The Immunomodulatory Effects of E. coliHeat-Labile Enterotoxins on the Respiratory Immune

Abstract Heat-Labile Enterotoxins (HLTs) are bacterial secreted proteins with unique immunomodulatory properties. Type II HLTs are produced by E. coli and have been found to possess mucosal and systemic adjuvant properties, but they have never been investigated in respiratory-specific responses. The goal of this study was to determine the impact of three Type II HLTs (LT-IIa, LT-IIb, LT-IIc) on the respiratory-specific immune response. Cell cultures of the human lung epithelial cell line, A549, were stimulated with LT-IIa, LT-IIb, or LT-IIc, in the presence or absence of lipopolysaccharide (LPS), a known immunostimulant. Cell viability was measured by flow cytometry, with cell morphology observations via light microscopy. Immune responses were evaluated via quantitative polymerase chain reaction (qPCR) to quantify RNA expression or enzyme-linked immunosorbent assay (ELISA) for cytokine secretion at 6 and 18 hours post treatment. HLT treatment resulted in no significant changes to cell viability, although cytoplasmic inclusions were consistently visible in LT-IIc-treated cells. IL-6, a marker of inflammation, showed reduced RNA expression in HLT-treated cells at 6 hrs with a return to baseline by 18 hrs. IL-6 cytokine secretion in HLT-treated cells was also decreased, with the greatest reduction at 18 hrs of treatment. Thus, immune responses triggered by HLTs are potentially less inflammatory than those induced by LPS, although a more in-depth characterization of the expression of other cytokines is needed. These results demonstrate that Type II HLTs drive a modified, and potentially less inflammatory, respiratory immune response compared to LPS, which could have significant impacts in the treatment of inflammatory lung conditions. Acknowledgement: This work was funded in part by a Transformative Seed Grant from the Health Sciences Center at Prisma Health and a University of South Carolina School of Medicine Greenville Summer Research Scholars Fellowship awarded to MR.