IL-17A increases MHC class I expression and promotes T cell activation in papillary thyroid cancer patients with coexistent Hashimoto’s thyroiditis

The incidence of papillary thyroid cancer (PTC) increases continuously over the last few decades, making PTC the fastest-growing cancer in most areas of the world [1]. In the meanwhile, Hashimoto’s thyroiditis (HT) is being frequently observed in PTC patients [2‐4]. Extensive attention has been paid to PTC and coexistent HT to elucidate their association. Most of these studies argued patients with HT are more likely to have favorable prognosis, including one study completed at our institute [2, 5, 6]. However, contradictory views also exist [7, 8].

Multiple cytokines are involved in HT pathogenesis and they can consequently affect the course of the disease [9]. According to previous researches, cytokines are decisive regarding tumor growth and metastasis in various cancers, including PTC [10, 11]. Of special interest is interleukin 17A (IL-17A), whose expression is reported to be elevated in autoimmune diseases [12, 13]. Moreover, there were evidences linking IL-17A to cancerogenesis through immune activation in lung cancer and human papillomavirus (HPV) related epithelial cancers [14, 15]. In PTC and coexistent HT, the role of IL-17A remains undefined.

Major histocompatibility complex (MHC) class I molecules present endogenously derived peptides and elicit cytotoxic T lymphocytes (CTL), which is a crucial step in T-cell mediated antitumor immune response. However, tumor cells often downregulate MHC class I expression to achieve immune escape [16, 17]. Novel immunotherapy approaches are being explored to restore MHC class I expression. In this paper, we concentrate on lymphocyte infiltration. Because it is both the foundation for CTL killing and pathologic characteristic of HT. Studies have proved that enhanced CTL could augment immunotherapy [18]. Therefore, we hypothesized that HT alters MHC class I expression in PTC, which consequently changes patient outcome.

The aim of this study is to determine the role of IL-17A in PTC patients with coexistent HT, evaluate the changes in PTC immune antigenicity afterwards and investigate possible underlying mechanism.

In the current study, we identified an elevated expression of IL-17A in PTC with coexisting HT. Administration of IL-17A could effectively induce MHC class I expression in K1 and TPC-1 cells in vitro, which led to increased T cell activation and IL-2 production by PBLs cocultured with IL-17A pretreated PTC cell lines. Downregulation of IFN-γ and PD-1 was also observed along with T cell activation. Combined together, our study showed IL-17A enhanced T cell activation in PTC with coexistent HT, possibly through PD-L1/PD-1 pathway.

The coexistence of HT and PTC has continuously drawing interests since Dailey et al. first reported the phenomenon [19]. A previous work of our institution found the presence of HT was a protective factor for central compartment lymph node metastasis in PTC [20]. Actually, the current study did not come to the same conclusion. This may due to our small sample size. Notably, our data showed PTC patients with HT were more vulnerable to multifocal lesions, a recognized risk factor for worse prognosis [21]. Even identified with risk factors, there were no differences as to the TNM stage between two groups, suggesting HT might be a protective factor. Yet this conclusion needs to be further validated by studies with a larger scale of patients.

HT triggers immune response specific to thyroid, leading to lymphocyte infiltration, cytokine production and the destruction of normal thyroid tissue [9]. Former researchers have established that IL-17A, which is a pro-inflammatory cytokine in autoimmune diseases, could also participates in tumor development [12‐15]. In breast cancer, elevated IL-17A expression results in polarization of neutrophils which suppress CTLs and eventually, promoting metastases [22]. Gomes et.al. showed blocking IL-17A axis prevents hepatocellular carcinoma [23]. Although its role in cancer has been widely described as pro-tumorous, there also exist studies proving the antitumor effect of IL-17A. Injecting recombinant Lactococcus lactis strain secreting IL-17A into a mouse allograft model of HPV-induced cancer effectively prolonged the disease free survival in contrast to control mice treated with the wild type strain of L. lactis [24]. In PTC, an analysis of Korean population revealed IL-17A SNP rs2275913 was significantly associated with lack of multifocality [25]. Considering that rs2275913 is a promoter SNP in IL-17A, their results strongly suggest the role of IL-17A in the cancerogenesis of PTC. The underlying mechanism needs to be further investigated. To our best knowledge, this is the first study to evaluate IL-17A in the context of coexisting PTC and HT. We hypothesized that IL-17A expression would be elevated in PTC + HT because of coexistent HT. Our results came confirmative. More interestingly was that the clinical data showed patients with high IL-17A expression had less lymph node metastasis, indicating its protective function in PTC.

Loss of MHC class I expression is a frequent mechanism of immune escape in PTC [17]. Restoration of MHC class I has been proved to be a promising mechanism to enhance immunotherapy efficacy in melanoma, hepatocellular carcinoma and other malignancies [26, 27]. In the current study, an increased expression of MHC class I was detected in PTC patients with HT, indicating suppressed immune escape in these patients. IL-17A expression was reported to elevate in Treg-decreased patients with unresectable pancreatic cancer after chemotherapy, suggesting its role in immune escape [28]. By in vitro administration of IL-17A, we successfully proved that MHC class I expression was induced in PTC cell lines (K-1 and TPC-1). Elevated expression of MHC class I could strengthen T cell mediated cytotoxicity. In the current study, increased CD25⁺% within CD3⁺T cells and IL-2 secretion were observed in cocultures of isolated PBLs and IL-17A stimulated PTC cells, indicating IL-17A induced MHC class I expression was accompanied by increased antigenicity in PTC.

IFN-γ production by the PBLs cocultured with pretreated PTC cells were downregulated in our study. IFN-γ is widely thought to be a representative antitumor cytokine, however it also has been proved to induce PD-L1 expression and impair tumor immunity [29]. Activation of costimulatory molecules programmed death 1 (PD-1)/ programmed death ligand 1 (PD-L1) facilities immune escape in multiple malignancies including PTC [30, 31]. Our data showed downregulation of IFN-γ was accompanied by decreased PD-1 expression, suggesting the immune escape suppressed by IL-17A may be linked to PD-1/PD-L1 pathway.

There are also unanswered questions and limitations in the present study. A major flaw is that our findings of IL-17A, which were interesting and may provide new insights in immune therapy, could not fully represent HT. The role of PD-1/PD-L1 pathway in PTC and coexistent HT has not been thoroughly examined. Statistical analysis of clinical data may also be compromised by the small sample size. Future researchers need to take circulating cytokines into consideration as well.

We successfully demonstrated IL-17A induced MHC class I expression and promotes tumor antigenicity in PTC, and may be related to PD-1/PD-L1 pathway. This result may facilitate immunotherapy in cancers with downregulated MHC class I expression.