Mannose Receptor-positive Macrophage Infiltration Correlates with Prostate Cancer Onset and Metastatic Castration-resistant Disease

Abstract

Background: M2 tumor-associated macrophages (M2-TAMs) can suppress inflammation in the tumor microenvironment and have been reported to modulate cancer progression. We and others have previously reported M2-TAM infiltration in metastatic castration-resistant prostate cancer (mCRPC).

Objective: To determine whether the extent of M2-TAM infiltration correlates with PC aggressiveness.

Design, setting, and participants: Normal prostate tissue, localized PC, and mCRPC samples from 192 patients were retrospectively analyzed.

Outcome measurements and statistical analysis: We analytically validated an immunohistochemistry assay for detection of the human mannose receptor (CD206) to assess M2 macrophage involvement.

Results and limitations: Multiplex immunofluorescent staining showed that a small fraction of CD206 staining co-localized with the endothelial cells of lymphatic vessels, while the vast majority of staining occurred in CD68-positive macrophages. The area fraction of staining for CD206-positive macrophages increased in a stepwise fashion from normal (ie, no inflammation) prostate tissue, to primary untreated carcinomas, to hormone-naïve regional lymph node metastases, to mCRPC. Complementary studies using flow cytometry confirmed CD206-positive M2-TAM infiltration. Limitations include the small number of rapid autopsy samples and the lack of neuroendocrine PC samples.

Conclusions: Our results revealed a progressive increase in CD206-positive macrophages from normal prostate to mCRPC. Given the immunosuppressive nature of macrophages and the lack of clinical success of immunotherapy for PC patients, our results provide a rationale for therapeutic targeting of macrophages in the PC microenvironment as a potential method to augment immunotherapeutic responses.

Patient summary: In this report we used 192 prostate cancer samples to determine if M2 macrophage infiltration is correlated with castration resistance in prostate cancer.

Keywords: Castration-resistant prostate cancer; M2 tumor-associated macrophages; Mannose receptor; Prostate cancer.

Fig. 1 -

Analytical validation of immunohistochemistry detection of CD206 expression. (A) Human M2 macrophages were stained for CD206 as a positive control and (B) PC3 cells were stained as a negative control. (C–H) Normal human prostate tissue and prostate cancer tissue were assessed for CD206 positivity. (C) Normal prostate tissue. (D) In benign regions with somewhat higher levels of inflammatory infiltrates (predominantly mononuclear) there was an increase in CD206-positive cells. (E) Normal prostate. (F) Gleason 3 + 4 + 7 core. (G) Lymph node metastasis. (H) Distant metastasis.

Fig. 2 -

Immunofluorescent staining for CD206-positive macrophages. Deparaffinized prostate sections were stained for three markers. (A) Infiltration of cells with dual CD206/CD68 positivity cells (arrows) within the prostatic stromal compartment and the lumen (Lu); CD206-negative cells (short arrow) were also present. (B) D240 was used as a marker for the lumen of lymphatic vessels. Cells with dual CD206/D240 positivity (arrows) surround the lymphatic lumen; CD206-positive/D240-negative cells (short arrows) were present but not fused to the lymphatic vessel wall. 4′,6-Diamidino-2-phenylindole (DAPI) staining was also used for nuclei in (A) and (B).

Fig. 3 -

Mannose receptor expression was greater in prostate cancer metastasis relative to normal benign prostate tissue. Using prostate cancer tissue microarrays and immunohistochemistry, MRC1/CD206 expression was quantified and compared for normal human prostate tissue and prostate cancer tissue. Comparisons were performed using a Mann-Whitney test and/or t test as appropriate. Mets = metastases.

Fig. 4 -

Dual mannose receptor and scavenger receptor expression in metastatic castration-resistant prostate cancer (mCRPC) rapid autopsy samples obtained 0–4 h after death. M2 macrophage infiltration was determined via flow cytometry. Coronal lymph node in mCRPC with (A) CD163 staining and (B) CD206 staining. Paratracheal lymph node in mCRPC with (C) CD163 staining and (D) CD206 staining. First right rib in mCRPC with (E) CD163 staining, (F) CD206 staining, and (G) dual CD206 and CD163 staining.