ER-stressed macrophages promote, reinforce, swap and in some cases ameliorate disease outcomes. One of the reasons behind the overwhelming presence of ER stress response in macrophages is that both pattern-recognition receptors, Toll-like receptor (TLRs) and nucleotide-binding oligomerization domain-like NOD1 and NOD2 receptors, directly engage IRE1 activation to fully execute a resolutive pro-inflammatory response [37,38]. ER stress inhibitors during Brucella abortis infection limit abortion of disease by restraining IL-6 production in an IRE1- and NOD1/NOD2-dependent manner [37]. Similarly, TLR2- and TLR4-dependent splicing of XBP1 occurs upon acute infection with Francisella tularensis and essentially contribute to bacteria clearance by directly promoting the production of IL-6 and TNFα [38]. In addition, IRE1 activation in macrophages also promotes inflammasome activation that is essential to foster IL-1b production and to clear bacterial infection [39]. These pioneering discoveries opened the road to a large number of studies that further confirmed the involvement of IRE1/XBP1 in the pathophysiology of macrophage function during acute infection. Many more studies have been then performed in the chronic setting and revealed that persistent ER stress might instead be deleterious, rather than protective, for the progression of diseases. In a model of chronic inflammatory arthritis, myeloid-specific ablation of IRE1 protected the mice from the disease, attenuating the release of pro-inflammatory cytokines [40]. Interestingly, in the same study, the authors mechanistically uncovered that TLR4 promotes TRAF6-dependent ubiquitination of IRE1 that impedes the binding of the phosphatase PP2a, therefore favoring IRE1 phosphorylation [40]. In other chronic diseases, such as obesity and atherosclerosis, ER-stressed macrophages contribute to onset and progression of disease [41,42]. More specifically, it has been shown that in high-fat diet (HFD)-induced obesity, IRE1-depleted macrophages alleviate pathological symptoms by favoring the switch of pro-inflammatory macrophages towards anti-inflammatory [43]. In support of these findings, in a genetic model of HFD-induced NASH (nonalcoholic steatohepatitis) and HCC (hepatocellular carcinoma), treatment with ER stress alleviators PBA and TUDCA, improved disease outcomes by limiting the release of TNF by inflammatory macrophages [44]. TUDCA and 4-PBA are chemical chaperones that work by lowering the burden of unfolded protein within the ER [45]; 4-PBA has also been successfully employed in the treatment of atherosclerosis in mice. In the atherosclerotic plaques, lipid-laden macrophages undergo changes in lipid composition that induce ER stress and engage PERK/CHOP-mediated apoptosis; 4-PBA was able to relieve macrophages from PERK activation, prevent apoptosis and preserve their functions [46]. Accordingly, in a similar model of atherosclerosis, treatment with the IRE1 inhibitors STF083010 or 4μ8c decreased plaque area by reducing macrophage infiltration by limiting IL-1β and the recruiting chemokine CCL2 [47].

Both atherosclerosis and obesity are mainly characterized by a lipid-enriched microenvironment, and macrophages tend to uptake these metabolites acquiring the status of “foam cell”. Therefore, in these cells, beside UPR or in combination with it, ER stress sensors are mainly activated by changes in the lipid composition and/or saturation of the membranes. It has been recently demonstrated that in adipose-tissue macrophages isolated from obese mice, macrophage-specific depletion of phosphocholine cytidylyltransferase A by limiting the turnover of PC favors the integration of polyunsaturated fatty acid within the ER membrane, therefore reducing ER stress and retaining inflammation [48]. In cancer, tumor-associated macrophages (TAMs) engage the IRE1-dependent ER stress response upon synergistic action of IL-4, IL-6 and IL-10 that promotes cathepsin secretion and increased pro-metastatic phenotype [49]. Interestingly, in this paper the authors found that IL-6/IL-10-dependent STAT3 phosphorylation is upstream of IRE1 activation, adding other level of complexity to the reciprocal regulation of IRE1/STAT3. Beside pro-tumoral macrophage, MDSCs (myeloid-derived suppressive cells) have been recently described as cells expressing both macrophages and granulocyte markers that contribute to the establishment of an immunosuppressive tumor microenvironment, therefore blunting the anti-tumoral immune response [50]. In these cells, tumor-derived stress factors drive CHOP activation in a PERK-dependent manner. By boosting IL6 production, CHOP favors the instauration of an immunosuppressive microenvironment. MDSC-specific deletion of CHOP synergizes with immune checkpoint blockade inhibitors to cure lung, melanoma, thymoma and colon tumors [51]. These findings extensively prove that ER stress is actively involved in pathophysiological mechanisms, having a protective role in acute macrophage response and adverse effects in chronic disease (Figure 1). However, these studies have also opened new fields of investigation that aim to clarify when and how the switch occurs and, most importantly, how the ER stress players are specifically activated in different contexts.

