Abstract
The Type 2 immune response has evolved to provide host immunity against large, extracellular helminth parasites and provide homeostatic functions such as tissue repair and regeneration. When dysregulated, Type 2 immune responses mediate pathologic responses towards allergens that can result in allergic diseases including atopic dermatitis, asthma, rhinitis, and food allergies. At the centre of Type 2 immunity are CD4+ T helper (Th) 2 cells, which play a pivotal role in all phases of the Type 2 immune response including sensitisation, effector, resolution, and memory. Th2 cells are activated in the draining lymph node (dLN) after primary sensitisation with a Type 2 stimuli, before migrating to the site of antigen exposure where they perform effector functions. Th2 cells express hallmark Type 2 cytokines including interleukin 4 (IL-4) and IL-13, which have distinct effector functions in mediating the Type 2 immune response. IL-4 acts primarily in secondary lymphoid tissues and is essential for Th2 differentiation and promoting the humoral response. In comparison, IL-13 acts primarily in peripheral tissues at the site of antigen exposure and plays a critical role in mediating effector responses of non-haemopoietic cells. Despite the fundamentally distinct functions of IL-4 and IL-13, the heterogeneity of Th2 cells regarding the cytokines and effector molecules expressed is not well appreciated. This thesis dissects the intricate roles of Th2 effector (Th2eff) cells in the tissue by determining the cellular origin of skin Th2eff cells; characterising the heterogeneity between distinct skin Th2eff subsets; determining the developmental trajectory of unique Th2eff subsets; and finally, by exploring the role of memory CD4 T cells in a secondary Th2eff response that initiates allergic pathology in the skin.
By using a novel mouse strain that lacks T follicular helper (Tfh) cells this thesis shows that skin Th2eff cells are derived from non-Tfh cellular precursors in the dLN and intriguingly shows that the lack of the Tfh master transcription factor, BCL6, in CD4 T cells increases the recruitment of eosinophils and basophils to the skin tissue but does not affect the transcriptional signature of Th2eff cells. A central hypothesis of this thesis is that the cytokine expression profile of a Th2 cell describes distinct Th2 subsets that have unique functional roles in mediating the Type 2 immune response. To interrogate this hypothesis, I used dual IL-4/IL-13 reporter mice and performed RNA-sequencing on skin Th2 cells that solely produce IL-4, produce IL-4 and IL-13, or solely produce IL-13 after being sensitised intradermally by either an allergen (house dust mite, HDM) or a helminth (Nippostrongylus brasiliensis, Nb). Analysis confirmed my hypothesis that the cytokine expression profile of a Th2eff cell dictates unique Th2eff subsets with distinct functional phenotypes. I report a novel finding that IL-13+ cells represent a pro- inflammatory Th2eff subset that likely drives pathological disease, whilst IL-4 producing cells that do not express IL-13 represent a regulatory Th2eff subset that likely functions to mitigate pathology and promote resolution and tissue repair. Given that IL-4+ anti- inflammatory Th2eff cells are significantly more abundant after Nb sensitisation, this may explain the difference in pathology between a skin-penetrating helminth infection compared to allergic skin disease. This thesis presents a second novel finding, which reveals that IL-13+ pro-inflammatory Th2eff cells develop via a non-canonical Th2 differentiation pathway. I report a partial role for both IL-7 and IL-10 in promoting the development of the IL-13+ Th2eff subset and explore a previously unrecognised mechanism whereby effector CD4 T cells can undergo further maturation in the tissue, via a secondary TCR restimulation. I report that a second TCR stimulation alone is sufficient to drive the expression of IL-13 in Th2eff cells and implore future research to continue investigating this pathway mediating Th2eff cell maturation. Finally, this thesis investigates the Type 2 memory response after multiple exposures to HDM and shows that tissue-resident memory (Trm) cells are critical in mediating the secondary challenge response. This secondary memory response is responsible for symptoms of allergic skin pathology, such as skin barrier dysfunction that is noted in patients with atopic dermatitis.
Together, this highlights the broad role of Th2 cells and provides vital insight into how Th2 cells mediate a diverse array of immunological responses. This work enhances our foundational knowledge of Th2 cells and improves our understanding of how Type 2 immune responses are initiated and maintained, which will be critical for the development of novel therapeutics in the future.