Abstract
Photodynamic therapy, unlikely to elicit drug-resistance, deserves attention as a strategy to counter this outstanding problem common to the chemotherapy of all diseases. Previously, we have broadened the applicability of this modality to photodynamic vaccination by exploiting the unusual properties of the trypanosomatid protozoa, Leishmania, i.e., their innate ability of homing to the phagolysosomes of the antigen-presenting cells and their selective photolysis therein, using transgenic mutants endogenously inducible for porphyrin accumulation. Here, we extended the utility of this host-parasite model for in vitro photodynamic therapy and vaccination by exploring exogenously supplied photosensitizers. Seventeen novel phthalocyanines (Pcs) were screened in vitro for their photolytic activity against cultured Leishmania. Pcs rendered cationic and soluble (csPcs) for cellular uptake were phototoxic to both parasite and host cells, i.e., macrophages and dendritic cells. The csPcs that targeted to mitochondria were more photolytic than those restricted to the endocytic compartments. Treatment of infected cells with endocytic csPcs resulted in their accumulation in Leishmania-containing phagolysosomes, indicative of reaching their target for photodynamic therapy, although their parasite versus host specificity is limited to a narrow range of csPc concentrations. In contrast, Leishmania pre-loaded with csPc were selectively photolyzed intracellularly, leaving host cells viable. Pre-illumination of such csPc-loaded Leishmania did not hinder their infectivity, but ensured their intracellular lysis. Ovalbumin (OVA) so delivered by photo-inactivated OVA transfectants to mouse macrophages and dendritic cells were co-presented with MHC Class I molecules by these antigen presenting cells to activate OVA epitope-specific CD8+T cells. The in vitro evidence presented here demonstrates for the first time not only the potential of endocytic csPcs for effective photodynamic therapy against Leishmania but also their utility in photo-inactivation of Leishmania to produce a safe carrier to express and deliver a defined antigen with enhanced cell-mediated immunity.
Highlights
Photodynamic therapy (PT) eliminates diseased cells/pathogens by using photosensitizers (PS) that are excitable by light to produce cytotoxic reactive oxygen species (ROS) in the presence of oxygen [1]
The results indicate that foreign antigens can be expressed by Leishmania for csPc-mediated photolytic delivery to antigen-presenting cells (APC) for presentation to activate epitope-specific T-cells in vitro. This is the first report showing that both stages of Leishmania are intrinsically susceptible to the photolytic activities of soluble and cationic Zn2/Si-Pcs examined (Figure 1 and 2 [A–B])
Since the axenic amastigotes are closer to the disease-causing stage of Leishmania, their intrinsic and irrevocable susceptibility to csPcmediated cell death is especially relevant in considering csPcs as agents for therapeutic PT against cutaneous leishmaniasis
Summary
Photodynamic therapy (PT) eliminates diseased cells/pathogens by using photosensitizers (PS) that are excitable by light to produce cytotoxic reactive oxygen species (ROS) in the presence of oxygen [1]. Effective drugs have never been developed for this and related diseases, i. Resistance has developed from the continuous use of ineffective drugs, e. Clinical management of these diseases is difficult [15,16], while vaccines are still under development [17,18]. All pathogenic Leishmania spp. show the homing specificity to parasitize mononuclear phagocytes, e.g. macrophages (MC) and dendritic cells (DC) [19,20,21]. How PS can be targeted to this site against Leishmania with specificity is a challenging issue
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