Targeted Intracranial Delivery of Secreted Immune Molecules to Investigate Neuroimmune Interactions.

Although secreted immune proteins have increasingly been recognized to influence nervous system function, technical barriers persist in defining their spatiotemporal dynamics within the brain's complex environment. For example, microglia, a population of key immune cells in the brain, release a variety of factors that interact with many diverse cellular targets. Microglia-derived proteins can signal to other cell types via mechanisms such as surface receptor binding without internalization, passive membrane transport, phagocytic uptake, or endocytosis-mediated uptake. Here, a method for localized delivery of a microglia-derived secreted protein is presented to dissect mechanisms of cellular interaction and downstream functional outcomes in vivo. We describe how to prepare protein samples for delivery, how to perform stereotaxic administration of protein to target brain regions, and best practices for the collection and preparation of brain tissue to visualize fixation-sensitive proteins by immunohistochemistry. This approach provides a versatile framework for spatiotemporally controlled studies of neuroimmune communication, enabling mechanistic exploration of how immune-derived proteins dynamically regulate neuronal and brain function.