Functionalized Liposomes as Vaccines against Vector-borne Diseases
Federizon, Jasmin Fe
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Liposomes, which are bilayered vesicles formed by self-assembly of phospholipid molecules in an aqueous medium, provide a versatile vaccine platform for delivering vaccine antigens due to its amphiphilic construct. The unique bilayer structure allows encapsulation of hydrophilic substances in the inner aqueous compartment, entrapment of hydrophobic substances within the lipid bilayer, and incorporation of amphiphilic substances at the interfaces. A non-covalent approach in antigen attachment using cobalt porphyrin phospholipid (CoPoP) enables surface functionalization of liposomes using polyhistidine tag as anchor and allows natural presentation of antigenic epitopes on the liposomal surface without chemical modification. Spontaneous association of antigens to pre-formedCoPoP liposomes occurs by insertion of histidine tag into hydrophobic bilayer and subsequent coordination of imidazole moiety to the metal center. To evaluate significance of particleization in cobalt tetrapyrrole immunization, a cobalt-containing corrin macrocycle, aquocobalamin, is utilized to mimic the antigen attachment method in the non-particleized form using histidine-tagged Pfs25, a sexual stage antigen that is the primary target of malaria transmission-blocking vaccine. Serum stability assay indicated weak association of Pfs25 to cobalamin but stable protein binding to CoPoP, underscoring the advantage of a sheltered binding site in CoPoP liposome. Immunization with Pfs25 bound to liposome, not cobalamin, elicited antibodies which recognized ookinetes and exhibited transmission-blocking activity. This emphasizes the impact of particleization in differential uptake by macrophages and in modulating adjuvant efficacy.As a proof-of-concept study, a non-lipidated form of outer surface protein A (OspA) derived from B. burgdorferi B31 is utilized to exhibit the adjuvanticity of CoPoP liposome in modulating the immune response against Lyme disease. OspA was purified to homogeneity and then conjugated to a liposomal vaccine adjuvant containing both CoPoP and PHAD, a synthetic monophosphoryl lipid A, by simple aqueous incubation. Prime-boost immunization with nanogram antigen dosing in mice elicited Th1-biased OspA antibodies with year-long durability. Based from ELISA, CoPoP/PHAD liposomes induced higher antibody response compared to other adjuvant systems. Antibodies were reactive with B. burgdorferi in an immunofluorescence assay and induced potent complement-mediated bactericidal activity in vitro . These results demonstrated applicability of the CoPoP/PHAD liposomes as a vaccine platform for future development of Lyme disease vaccines.Different strategies in Zika virus (ZIKV) subunit vaccine design has been used in this study to further explore utility of the CoPoP liposome as a vaccine adjuvant platform. Initial strategy involves association of the ZIKV envelope (E) protein lacking the stem region and transmembrane anchor to the CoPoP/PHAD liposomal system. Immunization studies indicated stimulation of robust immune response but with high cross-reactivity to the E protein of dengue virus, a close homologue of ZIKV. The next approach used in ZIKV vaccine development involves attachment of ZIKV E domain III to the liposomal scaffold to improve specificity of vaccine-induced antibodies. The last vaccine strategy involves novel membrane-proximal biomimetic stem portions of ZIKV. To prevent post-binding liposome aggregation, a liposome variant containing the monounsaturated lipid, DOPC, instead of DPPC, was formulated. Incorporation of QS-21, a saponin adjuvant, into DOPC-based liposomes enhanced immunogenicity and enabled neutralization, albeit weak, in PRNT assay.