Our study aimed to illustrate the causal effects between CD3 + CD8 + TD T cell and colorectal cancer. We used MR analysis to investigate the association between CD3 + CD8 + TD T cell and colorectal cancer based on existing large publicly available genetic data and to demonstrate whether the causal relationship between them is mediated through VEGF-A. Our results suggested that genetically predicted CD3 + CD8 + TD T cell was associated with an increased risk of CRC (5.3% increased risk of CRC for every 1 SD increase in CD3 + CD8 + TD T cell), and 6.17% of this effect was mediated through VEGF-A.To date, we are the first to investigate the causal relationship between CD3 + CD8 + TD T cell and the risk of colorectal cancer by Two-step MR methods, while also demonstrating VEGF-A as their mediator.
Tumor immune microenvironment (TIME) include tumor cells, immune cells, cytokines,etc, the interactions between these components, which are divided into anti-tumor and pro-tumor, determine the trend of anti-tumor immunity(B et al., 2022).Although the immune system can eliminate tumor through the cancer-immune cycle, tumors appear to eventually evade from immune surveillance by shaping an immunosuppressive microenvironment.Tregs play a key role in maintaining immune homeostasis and peripheral tolerance, The Tregs acts as a feedback mechanism to inhibit anti-tumor immune response in the TIME through production of immunosuppressive cytokines, such as TGF-b, IL-10 and IL-35 (X et al., 2017),furthermore,Tregs inhibit the generation of memory CD8 + T cell through CTLA-4,and can induce NK and CD8 + T cell death in a granzyme B and perforindependent manner(E et al., 2023).In addition,Metabolism in the microenvironment, such as nutrient consumption, increased oxygen consumption, and production of reactive nitrogen and oxygen intermediates, significantly influences antitumor immune responses(L et al., 2021).
It has been vigorously debated as to whether the immune system can detect and eliminate malignant cells, prevented cancer, and referred to as “cancer immunosurveillance”,whether the immune system and especially CD8 + T cells survey and/or edit developing tumours remains an open question.CD8 T cells play crucial roles in immune surveillance and defense against infections and cancer. After encountering antigenic stimulation, naïve CD8 T cells differentiate and acquire effector functions, enabling them to eliminate infected or malignant cells(H et al., 2023).Traditionally, cytotoxic T cells, characterized by their ability to produce effector cytokines and release cytotoxic granules to directly kill target cells, have been recognized as the constituents of the predominant effector T-cell subset. However, emerging evidence suggests distinct subsets of effector CD8 T cells that each exhibit unique effector functions and therapeutic potential(CH et al., 2023).When tumours progress and sufficient tumour antigen is presented in draining lymph nodes, tumour- specific T cells can be activated in the draining lymph node or within tumour tissue in a largely non- inflammatory, non- stimulatory context, inducing an anergic, early dysfunctional T cell state (phase 1). The tumour continues to progress, inducing an immunosuppressive microenvironment. Persistent tumour antigen and microenvironmental signals drive tumour- specific T cells into a late dysfunctional state (phase 2)(M and A, 2021).Tumour- induced CD8 + T cell dysfunction was previously thought to be established late during cancer development, induced by the immunosuppressive TME of established solid tumours(KG et al., 2017)(comprising, for example, myeloid- derived suppressor cells (MDSCs), FOXP3 + CD4 + regulatory T cells, interleukin-10 (IL-10)) .However,cancers in humans usually derive from a single transformed cell and develop slowly, sometimes over several years, through clonal evolution(JRM and N, 2021). Between the initial transformation event in a normal cell and the eventual progression to a clinically detectable established tumour,cancers are present as undetectable lesions, which must evade T cell recognition and elimination. In patients,when and how during tumorigenesis tumour- specific T cells differentiate to a hyporesponsive state and how this state evolves over time remain incompletely Understood(S and TF, 2018).T cells follow a triphasic distinct pathway of activation, proliferation and differentiation before becoming functionally and phenotypically “exhausted” in settings of chronic infection,autoimmunity and in cancer. Exhausted T cells progressively lose canonical effector functions, exhibit altered transcriptional networks and epigenetic signatures and gain constitutive expression of a broadcoinhibitory receptor suite(DJ et al., 2020).Although MSI (microsatellite instable)/MSS(microsatellite stable) status, TMB (tumor mutational burden),POLE/POLD1 mutation, and MSI-like gene signature are widely used as indications of whether CRC a patient should receive immunotherapy, these features cannot fully explain anti-PD1 resistance and exhibit limited accuracy in predicting the response to immune checkpoint inhibitor treatment, due to they only directly or indirectly indicate the potential of high quantity of tumor-infiltrating CD8 + T cells in tumor samples while ignoring the exhaustion state of CD8 + T cells (AF et al., 2019). Therefore, dissecting the TEX status in the tumor microenvironment seems to occupy a more important position in predicting the response to immunotherapy in CRC(JT et al., 2023).Sun Tian et al(S et al., 2021)developed a TMEPRE method that models signatures of CD8 + T-cell inltration and CD8 + T-cell exhaustion states in the tumor microenvironment of colorectal cancer,TMEPRE showed predictive power in three datasets of anti-PD1-treated patients(p = 0.056, 0.115, 0.003). CD8 + T-cell exhaustion component of TMEPRE model correlates with anti-PD1 responding progenitor exhausted CD8 + T cells in both tumor and viral infection (p = 0.048, 0.001).Alexandros Lalos et al(A et al., 2021)found that the combined high CD8 + T-cell infiltration and expression of SDF-1 shows a favorable 5-year overall survival rate (66%; 95% CI 48–79%) compared to tumors showing a high expression of CD8 + T-cell only (55%; 95% CI 45–64%; p = 0.0004). The combined high expression of SDF-1 and CD8 + T-cell density was an independent, favorable, prognostic marker for overall survival (HR = 0.34, 95% CI 0.17–0.66; p = 0.002 and HR = 0.45, 95% CI 0.23–0.89; p = 0.021, respectively),The combination of high CD8 + T-cell density and expression of SDF-1 represents an independent, favorable, prognostic condition in CRC, mostly in patients with stage III disease.In our study,CD3 + CD8 + TD T cell played an immune-suppressing role and increasing risk of colorectal cancer,This suggested that different subsets of effector CD8 + T cells have unique effector functions and therapeutic potential, as well as the complexity of tumor immune escape mechanisms.
