title: Modulation of dendritic cell signalling and function by redox regulators
Creators: name: Abbas Al-Huseini, Laith
Publisher: name: University of Liverpool Repository
Year: 2525

description: Dendritic cells (DCs) are antigen-presenting cells crucial for the initiation and coordination of primary adaptive immune responses. Immature DCs (iDCs) express low levels of MHC class II and co-stimulatory molecules such as CD80, CD86, and CD40, with high phagocytic capacity and limited ability to induce antigen-specific T cell activation. DC maturation is associated with up-regulation of co-stimulatory molecules and cytokine production, rendering the DCs competent in T cell activation and the elicitation of an immune response. DC function and co-stimulatory receptor gene expression are known to be regulated by intracellular redox status, NF-kB and MAPKs signalling pathways. Intracellular reactive oxygen species (ROS) levels influence DC maturation and function. The transcription factor, Nrf2, is essential for maintaining intracellular redox homeostasis. In response to oxidative stress, Nrf2 induces the transcription of a set of cytoprotective and antioxidant genes, including heme oxygenase-1 (HO-1), that are required for detoxification of xenobiotics and their reactive metabolites and the nullification of oxidative insult. It is now emerging that Nrf2, and its gene product, HO-1, play pivotal roles in regulation of immune responses. However, the key signalling mechanisms involved in Nrf2 and HO-1-mediated altered DC function has not been fully elucidated and requires further investigation. In addition, the role of ROS in the absence of Nrf2 or HO-1 activity, in DC activation and function has not been investigated. Using immature bone marrow-derived DCs (iDCs) from Nrf2 +/+ and Nrf2 -/- mice, we demonstrate in the first part of the work presented in this thesis, that Nrf2 deficiency in iDCs resulted in increased ROS levels, enhanced iDCs co-stimulatory receptor expression, and increased iDC-mediated antigen-specific CD8 T cell stimulatory capacity in response to an antigenic peptide. Using antioxidant vitamins to reset ROS levels in Nrf2 -/- iDCs, we show that elevated ROS was not responsible for the altered phenotype and function of these DCs. Additionally, using appropriate pharmacological inhibitors, we demonstrate that the altered Nrf2 -/- iDC phenotype and function did not require NF-kB, ERK or JNK activity but was dependent on p38MAPK-CREB/ATF1 activity. Based on these experimental results, we conclude that Nrf2 regulates DC maturation and function by modulating intracellular signalling pathways independent of intracellular ROS levels. In the second part of the study, we demonstrate that inhibition of HO-1 activity in iDCs resulted in DCs with raised intracellular ROS levels, a mature phenotype, impaired phagocytic and endocytic function, and increased capacity to stimulate antigen-specific CD8 T cells. Interestingly, our results reveal that the increased ROS levels following HO-1 inhibition did not underlie the changes in phenotype and functions observed in these iDCs. Importantly, we show that the p38MAPK-CREB/ATF1 pathway was involved in the mediation of the phenotypic and functional changes arising from HO-1 inhibition. Furthermore, up-regulation of HO-1 activity rendered iDCs refractory to lipopolysaccharide-induced activation of p38MAPK-CREB/ATF1 pathway and DC maturation. Finally, we demonstrate that treatment of iDC with the HO-1 substrate, heme, recapitulated the effects that result from HO-1 inhibition. Based on these experimental results, we conclude that HO-1 regulates DC maturation and function by modulating the p38MAPK-CREB/ATF1 signalling axis.Collectively, the work described in this thesis highlights the importance of the redox regulators, Nrf2 and HO-1, in controlling DC immune functions. This work supports the basis for utilisation of Nrf2 and HO-1 as potential molecular targets for pharmacological intervention in disease states that arise from dysregulated redox function and the design of new pharmacological strategies aimed at modulating DC function in the therapy of immune diseases.