REVIEW published: 09 June 2020 doi: 10.3389/fonc.2020.00899 Frontiers in Oncology | www.frontiersin.org 1 June 2020 | Volume 10 | Article 899 Edited by: George S. Karagiannis, Albert Einstein College of Medicine, United States Reviewed by: Fransisca Leonard, Houston Methodist Research Institute, United States Laetitia Aymeric, INSERM U1232 Centre de Recherche en Cancérologie et Immunologie Nantes Angers (CRCINA), France *Correspondence: Panagiotis Papageorgis p.papageorgis@euc.ac.cy Specialty section: This article was submitted to Molecular and Cellular Oncology, a section of the journal Frontiers in Oncology Received: 13 March 2020 Accepted: 07 May 2020 Published: 09 June 2020 Citation: Neophytou CM, Pierides C, Christodoulou M-I, Costeas P, Kyriakou T-C and Papageorgis P (2020) The Role of Tumor-Associated Myeloid Cells in Modulating Cancer Therapy. Front. Oncol. 10:899. doi: 10.3389/fonc.2020.00899 The Role of Tumor-Associated Myeloid Cells in Modulating Cancer Therapy Christiana M. Neophytou 1,2 , Chryso Pierides 3 , Maria-Ioanna Christodoulou 2 , Paul Costeas 3,4 , Theodora-Christina Kyriakou 2 and Panagiotis Papageorgis 1,2 * 1 European University Research Centre, Nicosia, Cyprus, 2 Department of Life Sciences, School of Sciences, European University Cyprus, Nicosia, Cyprus, 3 The Center for the Study of Haematological Malignancies, Nicosia, Cyprus, 4 The Cyprus Cancer Research Institute, Nicosia, Cyprus Myeloid cells include various cellular subtypes that are distinguished into mononuclear and polymorphonuclear cells, derived from either common myeloid progenitor cells (CMPs) or myeloid stem cells. They play pivotal roles in innate immunity since, following invasion by pathogens, myeloid cells are recruited and initiate phagocytosis and secretion of inflammatory cytokines into local tissues. Moreover, mounting evidence suggests that myeloid cells may also regulate cancer development by infiltrating the tumor to directly interact with cancer cells or by affecting the tumor microenvironment. Importantly, mononuclear phagocytes, including macrophages and dendritic cells (DCs), can have either a positive or negative impact on the efficacy of chemotherapy, radiotherapy as well as targeted anti-cancer therapies. Tumor-associated macrophages (TAMs), profusely found in the tumor stroma, can promote resistance to chemotherapeutic drugs, such as Taxol and Paclitaxel, whereas the suppression of TAMs can lead to an improved radiotherapy outcome. On the contrary, the presence of TAMs may be beneficial for targeted therapies as they can facilitate the accumulation of large quantities of nanoparticles carrying therapeutic compounds. Tumor infiltrating DCs, however, are generally thought to enhance cytotoxic therapies, including those using anthracyclines. This review focuses on the role of tumor-infiltrating and stroma myeloid cells in modulating tumor responses to various treatments. We herein report the impact of myeloid cells in a number of therapeutic approaches across a wide range of malignancies, as well as the efforts toward the elimination of myeloid cells or the exploitation of their presence for the enhancement of therapeutic efficacy against cancer. Keywords: myeloid cells, dendritic cells, macrophages, immunotherapy, Tumor-associated myeloid cells, nano-immunotherapy INTRODUCTION Immunity is the result of an intricate interaction between the innate and adaptive immune system. Innate immunity is the initial immunological response against an invading pathogen and has no immunological memory. On the other hand, adaptive immunity involves the development of immunological memory and enables the host to respond more efficiently to future exposure to the antigen. Following antigen processing, the degraded peptides associate with major histocompatibility complex (MHC) molecules within the interior of an antigen-presenting cell