Dendritic cells (DCs) represent the connection arm between innate and adaptive immunity. DCs are able to cross-present antigens to CD8⁺ and CD4⁺ T cells, unleashing their activation and the engagement of antigen-specific immune responses. Based on location and functions, many subsets of DCs exist, varying from professional antigen-presenting cells to inflammatory DCs. Features and differences of DCs subsets have been nicely reviewed elsewhere [52]. As other innate cells, DCs have myeloid origin and are activated by DAMPs and PAMPs through the expression of PRRs receptors. Once activated, antigen-loaded DCs migrate to draining lymph nodes where they encounter naïve T cells. Binding of the MHC/antigen complex to the T cell receptor in T cells leads to their activation and migration to the site of infection. The molecular mechanism orchestrating the process of antigen presentation is complex and to some extent still not completely uncovered [53]. The ER host the loading of antigens on MHCI or MHCII complexes and for this reason many reports have demonstrated that the ER guardians are indirectly involved in the antigen processing and presentation [54]. In one of the first papers showing the connection between ER stress and DCs, it was drastically proven that XBP1 deletion in the hematopoietic compartment impairs development and survival of dendritic cell lineage [55]. Immature DCs constitutively activate XBP1, which in turn regulates their differentiation in a cell intrinsic manner. However, the molecular mechanism behind this crucial role of XBP1 was not elucidated. Later, more studies clarified that the severity of the phenotype upon genetic loss of XBP1 is different in distinct subsets. Splenic dendritic cells show impaired antigen presentation associated with a disturbed ER architecture. IRE1/XBP1 signaling is constitutively activated in splenic dendritic cells and promotes cross-presentation by directly regulating the expression of proteins crucial for the antigen loading into MHC complex [56]. In contrast, conditional knockout of XBP1 under the CD11c promoter results in ablation of lung DCs via CHOP-mediated apoptosis. In the same mouse model, mucosal DCs survived. However, upon pharmacological blockade of IRE1 activity (which is overactivated in XBP1 KO), mucosal DCs also undergo cell death, suggesting a different threshold of IRE1 activation in different tissues [57]. These studies have been performed in absence of infection and therefore are limited to the physiological status of DCs. However, as it occurs for macrophages, upon TLR activation, ER stress pathways are engaged in DCs and contribute to their function. Strikingly, treatment of DCs with TLR4 and TLR8 agonists induces CHOP activation and its direct binding to the promoter of the pro-inflammatory cytokine IL-23 [58] (Figure 2). Similarly, it has been recently published that palmitic acid or HFD are able to reinforce the effect of TLR7/8 agonist IMQ by boosting IL-23 production in DCs, upon activation of CHOP and sXBP1-mediated ER stress response [59]. In cancer, dendritic cells are loaded with lipid that undergo peroxidation and induce an XBP1-mediated ER stress response. Conditional knockout mice harboring XBP1 deletion in dendritic cells were strongly protected against tumor growth compared to WT mice. Mechanistically, XBP1 boosts triglyceride production and further widens the overloading of lipids, therefore disturbing the machinery of antigen presentation and blunting anti-tumor response [60]. In contrast, a study performed in human graft vs. host disease (GVHD), has shown that DCs-specific sXBP1 deletion strongly suppress alloreactive CD4 T cells, and pharmacological block of spliced XBP1 protects against rejection. Moreover, the authors provided evidence that this phenomenon is restrict to GVHD, because intratumoral T cell response was not affected by XBP1-expressing DCs [61]. In conclusion, by directly or indirectly controlling antigen presentation in combination with TLR signaling, ER stress acts as major hub in regulating DCs functions and therefore represent an attracting therapeutic target. However, more studies are needed in order to fully understand how duration and intensity of inflammation and priming are interplaying with the ER guardians to balance physiologic versus pathologic DC responses.