TREM-1, as a novel inflammatory receptor, played an important role in inflammatory and immune responses in the tumor microenvironment(Bouchon et al., 2000; Colonna, 2003). In the present study, we proved the expression of TREM-1 in GBM was much higher than that in normal brain tissue and high expression of TREM-1 in GBM predicted poor prognosis.
The results are consistent with previous studies in HCC(Liao et al., 2012), RCC(Ford et al., 2021) and breast cancer(Pullikuth et al., 2021). In NSCLC, high TREM-1 expression in TAMs and high soluble TREM-1 level were also the poor prognostic factor (Ho et al., 2008; Kuemmel et al., 2018). Both in human lung cancer xenografts and pancreatic cancer xenograft mouse models, TREM-1 inhibitors all could exhibit strong antitumor effect(Shen & Sigalov, 2017; Sigalov, 2014). In pancreatic cancer, the anti-tumor effect correlated significantly with increased survival and suppressed TAM infiltration. Blockade of TREM-1 significantly reduced the serum levels of IL-1α, IL-6 and macrophage colony-stimulating factor (M-CSF)(Shen & Sigalov, 2017).
Functional enrichment analysis showed TREM-1 was predominantly involved in inflammatory response, TNF signaling via NF-κB and JAK-STAT signaling pathways, which in consistent with the previous reports on the function and mechanisms of TREM-1. TREM-1 can stimulate neutrophil and monocyte-mediated inflammatory responses through the transmembrane adapter protein DAP12 and induces secretion of inflammatory chemokines and cytokines. After phosphorylation of DAP12, the production of chemokines and cytokines was induced, such as TNF-α and IL-1β(Carrasco et al., 2019; Duan et al., 2015; Tessarz & Cerwenka, 2008). NF-κB was a key orchestrator of innate immunity/inflammation and aberrant NF-κB regulation had been observed in many cancers(Karin, 2006). In both tumor and inflammatory cells, NF-κB was activated downstream of the toll-like receptor (TLR)-MyD88 pathway and of the inflammatory cytokines TNF-α and IL-1β. TREM-1 possesses the ability to amplify signaling by the TLR4 or TLR2(Arts et al., 2011). STAT3 was a point of convergence for numerous oncogenic signaling pathways(Yu et al., 2007). TREM-1 blockade could prolong survival of rats with polymicrobial sepsis and attenuate systematic inflammatory responses through the JAK2/STAT3 signaling pathway(Ford et al., 2021).
TNF played a major role in the proinflammatory cytokines. Tumor promotion by this cytokine could involve different pathways: TNF enhances tumor growth and invasion, leukocyte recruitment, angiogenesis and facilitate epithelial to mesenchymal transition(Balkwill, 2009; Kulbe et al., 2007). Caer et al. showed TNF correlated with TREM-1-expressing monocytes and TREM-1 pathway may act in concert with other factors in the intestine of Crohn’s disease patients, such as TNF, to influence the inflammatory environment (Caer et al., 2021).
Inflammasome-dependent release of cytokines and antigen can activate, shape or even inhibited adaptive immune responses(Deets & Vance, 2021). So, the relationship between TREM-1 and tumor-infiltrating immune cells, ICGs were further explored in GBM.
Macrophages, as important antigen presenting cells, can determine the preference of T cell response, and the cytokine pattern secreted by activated T cells is determined by the cytokine pattern secreted by M1/M2(Aras & Zaidi, 2017). TAMs are closely related to immune response in tumor microenvironment and recognized as antitumor suppressors. TIICs analysis showed M2 Macrophages was the highest immune cell in GBM. High macrophage infiltration can mediate the tumor immunosuppressive microenvironment in a variety of ways to promote tumor genesis, development and metastasis(Aras & Zaidi, 2017). Up-regulation of TREM-1 was observed frequently in dendritic cells and occasionally in TAMs(Ho et al., 2008; Klesney-Tait et al., 2006). Additionally, TREM-1 can mediate macrophage polarization by regulating the PI3K/AKT signaling pathway in HCC(Chen et al., 2021). But, in the present study, TREM-1 expression was positively associated with the infiltrating level of Macrophage (TIMER) and negatively associated with the M1 Macrophages (CIBERSORT algorithm), but not the M2 Macrophages. So, the relationship between TREM-1 and M1/M2 Macrophages in GBM needs to be further verified in tissue specimens.
In addition to immune cells, the immune microenvironment also includes non-immune stromal components, which also closely associated with oncogenesis and malignant behaviors of tumors(Bremnes et al., 2011). TREM-I was positively associate with the stromal score and immune score. In the TCGA-GBM cohort, the survival time of both high stromal score and immune score groups were shorter than that in the low groups. But there was no statistical difference(Jia et al., 2018). Zeng’s study showed that immune score was linked to favorable OS, while stromal scores were linked to unfavorable OS, and patients with low immune score and high stromal score exhibited the worse survival time(Zeng et al., 2021). Ren et al reported that interaction between stromal cells and epithelial cancer cells affected the cancer progress in pancreatic cancer(Ren et al., 2018).
