This study demonstrates that BALB/c mice are more able to limit ascending genital tract infection with C. trachomatis
NI1 than are C3H mice and suggests that this control is associated with the recruitment of MHC class II cells into uterine tissue early in infection. Previously, we demonstrated that introduction of the organism directly into the uterus produces severe salpingitis and infertility in C3H mice but milder inflammation and no impact on fertility in BALB/c mice (32
). In the current study we compared ascending infection following i.vag. inoculation to mirror natural infection and found evidence for differences in susceptibility. Spread of the organism into the uterus occurred early in infection in both strains, but in the BALB/c mice the organism was cleared from the tissues with concomitant resolution of inflammation. In contrast, in C3H mice the organism or components of it persisted for at least 3 weeks after infection and the inflammation did not resolve.
A comparison of ascending infection in wild-type BALB/c and athymic BALB/c nu/nu
mice illustrated the importance of the host immune response and demonstrated the ability of C. trachomatis
NI1 to persist in BALB/c tissue and provide an inflammatory stimulus in the absence of such a response. Shedding of bacteria, isolation from uterine homogenates, and histological changes were all greater in nude mice. This is consistent with the earlier finding (34
) that, after unilateral i.u. injection, spread to the contralateral uterine horn is more common in nude mice. These data suggest that the control of ascending infection involves T-cell-dependent mechanisms, which is consistent with findings in other models of genital chlamydial infection. We are unable to conclude that inflammatory disease is T-cell independent in our model since low numbers of both αβ and γδ T cells were detected in the uterine tissue of our infected nude mice. Oviduct inflammation was only noted in the immunodeficient BALB/c nu/nu
mice during the 3-week period in this study. We chose a limited time course to enable the estrus cycle to be halted throughout without the need for additional hormone injections, which can induce genital tract pathology (unpublished observation). With a longer time course oviduct disease may also have developed in the C3H mice.
The findings that the host immune response is essential to limiting ascending infection and that BALB/c and C3H mice differ in their susceptibilities suggested that there may be a difference in the immune response of these two strains. We have attempted to identify this difference. In lung infection experiments with the mouse pneumonitis (MoPn) biovar of C. trachomatis
, mouse strain differences in susceptibility have been identified (39
), but in this case BALB/c display high susceptibility. Differences correlate with differences in cytokine production: susceptibility is associated with a shift away from the production of IFN-γ and towards the production of IL-10 (39
). Treatment with anti-IL-10 increases clearance of MoPn. In our model of ascending genital tract infection there was no evidence for differences in cytokine production. Draining lymph node cells from both BALB/c and C3H mice yielded predominantly IFN-γ and TNF-α with a little IL-10. Although IFN-γ and TNF-α have protective effects (8
) and have also been implicated in pathology (1
), production by BALB/c and C3H lymph node cells was similar. Darville et al. have recently reported more severe genital tract disease in C3H mice than in C57 mice despite a dominant Th1 response in both strains (9
). It remains possible that local differences in genital tract tissue, which are not reflected in the draining nodes, could contribute to this variation in response. The differences between findings in the genital tract and those in the MoPn lung model are probably related to the route of infection since intravaginal inoculation of BALB/c mice with MoPn induces a Th1 response (4
Within 1 week of i.vag. inoculation there was evidence of spread of the organism into the uterus of both BALB/c and C3H mice and the accumulation of CD45+
MC in the tissue. When individual mice were examined there was a strong association between ascending infection and inflammatory changes in uterine tissue, suggesting that the accumulation of CD45+
MC represents a reaction to the presence of the organism in the upper genital tract (29
). The amounts of this early CD45+
MC infiltrate were similar in BALB/c and C3H mice, but there was a difference in composition; in BALB/c mice the infiltrate comprised a population of small agranular cells and a population of larger cells present in approximately equal numbers. In contrast, the infiltrate in C3H uteri lacked these larger cells. Phenotypic analysis revealed similar T-cell and B-cell infiltrates in the two mouse strains but only one-third the number of MHC class II-positive cells in C3H mice. This population corresponded to the large cells identified by light scatter. A comparison of infected and control BALB/c mice indicated a selective increase in large cells following i.vag. infection (a five-fold increase as opposed to a three-fold increase in small CD45+
MC), and these cells were not removed by perfusion of the genital tract tissue before isolation of MC (data not shown). Large CD45+
cells were only a minor population of BALB/c peripheral blood MC (data not shown). Collectively, these data suggest that there is a selective accumulation of large MHC class II-expressing cells in the uterine tissue of BALB/c mice which is absent or significantly reduced in the more susceptible C3H mouse strain. Blander and Amortegui (2
) reported that immunization of BALB/c mice with a detergent extract of MoPn induced a splenic T-cell response and enhanced infiltration of the genital tract upon infection, notably with eosinophils. However, this response was not protective, again suggesting that the composition of the infiltrate is important in determining the degree of protection. There may be additional differences in the composition of the infiltrates that are not revealed by this study, since the panel of markers we used only identified a maximum of approximately 70% of the CD45+
cells (Fig. ).
Expression of the costimulatory molecules CD86 and CD40, in addition to MHC class II antigens, suggests the large CD45+
MC in BALB/c uteri are a population of APC. They were able to activate naive allogeneic T cells in an MLR, albeit less potently than mature splenic DC. These cells may be equivalent to the MHC class II+
DC population identified histologically in the murine cervix and vagina (23
). Antigens introduced into the genital tract are believed to elicit systemic immune responses after acquisition by DC and transportation to the draining lymph nodes (22
). Studies with airway epithelia suggest that additional DC are recruited early in inflammatory responses (18
) in response to chemokines and chemoattractants (17
). They form part of a specialized surveillance system designed to alert the immune system to the presence of pathogens at mucosal surfaces. Infection in C3H mice may fail to induce these signals, or their DC (or DC precursors) may be poorly responsive to the signals induced.
We are currently unable to conclude with certainty that a difference in recruitment of uterine APC is the single critical factor which underlies the difference in disease susceptibility of BALB/c and C3H mice; there may be a number of factors involved. However, the failure to find differences in other aspects of the host response already demonstrated to be important in the control of chlamydial infection supports the notion that this recruitment is important. We are currently attempting to identify specific signals involved in the recruitment of DC into genital tissue during chlamydial infection in order to target these experimentally. The uterine APC population identified in BALB/c mice could play a number of important roles in the control of genital infection. These include the initiation of a systemic immune response, local presentation of chlamydial antigen in the tissue, production of cytokines or other compounds with antichlamydial effects, or an influence on the growth or differentiation of B cells. IL-12, which has a role early in chlamydial infection (25
) and drives the development of a Th1 response (reviewed in reference 25
), is produced by macrophages and DC (16
). Stimulation via CD40 is potent at inducing IL-12 production by DC (5
), and the cells of the uterine population we identified were CD40+
. In addition to their influence on T-cell responses, DC generated from progenitors in vitro can interact with activated B cells to potentiate isotype switching towards immunoglobulin A production (10
The model we have described here closely mirrors the insidious nature of human infection with a low inoculum dose, ascending infection into the upper genital tract, and a low bacterial burden. Our findings are consistent with the importance of Th1 T-cell responses described in other models but suggest that the immunological control of genital infection is more complex: the ability to clear antigen from the tissue and therefore protect against immunopathology requires recruitment of APC as well as the appropriate T-cell response.