Understanding the Generation and Regulation of IL-4 Producing T Helper 2 Cells
A keenly sought therapeutic approach for the treatment of allergic disease is the identification and neutralization of the cytokine that regulates the differentiation of Th2 cells. Th2 cells are CD4 T cells differentiated to secrete IL-4. These cells are exciting new targets for asthma therapies due to the key role they play in allergic airway diseases. Recently the cytokine IL-25 has been shown to enhance Th2 cell activity and play important roles in mediating allergic inflammatory responses. To investigate this further we crossed IL-25 deficient mice with GFP-IL-4 reporter mice and developed an assay of in vitro and in vivo IL-4 independent Th2 differentiation. These assays were used to determine whether IL-25 was critical for the formation of Th2 cells. We found there was no physiological role for IL-25 in either the differentiation of Th2 cells or their development to effector or memory Th2 subsets. In the strong Th2 setting of a helminth infection the absence of IL-25 resulted in no defects in the effector type 2 responses associated with T helper type 2 immunity including, mucous hyperplasia, class switching to IgE and protection against challenge infections. Importantly this data challenges the newly found and growing status of the cytokine IL-25 and its proposed role in promoting Th2 responses. The second part of this thesis investigated whether the genomic organisation, which reflects commitment to Th2 cytokine expression, could provide a clearer definition of a functional in vivo Th2 cell. Two distinct IL-4 reporter mice were crossed and Th2 in vivo assays were developed that allowed tracking of the individual alleles of IL-4 in a variety of tissue types and Th2 subsets. Interestingly in vivo expression of IL-4 is mostly monoallelic yet there is a small highly activated population of biallelic IL-4 expressing Th2 cells. Physiologically each allele of IL-4 was required for a functional Th2 response with total serum IgE titres up to 4 fold lower in IL-4+/- heterozygous compared to the IL-4+/+ sufficient animals and a significant loss in protective immunity against challenge infections with helminths occurred in the IL-4+/- heterozygous animals. The similarity in deficiencies in Th2 immunity in the IL-4+/- heterozygous and IL-4-/- deficient compared to the IL-4+/+ sufficient animals lead to the proposal that the generation of biallelic Th2 cells may be required for specialised cell-to-cell mediated delivery of tailored activation signals and higher quantities of IL-4 required to mediate fully developed Th2 immune responses.