We constructed an OS-related FRLncs risk prognostic model through joint analysis of the TCGA database and the GEO database, including 13 FRLncs: 5 high-risk FRLncs (AP000757.1, AL035530.2, AC006160.1, PRR34-AS1, and LINC01719) and eight low-risk FRLncs (AC090559.1, AC100847.1, MIS18A-AS1, ITCH-IT1, AL031722.1, AC027575.2, AC104561.1, and NBR2). The models were subsequently subjected to risk curve analysis, survival analysis, ROC curve analysis, and independent prognostic analysis, indicating that the 13 FRLncs in the model could guide the prognosis of OS. Model validation of clinical groupings showed that risk-prognostic models were also applicable to clinical traits such as age, gender, tumor metastasis and primary tumor site. Further immune correlation analysis was performed on the risk prognosis model, and it was found that B cells, macrophages, neutrophils, natural killer (NK) cells, plasmacytoid dendritic cells (pDCs), helper T cells, tumor infiltrating lymphocytes (TIL) and regulatory T cells(Treg) were all down-regulated in the high-risk group. In terms of immune function, APC-co-stimulation, check-point, cytolytic-activity and T cell-co-inhibition were all down-regulated in the high-risk group. We also obtained 17 immune checkpoint-related genes that differed between high and low risk groups, of which LGALS9, BTLA and TNFSF15 had extremely high statistical significance.
The study found that AL035530.2 can be used as pyroptosis-related lncRNAs (PRLncs) to evaluate the prognosis and immunotherapy of endometrial carcinoma (EC) (21). PRR34-AS1 can not only promote the development of hepatocellular carcinoma by adsorbing miR-296-5p and up-regulating E2F transcription factor 2(E2F2) and SRY-box transcription factor 12(SOX12) to regulate the Wnt/β-catenin pathway(22), but also aggravate the progression of hepatocellular carcinoma by up-regulating forkhead box O3(FOXO3) by adsorbing miR-498(23). AC090559.1 can not only serve as an autophagy-related lncRNA (ARLncs) to guide the prognosis of lung adenocarcinoma(24), but also act as an FRLncs to correlate with the immunological research, treatment and prognosis of lung adenocarcinoma(25). ITCH-IT1 not only acts as a competitive endogenous RNAs (ceRNA) and is associated with the pathogenesis of colon adenocarcinoma, but also correlates with the overall survival of colon adenocarcinoma, which is a new important prognostic factor and potential therapeutic target for colon adenocarcinoma(26). AL031722.1 is differentially expressed in glioma and is associated with glioma prognosis(27). NBR2 inhibits tumor development by regulating the activation of adenosine monophosphate–activated protein kinase (AMPK)(28). The study found that the expression of NBR2 is down-regulated in OS, NBR2 inhibits epithelial-mesenchymal transition by regulating Notch1 signaling in OS cells, and acts as a tumor suppressor gene to inhibit the proliferation, invasion and migration of OS cells(29). In this study, we found that NBR2 is a low-risk FRLncs for OS, and the higher the expression of NBR2 in OS, the lower the risk of OS patients. This is consistent with previous findings that NBR2 is a tumor suppressor gene in OS. Among 13 FRLncs that can guide OS prognosis and immunity, AL035530.2, PRR34-AS1, AC090559.1, ITCH-IT1, AL031722.1 and NBR2 are all associated with tumor prognosis, among which NBR2 acts as a tumor suppressor gene to suppress OS cells proliferation, invasion and migration. This also reflects the reliability of the results of this study from the side. The related studies of AP000757.1, AC006160.1, LINC01719, AC100847.1, MIS18A-AS1, AC027575.2 and AC104561.1 have not been clearly reported.
