Endoplasmic reticulum (ER) stress, a feature in the tumor microenvironment, refers to a condition when unfolded proteins accumulate in the ER lumen. Intrinsic and extrinsic perturbations, such as gene mutations, protein overload, and hypoxia, may elicit ER stress and induce the downstream unfolded protein response (UPR). Previous studies have revealed the importance of the UPR in tumor growth and metastasis, as well as its physiological functions in immune cell regulation.

In a study in Cell Metabolism, a research team led by Dr. WANG Likun at the Institute of Biophysics of the Chinese Academy of Sciences reported that tumor cells experiencing ER stress non-cell-autonomously suppressed anti-tumor immunity to facilitate tumor growth by secreting cholesterol via small extracellular vesicles (sEVs) and fostering the expansion and activation of myeloid-derived suppressor cells (MDSCs).

By employing B16 mouse melanoma cells or MC38 mouse colon adenocarcinoma cells that have XBP1 knocked down (shXBP1), the researchers found that shXBP1 greatly retarded the tumor growth in immunocompetent, but not immunodeficient mice, suggesting that tumor-intrinsic XBP1 may regulate immune response. Using mass cytometry and flow cytometry, they revealed a decreased proportion of MDSCs in tumor-infiltrating leukocytes (TILs), accompanied by increased level of CD8⁺ T cells, in shXBP1 tumors compared with its wild-type (shNT) counterparts, and the depletion of MDSCs in mice bearing shNT tumors also elevated the ratio of CD8⁺ T cells and suppressed tumor growth.

Besides, transcriptome analysis on tumor tissues identified genes encoding enzymes of cholesterol biosynthesis whose transcription levels were downregulated in tumors upon XBP1 depletion, and a decreased level of cholesterol in tumor tissues was observed. In vitro and in vivo experiments showed that cholesterol can stimulate the expansion and activation of MDSCs. On the other hand, genetic or pharmacological disruption of cholesterol production in tumor cells decreased the proportion of MDSCs and increased CD8⁺ T cells in TILs, and reduced tumor growth rate. Remarkably, XBP1s binds to the promoter of HMGCR, a rate-limiting enzyme-encoding gene for cholesterol biosynthesis, and directly promotes the transcription of HMGCR. Thus, the UPR protein XBP1s drives cholesterol production in tumor cells to suppress anti-tumor immunity and facilitate tumor growth.

Moreover, the researchers investigated the mechanism for how cholesterol propagates ER stress signals from tumor cells to MDSCs. They found that shXBP1 reduced the amount of small extracellular vesicles (sEVs) secreted by tumor cells, and decreased the cholesterol content in sEVs, and sEVs derived from shXBP1 tumor cells were less effective in promoting MDSCs expansion and activation compared with the ones from shNT tumor cells. Inhibition of macropinocytosis on MDSCs limited the sEVs-induced proliferation and activation of MDSCs. These results indicated that cholesterol is delivered from tumor cells via sEVs, which are then received by MDSCs through macropinocytosis.

Finally, the researchers proposed a strategy in tumor therapy, in which inhibiting IRE1α/XBP1 pathway by using KIRA8 greatly revived anti-tumor immunity and reduced tumor growth. Such inhibitory effect can be further improved when combined with anti-PD-1 checkpoint blockade.

This work reveals an interesting function of tumor-intrinsic UPR, which dictates the production of cholesterol and leads to immunosuppression by orchestrating MDSCs in a cell non-autonomous way, and provides insight into tumor therapy.