In this study, we established a scoring model (PR) that can effectively predict prognosis and explored the potential relationship between prognosis and the TME. The H-PR group, which had poor prognostic features, showed four main characteristics: 1. comprehensive weakening of the immune system, 2. an increased ratio of tumor-promoting immune cells, 3. an increased number of CSCs, and 4. an increased TMB (Fig. 11).
The importance of the TME in tumor progression and immunotherapy has been recognized. TC is an inflammation-driven cancer (8, 24, 25). Our research confirmed the enrichment of inflammatory features in PTC, which was our initial clue to examine the TME. In recent years, remarkable progress has been made in the characterization of several immune cell types (but not all) in the TME of different TCs (9, 24–34). In different stages of PTC, tumor-promoting and antitumor immune cells engage in confrontation. However, many aspects of the immune-related molecular mechanisms of PTC remain unclear, and the relationship between immune cells and prognosis has yet to be discovered. Our research provides adequate data for such an assessment. Studies have reported differences in the TME between PTC and nontumor samples, but no systematic study to explore the characteristics of the TME and progression in PTC has been carried out. We have conducted a series of studies on the above issues and obtained abundant interpretive results.
To develop a simple and convenient signature with which to monitor the immune status of PTC patients and suggest clinical outcomes, we created a prognostic model based on 13 VDEGs. Due to the excellent prognosis of PTC, the number of OS and DFS events in PTC are small, and metastasis and recurrence have become the main in its clinical management. Therefore, we chose PFS as the ending indicator. In recent years, models that predict the prognosis of PTC have developed rapidly. The 35 gene-based prognostic scoring system developed by Pak et al. (35) shows strong discriminatory power in the prediction of event-free survival. Lin et al. (36) used DEGs to develop a risk index model to predict the prognosis of PTC. Liu et al. (37) established a prognostic model based on two microRNAs. You et al. (38) defined a signature to predict the prognosis of PTC consisting of three long noncoding RNAs (lncRNAs). A competing endogenous RNA (ceRNA) network for the prediction of tumor recurrence has also been reported (39). Our study was performed not only to establish an accurate and reliable prognostic model, but also to compare the differences between prognostic groups and to explore the potential mechanisms that cause differences in PTC prognoses.
By ESTIMATE analysis, we found that a high PR indicated a lower degree of immune cell and stromal cell infiltration and a higher tumor purity, and these samples showed that overall immune infiltration was decreased in the poor prognosis group. ssGSEA and TIMER analysis confirmed an overall decrease in immune cell abundance in the H-PR group. Moreover, the comprehensive increase in immune-related gene expression levels in the L-PR group confirmed a highly active immune system (with both tumor-antagonizing and tumor-promoting effects) in the group with a good prognosis. This finding is different from the general conclusion of previous TC studies that anticancer immune cells are positively correlated with prognosis, while cancer-promoting immune cells are negatively correlated with prognosis (34, 40–47). Our previous research also found that the immune system as a whole has a high degree of consistency in the occurrence and development of PTC. Immune escape as a tumor hallmark has been shown to be associated with poor prognosis in a variety of tumors (48, 49). The immune editing hypothesis was proposed by Dunn et al. (50) and divided the interaction between the immune system and the tumor into three stages: 1. elimination, in which immune effector cells (NK cells, CTLs and γδ T cells) recognize and destroy transformed tumor cells; this stage is also called immune surveillance; 2. equilibrium, in which the immune system eliminates tumor cells with high immunogenicity, while tumor cells with low immunogenicity survive; and 3. escape, in which tumor cells eventually escape the body's immune surveillance. The entire process results in decreased tumor immunity (51). In our study, the L-PR group showed the characteristics of the "elimination" stage, while the H-PR group showed more characteristics of the "escape" stage. Poschke et al. (52) suggested that tumors have two methods of immune escape: camouflage and sabotage. Camouflage refers to the malformation and loss of the major histocompatibility complex class I (MHC-I) molecules on the surfaces of tumors, allowing tumors to escape the detection of the immune system. The MHC in humans is also called HLA, and our results showed that HLA gene expression was lower in the H-PR group than in the L-PR group. Sabotage refers to the ability of some tumors, including PTC, to manipulate part of the immune system to fight against the body's immune response to protect themselves. In this process, tumors attract and even mediate some immune cells, such as TAMs, MDSCs, Tregs, and mast cells (MCs). In general, MDSCs, Tregs, and MCs respond to uncontrolled inflammatory reactions in the body, but during tumor progression, they create an immune microenvironment that allows this process.