TREM-1 was highly expressed in GBM, in which immune therapy were less effective due to the immune suppression microenvironment (Bagley et al., 2018). Study found that in HCC, blocking the TREM-1 pathway can not only inhibit tumor development, but also improve the efficacy of PD-L1 by reducing the recruitment of CCR6 + Foxp3 + Tregs (Wu et al., 2019). TREM-1 inhibitors may be an effective adjuvant that enhances anti-PD-1-mediated immunogenic cell death in microsatellite stable (MSS) colorectal cancer (CRC)(Roh et al., 2021). These studies suggest that TREM-1 may be involved in the resistance of ICIs. And if there were relationships between TREM-1 expression and immune checkpoint genes, as well as PD-L1? In our analysis of the TCGA GBM cohort, we observed TREM-1 significantly correlate with not only PD-L1(CD274), but also multiple ICGs.
Among the ICGs, CD80(CD86)/CD28/ ICOS (the receptor of B7h), PD-L1/PD-L2(PDCD1LG2) and TMIGD2(the receptor of HHLA2) were the members of B7-CD28 family and belonged to groups 1, 2 and 3 respectively(Janakiram et al., 2017). It was well established that engagement of CD28 by its ligands CD80 or CD86 provides key costimulatory signals to activate the T cell activation(Janakiram et al., 2017). B7-1/B7-2/CD28/CTLA-4 pathway was important in modulating central immune tolerance while PD-L1/PD-L2/PD-1, B7-H3, B7x, and HHLA2 were important in peripheral immune regulation(Janakiram et al., 2017). ICIs targeting CTLA4 and PD-1/PD-L1 been explored in GBM(Cloughesy et al., 2019; Schalper et al., 2019; Youssef & Dietrich, 2020).
In the correlation analysis, TREM-1 was significantly associated with the expression of CD86, as well as in the PPI network. Previous study showed that TREM-1 inhibitory peptide can attenuate proinflammatory subtype transition of microglia, which was evidenced by the decreased levels of markers including CD68, CD16, CD86(Wu et al., 2021).
TNFR/TNF family had a diverse and complex set of interactions with the immune system and could influence T cell responses in a number of ways, and many inhibitors targeting TNF family had been explored(Croft et al., 2013). Among the significant associations of TREM-1 with ICGs, TNFSF9, TNFRSF9, TNFSF14, TNFRSF14, TNFRSF18, TNFSF15, CD40 and CD70 were the members of TNFR/TNF family. TNFRSF14 expression in GBM is associated with worse overall survival and disease‑free survival and a lower Th1 response(Lombardo et al., 2020). TNFRSF9 is strongly overexpressed in human diffuse gliomas as compared with the normal central nervous system(Blank et al., 2015) and the antitumoral effects and prolonged survival were observed(Kim et al., 2001). In lung cancer, low TNFRSF9 expression level in Tregs was associated with enhanced overall survival rate and response to anti-PD-1 immunotherapy, proposing that TNFRSF9 promotes immune suppressive activity of Tregs in tumor (Cho et al., 2021).
CD70 had also been identified have (Wischhusen et al., 2002) the ability to induce immunosuppression
through tumor-induced apoptosis of T-lymphocytes in glioblastoma. Different from the above members of TNFR/TNF family, high CD40 expression was recognized as a poor clinical outcome, but a good performance in predicting ICI response(Yan & Richmond, 2021). High expression level of TREM-1 indicated a poor response to anti-TNF drugs, which may be associated with the decreased differentiation to M2 type regulatory macrophages inflammatory bowel disease (IBD)(Prins et al., 2021). These studies suggest that TREM-1 may have staggered interactions with TNF signaling pathway, which was consistent with our previous functional enrichment analysis.
In addition to those mentioned above, several related ICGs were involved in immunosuppressive responses, such as HAVCR2(TIM-3), LGALS9, CD48(SLAMF2) and ligands CD244(SLAMF4), VSIR and LAIR1. The ICGs could exert immunosuppressive effect through different functions and were new research targets of cancer immunotherapy(Katabathula et al., 2022; McArdel et al., 2016; Peng et al., 2020; Solinas et al., 2019; Sun et al., 2021; Wang et al., 2020). The correlation between TREM-1 and multiple ICGs in tumor microenvironment suggested the potential immunosuppressive effect of TREM-1 in GBM. Combined blocking of TREM-1 and other ICGs may be a strategy for enhancing GBM immune response.