Prior to the advent of ICIs, surgery and platinum-based systemic chemotherapy were the standards of care. However, only 20% of IMBC patients are fit to receive chemotherapy and almost half of them may have sequelae with poor prognosis [24]. About 30% − 50% of metastatic IMBCs cannot receive cisplatin due to comorbidities and until recent years platinum-based regimes were still the only treatment option, with no inspiring effect in clinical outcomes[25]. Nowadays, immunotherapy represented by ICIs has gradually revolutionized the treatment paradigm of BLCA. Indeed, the intravesical use of bacillus Calmette–Guérin (BCG) for treating NMIBCs has proven the immune response characteristic of BLCA since the 1970s [26]. Tumorigenesis is a process of mutation accumulation. BLCA has the third-highest level of somatic mutations and is highly antigenic which may facilitate the immunological recognition[27]. Consequently, it is crucial to fully understand and explore the role of TMB in the immunotherapy of BLCA.
In the present study, we found that BLCA patients with higher TMB had more survival benefits. This is consistent with the results of some previous studies, such as cutaneous melanoma and ovarian carcinoma [28]. For patients treated with ICIs, high TMB has been confirmed to be associated with good response and better OS [29]. However, Cao et al. made a pooled analysis of 103,078 patients to evaluate the predictive efficiency of TMB and found the prognostic effect of TMB varied in patients with or without ICIs treatment, which might be related to tumor types [30]. Another research also indicated that the patients without ICIs treatment in some tumors may suffer a worse prognosis despite having high TMB [31]. Therefore, for the real application of TMB, it is still necessary to determine and assess its exact role in different tumors by more clinical trials. Moreover, meaningful GSEA results were all enriched in the low TMB group and most were immune-related, indicating that low TMB might enhance the tumor heterogeneity in BLCA. Of the GSEA results, ECM receptor interaction plays a crucial role in regulating invasion and progression of tumors. Cancer cells in the ECM of TME can release signals to mislead immune cells to avoid being attacked [32]. Besides ECM, the overactivation of focal adhesion pathway can result in dysfunction of cell migration and influence immune cells chemotaxis, which lead to tumor metastasis [33].
In terms of immune cell infiltration, the high proportion of CD4+ T cell, CD8+ T cell and Tfh may be an important factor leading to better OS in the high TMB group. Tumor-infiltrating immune cells are a hallmark of immune surveillance and an integral part of complex microenvironment regulating tumor progression [34]. CD8+ T cells have strong cytotoxic activity for killing cancer cells, being considered as main drivers of anti-tumor immunity. The depletion in numbers and dysfunction of CD8+ T cells in TME create a favorable condition for cancer cell proliferation and metastasis [35]. CD4+ T cells and Tfh are also required and play a prominent role in anti-tumor immunity. CD4+ T cells can directly eliminate tumor cells through cytolysis or indirectly regulate TME to target tumor cells [36]. And Tfh may conduce to the formation of tertiary lymphoid structures in primary site and thereby promote intertumoral immune response of CD8+ T cells and B cells [35]. ICIs can help T lymphocytes restore activity and break through the physical barrier of TME to promote T cell homing, thus activating anti-tumor immunity and improving the effect of immunotherapy [37].
After strict screening, NTRK3 was finally selected as a potential biomarker due to its excellent prognostic value and immune infiltration correlation. It encodes TRKC protein, a member of neurotrophic tropomyosin receptor kinases (TRK) family which regulates many aspects of neuronal development and function. After binding to the ligand neurotrophin-3 (NT-3), TRKC autophosphorylates and motivates various signaling pathways such as MAPK and PI3K/AKT pathways which can regulate cellular growth and differentiation [38]. In recent years, many studies have reported that TRK pathway aberrations such as single nucleotide variation, gene fusion and gene overexpression are involved in the pathogenesis of many cancers, among which NTRK3 gene fusion is the most fully verified carcinogenic event [39]. Except for the functional relevance of TRKC in the nervous system, the overexpression of TRKC is observed in many types of tumors, including neuroblastoma, breast cancer, hepatocellular carcinoma and metastatic melanoma. TRKC plays an important role in regulating angiogenesis, inducing tumor growth, preventing apoptosis and promoting metastasis. Abnormal activation of NTRK3 and its fusion proteins may regulate the epithelial-mesenchymal transition (EMT) process, tumor growth rate and tumorigenicity by activation of several signaling pathways [40].
In our results, the expression level of NTRK3 was significantly positively correlated with the patients’ immune/stromal scores, suggesting that NTRK3 may be inherently regulated by the TME. Moreover, NTRK3 also showed good correlation with a variety of immune lymphocytes. Among the related immunomodulators, we are particularly interested in CCL14 and CXCL12, both of which were strongly positively co-expressed with NTRK3 and had prognostic value. CCL14 represents a C-C type chemokine with high concentrations in human plasma, mainly involved in the transport of lymphocytes and inflammatory cells [41]. The role of CCL14 in tumor progression is unclear and has been poorly reported, especially in bladder cancer. Yu et al. found that high expression of CCL14 played a protective role which can promotes apoptosis of cancer cells and improve survival time in hepatic carcinoma[42]. However, Li et al. found that inhibiting the expression of CCL14 could effectively suppress the metastatic potential and angiogenesis of breast cancer [43]. In addition, there is a very interesting theory. Tumors can actively release chemokines to regulate the microenvironment so as to transfer the host immune response from immunogenic to tolerogenic, thereby achieving immune escape and promoting tumorigenesis and metastasis. Shields et al. expounded this view in their research on CCL21 [44]. Thus, the underlying mechanism of the interaction between CCL14 and NTRK3 and how they jointly affect the progression of BLCA still needs more studies to explore. As for CXCL12, it has been extensively studied in the field of cancers. CXC12, also known as stromal cell derived factor-1 (SDF-1),was first characterized as a pre-B cell growth factor and combines its receptor CXCR4 as a signaling axis which mainly participated in a lot of physiological processes such as hematopoiesis, immune responses and vascular formation [45]. Studies have shown that cancer cells in TME can induce overexpression of CXCL12 by autocrine or paracrine, and then activate downstream pathways to affect immune status and promote cancer cell proliferation and distant metastasis. Blocking CXCL12/CXCR4 signaling axis may inhibit tumor growth and provide new ideas for immunotherapy [46].
The current study exists several limitations. First, our study is retrospective based on public databases and problems such as insufficient and limited data are inevitable. Second, the relationship between TMB and the response to ICIs cannot be evaluated in BLCA due to a lack of immunotherapy information of patients. Third, our results need to be verified by other independent patient cohorts with mutation information and by laboratory or clinical experiments.