As one of the most common malignancies worldwide, CRC poses a major risk to human health due to its escalating death rate [1]. Tumor-infiltrating immune cells are linked to the anti-tumor effects produced by different chemotherapy regimens. Our study evaluates gene features related to immune response using the ESTIMATE algorithm [21], calculates the immune cell's makeup in tumor tissue using CIBERSORTx [17], analyzes different patterns of tumor immune cell infiltration under different chemotherapy regimens, and examines their association with the response, PFS, and OS of CRC patients after chemotherapy. Importantly, existing literature provides evidence for the reliability of this study.
In numerous studies prior to this, it has been demonstrated that immune cells infiltrating in tumor microenvironment participate in the tumor immune response [26–29]. Similarly, in the microenvironment of CRC, numerous immune cells, for example, dendritic cells, natural killer cells, macrophages, CD4+ T cells, and CD8+ T cells, are known to infiltrate tumors [28] [30–32]. Moreover, various immune cells have distinct functions inside the tumor microenvironment. Tumor-associated macrophages exert inhibitory phagocytosis by expressing PD-1; thus PD-1 therapy may exert anti-tumor effects by directly acting on macrophages [33]. The content of tumor-infiltrating plasmacytoid dendritic cells (pDCs) is substantially positively connected with CRC patients' prognosis [13]. When CRC patients have highly infiltrative effector memory T cells, pathological signs of early metastatic invasion (VELIPI) are less likely to occur. Furthermore, a better clinical outcome is linked to a greater level of CD45RO+ cell infiltration [34]. Galon et al. showed that the prognosis of patients with CRC is strongly connected with the high infiltration of effector and memory T lymphocytes [12]. A lower CD4+/CD8+ ratio can significantly improve CRC patients' 5-year survival rate [35].
In the current study, we utilized publicly available CRC databases to evaluate immune response-related gene features by ESTIMATE algorithm, quantified the relative proportions of 22 tumor-infiltrating immune cell subtypes in each dataset using CIBERSORTx, and systematically analyzed the effect of immune cells that infiltrate tumors on CRC patients' prognosis after chemotherapy. The results indicate that under the FOLFOX chemotherapy regimen, M1 macrophages are negatively correlated with the drug response rate. Typically, M1 macrophages are considered to have anti-tumor effects because they can secrete pro-inflammatory factors and chemokines with anti-tumor cell activity [36, 37]. Compared to the results of our study, Edin et al. found that as M1 macrophage infiltration increases, patient survival rates also tend to increase [38]. However, several investigations have demonstrated a link between macrophages and a negative prognosis for patients. Research has identified a unique way that CXCL12 influences macrophages to encourage tumor growth through GMCSF/HB-EGF paracrine signaling [39]. A negative correlation has been observed between the prognosis for patient survival and tumor-associated macrophages [40, 41]. Furthermore, we discovered that resting dendritic cells and eosinophils, two immune cells linked to patient prognosis, were linked to a poor PFS, which was in line with the findings of the univariate analysis.
Similarly, under the FOLFIRI chemotherapy regimen, the medication response rate has an adverse relationship with M0 macrophages, while activated natural killer cells are connected to patients' low PFS and OS. In the multivariable regression model, resting mast cells and CD8+ T cells were also negatively correlated with drug response rates. Cao et al. discovered a substantial correlation between low OS and high levels of M0 macrophages [11]. Additionally, an essential part of the tumor immune response is played by CD8+ T cells; there are also many studies exploring their immune function in CRC. Wang et al. showed that Th17 cells reduce CD8+ T cells in late-stage CRC patients through the IL-17A/STAT3/CXCR3 axis, but contrary to these study results, survival analysis revealed that higher OS is predicted by increased expression of CD8 [42]. A study in 2021 found that a favorable patient survival prognosis is linked to both high levels of fatigue and low residency programs in NK cells, as well as high levels of exhaustion and low TGFb signaling in CD8+ T cells and NK cells [43]. Therefore, our study results are supported to some extent by previous research findings.
Consensus clustering analysis was carried out on all samples' immune cell components, resulting in the identification of three immune clusters. Among them, resting memory CD4+ T cells, resting B cells, and M0 macrophages were more prevalent in the third immune cluster, leading to a higher drug response rate and improved OS after FOLFOX regimen treatment. Conversely, poor OS was linked to a higher proportion of activated mast cells and a lower proportion of M0 macrophages. A study identified immune cell infiltration traits (activated mast cells, M0-M2 macrophages, and resting memory CD4+ T cells) linked to the prognosis of CRC, with these five immune cells serving as prognostic factors for CRC. Among them, M0-M1 macrophages and resting memory CD4+ T cells are safeguarding elements, while M2 macrophages and activated mast cells are harmful ones [44]. Consistent with our study, M0 macrophages and resting memory CD4+ T cells are linked to a favorable prognosis. According to a study, IL-17A could stimulate the expansion of tumors by preventing the infiltration of CD4+ and CD8+ T cells [45].
Undoubtedly, our study has certain limitations. Firstly, there is a considerable amount of missing PFS information for patients receiving the FOLFOX chemotherapy regimen in our study; as a result, fewer people are included, which may lead to some degree of bias in the results. Secondly, the estimation of tumor-infiltrating immune cell immune content and relative proportions using algorithms in our study may not be as precise as experimental measurements. Likewise, the absence of covariates may lead to a decrease in statistical power in multivariate models. Lastly, to ensure greater reliability of the results, additional biological trials and confirmation in separate cohorts are required.
In summary, our study identified many immune cell subtypes, such as M0, M1 macrophages, resting mast cells, and activated natural killer cells associated with CRC patients’ prognosis who received FOLFOX and FOLFIRI chemotherapy. Furthermore, we discovered an immune cluster characterized by a high proportion of M0 macrophages, resting memory CD4+ T cells, and resting B cells associated with the survival of CRC patients receiving FOLFOX chemotherapy, improving the OS of CRC patients. Our findings can provide important clues for understanding the differences in adjusting the tumor microenvironment and immunity response with different chemotherapy regimens, aiding in the realization of personalized treatment, and offering more accurate treatment options for clinical practice.