Described herein are biomimetic Janus particles useful as artificial antigen presenting cells capable of activating T cells in vitro. “Bull's eye” ligand patterns mimicking either the native or reverse organization of the T cell immunological synapse are provided on the surface of nano- or micro-sized particles. Methods for activating T cells in vitro using biomimetic Janus particles described herein are also provided. T cells activated by the biomimetic Janus particles can be used in adoptive immunotherapies for treating cancer, tolerance induction in autoimmune disease, autologous immune enhancement therapy, and viral infection immunotherapy. Also described herein are methods for producing a biomimetic Janus particle.