Characterisation of eosinophil heterogeneity in models of murine and human helminth infection

Eosinophils are innate immune cells typically associated with Type 2 immune responses to parasite infections and allergic diseases. With recent scientific advances, researchers have been able to probe eosinophil function in more depth, with accumulating evidence suggesting that eosinophils are in fact multifaceted leukocytes involved in a variety of functions. These include mediating various homeostatic processes, roles in host defence against parasitic, bacterial, and viral infections, and participating in pathologic eosinophil-associated diseases. This functional diversity highlights the heterogeneity of eosinophils, prompting investigation into the existence of multiple, diverse eosinophil subsets.

The first aim of this thesis was to examine eosinophil heterogeneity and subtypes across multiple tissue sites, utilising murine and human models of helminth infection to drive robust eosinophil responses. In this thesis I show that helminth infection alters the recruitment potential, activation, and metabolic profiles of circulating eosinophils. I demonstrate that helminth infection-induced eosinophilia is primed at the level of the bone marrow (BM), with helminth infection inducing eosinophil maturation in the BM. In the lung compartment I characterise two distinct subsets of eosinophils, phenotypically defined as CD101^low and CD101^high eosinophils, which are phenotypically and metabolically distinct and have distinct anatomical localisation within the lung. Furthermore, I investigate eosinophil heterogeneity within the small intestine (SI), which is where the majority of eosinophils reside during homeostasis and is the site of helminth infection. I report that SI eosinophils display a unique tissue-specific phenotype that is maintained during helminth infection.

The second aim of this thesis was to delineate the signals that mediate eosinophil responses during helminth infection. I report that the Type 2 cytokines, interleukin (IL)-4, IL-5, IL-13 and eotaxin, play differential roles in directing eosinophil immune responses. Specifically, I show that IL-5 drives eosinophilia systemically and within the lung, while IL-13 is required for the induction of CD101^high lung tissue-infiltrating eosinophils.

The final aim of this thesis was to expand the clinical relevance of my data by identifying clinical biomarkers of eosinophil tissue infiltration and activation within the intestine. The identification of eosinophil-associated biomarkers is critical for progressing our understanding of the roles of eosinophils in health and disease, as well as informing the development of therapeutics for eosinophil- associated diseases. I report that levels of the eosinophil degranulation proteins, eosinophil cationic protein (ECP) and eosinophil-derived neurotoxin (EDN), and the biomarker, faecal calprotectin (fCal), are elevated in the faeces during human hookworm infection. Accordingly, these proteins represent potential clinical biomarkers of eosinophil accumulation and activation within the intestine. I also investigate the associations between these biomarkers with clinical outcomes of controlled human hookworm infection. My results show that fCal levels are significantly associated with hookworm infection burden and hookworm infection-associated symptoms, indicating that fCal could potentially act as a clinical biomarker of hookworm infection burden and responses. Overall, this thesis presents a comprehensive characterisation of eosinophil heterogeneity across tissues, identifies distinct eosinophil subsets, and provides insight into eosinophil immune responses and the mechanisms that drive them.