Oral administration of low doses of plant-based HBsAg induced antigen-specific IgAs and IgGs in mice, without increasing levels of regulatory T cells

Plant-based oral vaccines run the risk of activating regulatory T cells (Tregs) and suppressing the antigen-specific immune response via oral tolerance. Mice humanized for two HLA alleles (HLA-A2.1 and HLA-DR1) were used to measure changes in Tregs and antigen-specific immune responses induced by the oral administration of tobacco (Nicotiana tabacum), expressing the hepatitis B surface antigen (HBsAg). Antigen-specific CD8+ T cell activation was not detected, but the plant-based oral immunization, without adjuvant, resulted in humoral responses comparable to those obtained by adjuvanted DNA immunization. Treg titers did not increase with DNA immunization. In contrast, with plant immunization, Tregs increased linearly to reach a plateau at high antigen doses. The highest humoral IgA and IgG responses correlated with the lowest plant antigen dose (0.5 ng), while for DNA immunization the best antibody responses were obtained at higher antigen doses. These experiments suggest that plant-based oral vaccines could be adjusted to minimize tolerance, while still inducing an immune response. Oral tolerance and adjuvant engineering in plants are discussed.