Three-Dimensional Culture Models to Study Innate Anti-Tumor Immune Response: Advantages and Disadvantages

SIMPLE SUMMARY: The classical approach to study the immune response against a tumor was mixing immune cells with tumor cell suspensions in several experimental settings. These models lack the appropriate tissue architecture in which the immune response takes place and do not consider other cellular and extracellular players of the tumor microenvironment essential to understand the anti-tumor immune response. Thus, to confirm in vitro data, in vivo experiments have been extensively performed, using animal models that may not fully reproduce what happens in humans. Indeed, in animal-based studies, tumors are artificially generated in a short time, and immune cell subsets and receptor-ligands pairs, involved in tumor cells recognition by the immune system, are often different from human counterparts. To reduce the number of animals used, and possibly replace animal models, alternative methods of culture have been developed. Herein, some of these approaches will be described, highlighting their advantages and disadvantages, focusing on natural killer cells as the first line of anti-tumor effector cells able to contrast tumor growth. ABSTRACT: Several approaches have shown that the immune response against tumors strongly affects patients’ clinical outcome. Thus, the study of anti-tumor immunity is critical to understand and potentiate the mechanisms underlying the elimination of tumor cells. Natural killer (NK) cells are members of innate immunity and represent powerful anti-tumor effectors, able to eliminate tumor cells without a previous sensitization. Thus, the study of their involvement in anti-tumor responses is critical for clinical translation. This analysis has been performed in vitro, co-incubating NK with tumor cells and quantifying the cytotoxic activity of NK cells. In vivo confirmation has been applied to overcome the limits of in vitro testing, however, the innate immunity of mice and humans is different, leading to discrepancies. Different activating receptors on NK cells and counter-ligands on tumor cells are involved in the antitumor response, and innate immunity is strictly dependent on the specific microenvironment where it takes place. Thus, three-dimensional (3D) culture systems, where NK and tumor cells can interact in a tissue-like architecture, have been created. For example, tumor cell spheroids and primary organoids derived from several tumor types, have been used so far to analyze innate immune response, replacing animal models. Herein, we briefly introduce NK cells and analyze and discuss in detail the properties of 3D tumor culture systems and their use for the study of tumor cell interactions with NK cells.