Although epidemiological, genetic, and biological studies have firmly established that the inflammation-immune response was associated with BCa, they have not been able to capture causal link and the whole picture. Here, we designed a MR analysis using aggregated data of inflammation-related cytokines and immune cells from two GWASs. We identified that 4 inflammation-related cytokines and 17 immune cells play an important role in the development of BCa, and contribute to a poorer prognosis in BCa patients. These potential biomarkers may play an vital role in the progression of BCa, providing new clues for the prevention and treatment of BCa.
4.1 Inflammation-related cytokines are one of the important mechanisms of BCa
Inflammatory-immune response is a complex regulatory network involving the production and release of cytokines and effector cells, and plays a crucial role in the occurrence and development of malignant tumors [7, 8, 16, 19]. As show in Fig. 1, vascular endothelial cells initially release many factors such as growth factors, Ep-CAM, E-selectin, and etc. which promote the recruitment and differentiation of inflammatory cells, resulting in a significant increase in the production of inflammatory cells [35–37]. Simultaneously, activated inflammatory cells secrete a range of inflammatory mediators, including ILs, reactive oxygen species, free radicals, and 5-HT, which act on tumor tissues and cause the TME to be in a state of chronic inflammatory response [38]. Tumor and inflammatory cells secrete a variety of inflammation-related cytokines in bone marrow, affecting the differentiation of bone marrow hematopoietic stem cells (HSCs) into myeloid cells and lymphocytes [39]. These cytokines also contribute to the production of myelosuppressive cells, which migrate to the tumor site and play a tumor immunosuppressive role, leading to the development of a special microenvironment in the bone marrow [40]. In addition, bone marrow mesenchymal stem cells (BMSCs) can inhibit the activity of immune cells by reducing the production of pro-inflammatory cytokines such as IL-12 and prostaglandin E2 (PGE2) [41].
ILs are important communication tools of innate and adaptive immune cells, and they play a crucial role in the occurrence, development, and regulation of cancers [35, 42]. With the deepening of research, it has been found that IL-2 exhibits dual activities. High-dose IL-2 stimulates the growth of effector T cells, thus exerting anti-cancer effects [43]. Low-dose IL-2 selectively activates Treg cells and suppresses immune activation [44]. Previous studies have shown that IL-2 and IL-2-activated NK cells are significantly high expressed in both low-grade and high-grade BCa, and are associated with better prognosis [45]. Therefore, IL-2 immunotherapy has been explored as a potential treatment for BCa. In addition to IL-2, other members of the IL family, such as IL-1, IL-4, IL-6 and IL-10, have also been found to be related to the occurrence, development, and treatment of BCa [46–48]. In this study, we found that IL-2Rβ was positively correlated with the occurrence of BCa. However, we did not found a statistically significant association between other cytokines of the IL family and BCa progression in our MR analysis, which may be due to the small sample size of the included studies and the statistical methods used. In future studies, it will be necessary to include larger sample sizes to further investigate the specific role of various cytokines of the IL family in BCa using MR analysis.
NT-3 is a member of the family of neurotrophic factors, which can promote the growth and development of central nervous system and peripheral neurons, and help repair damage [49]. Although NT-3 is primarily associated with the nervous system, recent studies have discovered that they were also involved in tumor development [50, 51]. Previous studies have indicated that NT-3 levels were elevated in the urine of patients with interstitial cystitis and BCa, which was consistent with certain clinical and pathological features [52]. In our study, we also observed that NT-3 served as a risk factor for BCa and can contribute to its development. It is hoped that in the future, through further studies of large clinical cohorts and the underlying mechanisms, NT-3 levels could be used as a novel target to assist in the diagnosis, monitoring, and treatment of BCa.
5-HT is another important cytokine found in the nervous system and is synthesized mainly in the brain and gastrointestinal tract [53]. Recent studies have demonstrated that different 5-HT receptors were significantly involved in the progression of malignancies, including breast cancer, prostate cancer, colon cancer, and BCa [54–56]. It has been observed that 5-HT can stimulate the proliferation and metastasis of BCa, and the use of 5-HT antagonists can significantly inhibit the growth of BCa cells [57, 58]. In addition, tumor-related inflammatory responses stimulate the release of 5-HT by immune cells, leading to elevated 5-HT expression in BCa [55]. In this study, we found that 5-HT was a positively correlation risk factor for BCa through MR analysis, which is consistent with the results of previous studies.
E-selectin is an inducible member of the selectin adhesion receptor family and is expressed only in endothelial cells stimulated by pro-inflammatory cytokines such as TNFα and IL-1β [59]. E-selectin can bind to other cell adhesion molecules and promote extravasation into inflammatory tissues [60]. Multiple studies have consistently shown that E-selectin played a pivotal role in tumor metastasis, such as colon cancer and BCa, and its binding efficiency was directly related to its metastatic potential [61, 62]. Higher serum E-selectin level in patients with BCa was found to be associated with poor prognosis [61]. In order to further investigate this phenomenon, we conducted a MR analysis, and the results revealed that E-selectin level was significantly positively correlated with the occurrence of BCa, which indicated that E-selectin level can be used as a valuable indicator for auxiliary diagnosis and prognostic evaluation of BCa.
