Effect of Inflammatory Response on In Vivo Competition between Two Chlamydial Variants in the Guinea Pig Model of Inclusion Conjunctivitis

Abstract

In order to study the interaction of variants in in vivo infection, we employed an azithromycin-resistant mutant (AZ 2 ) and its wild-type parent (SP 6 ) in the guinea pig model of Chlamydia caviae conjunctival infection. When each strain was inoculated individually into conjunctiva, both attained the same level of growth, but AZ 2 elicited less pathology. However, when equal numbers of the two strains were inoculated together into the guinea pig conjunctiva, SP 6 produced a significantly greater number of inclusion-forming units than AZ 2 , and the pathology reflected that of a SP 6 monoinfection. The goal of this study was to further characterize the dynamics of concomitant infection of these two distinct variants, with particular emphasis on the impact of the host response on the in vivo growth of each organism and the development of pathology. Animals infected with AZ 2 had reduced conjunctival infiltration with CD45 ؉ cells and neutrophils as well as a reduced interleukin-8 (IL-8) response. Gene expression of gamma interferon (IFN-␥), tumor necrosis factor alpha (TNF-␣), CCL2, and CCL5 was also significantly lower in AZ 2 -infected animals. The lower inflammatory response induced by AZ 2 was associated with its decreased ability to activate NF-B via Tolllike receptor 2 (TLR2). In general, the inflammatory response in animals infected with both variants was greater than in infection with AZ 2 alone, resulting in lower numbers of AZ 2 than those of SP 6 in the mixed infection. Our results suggest that the ability to elicit an inflammatory response is an important factor in the dynamics of mixed infection with strains that display different pathological phenotypes.

Chlamydial Variants Differ in Ability To Ascend the Genital Tract in the Guinea Pig Model of Chlamydial Genital Infection

“…Even though AZ2 has a lower burst size and a lower growth rate than SP6 in vitro, no difference in the course of the infection was seen between the two variants when assessed by two-factor (days and group) analysis of variance (ANOVA) with repeated measures (Fig. 1), similar to our observations in conjunctival infection (9). We had previously observed that AZ2 elicits a weaker inflammatory response than SP6 in the conjunctiva as a result of a decreased proinflammatory cytokine response in the AZ2-infected animals.…”

“…Nevertheless, the outcome of our experiments marks a starting point for understanding how chlamydial variants compete within the context of an ascending genital tract infection. Similar to what we observed in the conjunctival model (9), the total number of chlamydiae in the cervix in the first 9 days postinfection in a mixed infection with SP6 and AZ2 was comparable to those in a monoinfection of each variant alone. However, of interest was our observation that SP6 outcompeted AZ2 in the mixture and by 9 days postinfection represented 92% of the total number of chlamydiae in the cervix.…”

“…The data from these studies strongly suggest that an inflammatory response is essential for ascending infection to occur and that in vivo, PMNs are an integral component of the chlamydial strategy for spread to new host cells and distribution to new sites. The inability of AZ2 to elicit a robust inflammatory response that is associated with decreased activation of the TLR2 pathway could explain its decreased ability to ascend the genital tract (9). The lower growth rate of AZ2 could also be a factor in reduced ascending infection; however, the kinetics of AZ2 infection versus SP6 infection in the cervix do not support this explanation.…”

“…The number of AZ2 IFU in the mixed infection was significantly lower than that of AZ2 at 9 days in a monoinfection; thus, it is unlikely that the growth rate of AZ2 alone can account for their decreased number in the mixed infection. We also observed in the conjunctival model that within the mixed-infection group, SP6 was always significantly more numerous than AZ2, although there was no difference in monoinfections of each (9). It would appear that the presence of SP6 in the mixture was a key factor in the reduced number of AZ2 IFU.…”

“…Ramsey's work clearly suggests that a selection process occurs within the host, but is that selection solely host dependent or is there competition among variants based on variant-specific parameters? We have previously reported that a virulent clonal variant of C. caviae strain SP6 and an isogenic attenuated variant, AZ2, individually produce similar courses of infection in guinea pig conjunctiva (9). However, SP6 elicits significantly more inflammation than AZ2.…”