Abstract Abstract 1035 Introduction: Distinct macrophage subsets have been linked with either protective or pathogenic roles in cancer. M2 macrophages have been shown to suppress adaptive tumour-specific immune responses and promote tumour growth, while M1 macrophages eliminate neoplastic cells and activate tumour-killing mechanisms. Macrophages isolated from solid tumours (tumor-associated macrophages, TAM) reveal a suppressive and tumor promoting M2-like phenotype. However, it remains unclear which tumoricidal effector mechanisms are compromised in theses immune cells. While infilatration of macrophages is a characteristic morphological hallmark in Burkitts' lymphoma (BL), the phenotype and functional properties of TAM as part of the stroma are poorly understood. In this study we investigated the phenotype of lymphoma associated macrophages (LAM) and the mechanisms by which macrophages are able to modulate the growth of tumor cells. Methods: We included 19 patients diagnosed with BL and 20 patients with benign reactive lymphadenopathy as a control group. Phenotypic characterizations of LAM were evaluated by immunohistochemistry and by qPCR in paraffin embedded tissues. To investigate anti lymphoma cytotoxicity of distinct macrophage subsets we generated macrophages from PBMC either by GM-CSF to create the M1 phenotype, or by M-CSF to obtain the M2 phenotype. M1 and M2 macrophages were coincubated them with several BL cell lines and cytotoxicity was analyzed by flow cytometry, qPCR and immunofluorescence. Results: First, we could demonstrate that BL infiltrating macrophages displayed an anti-inflammatory M2-phenotype characterized by expression of surface markers such as CD68 and CD163. Second, we identified impaired vitamin D metabolism in LAM and M2 macrophages as demonstrated by low expression of vitamin D receptor and Cyp27B1, and increased expression of Cyp24A1. Third, macrophages revealed a reduced cytotoxic potential towards lymphoma cells which was dependent on the expression of the vitamin D dependent peptide cathelicidin. Finally, we could demonstrate that excess supplementation of calcitriol led to increased cytotoxicity of M1- and M2 macrophages towards BL cells. Conclusions: These results suggest a mechanism in which vitamin D is required for innate immunity to overcome the ability of lymphoma cells to evade macrophage-mediated antitumoral responses. The present findings underscore the importance of vitamin D for sustaining innate immunity and imply that the therapeutic activation of the vitamin D pathway may even result in triggering tumoricidal effector mechanisms of LAM. Disclosures: No relevant conflicts of interest to declare.