4.2 Immune cells represent a double-edged sword for BCa
4.2.1 Some granulocytes have been linked to an increased risk of BCa
Immune cells secrete chemokines when inflammation is activated [63]. Peripheral immune cells, such as monocytes, neutrophils, mast cells, T cells, and B cells, regulate TME by secreting various factors and indirectly promote tumor growth [64]. Simultaneously, myeloid-derived suppressor cells (MDSCs) are mobilized into TME to adapt to local inflammation [65]. Our MR analysis revealed that MDSCs, monocytes, neutrophils, basophils, eosinophils, T cells and their different types were significantly associated with BCa.
Monocytes, eosinophils, and neutrophils are all leukocytes that play crucial roles in the immune system. Monocytes have become an important regulator of cancer occurrence and progression [66], and some subtypes of monocytes can promote tumor growth and some subtypes can inhibit tumor cell metastasis [67]. Eosinophils possess immunoregulatory and homeostasis functions, and tumor-associated tissue eosinophilia (TATE) has been found to be associated with a variety of hematologic and solid malignancies, including colorectal cancer, breast cancer, nasopharyngeal cancer, oral cancer, gastric cancer, and head and neck cancer [68, 69]. Basophils have also been identified in many types of human cancers, including lung cancer, gastric cancer, pancreatic cancer, and ovarian cancer [70, 71]. In this study, we found that these granulocytes were associated with a higher risk of BCa through MR analysis. These associations may be due to an increase in various stimulatory factors secreted by tumor cells that promote the differentiation and maturation of these immune cells [19, 72]. Therefore, in clinical practice, the increase of these cells could be served as a crucial reference indicator for auxiliary diagnosis, risk prediction, and efficacy evaluation of BCa. However, further studies are needed to validate these findings.
Granulocytic myeloid-derived suppressor cells (GMDSCs) are pathologically activated neutrophils and monocytes with strong immunosuppressive activity [73]. These cells play a crucial role in regulating immune responses under various pathological conditions and are strongly associated with adverse clinical outcomes in cancer [74, 75]. Previous studies have shown that the reduction of GMDSCs may be a significant factor in the improvement of BCa through chemotherapy [76]. In our study, we observed a significant association between GMDSCs and an increased risk of BCa, including various CD33 subtypes on IMDSCs. These results suggested that GMDSCs could serve as a potential marker for personalized treatment of BCa patients and should be thoroughly screened in clinical practice. The development of effective novel techniques targeting GMDSCs and their subtypes is expected to improve the effectiveness of tumor immunotherapy.
4.2.2 HLA DR + T cells may have a protective effect against BCa
Cellular immune responses, such as T cells and B cells, play a crucial role in the immune response [77]. T cells carry out this function through different subpopulations. CD4, CD8, CD14, CD33, and HLA-DR are commonly used biomarkers to identify different subtypes of T cells [78]. CD4 + T cells, also known as helper T cells (Th), play a crucial role in supporting humoral and cellular immunity [79, 80]. CD8 + T cells, also known as cytotoxic T cells (CTLs), have the function of directly eliminating target cells [81, 82]. CD33 is a myeloid cell differentiation antigen that can be expressed on a variety of cell types, including normal pluripotent stem myeloid precursor cells (hematopoietic stem and progenitor cells), unipotent stem colony-forming cells, mature granulocytes, and monocytes [83]. HLA-DR belongs to human major histocompatibility complex class II (MHC-II) antigens, which are uniformly expressed on the surface of B lymphocytes, monocytes, and macrophages, and its main role is to participate in antigen presentation to CD4 + T cells [84, 85].
HLA-DR has been shown to activate T cells in the presence of viral infection [86], and HLA-DR expression is consistent with the presence of monocyte infiltration on the bladder wall, consisting mainly of activated helper T lymphocytes [87]. In this study, we observed that HLA DR + T cell AC, HLA DR + T cell %T cell, HLA DR + CD4 + T cell AC, HLA DR + CD4 + T cell %lymphocyte, HLA DR + CD8 + T cell AC and HLA DR + CD8 + T cell %T cell were negatively associated with the risk of BCa. Previous studies have shown that HLA-DR was associated with the pathological stage and prognosis of BCa [88, 89]. Our MR analysis was consistent with these findings, indicating that HLA-DR had a significant immunoregulatory role in BCa. These results suggested that HLA DR + T cells, HLA DR + CD4 + T cells, and HLA DR + CD8 + T cells may be potential protective factors for BCa and can be used as biomarkers of T cell immune status and potential therapeutic targets in the future.
Conversely, CD33-positive immune cells have been identified as a risk factor for the development of BCa due to their involvement in immune evasion or suppression. These cells included CD33 + HLA DR + CD14dim AC, CD33dim HLA DR AC and CD33 on IMDSCs. CD33 activation in early myeloid cells is considered a risk factor for poor prognosis in BCa, making it a potential therapeutic target [90]. Currently, several antibody-drug conjugates targeting CD33 have been used in clinical treatment of hematological tumors [91, 92]. With further research, these drugs are expected to play significant roles in the immunotherapy of solid tumors. In summary, the era of precision medicine emphasizes personalized diagnosis and treatment protocols, which require advanced patient immune assessment and further subdivision of lymphocyte subpopulations, paving the way for precise and individualized treatment approaches.