Background
Macrophages that infiltrate tumor tissues are referred to as tumor-associated macrophages (TAMs) and are closely involved in tumor progression by inducing angiogenesis, immunosuppression, and invasion [
1]. The protumoral role of TAMs is supported by many clinical studies of carcinomas, including breast, prostate, endometrial, and bladder carcinomas, and malignant lymphomas, which showed a correlation between a large number of macrophages and poor prognosis [
2],[
3]. On the other hand, dendritic cells (DCs) that infiltrate tumor tissues are involved in tumor suppression via immune responses. A large number of DCs is related to better survival in a variety of malignant tumors such as melanoma, breast carcinoma, hepatocellular carcinoma, and lung adenocarcinoma [
4].
Thymic epithelial tumors are rare mediastinal tumors and can be classified into thymoma and thymic carcinoma. Compared with other organs, the human thymus is a lymphoepithelial organ, and macrophages and DCs, as well as epithelial cells, are the cellular components; however, there have been very few reports [
5] concerning TAMs and DCs in thymic epithelial tumors. Here, we immunohistochemically examined tumor tissue samples to characterize TAMs and DCs in thymoma and thymic carcinoma by comparing them with those in normal thymic tissues.
Discussion
Macrophages are found in the cellular microenvironment of many carcinomas, and these TAMs represent a heterogeneous population of functionally distinct cells [
2] that may affect the neoplastic process. Different phenotypes, as well as different cytokine secretion profiles, have suggested a distinction between ‘proinflammatory M1’ and ‘immunosuppressive M2’ macrophages [
9]. Although it is now acknowledged that the binary M1/M2 model is oversimplified [
10],[
11] and that there is a spectrum of intermediate macrophage phenotypes in response to various local microenvironmental signals [
12],[
13], TAMs most often seem to exhibit M2 features [
14]. However, at present, there is no single marker for macrophage polarization [
15].
In the tumor microenvironment, TAMs play a key role in carcinoma-associated inflammation and affect the progression and prognosis of various tumor types [
16],[
17] other than colorectal-gastric carcinoma and osteosarcoma [
1], and a dense macrophage infiltrate is associated with enhanced nodal metastases, distant metastases, and reduced recurrence-free survival [
18]. On the other hand, there have been only a few reports comparing the TAMs in malignant tumors with those in benign tumors arising in the same organs. In ovarian tumors, the number of CD68+, CD163+ TAMs is reported to show a stepwise increase from benign, borderline to malignant [
19]. In thyroid tumors, the number of CD68+ TAMs is also higher in papillary carcinoma than in follicular adenomas [
20]. In thymic tumor, although there has been no report comparing TAMs between thymoma and thymic carcinoma, in this study, we confirmed previous findings for thymic epithelial tumor. Although the percentage of CD68+ TAMs was not significantly different between thymoma and thymic carcinoma, in thymic carcinoma, which is associated with more frequent invasive growth and distant metastases, a higher percentage of CD163+ TAMs was found. These observations suggest that malignant tumors harbor a higher percentage of CD163+ TAMs than benign tumors in their microenvironment, which is a reasonable finding considering the roles of TAMs in malignant tumors.
CD68 is a glycoprotein used as a macrophage marker but is nonspecific. On the other hand, CD163 is a member of the scavenger receptor family and is specific for macrophages [
7]. Immunohistochemical studies of TAMs demonstrated the superiority of CD163 over CD68 in predicting the clinical outcome [
17]-[
19]. We found a higher percentage of CD163+ TAMs than of CD68+ TAMs in both thymoma and thymic carcinoma samples, which was consistent with previous observations in Hodgkin’s lymphoma [
7], malignant melanomas [
20], and leiomyosarcomas [
21]. Thus, the use of CD68 may lead to underestimation of the true percentage of TAMs [
18].
DCs, i.e., specialized antigen-presenting cells, play a critical role in innate and adaptive immune responses [
22]. In the tumor microenvironment, DCs are the most potent antigen-presenting cells that induce antigen-specific immune responses by engulfing dying tumor cells [
23]. Among several DC markers, many investigators have used S100 as a valuable marker of DCs, because S100 show adequate immunohistochemical staining on paraffin tissue sections [
24]. Several studies showed that the number of S100+ DCs in colon carcinoma [
25], human gastric carcinoma [
26], and esophageal carcinoma [
27] negatively correlates with lymph node metastases, size of tumor, and survival time, that is, the larger the number of DCs, the better the patient’s prognosis. In addition to these clinicopathological parameters, a recent study of uterine endometrioid adenocarcinoma demonstrated the inverse correlation between a large number of S100+ DCs and the histological grade for malignant potential [
28].
S100 can also be used as a useful marker of thymic DCs [
29]-[
31]. In thymic tumor, although there has been no report showing the relationships between the percentage of DCs and prognosis, the percentage of S100+ DCs was found to be lower in thymic carcinoma than in thymoma [
5]. On the other hand, using fascin as a mature DC marker, DCs were reported to appear more frequently in benign thymic neoplasms [
32]. In this study, we also confirmed the previous finding of a lower percentage of S100+ DCs in thymic carcinoma than in thymoma, indicating that malignant thymic epithelial tumor is associated with a paucity of DCs in their microenvironment.
Few studies examining the interaction between TAMs and DCs in malignant tumor are available. In Hodgkin’s lymphoma cases, a high percentage of TAMs and a low percentage of DCs were reported to be associated with adverse prognostic parameters [
4]. In a comparison between benign and malignant skin tumors, malignant transformation of keratinocytes was found to be associated with infiltration of TAMs and loss of DCs [
33]. Similarly, we found the same correlation, and this is the first report showing the association of thymic carcinoma with the increase in the percentage of TAMs and loss of DCs. Regarding the differentiation of monocytes, the CD115 (macrophage colony-stimulating factor receptor or CSF-1 receptor) pathway stimulates their survival and differentiation into macrophages rather than into DCs [
34],[
35], and the CD115/CSF-1 pathway is reported to play a central role in tumor progression through its effects on the differentiation of TAMs [
36]. Concerning the therapeutic aspects focusing on TAMs and DCs, recent studies revealed that some anti-CD115 monoclonal antibodies inhibit monocyte differentiation to TAMs, thereby skewing TAM differentiation toward DCs, and contributing to the generation of more efficient anti-tumor immune responses [
37]. This study demonstrated an idea for future targeted therapeutic strategies for thymic carcinoma using such antibodies competing with CSF-1 binding to CD115.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
MO and TK designed the study, analyzed the data and drafted the manuscript. TM and AS assisted with the design of the study. TN carried out the immunohistochemistry. AM, AK, TS and MK collected the patients samples. All authors read and approved the final manuscript.