cholerae (rVCG)-based multiepitope subunit vaccine (rVCG-MECA) elicited robust long-lasting, cellular, and humoral immune effectors that protected mice against transcervical (TC) challenge infection with the homologous C. trachomatis (CT) serovar D. In this study, we evaluated the ability of this vaccine to confer cross-protection against primary and secondary infection with the heterologous CT serovars F and J and induce cross-reactive immunological memory following reinfection. Thus, groups of female C57BL/6 mice were immunized and boosted twice IM or IN 2 weeks apart with rVCG-MECA or PBS or unimmunized (naïve) controls. Vaccine immunization elicited serum immunoglobulin G (IgG) antibodies with moderate to high avidity. Four weeks postimmunization, mice were challenged transcervically with 1 × 106 IFU of CT serovar F or J and rechallenged 10 weeks after initial challenge. The humoral and cell-mediated immune responses elicited at weeks 1 and 4 following rechallenge were evaluated. Irrespective of route of immunization, all immunized mice had significantly (p < 0.0001) reduced genital bacterial burden beginning as early as day 3 following primary challenge infection with the heterologous serovars F and J and completely the infection by day 21. However, PBS control mice still shed high numbers of Chlamydia at this time point. Also, mice rechallenged with serovars F and J 70 days after the primary challenge completely cleared the secondary infection by day 15. Furthermore, rechallenge of mice with serovars F and J resulted in the elicitation of high levels of chlamydial-specific IgG2a and IgA antibodies in sera and vaginal secretions as well as the production of high concentrations of the T helper-1 (Th1) cytokines, interferon-γ (IFN-γ) and interleukin-12 (IL-12), and low levels of the Th2 cytokine, IL-4, in the spleen and draining lymph nodes. These findings demonstrate that rVCG-MECA induces durable, cross-protective immunity against multiple CT serovars, indicating its potential as a promising universal vaccine candidate for the prevention of genital chlamydial infections.