The main objective of this research project was to decipher the function of the unfolded protein sensor IRE1 in dendritic cells (cDC). Earlier studies from my lab had revealed that IRE1 shows a high basal activity in cDC1s, one of the two major subsets of conventional DCs that play an important role in tolerance and anti-viral and anti-tumor immunity. Of note, IRE1 activity was not reflected by activation of XBP1, the transcription factor that is typically activated downstream of IRE1 signaling. This made us wonder how and why IRE1 was active specifically in cDC1s.
A transcriptional profiling analysis performed before the start of the ERC revealed that absence of IRE1 in cDC1 led to a decrease in expression of maturation genes, genes associated with efferocytosis and genes implicated in cholesterol metabolism. This led to the hypothesis that IRE1 could be a regulator of apoptotic cell engulfment and homeostatic DC maturation, which formed the basis of the current DC-RIDDLE project. More concrete, we formulated 4 main working hypotheses: In WP1, we wanted to test the assumption that IRE1 main’s actions in dendritic cells were not explained by XBP1-dependent transcriptional activity, but rather by IRE1-dependent RIDD activation and degradation of miRNAs, in WP2, we postulated that IRE1 would be important for apoptotic cell engulfment in DCs, in WP3, we postulated that IRE1 might play a role in tolerogenic antigen presentation, and finally, in WP4, we wanted to investigate the role of IRE1 in viral infections.
Early during the DC-RIDDLE project, we noticed that cDC1s that are deficient for IRE1 no longer matured in homeostatic conditions, while immunogenic maturation induced by pIC did not appear to be affected.
For many years, the upstream signals driving the homeostatic maturation of DCs have remained enigmatic and seeing the crucial role of IRE1 in homeostatic not immunogenic DC maturation gave us a tool to find out the mechanisms underlying this homeostatic maturation program. The results of this study can be found in (Bosteels et al., Sci Imm, 2023). In brief, we showed that apoptotic cell engulfment and concomitant lipid influx is a major driver for cDC1 homeostatic maturation. Homeostatic maturation is associated with cholesterol efflux, while in immunogenic conditions cholesterol levels are retained, showing a tight link between cholesterol metabolism and the balance between immunity and tolerance.
Later, we found that IRE1 in cDC1s is triggered by the cholesterol influx during engulfment in cDC1s, and that in response, IRE1 regulates the cholesterol efflux. In absence of IRE1, free cholesterol accumulates at the ER, inducing lipotoxicity and cell death of the late immature and mature cDC1 substages (Bosteels et al., in revision).
