Silk fibroin-based hydrogel as injectable carrier for prolonged immunization of plant-based COVID-19 subunit vaccine

Silk fibroin (SF)-based hydrogel was prepared to serve as an injectable carrier, aimed at prolonging the immunization effect of the plant-derived vaccine for the Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV-2). This vaccine consists of the Receptor-Binding Domain (RBD) linked with the Fragment Crystallizable (Fc) region of Human Immunoglobulin G 1 (IgG1), also referred to as the RBD-Fc vaccine. The formulation also includes Aluminium hydroxide gel (Alum). Ex-vivo experiments demonstrated that both the SARS-CoV-2 RBD-Fc-loaded SF hydrogel (RBD-Fc/SF hydrogel) and the Alum-adjuvanted SARS-CoV-2 RBD-Fc-loaded SF hydrogel (Alum-RBD-Fc/SF hydrogel) indicated localized retention. This resulted in a controlled and prolonged release of 75% and 25% over a period of 60 days, respectively. This facilitated continuous interaction between the immune system and the viral peptide, specifically the RBD, which could have significant implications for geometric mean titer (GMT). In immunization studies, the RBD-Fc/SF hydrogel and Alum-RBD-Fc/SF hydrogel had induced a more prolonged and stronger immunogenic response of mouse-RBD-specific IgG titer and IgG subtypes (IgG1 and IgG2a). The specific IgG1 and IgG2a titers of RBD-Fc/SF hydrogel (with GMTs of 67,559 and 1,056, respectively) and Alum-RBD-Fc/SF hydrogel (with GMTs of 155,209 and 1,838, respectively) were superior to the RBD-Fc vaccine without SF hydrogel (with GMTs of 3200 and 200, respectively). Furthermore, they were comparable to the responses elicited by commercial Alum-adjuvants currently in use (with GMTs of 51,200 and 4,850, respectively). Additionally, a concentration of 0.1%w/v SF exhibited significant adjuvant properties by augmenting the secretion of various cytokines. For instance, it enhanced the secretion of Interleukin-6 (IL-6) to 88.02 pg/mL, which is comparable to the secretion induced by commercial Alum adjuvants (39.48 pg/mL). These findings suggest that SF hydrogel-based vaccine formulations could enhance humoral immunity and induce long-lasting immune responses. However, further investigations are required to assess the neutralizing potential of the antibodies produced against SARS-CoV-2 and its variants. The prospects and outcomes of SF hydrogel-based vaccine formulations indicate their potential as a next-generation vaccine platform. Such innovative strategies could be pivotal in the fight against COVID-19 and other infectious diseases.