The present disclosure relates to the field of biotechnology, and more specifically, to single-chain and multi-chain chimeric polypeptides having a linker domain positioned between two target-binding domains that are useful for a variety of applications including, without limitation, stimulating an immune cell, inducing or increasing proliferation of an immune cell, inducing differentiation of an immune cell, or treating a subject in need thereof (e.g., a subject having cancer or an aging-related disease or condition).