Macrophages make up the bulk of the leukocyte infiltrate in most solid tumors. These so-called tumor associated macrophages (TAMs) are thought to be recruited by tumor-derived chemokines and converted to a tumor-permissive state. TAMs are called M2, or alternatively-activated macrophages, to distinguish them from M1, or classically-activated macrophages. Of interest, these TAMs are not able to fight the tumor; instead, they play a tumor-supporting role via their ability to promote angiogenesis and metastases. In Burkitt's lymphoma infiltration of the tumor masses by macrophages is a characteristic morphological hallmark, while the phenotype and the relevance of TAMs as part of the stroma are poorly understood. Using a murine model of spontaneous Burkitt's lymphoma (λ-hu-c-myc transgenic mice), we therefore characterized macrophages infiltrating lymphoma masses and show here that infiltrating macrophages display an anti-inflammatory M2-phenotype characterized by the expression of surface markers, such as CD206. In addition, they secrete TGFβ and IL-10, which are known not only to support B-cell growth, but suppress T-cell immunity at the tumor site. In vitro analysis of human macrophages generated from PBMC either by GM-CSF to generate the M1 phenotype, or by M-CSF to obtain the M2 phenotype, revealed that both types display a distinct cytotoxic potential. When incubated together with different Burkitt's lymphoma cells, M2 macrophages did not efficiently lyse lymphoma cells. In contrast, M1 macrophages revealed a strong cytotoxic activity towards lymphoma cells, in a TNF, TRAIL, FasL and NO independent manner, as shown by antibody blocking studies. Investigating this mechanism further, we found that M1 macrophages, in contrast to M2 macrophages, express high levels of the Vitamin D inducible Cathelicidin antimicrobial peptide. Strikingly, we demonstrate that cathelicidin has a cytotoxic effect on lymphoma cells, but not on non-malignant B-cells.
Of note, excess supplementation of Vitamin-D increased the cytotoxicity of M2 macrophages against lymphoma cells. We conclude that Vitamin D may therefore point into a therapeutic option to influence a tumor stromal component that greatly impacts on survival and immunogenicity of malignant B-cells.
Mackensen:Micromet Inc.: Research Funding.
Asterisk with author names denotes non-ASH members.