Repurposing a Detrimental Antibody Epitope as Targeted Therapeutics for Sepsis and Rheumatoid Arthritis.

Sepsis and rheumatoid arthritis (RA) are distinct yet mechanistically related conditions commonly driven by dysregulated inflammatory responses. Here, we explored the counterintuitive hypothesis that an epitope from a deleterious anti-tetranectin (TN) antibody (mAb9) could hold unforeseen therapeutic potential. By mapping mAb9's epitope to P2 (residue 55-70), a region crucial for TN's protective functions, we developed P2-1, a water-soluble derivative as a targeted therapy. P2-1 significantly improved survival and reduced systemic inflammation in a sepsis model, and attenuated arthritis severity and pain sensitivity in a RA model, even with therapeutic administration after disease onset. Mechanistically, P2-1 exhibited high-affinity binding to high mobility group box 1 (HMGB1) and selectively suppressed HMGB1-induced Ctsl mRNA upregulation and procathepsin L (pCTS-L) secretion from human immune cells, crucially without perturbing other HMGB1-induced cytokines and chemokines. We further validated pCTS-L as a therapeutic target by demonstrating that a neutralizing antibody conferred potent anti-arthritic effects, reducing joint inflammation, pain, and structural damage. Our findings introduce a paradigm-shifting drug discovery strategy that transforms insights from paradoxical antibody action into targeted therapeutics for the HMGB1-pCTS-L axis, not only delivering P2-1 as a potent therapy but also establishing pCTS-L as crucial mediator of inflammatory diseases like sepsis and RA.