Type 3 fimbrial regulation underpins anti-MrkA immunotherapeutic efficacy in experimental <i>Klebsiella pneumoniae</i> infection

BACKGROUND: Klebsiella pneumoniae (KP) is a critical-priority organism due to prevalent last-line antibiotic resistance. Alternative treatments, including vaccines and monoclonal antibodies (mAb), depend on antigen (Ag) expression at infection sites for immunotherapeutic activity. However, the relationship between genome-encoded Ag presence and Ag expression is often overlooked. Here, we use the KP type 3 fimbrial (T3F) subunit MrkA as a prototype to build a generalisable framework to assess Ag expression and its correlation with in vivo immunotherapeutic efficacy. METHODS: We perform genomic analysis of 1649 KP genomes for T3F genes, including structural and regulatory components. We generate isogenic mutants with absent, normal or overexpressed MrkA and profile MrkA expression at single-cell level from murine pneumonia and bacteraemia models. We compare anti-MrkA mAb efficacy in vivo against strains with normal and enhanced MrkA expression. RESULTS: T3F structural and regulatory genes are highly conserved, however, regulatory gene disruption (mrkH) is more common than structural gene disruption and, in both cases, MrkA Ag is not expressed. In vivo Ag profiling revealed site-specific differences in MrkA expression, with ∼20% of KP cells expressing MrkA in the lung versus ∼5% in the bloodstream. Anti-MrkA mAb activity was dependent on MrkA abundance, with significantly enhanced efficacy following infection with MrkA-overexpressing KP. CONCLUSIONS: Regulatory genes are as important to characterise as structural gene presence when evaluating antigen candidates in clinical isolates, and Ag expression can vary by anatomical context. For MrkA, Ag abundance determines anti-MrkA mAb activity, suggesting infections with high MrkA expression will respond better to therapy.