Plant-produced STI vaccine antigens with special emphasis on HIV-1 p24

Abstract: Objective: To establish stable transgenic non-toxic plants as a platform for plant-based vaccine production as well as potential oral delivery system of vaccine antigens for sexually transmitted infections (STIs). The concept is to immunize the mucosal immune system present in the gut-associated lymphoid tissues (GALT). HIV-1 p24 subtype C protein has been used as the main antigen model, in parallel with an engineered unique chimeric MOMP antigen from Chlamydia trachomatis serovar E.Methods: Chimeric MOMP and p24 vaccine antigens were successfully inserted into the nuclear genomes of Arabidopsis thaliana and Daucus carota via Agrobacterium-mediated gene transfer. The characteristics of the genetic inserts and corresponding mRNAs and recombinant proteins in planta were described using several methods, including northern, Southern, and western blotting, ELISA, and a commercial HIV Ag/Ab combination assay. Immunogenicity of the antigens was studied in mice models.Results: Transgenes of both plant species expressing p24 or chimeric MOMP were successfully generated. Additional HIV-1 vaccine antigen candidates were introduced and the genetic inserts have been confirmed in Arabidopsis thaliana. The Arabidopsis thaliana expressing p24 and chimeric MOMP were demonstrated to be stable over generations and antigenicity analyses showed that plant-derived HIV-1 p24 and chimeric MOMP retained immunological epitopes when they were expressed in planta. Oral administration of transgenic plant material generated a priming effect of the immune competent cells present in the GALT, shown by the presence of antigen-specific-IgG in mice sera after boosting. Mice immunized with plant-derived HIV-1 p24 antigen were also analyzed for antigen-specific faecal IgA as well as cellular immune responses. However, detectable levels of the two latter immune responses were not observed. The Chlamydia trachomatis chimeric MOMP antigen was further evaluated for its potential as a vaccine antigen candidate, with positive results indicating a more rapid clearance of the Chlamydia trachomatis infection post immunization.Conclusion: Stable non-toxic transgenic plants expressing either HIV-1 p24 or a novel  Chlamydia trachomatis chimeric MOMP antigens have successfully been developed. The two plant-produced STI vaccine antigens have in initial mice feeding studies provided important proof-of-concept for the oral vaccination approach. Now, immunization studies to expand, en-hance, and improve knowledge of the immune responses generated by the orally delivered transgenic plants are of high priority.