Spatial and temporal regulation of cytokine expression in Type 2 immune responses
Authors:   , Kyle Ryan 
Published: November 2021

<p>Type 2 immune responses are generated to provide protection against parasitic helminth infections, however these responses also cause the pathologies associated with allergic inflammation. Studies of the cell types and signalling pathways that mediate Type 2 immune responses have been previously undertaken with the goals of efficient development of vaccines against helminths, and identification of pathways that can be inhibited to decrease the damage caused by allergic inflammation.  The cytokines interleukin-4 (IL-4) and interleukin-13 (IL-13) mediate many of the downstream effector functions of the Type 2 immune response. To study the mechanisms that control expression of these two cytokines I have used a novel dual cytokine IL-4 and IL-13 transgenic reporter mouse. Utilising this tool along with other IL-4 reporter mice I have discovered that the amount of T cell receptor (TCR) signalling modulates the allelic expression of IL-4 by CD4⁺ T cells. The transgenic IL-4 reporter mouse has for the first time allowed independent measurement of the effects of IL-4 deficiency on the expression of IL-4 in vivo. Using this system I have found that IL- 4 is not required for the in vivo generation or expansion of IL-4 producing CD4⁺ T cells. Th2 differentiated CD4⁺ T cells also expresses IL-13, however the dual reporter mice have demonstrated that IL-13 is expressed consistently later than IL-4 in vitro, and IL-13 requires constant, or multiple exposures to TCR stimulus for expression to be induced. IL-13 expression is absent from lymph node CD4⁺ T cells during exposure to allergens or helminth infection. Sequestration of CD4⁺ T cells in the lymph node does not impact the number of IL-13 expressing CD4⁺ T cells in the lung during a helminth infection, indicating that adaptive immune cell derived IL-13 may be entirely produced by lung resident cells not requiring transit through the lymph node.  I have characterised a population of innate lymphoid cells (ILCs) within the skin and found that the proportion of these cells that constitutively express IL-13 decreases with age. These cells did not drastically change in numbers or IL-13 responses in a range of inflammatory conditions including a model of atopic dermatitis. Basophils were found to respond to the atopic dermatitis model by migrating specifically to the treated skin site and draining lymph node, and producing IL-4 in a thymic stromal lymphopoietin dependant manner.  Treatment with exogenous cytokines induced IL-13 expression from group 2 ILCs (ILC2s) in the lung and these cells promoted protective immune responses against Nippostrongylus brasiliensis infection. The immune response generated during a primary infection by Nippostrongylus brasiliensis provides protection from re-infection. Long-term protection is dependent on CD4⁺ T cells but when sufficiently stimulated by cytokine, ILC2s can rescue the protection lost by the depletion of CD4⁺ T cells.  This thesis has shown that CD4⁺ T cells and populations of innate immune cells differentially regulate the expression of the closely related Type 2 cytokines IL-4 and IL- 13. These discoveries will help direct future research aiming to boost the effectiveness of anti-helminth vaccines, or decrease the pathology caused by allergic diseases by targeting specific cytokine expression.</p>