Oral immunization with simian immunodeficiency virus p55^gag and cholera toxin elicits both mucosal IgA and systemic IgG immune responses in nonhuman primates

Rhesus macaques were orally immunized with a mucosal vaccine consisting of two different concentrations (1 mg vs 250 μg) of recombinant SIV p55^gag (p55) with or without cholera toxin (CT, 50 μg) as a mucosal adjuvant. The plasma from macaques receiving the higher dose of p55 (1 mg) and CT had higher p55-specific IgG and IgA Ab titers compared with macaques that received the lower dose of p55 (250 μg) and CT. Further, high levels of p55-specific IgG and IgA Abs were present in external secretions from both groups. The level of p55-induced T cell responses was elevated in PBMCs isolated from the high dose group compared with the low dose group. When culture supernatants from these p55-stimulated PBMCs were examined for Th1 (IFN-γ) and Th2 (IL-4 and IL-10) cytokines, both IFN-γ and IL-10 were present, but IL-4 was absent. CD4⁺ T cells isolated from these p55-stimulated PBMCs contained IFN-γ spot-forming cells (SFCs) but not IL-4 SFCs. These results were further confirmed by cytokine-specific reverse transcriptase PCR analysis, where p55-specific CD4⁺ T cells expressed mRNA for IFN-γ, IL-6, and IL-10, but not IL-4. These findings suggest that oral immunization of nonhuman primates induced both IFN-γ-secreting Th1 and select Th2 cytokine (e.g., IL-6 and IL-10)-producing CD4⁺ Th cells, which accounted for the generation of p55-specific systemic and mucosal Ab responses.