Th2 and Th17‐associated immunopathology following SARS‐CoV‐2 breakthrough infection in Spike‐vaccinated ACE2‐humanized mice

Abstract

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.