Single shot dendritic cell targeting SARS-CoV-2 vaccine candidate induces broad, durable and protective systemic and mucosal immune responses in mice.

Current COVID-19 vaccines face limitations including waning immunity, immune escape by SARS-CoV-2 variants, limited cellular response, and poor mucosal immunity. We engineered a Clec9A-Receptor Binding Domain (RBD) antibody construct that delivers the SARS-CoV-2 RBD to conventional type 1 dendritic cells. Compared to non-targeting approaches, single dose immunization in mice with Clec9A-RBD induced far higher RBD-specific antibody titers that were sustained for up to 21 months post-vaccination. Uniquely, increasing neutralizing and antibody-dependent cytotoxicity activities across the sarbecovirus family was observed, suggesting antibody affinity maturation over time. Consistently and remarkably, RBD-specific follicular T-helper cells and germinal center B cells persisted up to 12 months post-immunization. Furthermore, Clec9A-RBD immunization induced a durable mono- and poly-functional TH1-biased cellular response that was strongly cross-reactive against SARS-CoV-2 variants of concern, including Omicron subvariants, and with robust CD8+ T cell signature. Uniquely, Clec9A-RBD single-shot systemic immunization effectively primed RBD-specific cellular and humoral immunity in lung and resulted in significant protection against homologous SARS-CoV-2 challenge as evidenced by limited body weight loss and approx. 2 log10 reduction in lung viral loads compared to non-immunized controls. Therefore, Clec9A-RBD immunization has the potential to trigger robust and sustained, systemic and mucosal protective immunity against rapidly evolving SARS-CoV2 variants.