Abstract
Purpose :
The polysaccharide capsule of Streptococcus pneumoniae is an important factor in the pathogenesis of pneumococcal diseases such as pneumonia; however, the capsule is not required for bacterial conjunctivitis and its role in keratitis is poorly understood. The aim of this study was to investigate the importance of capsule using an in vitro corneal cell binding assay and a topical infection model of pneumococcal keratitis.
Methods :
Adhesion to human corneal epithelial cells (HCECs) was determined for a keratitis strain of S. pneumoniae, K1544, and its isogenic capsule-deficient mutant, K1544Δcps. Each strain was inoculated onto the scratched corneas of 6-week-old A/J mice for acute experiments evaluating bacterial recovery (4 time points) and cytokine quantitation (2 time points) and long-term experiments evaluating clinical severity (1, 2, 3, 6, 8, and 10 days) and cytokine quantitation (10 days). Forty cytokines were quantified using antibody array densitometry.
Results :
K1544Δcps binding to HCECs was significantly higher than that of K1544 (p = 0.004). Mouse corneas infected with K1544Δcps had significantly higher mean clinical scores than those infected with K1544 (p = 0.033; n ≥ 22 per group) 24 hours post-infection. Beyond 24 hours and ending on day 10, there were no significant differences in clinical scores. All eyes were sterile by day 10 regardless of strain. Four and 8 hours post-infection, the mutant maintained significantly higher loads than the parent strain (p = 0.014 and 0.024, respectively). Bacterial loads were not significantly different at 12 or 24 hours. Protein levels of ocular IL-9, IL-10, IL-12-p70, MIG, and MIP-1-alpha were significantly higher for mice infected with K1544Δcps 24 hours post-infection compared to scratch controls or those infected with K1544 (p ≤ 0.043; n = 3 per group). Cytokine levels were not significantly different 8 hours or 10 days post-infection.
Conclusions :
Enhanced binding to corneal epithelial cells, retention of bacterial loads early in infection, and slightly higher clinical scores combined with an increase in cytokines one day after infection, suggest that non-encapsulated S. pneumoniae may have enhanced virulence in the cornea in the early stages of keratitis.
This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.