The Invention provides in vivo selection methods for identifying modulating agents (e.g., antibodies or polypeptides) that promote cellular differentiation and migration. The methods utilize a combinatorial agent library (e.g., antibodies expressed via lentiviral vectors) that are expressed in a population of to-be modulated cells (e.g., stem cells), which are then introduced into, the body of a non-hum an animal (e.g., mouse). This is followed by examining an organ or tissue of interest (e.g., brain) of the manipulated animal for the presence of a modulating agent and/or a specific phenotype. The; invention also provides specific antibody agent, that can induce differentiation of stem cells into microglia and migration into the brain. Further provided in the invention are therapeutic applications of the microglia-inducing antibodies.