The invention provides conjugates comprising an immune co-stimulatory polypeptide and an antigen or infectious agent. The conjugates are useful for generating or enhancing an immune response against the antigen or infectious agent. The invention also provides immune cells modified with a conjugate that are useful for generating or enhancing an immune response to an antigen or infectious agent. The invention also provides immunostimulatory moieties comprising an immune co-stimulatory polypeptide that are useful for stimulating an immune response. The invention also provides immunotherapy methods and methods of treating or preventing infections.