The tumor microenvironment (TME) acquires immune resistance during the process of tumor formation. Recently, cancer immunotherapy has been attracting attention as a treatment modality, following the three major standard cancer treatments (surgical therapy, radiation therapy, and chemotherapy), for its potential to overcome such an immunosuppressive TME. Particularly, blocking antibodies against immune checkpoint molecules, such as PD-1 and CTLA-4 have caused a paradigm shift in cancer treatment. However, several patients do not respond to existing cancer immunotherapy; therefore, the establishment of a novel therapeutic target is essential. Macrophages are the most abundant cells in various tumors and are biased toward immunosuppressive forms. Therefore, research is ongoing globally to determine whether macrophages could be therapeutic targets. Interleukin-34 (IL-34) has been reported as a factor that biases macrophages to immunosuppressive forms. It is expressed in various types of cancer cells and plays important roles in multiple aspects of the TME. In this review, we comprehensively introduce the roles of IL-34 in the TME.
Keywords: Immunosuppression; Interleukin-34; Macrophages; Tumor microenvironment.