Oral squamous cell carcinoma (OSCC) is the most common cancer of the head and neck area. Despite recent advances in understanding the diagnosis, molecular biology and treatment of OSCC, the five-year survival rate has remained under 50% for the past 30 years, mainly due to metastasis or local incontrollable recurrence.[1] OSCC is mainly caused by tobacco use (chewing and smoking). Other risk factors include infections with certain types of human papillomavirus, certain workplace exposures and radiation exposure. The diagnosis is confirmed by tissue biopsy, with computed tomography and blood tests often used to check the degree of spread. Traditional treatments include surgery, chemotherapy and radiation therapy.[2, 3]
The immune system participates in anticancer activity; however, it is also involved in cancer development and progression.[4] The immunosuppressive status of OSCC has attracted increasing attention, and the effectiveness of immune checkpoint blockade and the overexpression of immune checkpoint molecules as OSCC therapy has been confirmed.[5]. Tumour cells reprogramme their surrounding cells to support cancer progression, tumorigenesis and invasion of adjacent tissues in the tumour associated environment (TME), [6] and 5–40% of the mass of solid tumours consists of macrophages.[7–9] The involvement of macrophages is now established and is becoming better understood. Macrophages serve as an interface between innate and acquired immunity, and they become polarised into M1 and M2 phenotypes depending on the expression of cytokines, receptors, and effector molecules.[10] Under physiological conditions, macrophages are normally polarised into the proinflammatory and antitumour M1 phenotype; however, tumour cells can induce macrophages to switch to the alternatively activated M2 phenotype via several pathways (CCL-2, IL-4, IL-6, IL-8, IL-10, TGF-β and PD-1/PD-L1(Programmed cell death-ligand 1)). The M2 macrophages, in turn, secrete high levels of cytokines, chemokines, enzymes, and growth factors, such as VEGF, PDGF, TGF-β, FGF and several matrix metalloproteinases; these upregulate inflammation while also promoting immunosuppression, angiogenesis, migration, tumour progression, metastasis and treatment resistance.[11, 12]
Tumour progression also responds to endoplasmic reticulum (ER) stress, a vital cellular response that maintains cell survival by activating the unfolded protein response. ER stress acts as a point of “protein quality control” in cells and is involved in several cellular functions, including protein folding and Ca2 + homeostasis, by processing nascent membrane and secretory proteins in a Ca2+-dependent manner.[13] ER stress is controlled by several ER stress–related proteins, including protein kinase R–like ER kinase (PERK), activating transcription factor 6 (ATF6), glucose-regulated protein 78 (GRP78) and inositol-requiring enzyme 1α (IRE1α). The initiation of ER stress has been reported in various tumours, and ER stress promotes further tumour progression.[14, 15] ER stress induces tumour cell escape from immunological surveillance, and activation of ER stress in immune cells affects the function of infiltrating immune cells. For example, ER stress increases a range of inflammatory factors, including interleukin (IL)-23 and IL-6, in macrophages.[16] ER-stressed tumour cells also modify immune cell functions by releasing ER stress–related molecules and subsequently promoting tumour survival, progression and metastasis.[17] However, the mechanisms by which ER-stressed tumour cells cultivate immune cells and suppress immune responses remain unclear.
One possible mechanism involves exosomes, which are 40 nm to 100 nm membrane vesicles involved in cell-to-cell communication. Exosomes are loaded with DNA, proteins, and coding and non-coding RNAs [18] and are released from living cells into the extracellular environment. Exosomes derived from cancer cells differ from the exosomes secreted by normal cells,[18] and the distinctive differences in exosome content in some cancer cells can be used as diagnostic or prognostic markers. For example, miR-21 is a well-known oncomiR that can be used as a diagnostic marker in ovarian cancer.[19] Exosomes released by cancer cells interact with myeloid-derived suppressor cells (MDSCs), tumour-associated macrophages (TAMs) or tumour-infiltrating T cells (TILs). They cause a phenotypic switch of stroma cells and tumour-infiltrating immune cells, thereby creating a tumour-permissive microenvironment. PD-L1 proteins were recently shown to be packaged in purified exosomes, indicating that these vesicles may deliver protein information to recipient cells.[20]
These findings suggest that PD-L1 is a vital molecule involved in exosome-mediated intercellular communication. However, few studies have examined whether ER stress affects the transfer of exosomal PD-L1 and whether exosomal PD-L1 affects OSCC tumour progression. The purpose of the current study was therefore to investigate whether ER-stressed OSCC cells can transmit PD-L1-enriched exosomes to macrophages and whether these exosomal PD-L1 modulate the immunologic functions of macrophages.