Killed but metabolically active microbes: a new vaccine paradigm for eliciting effector T-cell responses and protective immunity

We developed a new class of vaccines, based on killed but metabolically active (KBMA) bacteria, that simultaneously takes advantage of the potency of live vaccines and the safety of killed vaccines. We removed genes required for nucleotide excision repair (uvrAB), rendering microbial-based vaccines exquisitely sensitive to photochemical inactivation with psoralen and longwavelength ultraviolet light. Colony formation of the nucleotide excision repair mutants was blocked by infrequent, randomly distributed psoralen crosslinks, but the bacterial population was able to express its genes, synthesize and secrete proteins. Using the intracellular pathogen Listeria monocytogenes as a model platform, recombinant psoralen-inactivated Lm ∆uvrAB vaccines induced potent CD4 + and CD8 + T-cell responses and protected mice against virus challenge in an infectious disease model and provided therapeutic benefit in a mouse cancer model. Microbial KBMA vaccines used either as a recombinant vaccine platform or as a modified form of the pathogen itself may have broad use for the treatment of infectious disease and cancer. A major challenge for the international biomedical community is to develop vaccines for chronic diseases caused by intracellular pathogens. AIDS, malaria, tuberculosis and hepatitis are all established through chronic intracellular infections, and protection against the pathogens causing these diseases requires vaccines that elicit broad cellular immunity 1 . Vaccines based on either recombinant proteins or killed whole pathogens, although safe, typically induce weak cellular immunity. In contrast, vaccines based on live attenuated forms of a pathogen may have enhanced immunogenicity and elicit responses of increased breadth and durability, but they present safety risks, particularly among immunocompromised individuals. To address this dilemma in vaccine development, we sought to derive vaccine platforms based on whole organisms that elicited strong cellular immunity, yet retained the safety profile of killed or subunit vaccines. Killed but metabolically active (KBMA) vaccines are based on bacterial nucleotide excision repair mutants, and are inactivated by photochemical treatment with a synthetic highly reactive psoralen known as S-59. Psoralens form covalent monoadducts and crosslinks with pyrimidine bases of DNA and RNA upon illumination with long-wavelength ultraviolet (UVA) light 2 . The primary repair pathway for psoralen crosslinks is nucleotide excision repair 3 , a process initiated by the ABC excinuclease,