This study describes the distribution of tumor-infiltrating immune cells in BC based on gene expression data in the TCGA cohort, which clearly confirmed that immune infiltration was indeed involved in the occurrence of breast cancer. Compared with other treatment methods, cellular immunotherapy has the advantages of fewer side effects, stronger targeting, and synergistic effects with other treatments. Because of these advantages, immunotherapy has great potential to become the leading tumor therapeutic modality. However, targeting TIICs has only achieved good results in the treatment of melanoma. Melanoma has high expression of tumor neoantigens and a high mutational load, and its immune characteristics are quite different from those of other solid tumors. Although a large number of clinical studies have been carried out on immunotherapy for BC and other solid tumors, the reported clinical response rates are not very satisfactory, fluctuating between 10% and 30%. Therefore, an in-depth understanding of the immune status of the BC tumor microenvironment, to explore the function and distribution of TIICs in the tumor microenvironment, could be useful for improving immunotherapy.
BC has a high susceptibility and malignancy in women. Traditional surgical resection treatment has limited effect due to frequent distant metastasis and invasion, while chemotherapy and radiotherapy are very harmful to normal body tissues due to a lack of targeted treatment. Therefore, it is urgent to develop more promising treatment strategies. Cancer immunotherapy emerged as a breakthrough cancer treatment strategy and provide promising results for this aggressive cancer. Immune system-related biomarkers have important prognostic significance in many tumor types, including BC, and there are many ongoing clinical studies involving BC patients and various immunotherapy strategies. The most mature immunotherapy revolves around programmed cell death 1 (PD-1) receptor and its ligands, PD-L1 and CTLA-4. It has been found that TIICs play an important role in host defense mechanisms during BC adjuvant and neoadjuvant therapies. Although a growing number of studies have shown that TIICs have great potential to predict clinical outcomes and treatment response, studies on the impact of single TIICs on cancer prognosis have been lacking. It is worth noting that the study of the infiltration pattern of tumor immune cells based on a large number of genomic profiles from the public database and the discussion of the potential mechanism of the interaction of TIICs are further studies on genomics, which can bring the greatest possibility for the clinical cure of tumors and providing new insights into immunotherapy for BC.
The results of this study showed significant heterogeneity of TIICs within the tumor. According to the bar plot and heat map, M2 macrophages, M0 macrophages, resting memory CD4+ T cells, etc. were highly expressed in BC. Moreover, Figure 3 shows the statistical significance of the differential expression of the molecular markers of these three TIICs when comparing tumors and adjacent tissues, and the results were consistent. Through the correlation analysis of 22 TIICs, we found that CD8+ T cells were positively correlated with activated memory CD4+ T cells, and negatively correlated with M0 macrophages. In order to further explore the relationship between these TIICs and the prognosis of BC, we performed survival and Cox regression analysis. The results showed that M2 macrophages and mast cell activation were related to prognosis, and M2 macrophages could be used as an independent prognostic marker for BC. In-depth analysis of the relationship between 22 TIICs and clinical characteristics revealed a complex correlation between macrophages and CD4+ T cells and clinical results. Therefore, we concluded that macrophages, especially M2 macrophages, play an important role in regulating the occurrence, development, and prognosis of BC.
Macrophages have innate heterogeneity, which is closely related to the unique function of their local microenvironment, and demonstrate plasticity of functions in the pathological processes of various diseases, including infectious diseases, autoimmune diseases, and tumor progression.[26, 27] Macrophages are conventionally subdivided into three phenotypes: naïve (; also called M0), pro-inflammatory (M1), and anti-inflammatory (M2). Macrophages act as immune sentries to maintain the integrity of tissues and can sense, react, and quickly adapt to their local environment[29, 30] in order to maintain their tolerance and resist invading pathogens.  However, when the host is challenged for a long time (e.g., by long-term exposure to irritating particles or infection), macrophages may play a harmful role, leading to a low-grade inflammatory state that can lead to disease progression and even cancer.[32, 33] Many macrophage-centered treatment approaches are considered to be potential cancer treatments, including strategies to limit their infiltration or to exert their anti-tumor effects. In the vast majority of cancers, macrophages exhibit a global immunosuppressive phenotype, which is characterized by low levels of inflammatory molecules and increased transcriptional activity of alternatively activated M2 macrophages. In animal models, drugs that inhibit key signaling molecules of M2 macrophage polarization (such as IL-4, STAT3, or PI3- kinase) successfully limit the immunosuppressive function of tumor-associated macrophages (TAMs) and shrink tumors.[35, 36] However, M2 macrophages can also promote tumor angiogenesis, cancer cell migration/invasion, immunosuppression, and matrix remodeling.[37, 38] Our study confirmed and extended the results of previous studies; we found that M2 macrophages are a potential independent prognostic factor for BC, high expression of M2 macrophage markers is associated with poor prognosis.
In addition, it was found that high levels of CD8+ T cell in cancer cell nests heighten neoadjuvant chemotherapy sensitivity in breast cancers,[39-41] and Ali et al reported that CD8 T cells were associated with reduced risk of death in patients with either ER-negative or ER-positive/HER2-positive disease. The interleukin-17 A (IL-17A), a proinflammatory cytokine produced by activated CD4+ memory T cells has been found with a protective role in the development of breast cancer.[43, 44] Furthermore, mast cells (MCs) are a major source of pro-tumorigenic and antitumorigenic molecules, thus, their role on breast cancer is controversial. Some scholars believe that mast cell infiltration suggests a good prognosis of breast cancer.[46, 47] On the other hand, MCs as a proliferating factor, can stimulate the development of breast cancer and is related to the adverse reactions of neoadjuvant chemotherapy for breast cancer,[48, 49] which is consistent with the poor prognosis of high expression MCs found in our study. A large number of studies have shown that TIICs has prognostic value independent of many established prognostic signs, and single TIICs have the ability to predict clinical outcomes. These findings provide a reference for BC immunotherapy; however, owing to the complexity of this topic, further studies are required to validate our findings.