The immune environment of OS is mainly composed of T lymphocytes and macrophages, but also contains other subsets such as B lymphocytes and mast cells(30). The numbers of CD56bright natural killer cells, immature B cells, M1 macrophages and neutrophils were increased in non-metastatic OS tissues compared to metastatic OS tissues, whereas the numbers of M2 macrophages were decreased(31). An OS prognostic risk model study of immune-related genes found that the proportion of naive B cells and M0 macrophages in the high immune score group was significantly lower than that in the low immune score group, while the number of M1 macrophages, M2 macrophages, and resting dendritic cells were significantly higher than the low immune score group(32). The study of OS macrophage risk model found that macrophages may be involved in the regulation of OS metastasis, macrophages (especially M1 macrophages) are infiltrated in small amounts in metastatic OS, and macrophages can be used as candidate markers of metastatic OS prognosis and immune checkpoint therapy(33). High M2/M1 macrophage ratio favors OS metastasis (34). In OS, tumor-associated macrophages (TAMs) not only promote tumor growth and angiogenesis, upregulate tumor stem cell-like phenotype, but also inhibit OS metastasis(35). An immune study of children with metastatic OS finds differences in neutrophils between metastatic and nonmetastatic patients (36). An OS prognostic model study of hypoxia-related genes finds that neutrophils are downregulated in high-risk groups (37). Neutrophil/lymphocyte ratio has certain diagnostic and prognostic value in OS patients(38). The number of NK cells in the peripheral blood of OS patients is low, and NK cells can prevent the development of OS tumors(39).
pDCs can rapidly and massively produce type I interferon (IFN-I/α)(40). However, in cancer,pDCs have impaired response to Toll-like receptor7/9 (TLR7/9) activation, and IFN-α production is reduced or lost, which contributes to the establishment of immunosuppressive tumor microenvironment(41).pDCs play an important role in regulating the innate and adaptive immune systems and are a key player in cancer immunity(41). Studies on the interaction between immune cells and OS microenvironment show that cancer growth is most rapid when anti-tumor immune cells and cytokines including DCs, helper T cells, cytotoxic cells and IFN-γ from increase to decrease, while Treg from decrease to increase(42). Some studies have pointed out that the cytokine secretion ability and cell proliferation ability of T follicular helper (Tfh) are significantly reduced in OS patients, which may be the reason why the body cannot resist the development of OS(43). miR-138 inhibits the PI3K/Akt/mTOR pathway by targeting and negatively regulating pyruvate dehydrogenase kinase 1(PDK1) to alleviate Tfh dysfunction in OS(44). TILs against tumor cells are depleted in the OS microenvironment, thereby accelerating tumor recurrence(45). Adjuvant chemotherapy plus TIL therapy prolongs survival in OS patients with poor response to neoadjuvant chemotherapy (45). In the microenvironment of cancer, innate immunity is suppressed in immune tolerance, and one of the mechanisms of immune tolerance is the immune checkpoint mechanism, that is, suppressing T cells to prevent excessive immune responses (46). Immune checkpoint molecules also play an important role in Treg and are involved in suppressing the function of cytotoxic T cells(46).
Immune functions such as APC-co-stimulation, check-point, cytolytic-activity and T cell-co-inhibition were down-regulated in the high-risk group. The study found that the anti-EGFR antibody cetuximab can enhance the cytolytic-activity of NK cells on OS(47). Immune check-point inhibitors (ICIs) alter the course of many cancers by unleashing a breakdown in the immune system, allowing the host's immune system to fight the tumor(48). A study of hypoxic prognostic features associated with OS metastasis and immune infiltration found that immune check-point were downregulated in high-risk populations(37). The correlation of APC-co-stimulation and T cell-co-inhibition with OS has not yet been clearly reported.
Study finds Galectin-9, encoded by LGALS9, is altered in multiple cancers(49). LGALS9 is not only associated with mRNA levels in cervical cancer cells(49), but is also a potential prognostic biomarker in pancreatic cancer(50). BTLA modulates immune responses in the tumor microenvironment and may be an alternative target for new immunotherapies(51). The study found that OS expresses BTLA on TIL, and blocking BTLA can induce anti-tumor immunity(51). A study of programmed death ligand 1 expression in drug-resistant OS found that BTLA may be a potential biomarker for OS, and the immune checkpoint protein profile in OS may change as OS becomes more drug-resistant or metastatic(52). TNFSF15, a member of the tumor necrosis factor (TNF) superfamily, is present in endothelial cells and is an endogenous angiogenesis inhibitor that inhibits cell growth and migration(53). TNFSF15 acts through death receptor-3 (DR3) and decoy receptor-3 (DcR3)(53). Histone deacetylase inhibitors VPA and trichostatin A (TSA) increase the expression of TNFSF15, but have little effect on its receptor DR3, and induce apoptosis in OS cells through VEGI/DR3 autocrine and paracrine pathways(54).
However, this study also has certain limitations. First, the sample size of this study was relatively small, and the sampling method did not eliminate the effects of gender and underlying diseases. Second, the FRLncs obtained in this study, which can guide OS prognosis and immune microenvironment, have not been experimentally validated, but this will be the content of our next study.