These events, independently or collectively, can lead to a dysregulation of protein synthesis, folding, and secretion and the accumulation of misfolded proteins within the endoplasmic reticulum (ER), triggering a stress response termed the unfolded protein response (UPR). In addition, about 20% of tumors have a viral origin, and most (90%) solid tumors carry chromosomal abnormalities. The TME is home to environmental noxae such as hypoxia and nutrient deprivation. Because this phenomenon is considered at the root of the dysregulation of local adaptive T-cell immunity, much emphasis has been placed on identifying common mechanisms driving the acquisition of tumor-promoting properties by macrophages and dendritic cells in the TME. Among them, macrophages and dendritic cells-cells privileged with antigen presentation/T-cell activation functions-often acquire a mixed proinflammatory/immune-suppressive (IIS) phenotype, both in the mouse and in humans.

They infiltrate tumors in varying numbers depending on tumor types and display phenotypic and functional diversity. Myeloid cells in the tumor microenvironment (TME) are of central relevance to understand the dynamics of tumor progression. Thus, the IRE1α/Xbp1 axis drives the polarization of macrophages in the tumor microenvironment initiating a complex immune dysregulation leading to failure of local immune surveillance. RNA sequencing (RNASeq) analysis showed that bone marrow–derived macrophages with IRE1α deletion lose the integrity of the gene connectivity characteristic of regulated IRE1α-dependent decay (RIDD) and the ability to activate CD274 gene expression. Remarkably, we found a significant association between the IRE1α gene signature and CD274 gene expression in tumor-infiltrating macrophages in humans. Mice with IRE1α- but not Xbp1-deficient macrophages showed greater survival than controls when implanted with B16.F10 melanoma cells.

Macrophages in which the IRE1α/X-box binding protein 1 (Xbp1) axis is blocked pharmacologically or deleted genetically have significantly reduced polarization and CD86 and PD-L1 expression, which was induced independent of IFNγ signaling, suggesting a novel mechanism in PD-L1 regulation in macrophages. Here we report that the inositol-requiring enzyme 1 (IRE1α) branch of the UPR is directly involved in the polarization of macrophages in vitro and in vivo, including the up-regulation of interleukin 6 (IL-6), IL-23, Arginase1, as well as surface expression of CD86 and programmed death ligand 1 (PD-L1). The unfolded protein response (UPR) is emerging as the possible origin of these events. In the tumor microenvironment, local immune dysregulation is driven in part by macrophages and dendritic cells that are polarized to a mixed proinflammatory/immune-suppressive phenotype.

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