Human colonization by Streptococcus pneumoniae is a prerequisite for transmission and pneumococcal disease, causing 800,000 deaths/year. It is not fully understood how the bacteria transitions to cause invasive disease. The first line of defense occurs during interactions with the nasopharyngeal epithelium where carriage induces protective immunity. We have previously demonstrated using an experimental human pneumococcal challenge model (EHPC) and in vitro culture models, that pneumococci form surface micro-colonies and micro-invade nasal epithelial cells. Epithelial cell transcriptomic activation responses coincide with bacterial clearance in the EHPC, suggesting that the epithelium is central to the control of pneumococcal colonization.
In this study, we characterized the impact of a serotype 6B strain with deletions in pneumococcal biosynthesis genes (ΔX/PiaA and ΔY/PiaA) on epithelial-innate immune responses in vitro. We explored the hypothesis that these mutations influence pneumococcal interactions and the epithelial cell transcriptomic profile during micro-invasion events.
Using Detroit 562 cells, we observed a ten-fold reduction in both mutants’ association to the epithelium and a five-fold reduction in transmigration, compared to wild type colony forming unit counts. However, both mutants exhibited a higher micro-invasive index, and led to elevated epithelial secretion of IL-6 and IL-8. In contrast to the similarity between the mutant strains in the functional assays, the epithelial transcriptional responses following infection revealed a strong similarity between wild type and ΔX/PiaA strains, including induction expression of cytokine and chemokine genes and NF-κB transcriptional regulators. Although ΔY/PiaA induced these pathways to a lesser extent, we also discovered enhanced intracellular signaling, sorting and processing responses, which associated with elevated IL-33 protein secretion.
These data reveal a surprising uncoupling between the host cellular transcriptional response and the ability of pneumococcal strains to cross the epithelial barrier. Using the EHPC model, we will test the mutants’ ability to establish carriage and micro-invasion and, determine the epithelial transcriptomic signatures.