Existing evidence shows that B cells, CD8 + T cells, M1 macrophages, TH1 cells, NK cells, mDCs, and γδ T cells have tumor-antagonizing effects in the PTC TME, while Tregs, TH2 cells, M2 macrophages, MDSCs, iDCs, mast cells, monocytes, and neutrophils have tumor-promoting effects (9–11). Our previous research also showed that during the occurrence and development of PTC, the proportion of tumor-promoting immune cells (M2 macrophages, Tregs, monocytes, neutrophils, DCs, MCs, and M0 macrophages) increased, while the proportion of antitumor immune cells (M1 macrophages, CD8 + T cells, B cells, NK cells, Tfh cells, and γδ T cells) decreased (53). In this study, the H-PR group showed a high proportion of tumor-promoting immune cells (Tregs, monocytes and aDCs) and a lower proportion of antitumor immune cells (B cells, CD8 + T cells, M1 macrophages and γδ T cells), indicating that the tumor cells escaped the body's immune surveillance by "sabotage" and achieved immune escape. Based on the above conclusions, we speculate that the irreversible progression of tumors caused by immune escape in the PTC TME is a crucial cause of poor prognosis. The enrichment of immune-related pathways in the L-PR group by GSEA also supports the above view.
Tumor heterogeneity, one of the characteristics of malignant tumors, can cause differences in tumor growth rate, invasion and prognosis. Two models have been used to explain the heterogeneity of cancer cells (54). The first is the stochastic model, in which the development of cancer is triggered by the accumulation of gene mutations in a single cancer cell, followed by different subsequent genetic events in different subpopulations of cells. We found that the H-PR group had a significantly higher TMB than the L-PR group. In 2019, the TMB was included in the NCCN index as an emerging prognostic indicator in non-small cell lung cancer immunotherapy (12–15, 55). We found that PTC patients with a high TMB had a worse prognosis. Other studies have also reported that patients with a variety of cancer types with a high TMB can obtain a better prognosis after immunotherapy; however, without immunotherapy, patients with a high TMB show a poor prognosis (56). Our study fills the gap in knowledge of the role of TMB in PTC. The high TMB observed in patients with a poor prognosis suggests immunotherapy as a viable option for these patients.
The second model is the CSC model, which postulates that a small population of cells in the tumor is responsible for tumor initiation, growth, and recurrence (57). We found that the H-PR group had a higher CSC content than the L-PR group. The positive correlation between CSCs and recurrence and metastasis in our data can be explained well by the CSC model (6). CSCs in cancer biology are resistant to conventional therapies (such as surgery and radioactive iodine therapy), and TC usually relapses as CSCs recur (58). The combination of CSC-specific therapies (such as drugs that target the Notch or MEK and JNK pathways as therapeutics to eliminate CSCs) with conventional therapies has the potential to eradicate highly lethal cancers (58).
In addition, PTCs with high tumor heterogeneity have a higher frequency of weak immunogenic tumor cells, which will also accelerate the body's immune selection for tumor cells and exacerbate immune escape (59).
At present, PTC immunotherapy is an active research field, and the study of the tumor immune microenvironment is an important pillar of research on PTC management via immunotherapy (4). The results of this study provide a reference for subsequent research on the PTC immune microenvironment. The high level of immune escape and high TMB shown by the H-PR group suggest that immunotherapy is a viable option for patients with poor prognosis. The PR as a prognostic signature and an immune status indicator can assist in the clinical stratification of PTC patients to identify patients who respond to immunotherapy.
However, this study has some limitations. 1. Because the TCGA is the only public database with sufficient data on both PTC expression and prognosis, we did not use other independent cohorts to verify the prognostic value of the PR. However, we confirmed the negative correlation between the PR and immune infiltration in 5 GEO verification cohorts and verified the positive correlation between the PR and PTC progression and mutation in GSE60542 using clinical data. 2. The PR has a strong predictive effect on PTC prognosis, but there may still be slight differences in the TME between the PR grouping method and the actual different prognosis grouping method. However, related prognostic research is ongoing. 3. Cancer-related pathway enrichment in the L-PR group has not been well explained, but we suspect that this enrichment is related to increased immune infiltration. It was also found in triple-negative breast cancers that a group with a high level of immune infiltration showed a better prognosis, accompanied by the enrichment of cancer-related pathways (60).