Regulatory Functions of Hypoxia in Host-Parasite Interactions: A Focus on Enteric, Tissue, and Blood Protozoa

Body tissues are subjected to various oxygenic gradients and fluctuations and hence can become transiently hypoxic. Hypoxia-inducible factor (HIF) is the master transcriptional regulator of the cellular hypoxic response and is capable of modulating cellular metabolism, immune responses, epithelial barrier integrity, and local microbiota. Recent reports have characterized the hypoxic response to various infections. However, little is known about the role of HIF activation in the context of protozoan parasitic infections. Growing evidence suggests that tissue and blood protozoa can activate HIF and subsequent HIF target genes in the host, helping or hindering their pathogenicity. In the gut, enteric protozoa are adapted to steep longitudinal and radial oxygen gradients to complete their life cycle, yet the role of HIF during these protozoan infections remains unclear. This review focuses on the hypoxic response to protozoa and its role in the pathophysiology of parasitic infections. We also discuss how hypoxia modulates host immune responses in the context of protozoan infections.

A56 TUFT CELL RESPONSES DURING ACUTE- AND LATE-STAGE GIARDIA INFECTION

Background

Epithelial tuft cells can detect and respond to enteric infections and appear to help clear some intestinal parasites. Through tuft cell luminal surface receptors, tuft cells can sense ligands directly supplied by a parasite, or indirectly via excretory/secretory products. We hypothesize that microbiome alterations may also modulate tuft cell-derived gut responses. Tuft cells release the alarmin cytokine IL-25 which, upon acting on type 2 innate lymphoid cells (ILC2), ultimately lead to tuft and goblet cell hyperplasia. Upon helminth infections, tuft and goblet cell hyperplasia occurred concurrently, and coincided with the peak of infection. The role of tuft cells in infections with the enteric protozoan parasite Giardia sp. is unknown. The aim of our study is to characterize how tuft cells may be implicated in the pathophysiology of giardiasis, in an attempt to uncover novel regulatory pathways of intestinal physiology.

Aims

In this study, we aim to characterise the tuft cell response to Giardia infection during acute and late stages of infection and to assess goblet cell hyperplasia.

Methods

5–7-week-old C57BL/6 mice and tuft cell-deficient mice ( Pou2f3^-/-) were orally gavaged with 5x10⁴Giardia muris trophozoites and scarified at days 4, 11 and 21 post-infection. Parasite burden was assessed in the duodenum. Immunofluorescence (IF) staining of doublecortin-like kinase 1 (DCLK1) – a marker of tuft cells – was performed on C57BL/6 mice jejunum tissue sections and the number of tuft cells was quantified. Goblet cells were quantified in PAS/AB-stained jejunum sections. Quantitative PCR (qPCR) was performed on Dclk1, the epithelial secretory cell transcription factor Atoh1, and the mucus gene Muc2.

Results

G. muris infected C57BL/6 mice displayed high parasite load at days 4 ( p<0.05) and 11 ( p<0.05), with no or low parasite burden at day 21 ( p<0.05). Pou2f3^-/- mice showed less robust parasite burden at days 4 and 11, and similar low parasite burden at day 21, compared to WT mice. At day 21, tuft cell (DCLK1+) counts ( p<0.05) and Dclk1 mRNA expression levels were increased in Giardia infected mice in the jejunum. Goblet cell number and Atoh1 and Muc2 expression were increased at day 4 post-infection.

Conclusions

The data demonstrate that tuft cells expand late in Giardia infection, suggesting that upon parasite infection, tuft cells may possess roles in tissue repair or clearance of infection. Tuft cell-deficient mice ( Pou2f3^-/-) had lower parasite burden early in Giardia infection, counter-intuitively suggesting that tuft cells may facilitate trophozoite colonization, further highlighting the novelty of these findings. The crosstalk between tuft cells, Giardia, and other host responses during acute and late stages of infection remain to be fully characterised.

Funding Agencies

NSERC

Enteric Tuft Cells in Host-Parasite Interactions

Enteric tuft cells are chemosensory epithelial cells gaining attention in the field of host-parasite interactions. Expressing a repertoire of chemosensing receptors and mediators, these cells have the potential to detect lumen-dwelling helminth and protozoan parasites and coordinate epithelial, immune, and neuronal cell defenses against them. This review highlights the versatility of enteric tuft cells and sub-types thereof, showcasing nuances of tuft cell responses to different parasites, with a focus on helminths reflecting the current state of the field. The role of enteric tuft cells in irritable bowel syndrome, inflammatory bowel disease and intestinal viral infection is assessed in the context of concomitant infection with parasites. Finally, the review presents pertinent questions germane to understanding the enteric tuft cell and its role in enteric parasitic infections. There is much to be done to fully elucidate the response of this intriguing cell type to parasitic-infection and there is negligible data on the biology of the human enteric tuft cell—a glaring gap in knowledge that must be filled.

Giardia spp. and the Gut Microbiota: Dangerous Liaisons

Alteration of the intestinal microbiome by enteropathogens is commonly associated with gastrointestinal diseases and disorders and has far-reaching consequences for overall health. Significant advances have been made in understanding the role of microbial dysbiosis during intestinal infections, including infection with the protozoan parasite Giardia duodenalis, one of the most prevalent gut protozoa. Altered species composition and diversity, functional changes in the commensal microbiota, and changes to intestinal bacterial biofilm structure have all been demonstrated during the course of Giardia infection and have been implicated in Giardia pathogenesis. Conversely, the gut microbiota has been found to regulate parasite colonization and establishment and plays a critical role in immune modulation during mono and polymicrobial infections. These disruptions to the commensal microbiome may contribute to a number of acute, chronic, and post-infectious clinical manifestations of giardiasis and may account for variations in disease presentation within and between infected populations. This review discusses recent advances in characterizing Giardia-induced bacterial dysbiosis in the gut and the roles of dysbiosis in Giardia pathogenesis.