Viral vectors are provided for use as genetic immunotherapeutic agents, including preventive and therapeutic vaccines as well as compositions to enhance cellular immune responses. The vectors are particularly useful for treating or preventing cancer and infectious diseases. The vectors include lentiviral vectors that encode one or more antigens and an adjuvant, and optionally may encode one or more soluble checkpoint inhibitor molecules. The adjuvant is a fusion protein including latent membrane protein 1 (LMP1) from Epstein Barr virus with in which the intracytoplasmic domain has been replaced by human IPS1 or a variant thereof capable of activating the STING pathway. The vector-encoded sequences are codon optimized for human expression.