Chlamydia trachomatis (CT), an obligate intracellular bacterium that is the causative agent of chlamydia, invades and replicates within epithelial cells of the eye, pharynx, urogenital tract, and rectum. Termed the “silent killer,” CT hides within the host epithelial cell and causes chronic, often asymptomatic, infection and sub-clinical inflammation, which can lead to pelvic inflammatory disease, ectopic pregnancy, and tubal-factor infertility. To identify and treat asymptomatic infections in order to prevent complications, annual screening of all women younger than 25 is recommended.

Unfortunately, even with effective treatment, approximately 20% of women are re-infected with chlamydia within 6 months, and the rates of chlamydia within the U.S. and worldwide continue to rise and are at an all-time high. Although urgently needed, no vaccine is currently available, in part due to our poor understanding of how immune responses clear CT infection. In order to design and tailor a vaccine and novel antibiotics, we need to better understand how CT evades the immune system.

Nearly half of infected women are still infected with chlamydia a year after diagnosis, demonstrating how successful CT is at evading the immune system.  Approximately 20% of women appear to be able to naturally clear chlamydia in the 2-week interval between a positive chlamydia test (an RNA-based nucleic acid amplification test) and returning to clinic for treatment. This group of women is four times less likely to be re-infected within 6 months when compared with women who are unable to clear chlamydia without antibiotics, even after adjusting for all potential confounders. This suggests that natural clearance of CT infection may be a surrogate marker for effective immune-mediated clearance. Understanding the mechanisms of how this natural clearance occurs is a key step toward the development of novel antibiotics and a chlamydia vaccine.

Although multiple mechanisms of immune evasion are likely used by CT to avoid clearance, one major identified pathway is the ability of CT to escape clearance by synthesizing its own tryptophan, a required amino acid for CT growth. Elegant in vitro studies have found that the cytokine interferon-gamma (IFN-g), released by immune cells, signals intracellular depletion of tryptophan, which starves CT (a tryptophan auxotroph) and leads to CT cell death. To circumvent this pathway, urogenital CT strains contain a tryptophan synthase gene, which allows them to respond to tryptophan depletion by synthesizing their own tryptophan using the pre-cursor molecule indole. It is uncertain whether this pathway is involved in natural clearance of chlamydia in vivo.

To study whether the IFN-g-tryptophan-indole pathway influences natural clearance of chlamydia in humans, we enrolled women with and without natural clearance of chlamydia and collected vaginal secretions by cervicovaginal lavage (CVL). We then matched 72 women (36 women who naturally cleared CT matched to 36 women with persisting CT infection) by age, race, and co-infections and attempted to measure IFN-g, tryptophan, and indole. We hypothesized that women who naturally cleared CT infection would have lower CVL tryptophan and possibly higher IFN-g levels, compared with women with persisting chlamydia.

We found that women who naturally cleared chlamydia had significantly lower CVL levels of tryptophan. Indole was unable to be measured due to evaporation. IFN-g was slightly higher, though not statistically significant, in women with persisting infection. Though this appears counter to our hypothesis, it likely reflects the active inflammation still present in women with persisting chlamydia, which had resolved in women who cleared CT infection. Co-infection with bacterial vaginosis was strongly associated with lower tryptophan levels as well.

Our study confirms, for the first time in humans, that tryptophan metabolism likely plays an important role in natural clearance of CT infection. Future research to determine if tryptophan levels are the result of IFN-g signaling, inadequate tryptophan synthesis by CT, or possibly increased uptake by competing vaginal microbes (eg lower tryptophan levels were associated with BV co-infection) are needed. Studies are also needed to assess whether tryptophan supplementation influences CT clearance. It’s clear to us that CT survival is influenced by the vaginal milieu and that understanding the mechanisms of natural clearance may lead to the development of novel antibiotics and potentially an effective chlamydia vaccine.