Pulmonary delivery of a humanized heavy-chain antibody LcrV-X19-R1 confers complete protection against fatal pneumonic plague in mice.

Pneumonic plague is the most fatal form of plague, with a mortality rate approaching 100% within 24 h if untreated. The emergence of drug-resistant Yersinia pestis strains capable of human-to-human transmission poses a significant challenge to conventional antibiotic therapies. While therapeutic antibodies present a promising alternative, their intravenous administration is limited by low pulmonary bioavailability and suboptimal efficacy. Targeting LcrV, a key virulence factor of Yersinia pestis type III secretion system, we developed a humanized heavy-chain antibody designated LcrV-X19-R1, and formulated it for inhalation delivery. LcrV-X19-R1 exhibited a high affinity for LcrV (KD of 0.561 nM), demonstrated good thermal stability, produced inhalable aerosol particles with a mass median aerodynamic diameter of ≤5 μm after nebulization, and showed a favorable safety profile in vivo, with all these properties meeting the criteria for effective pulmonary deposition. In a lethal mouse model of pneumonic plague, pulmonary delivery of LcrV-X19-R1 achieved 100% survival, completely cleared the bacterial loads in organs, and markedly reduced tissue damage. The protective mechanism of LcrV-X19-R1 involves direct bacterial neutralization in tissues and enhanced clearance via antibody-dependent cellular phagocytosis, which together dampen harmful inflammation and promote tissue repair. Pharmacokinetically, pulmonary delivery achieved roughly 10-fold higher lung antibody levels and retention beyond 120 h, far surpassing intravenous administration. These advantages ultimately translated into a 70% survival rate, compared to 0% with intravenous treatment. This study establishes a mucosal therapeutic strategy against pneumonic plague and provides a versatile platform for developing antibody-based interventions against other respiratory infectious diseases.