Photodynamic vaccination: Novel strategies of vaccines delivery by intraphagolysosomal parasites and their oxidative inactivation for safe and effective applications.

Leishmania (trypanosomatid protozoa) have a number of advantages, which distinguish them favorably from other microbes/particles to serve as a universal vaccine carrier. Biotechnology is well-established to produce transgenic Leishmania for expressing peptide vaccines. Multiple vaccines can be expressed episomally at high capacity via its eukaryotic mechanisms of protein translation and post-translational modifications, e. g. N-glycosylation. Technology is also available to grow Leishmania in serum-free/autoclavable or chemically defined media that is expandable to industrial scale cost-effectively. The major attributes of Leishmania for vaccine delivery are their surface glyco-conjugates responsible for the following properties: Protection of Leishmania, hence their expressed vaccines from losses to the activities of humoral factors in the mammalian hosts; Targeting the vaccines to the phagosome-lysosome vacuolar system of macrophages and dendritic cells - a desirable destination for effective vaccine processing and presentation; and Serving as potential adjuvants for the natural vaccines to elicit the lasting immunity seen after spontaneous/therapeutic cure of human leishmaniasis. Significantly, the surface glycoconjugates retain these functional activities when Leishmania are photo-inactivated with reactive oxygen species (ROS). Photo-inactivation of Leishmania with ROS eliminates their immunosuppressive activities and releases vaccines in macrophages. A complete loss of Leishmania viability in vitro and in vivo ensues, especially when doubly photo-inactivated for cytosolic accumulation of uroporphyrin and endosomal uptake of exogenous cationic phthalocyanines. The delivery of ovalbumin (OVA) as a surrogate vaccine by such photo-inactivated Leishmania is more effective than by conventional methods in activation of OVA peptide-specific CD4+ and CD8+ T cells. Previously, evidence has been presented, showing that photodynamic vaccination of animals with Leishmania so inactivated produced effective immunity that is adaptively transferable to naïve individuals against experimental leishmaniasis. Immunotherapy of canine leishmaniasis with photo-inactivated Leishmania produced encouraging preliminary results. Work is also underway to use such carrier to deliver add-on vaccines against other infectious and malignant diseases