 When Signal 2 is not expressed, but T cells receive Signal 1, the antigen-specific T cells become anergic and do not perform effector function.Signal 3 is the APC secretion of stimulatory cytokines such as IL-2 which enhances T cell stimulation, though this is not required for T cell activation.Cell-based aAPCs have been produced by transfecting murine fibroblasts to express specific peptide-loaded HLA molecules with co-stimulatory signal B7.1, and cell adhesion molecules ICAM-1 and LFA-3.