CC is the second most common malignant disease among women in the whole world. More than 569,847 women are diagnosed with cervical carcinoma annually worldwide, resulting in nearly 311,365 deaths [1]. Traditional treatments for CC include surgery, radiotherapy and chemotherapy. Immunotherapy, as an emerging method of tumor treatment, is being popularized in clinic. TME is a complex system, and it is defined as a complex environment that supports cancer progression, proliferation of tumor cells, and invasion of adjacent tissues. Recent studies have highlighted the important roles of TME in the progression, invasion and metastasis of CC [26]. A comprehensive understanding of TME can provide research directions for finding novel immunotherapeutic agents. TAMs are critical component of TME, accounting for 30–50% of the TME cells. They promote tumor progression through various mechanisms, including therapeutic resistance, intravascular perfusion, angiogenesis, immune suppression and metastasis [27]. The multifunctional characteristics of TAMs in tumor progression suggest that targeting this group of immune cells may represent a new immunotherapeutic strategy.
According to stimulation in TME, TAMs are polarized into two types: M1 and M2. M1 macrophage polarization occurs mainly in the presence of interferon gamma (IFN-γ) or via exposure to microorganisms or its products such as lipopolysaccharide (LPS) [28–30]. Then, M1 secretes several proinflammatory cytokines such as IL-1 and IL-6, which are associated with activation of Th1 response and Th1 lymphocytes attraction [31]. Moreover, M1 macrophages can phagocytose and kill target cells [32]. M2 macrophage polarization is stimulated in response to IL-4 or IL-13 [33, 34], expressing abundant scavenger receptors and being associated with high production of IL-10, IL-1b, vascular endothelial growth factor (VEGF) and matrix metalloproteinases (MMP). Therefore, TAMs type conversion, that is, M1 to M2 polarization, plays a non-negligible role in tumor invasion and metastasis. In CC microenvironment, the transformation of TAMs belongs to transformation of the immune type, which plays an important role in the prognosis of CC [35].
Among specific surface molecules of TAMs, CD68 is the most commonly used. In our research, a total of three out of the eleven included studies [18, 24, 25] used CD68 as macrophage marker, while the other three studies [19, 21, 23] used CD68 in combination with CD163 for the detection of TAMs. CD163 served as a specific marker for TAMs with the M2 phenotype [36]. A total of five [15–17, 20, 22] out of the eleven studies individually used CD163 as macrophage marker.
In the included studies, CD68+ and CD163+ TAM density in cervical cancer were much enhanced than that in para-carcinoma or normal tissue. There was no relationship between CD68+ TAM density in cervical cancer and clinicopathological features, including age, histological grades and clinical stage. And there were no association between the stroma CD163+ TAMs density and clinicopathological features, such as age and histological grades. However, high stromal CD68+TAMs density was relevant to lymph node metastasis. Furthermore, high stromal CD163+ M2 TAM infiltration was found to be associated with more advanced FIGO stage. Both CD68+ and stromal CD163+ M2 TAM density were associated with lymph node metastasis in CC. However, as can be seen from the results of this meta-analysis, CD68+ TAM and CD163+ M2 TAM density in cervical cancer were much enhanced than that in para-carcinoma or normal tissue. And M2 TAM infiltration was associated with more advanced FIGO stage and lymph node metastasis. These findings indicate that number of TAM infiltration correlates with disease progression in CC. This was confirmed in our 100 experimental studies of CC. In our study, the intratumoral density of CD163-positive cells in FIGO stage (≥ IIB group) was significantly higher than that in FIGO stage (IA ~ IIA2 group) (p < 0.05). And the intratumoral density of CD68- and CD163-positive cells in cervical carcinoma with lymph node metastasis was significantly higher than that in non-lymph node metastasis group (both p < 0.05).
This is consistent with some studies mentioned in a review [37]. They suggested that a high number of TAMs is beneficial for tumor growth and associated with disease progression and poor prognoses for the patient. However, sometimes a high number of infiltrating TAMs correlated with better prognosis. For instance, in human papilloma virus (HPV) induced cervical intraepithelial neoplasia, TAM infiltration was found to correlate with disease progression [38]. Yet, in CC the number of stromal TAMs positively correlated with the intratumoral expression of IL-12p40, which itself was associated with a favorable overall survival (OS) of patients with CC [39]. Due to the limited data available, the stratified analysis to evaluate the correlation of infiltrating TAMs and prognoses was not performed.
Though we tried our best to perform a comprehensive analysis (experiment and meta-analysis) between CD68+ and CD163+ TAM infiltration and clinicopathological features in CC, there were still some limitations of the present study. First, the number of samples included our study and meta-analysis is relatively small and the sample size is relatively insufficient. Second, there was a potential risk of publication bias in the meta-analysis since we only included the publications in English and Chinese. And lack of gray literature may also result in missing negative results. Third, in the assessment of positive expression of CD68 and CD163 TAMs, Liu MM et al. [18] were scored based on TAMs staining intensity and cell number. Comprehensive evaluation of the number of positive cells ≤ 25% for low expression, > 25 for high expression. Chen XJ et al. [21] and Yan RM et al. [17] assessed the percentage of positive cells < 20% for low expression and ≥ 20 for high expression based on the proportion of positively stained cells. These studies have different threshold values in assessing TAM expression to distinguish between TAM high and low groups, ie patients with CD68+ TAM infiltration in the study do not necessarily belong to the same group. Patients with a low CD68+ TAM count in one study could be divided into high CD68+ TAM groups in another study due to different cut-off values. This makes the results of this study may be biased. Fourth, the main research object of this study is paraffin section of tumor tissue, para-carcinoma and normal tissue. However, IHC method may have certain technical bias affected by reagent, evaluation methods of positive result, skill of the operators and so on. In order to provide stronger evidence for evidence-based medicine in the progress of CC, more rigorous and high quality case-control studies and standardized methodology should be carried out to further confirm the correlation between the expression of TAM infiltration and clinicopathological features of CC.