Optimizing macrophage-targeted intracellular delivery systems for safe and effective immunotherapies

Recent studies increasingly highlight the crucial role of macrophages in tissue development, homeostasis and inflammation. Consequently, macrophages have become an attractive therapeutic target in the rapidly evolving field of immunotherapy, ranging from alleviating autoimmune diseases to modulating the tumor microenvironment. However, enhancing the functions of these cells typically requires ex or in vivo intracellular delivery of cell-impermeable functional molecules, such as nucleic acids. Traditionally, this is achieved using viral and non-viral carriers or methods involving membrane-disruption strategies. As a result, several techniques have been established for the intracellular delivery of functional molecules, resulting in e.g., chimeric antigen receptor-expressing macrophages. Nonetheless, it is becoming increasingly clear that the transfection technology itself perturbs cell homeostasis. Such off-target effects could significantly impact the therapeutic potential of the final cell product. In this review, we first discuss aspects of macrophage biology that are essential to understand macrophage activation and why these cells are of therapeutic interest. This is followed by a summary of the approaches that facilitate the intracellular delivery of functional payloads into macrophages and a brief discussion on how these approaches may potentially affect macrophage phenotype. The aim of this work is to allow informed selection of intracellular delivery methods towards more efficacious and safer macrophage-based therapies.