Mucosal defense mechanisms against amebic colitis

Entamoeba histolytica is an enteric parasite, the causative agent of amebiasis, and a significant cause of diarrhea in infants in low income countries. E. histolytica adheres to host cells by a parasite Gal/GalNAc lectin and disrupts the mucosal barrier via a unique process named amebic trogocytosis, penetrating underlying tissue and destroying cells. Host responses at the site of infection are critical for resistance to the ameba.

` IL-25 is a cytokine that is produced by intestinal epithelial cells in response to the gut microbiome and is known to help maintain gut barrier function in colitis due to Clostridium difficile. We discovered that IL-25 expression is decreased in colon biopsy tissue from patients with amebic colitis. We also observed decreased IL-25 in the cecum during E. histolytica infection in the mouse model of amebic colitis. We hypothesized that IL-25 protects the intestinal epithelium from invasion by E. histolytica. To test this hypothesis we administrated recombinant IL-25 in mice infected with ameba. We found that rIL-25 treated mice had a significantly lower infection rate as measured via culture, ELISA and quantitative qPCR. Histologically, there was significantly less epithelial disruption in rIL-25 treated mice. We further found that IL-25 mediated protection was eosinophil dependent. When eosinophils were depleted with anti-Siglec-F, IL-25 administration was no longer protective.

In order to understand how eosinophils protect, we tested for the impact of IL-25 on TNFα. We found that patients with amebic colitis had a greater amount of TNFα in the intestine as measured by immunohistochemistry. rIL-25 administration suppressed TNFα induction in E. histolytica challenged mice. Depletion of TNFα with monoclonal antibodies in mice resulted in resistance to ameba infection. Therefore, our research suggests that IL-25 may provide protection from amebiasis via two potential pathways, induction of protective Th2 responses, via eosinophils, and by suppression of inflammatory TNFα during infection.

We also considered whether antimicrobial peptide production was a mechanism of IL-25 mediated protection. Antimicrobial peptides play a crucial role in allowing epithelial cells to manage colonization with beneficial and pathogenic microorganisms. IL-25 is known to induce production of the antimicrobial peptide angiogenin-4. IL-25 mediated induction of angiogenin-4 was dependent upon IL-13 but not dependent on IL-22 or IL-17. During amebiasis, mice treated with IL-25 had increased IL-13. In these mice IL-13 neutralization abrogated angiogenin-4 production, however there was no change in susceptibility to amebiasis. We concluded that angiogenin-4 was not required for rIL-25 mediated protection against amebiasis.

In conclusion, this work identified a unique protection mechanism against amebiasis via IL-25 induced eosinophilia and TNF suppression. Understanding how the host immune response influences the infection outcome of amebiasis promises to provide new avenues to the treatment or prevention of this parasitic cause of diarrhea.