Adjuvant-driven epitope hierarchy correlates with the protective efficacy of FimA vaccine against Klebsiella pneumoniae.

Hypervirulent Klebsiella pneumoniae (hvKP) variants represent urgent multidrug-resistant threats against which no licensed vaccine exists. FimA is indispensable for KP mucosal colonization and is thus a promising vaccine target; however, its protective immunodominant B-cell epitopes in vaccines with different adjuvants against KP remain undefined. Here, we revealed how adjuvants shape different hierarchies of the immunodominant B-cell epitopes of FimA and studied the humoral anti-FimA response and protective efficacy in a murine model of acute pneumonia. BALB/c mice were intramuscularly immunized with recombinant FimA formulated in three clinically trialed adjuvants: AddaVax, AddaS03, and AlPO4, whereafter they were challenged with lethal doses of the hvKP YBQ strain. Under the experimental conditions used in this immunization-challenge model, AddaS03-immunized mice exhibited 100% survival and reduced pulmonary bacterial load, whereas AddaVax-immunized mice showed only 40% survival. Furthermore, FimA-immunized groups with different adjuvants exhibited different opsonophagocytic killing activities and inflammatory cytokine levels, likely explaining the variability in protective immunity. ELISA-based linear B-cell epitope mapping of FimA with different adjuvants revealed three novel immunodominant epitopes-their hierarchies were altered by different adjuvants. Further, a causal link might exist between FimA epitope hierarchy and protective efficacy in hvKP-infected mice. Immunization with a mixture of these FimA immunodominant epitopes plus AlPO4 achieved an 80% protection rate, where it exerted a potent therapeutic effect in combination with low-dose meropenem. Our findings revealed that adjuvants enhance anti-FimA immunity and modulate FimA epitope hierarchies across different vaccinated groups. Therefore, rational adjuvant selection may elicit effective FimA-induced responses in KP vaccines, wherein different FimA immunodominant epitopes induce protective humoral immune responses.