Choline-Retinoic Acid Ionic Liquid [Cho][Ra] as Potential Adjuvant to Enhance Humoral, Cellular, and Mucosal Immune Responses of SARS-CoV-2 RBD Antigen

All-trans retinoic acid (Ra) has been demonstrated to enhance the establishment of systemic and intestinal immunity for codelivered antigens; however, the extremely poor water solubility of Ra significantly limits its application in vaccines. Herein, leveraging the advantages of ionic liquids in modifying the physicochemical properties of small-molecule drugs, we synthesized a novel ionic liquid composed of choline and retinoic acid ([Cho][Ra]). Compared to Ra, [Cho][Ra] exhibited an 18165-fold increase in solubility at pH 7.4 and an acid-responsive Ra release behavior. The [Cho][Ra] was then formulated with the SARS-CoV-2 RBD antigen, and its adjuvant effects were comprehensively evaluated. The in vitro cellular studies indicated that at 50 μg Ra equivalent dose, the [Cho][Ra]@RBD showed significantly higher than RBD along in activation of BMDC and the expression of gut homing molecules, including C-C motif chemokine receptor 9 (CCR9, 10.55 folds), α4β7 integrin (13.29 folds), and lymph node migration molecule CCR7 (82.68 folds). Animal experiments showed that compared to RBD alone, [Cho][Ra]@RBD promoted the establishment of mucosal immunity in the intestines in mice at the early stage following subcutaneous immunization and elicited higher serum antibody levels. Furthermore, compared to RBD and Alum@RBD, [Cho][Ra]@RBD enhanced the formation of CD4⁺ T_CM and CD8⁺ T_CM cells, indicating stronger cellular immune responses. In summary, [Cho][Ra] showed potential adjuvant effects in enhancing the humoral and cellular immune responses, as well as in the establishment of intestinal mucosal immunity for RBD antigen without the need for delivery system, thereby offering promising prospects for applying Ra in vaccines.