Exploring TLR agonists as adjuvants for COVID-19 oral vaccines

Abstract

The COVID-19 pandemic underscored the importance of advancing technologies that enable the rapid development and distribution of more effective vaccines when required. Since SARS-CoV-2 enters the body through the nasal mucosa, optimising the induction of secretory IgA (sIgA) production, a key component of the mucosal immune response, is essential. It has long been known that the induction of sIgA occurs when a vaccine is administered through mucosal surfaces and the immune responses initiated at one mucosal site can influence immune activity at other mucosal surfaces. Consequently, we propose an oral vaccine formulation (Vacform) comprising the immunomodulator CL097, a TLR7/8 agonist, and the SARS-CoV-2 spike protein, both encapsulated within glucan particles (GPs). The studies demonstrated that Vacform induced ROS production in RAW 264.7 cells but not in human neutrophils. The concentrations of Vacform tested did not induce NO production in RAW 264.7 cells. While Vacform stimulated the production of TNF-α and IL-6 in mouse spleen cells, this effect was not observed in RAW 264.7 cells. Finally, Vacform stimulated the proliferation of human PBMCs. Thus, its immunomodulatory properties were evident in specific cells under certain in vitro conditions. The Vacform was subsequently tested in vaccination studies. C57BL/6 mice were initially immunized subcutaneously, followed by two oral boosts with Vacform every two weeks. The Vacform elicited both, humoral (serum IgG and mucosal sIgA) and cellular immune responses. A balanced Th1/Th2/Th17 immune profile was observed. In conclusion, the GPs:CL097 adjuvant system shows promise for eliciting robust immune responses against SARS-CoV-2 and provides a foundation for future studies on dose-response optimization and challenge models.