Implantable biomaterials have become a conventional solution for various human diseases and play a great role in biomedical research, including tissue engineering, drug delivery systems, and implants1,2. There is a wide range of implants made from various materials with different structures and functionalities, such as hip prostheses, stents, meshes, heart valves, or dental implants. Upon implantation, the tissue-implant contact provokes an immune response, followed by resolution, tissue remodeling, and homeostasis. These processes are influenced by the physical, chemical, and bioactive characteristics of the biomaterials used. These characteristics may affect the intensity and spectrum of pro- and anti-inflammatory responses, fibrotic capsule formation, tissue degradation, and healing phase3,4. In order to support and optimize the healing process and long-term implant integration, one emerging aspect of current research is to investigate and mediate the interaction between implant surfaces and immune cells.
Among other immune cells, macrophages, which are found throughout the body, are key players in inflammation and anti-pathogenic defense, as well as in healing processes and the maintenance of tissue homeostasis5,6. Based on their plasticity and the local tissue microenvironmental stimuli, macrophages are able to polarize into distinct functional phenotypes, which exhibit great differences in cell metabolism, cellular functions, and cytokine secretion profiles. Classically activated M1 phenotype can be distinguished by secretion of proinflammatory cytokines, such as IL-1β, IL-6, and TNF-α, and is involved in the initial and chronic inflammatory response to trauma and foreign biomaterials. In contrast, alternatively, activated M2 macrophages, which are triggered by cytokines such as IL-4 and IL-13, have characteristic features like the resolution of inflammation and the promotion of tissue healing. M2-polarized macrophages can be identified by the expression of cell surface markers such as CD206 and the production of cytokines like IL-10 and IL-47. Similarly, macrophages that have already been polarized can reprogram themselves in a new microenvironment.
Many studies on cell-biomaterial interactions have shown the importance of macrophages in the cascade of immunologic responses toward implantable biomaterials and in orchestrating processes involved in the healing of implant-related complications8,9,10. Even though biomedical engineering has made significant progress in recent years, further research is needed to understand how implants modulate macrophage behavior and polarization11,12,13.
In cell culture, monocyte-derived peripheral blood mononuclear cells (PBMCs) can be differentiated into adherent M0 macrophages followed by induced polarization towards M1 or M2 phenotypes using LPS and IFN-γ or IL-4, respectively. After in vitro incubation with new biomaterial specimens, it is possible to utilize the different cell surface receptors and cytokine profiles of M1 and M2 macrophages to detect the immunomodulatory potential of biomaterials in vitro14,15. This study aimed to develop an in vitro protocol that can be employed to investigate the polarization of MDMs in response to different implant surfaces. Gene expression analyses, microscopy techniques, and ELISA can be used to determine the phenotypic markers and specific cytokine profiles of M1 and M2 macrophages modulated by the biomaterial. Hence, the complex interactions between macrophages and biomaterial surfaces can be elucidated, and valuable information can be obtained to better understand macrophage-biomaterial interactions. Finally, a standardized in vitro protocol ensures reproducibility, reliability, and comparability of experimental results by minimizing variability in the experimental setup.