VEGF-A plays a critical role in promoting angiogenesis, a vital process for the sustenance and metastasis of tumors, VEGF-A enhances tumor growth and metastasis by activating receptors such as VEGFR-2, triggering signaling pathways like PI3K/Akt and MAPK that drive endothelial cell proliferation and migration(ZH et al., 2020). Furthermore, CD3 + CD8 + TD T cells contribute to this process by secreting pro-inflammatory cytokines, including TNF-α and IL-6, which activate signaling pathways in tumor and stromal cells, thereby promoting VEGF-A synthesis and release(I and N, 2021). Within the tumor immune microenvironment (TIME), these T cells interact with other cell subsets, such as macrophages, to regulate VEGF-A expression through both cytokine signaling and direct cell-to-cell contact, influencing angiogenesis(Y and RA, 2022). However, under certain conditions within the TME, CD8 + T cells can be reprogrammed to support tumor growth, including the enhancement of VEGF-A expression and angiogenesis, potentially via activation of immune checkpoint pathways like PD-1/PD-L1(Z et al., 2021). These T cells are also implicated in altering cellular signaling networks within the TME, involving direct cellular interactions or paracrine signaling through cytokines such as IFN-γ and TNF-α, which impact angiogenesis and tumor dynamics(AA et al., 2021). Lastly, the role of CD3 + CD8 + TD T cells in chronic inflammation further underscores their contribution to cancer progression, as chronic inflammation promotes VEGF-A expression and tumor angiogenesis through pathways like COX-2 and NF-rB(K et al., 2023). Therefore, CD3 + CD8 + TD T cells can increase the risk of colorectal cancer by mediating VEGF-A.However, this mediation accounts for only 6.17% due to the complexity of the tumor immune microenvironment (TIME). Other immune cells like regulatory T cells and myeloid-derived suppressor cells (MDSCs) within the TME can counterbalance the pro-tumorigenic actions of CD3 + CD8 + TD T cells(WJ et al., 2023). Additionally, cancer cells themselves, by altering immune responses and promoting angiogenesis, create a feedback loop that influences the behavior of immune cells, including CD3 + CD8 + TD T cells(RV and A, 2019). Furthermore, the extracellular matrix (ECM) and stromal cells contribute to the dynamic TME, affecting immune cell infiltration and function(TE et al., 2023). Thus, the impact of CD3 + CD8 + TD T cells is moderated by various factors within the TME, Understanding these intricate interactions is crucial for developing targeted colorectal cancer therapies, particularly those modulating immune responses and angiogenesis.
4.1 Limitations
This study conducted two-sample and two-step MR analysis based on the published results of large GWAS cohorts, so it has high statistical efficiency. The conclusions of this study are based on genetic instrumental variables, and causal inference is made using a variety of MR analysis methods. The results are robust and were not confounded by horizontal pleiotropy and other factors. Our study has several limitations. First, our analysis was performed using the European population, which limits its universality. Second,For sensitivity analysis and horizontal pleiotropy testing, more genetic variants need to be included as instrumental variables. Therefore, the SNPs used in the analysis did not reach the traditional GWAS significance threshold (P < 5 × 10− 8). Third, we used summary-level statistics in our study, not individual-level data. Therefore, we cannot further explore causal links between subgroups such as Colorectal cancer with different TNM stages. Fifth, our study demonstrates that genetic prediction of CD3 + CD8 + TD T cell mediated by VEGF is 6.17%, which is very low. Thus, more studies are needed to quantify other mediators.