Breast cancer is the most frequent malignancy among females worldwide. Traditional therapies against breast cancer have been designed to attack tumor cells themselves. Recently, studies have focused on targeting tumor microenvironment in order to reduce treatment resistance and improve patients outcomes [14].
Tumor-associated macrophages are crucial regulators of cancer cells and microenvironment. They modulate tumorigenesis and adjust the response to therapy. Several studies have reported that TAMs are related to poor prognosis in different tumors as hepatocellular carcinoma, gastric cancer, and lung cancer [15]. Several markers are used to label macrophages. CD68 identifies both tumoricidal M1 and tumor-promoting M2 macrophages, whereas CD163 is expressed principally by M2 macrophages [16].
Studies investigated TAMs infiltration within BC were remarkably variable. They used different markers and methods to assess macrophages. Most of them used CD68 alone to assess macrophages [13, 17], while others combined both CD68 and CD163 [18, 19]. Some studies assessed TAMs in different localization (stroma and nest) [13, 17], while others neglected the location of TAMs and counted total TAMs within the tumor [20, 21].
This study aimed to evaluate CD163 expression compared to the widely used CD68 pan-macrophage marker in breast invasive ductal carcinoma. In addition, it focused on assessing the significance of TAMs localization in relation to clinicopathologic parameters.
As regards TAMs infiltration within TN, the present work revealed that high density of CD163-positive TAMs within TN was significantly associated with high tumor grade and increased Ki-67 proliferation index. On the other hand, there was lack of significant associations between high density of CD68-positive TAMs infiltration and all included clinicopathologic parameters.
Gwak et al. and Jeong et al. reported that a high density of TAMs was related to high tumor grade and higher Ki-67 expression in both locations (tumor nest and tumor stroma) [13, 18]. Similarly, studies by Ni et al. and Sousa et al. revealed significant relations between high infiltration of both CD68-positive and CD163-positive TAMs, without addressing specific location, and high histologic grade and increased Ki-67 proliferation index [20, 21].
On the contrary, Ch'ng et al. and Yang and his colleagues demonstrated that increased TAMs in the stroma, not within tumor nest, were correlated with higher tumor grade [17, 22]. However, Yuan et al. noticed a lack of significant association between CD68-positive TAMs density and tumor grade [23].
It was proposed that high-grade tumors may elaborate higher levels of cytokines that recruit and modulate macrophages as monocyte colony-stimulating factors, interleukin-10, and/or transforming growth factor-β resulting in increased density of CD163-positive TAMs within high-grade tumors [21]. Moreover, TAMs may secrete different cytokines and growth factors that provide mitogenic signals to malignant cells [17].
As regards TAMs infiltration within tumor stroma, the present study reported significant associations between stromal TAMs infiltration and poor prognostic parameters. Infiltration of tumor stroma with high density of CD68-positive TAMs was significantly related to large tumor size, vascular invasion, and positive nodal metastasis, whereas, high density of stromal CD163-positive TAMs was significantly associated with large tumor size, positive vascular invasion, the presence of nodal metastasis, advanced stage, and ER-negative expression.
This was in accordance with several studies. Ch’ng et al. reported that only elevated stromal CD68-positive TAMs were associated with poor prognostic features [17]. Also, Medrek et al. showed that high density of only CD68- and CD163-positive stromal TAMs were related to larger tumors and inversely correlated with ER-positive expression and luminal A subtype [19]. Moreover, a study by Gwak et al. showed that high density of CD68-positive TAMs, in both tumor nest and stroma, was associated with aggressive histologic features [13].
Dealing with molecular subtypes, this study observed significant associations between CD163-positive TAMs in both TN and TS and molecular subtypes. Most of triple-negative tumors was associated with high density of CD163 positive TAMs. Whereas, luminal A tumors were accompanied by low levels of CD163 positive TAMs within both TN and TS. Similar results were reported by others [13, 18, 19]. A study by Stossi et al. has demonstrated that conditioned media from macrophages was able to stimulate different pathways inside BC cells that were important for the downregulation of ER expression [24].
Some studies evaluated total TAMs without considering TAMs localization and reported that TAMs were associated with an unfavorable prognosis [20, 23, 25]. The present work demonstrated that stromal TAMs were related to more aggressive behavior than that within the tumor nest. It was suggested that TAMs exert its function through regulating immune response within tumor stroma rather than by direct interaction with tumor cells [22].
The role of TAMs in infiltrating tumor nest differs by tumor type. High density of TAMs within tumor nest correlates with better prognosis in endometrial and gastric carcinomas. However, in malignant melanomas and esophageal cancers, TAMs within tumor nest was found to be associated with poor patient outcome [26]. In the tumor stroma, macrophages are recruited by cytokines produced by malignant cells. Most TAMs in the tumor microenvironment are mainly of M2-like phenotype that secretes high levels of cytokines and thus enhances tumor progression. Moreover, TAMs prevent infiltration and action of CD8+ cytotoxic T lymphocytes in attacking tumor cells [2].
In addition, TAMs express vascular endothelial growth factor (VEGF) and activate angiogenesis, and stimulate tumor cell proliferation and metastasis Moreover, TAMs can generate proteases that degrade extracellular matrix and thus enhance tumor cell invasion [27].
