Ocular immune responses, Chlamydia trachomatis infection and clinical signs of trachoma before and after azithromycin mass drug administration in a treatment naive trachoma-endemic Tanzanian community

Background Trachoma, caused by Chlamydia trachomatis, remains the leading infectious cause of blindness worldwide. Persistence and progression of the resulting clinical disease appears to be an immunologically mediated process. Azithromycin, which is distributed at the community level for trachoma control, has immunomodulatory properties. We investigated the impact of one round of oral azithromycin on conjunctival immune responses, C. trachomatis infection and clinical signs three- and six- months post treatment relative to three pre-treatment time-points. Methodology A cohort of children aged 6 to 10 years were recruited from a trachoma endemic region of northern Tanzania and were visited five times in a 12-month period. They were examined for clinical signs of trachoma and conjunctival swabs were collected for laboratory analysis. C. trachomatis infection was detected and the expression of 46 host genes was quantified using quantitative PCR. All community members were offered azithromycin treatment immediately after the six-month timepoint according to international guidelines. Findings The prevalence of C. trachomatis infection and inflammatory disease signs were significantly reduced three- and six- months post-mass drug administration (MDA). C. trachomatis infection was strongly associated with clinical signs at all five time-points. A profound anti-inflammatory effect on conjunctival gene expression was observed 3 months post-MDA, however, gene expression had largely returned to pre-treatment levels of variation by 6 months. This effect was less marked, but still observed, after adjusting for C. trachomatis infection and when the analysis was restricted to individuals who were free from both infection and clinical disease at all five time-points. Interestingly, a modest effect was also observed in individuals who did not receive treatment. Conclusion Conjunctival inflammation is the major clinical risk factor for progressive scarring trachoma, therefore, the reduction in inflammation associated with azithromycin treatment may be beneficial in limiting the development of potentially blinding disease sequelae. Future work should seek to determine whether this effect is mediated directly through inhibition of pro-inflammatory intracellular signalling molecules, through reductions in concurrent, sub-clinical infections, and/or through reduction of infection exposure. Author summary Trachoma, caused by conjunctival infection with Chlamydia trachomatis, remains the leading infectious cause of blindness. Repeated infection during childhood can trigger prolonged inflammation, which is the main risk factor for conjunctival scarring. Azithromycin is distributed globally for trachoma control, however it is also widely reported to have immunomodulatory properties. This report investigated the impact of one round of oral azithromycin for trachoma control on conjunctival immune responses, clinical signs and C. trachomatis infection in Tanzanian children. A large anti-inflammatory effect of azithromycin on conjunctival gene expression was observed 3 months post-treatment, however, gene expression patterns had mostly resumed to pre-treatment levels by 6 months. The effect was evident after adjusting for C. trachomatis infection and when analysis was restricted to uninfected individuals, however it was also observed to a lesser extent in individuals that did not receive treatment. These findings suggest that azithromycin may have a direct immunomodulatory effect on conjunctival gene expression but that it may also reduce inflammation by reducing exposure to C. trachomatis and other infections. This anti-inflammatory effect could have therapeutic potential in limiting the development of disease sequelae, that goes beyond its effect on the clearance of ocular C. trachomatis infection.