Th2 and Th17-Associated Immunopathology Following SARS-CoV-2 Breakthrough Infection in Spike-vaccinated ACE2-humanized Mice

Vaccines have demonstrated remarkable effectiveness in protecting against COVID-19; however, concerns regarding vaccine-associated enhanced respiratory diseases (VAERD) following breakthrough infections have emerged. Spike protein subunit vaccines for SARS-CoV-2 induce VAERD in hamsters, where aluminum adjuvants promote a Th2-biased immune response, leading to increased type 2 pulmonary inflammation in animals with breakthrough infections. To gain a deeper understanding of the potential risks and the underlying mechanisms of VAERD, we immunized ACE2-humanized mice with SARS-CoV-2 Spike protein adjuvanted with aluminum and CpG-ODN. Subsequently, we exposed them to increasing doses of SARS-CoV-2 to establish a breakthrough infection. The vaccine elicited robust neutralizing antibody responses, reduced viral titers, and enhanced host survival. However, following a breakthrough infection, vaccinated animals exhibited severe pulmonary immunopathology, characterized by a significant perivascular infiltration of eosinophils and CD4+ T cells, along with increased expression of Th2/Th17 cytokines. Intracellular flow cytometric analysis revealed a systemic Th17 inflammatory response, particularly pronounced in the lungs. Our data demonstrate that aluminum/CpG adjuvants induce strong antibody and Th1-associated immunity against COVID-19 but also prime a robust Th2/Th17 inflammatory response, which may contribute to the rapid onset of T cell-mediated pulmonary immunopathology following a breakthrough infection. These findings underscore the necessity for further research to unravel the complexities of VAERD in COVID-19 and to enhance vaccine formulations for broad protection and maximum safety. Significance statementThis research investigates the safety and efficacy of a Spike protein subunit vaccine adjuvanted with Alum and CpG in an ACE2-humanized mouse model, simulating SARS-CoV-2 breakthrough infections. The study reveals that despite robust protection against severe COVID-19, vaccinated mice exhibit substantial pulmonary immunopathology, including eosinophilia and enhanced Th2 effector immunity, following breakthrough infections. Surprisingly, the study also uncovers a significant systemic Th17 inflammatory response in vaccinated mice. This research sheds light on the potential risks associated with COVID-19 vaccine breakthrough infections and the need for a comprehensive understanding of vaccine-induced immune responses, emphasizing the importance of ongoing research, surveillance, and careful vaccine development for both protection and safety in the fight against the COVID-19 pandemic.