To effectively fend off bacterial invasions while evading unnecessary energy depletion and tissue damage caused by aberrant inflammation in the resting state, host organisms have evolved elaborate pathogen recognition and immune regulatory systems. The transcription factor NF-κB serves as the central hub for coordinating inflammatory responses. However, the molecular mechanisms that precisely control its activation and suppression, particularly the question of how hosts sense bacterial presence to toggle inflammation on and off, remain incompletely understood.

Published in the prestigious journal Science Immunology, a research team led by Prof. XIAO Wuhan from the Institute of Hydrobiology (IHB) of the Chinese Academy of Sciences, uncovers how Occludin-ellipsoid protein 1 (OCEL1), a critical immune modulator, senses bacterial infection to regulate inflammatory responses, shedding new light on the intricate balance between host defense and immune homeostasis.

Initially, the research stemmed from an unexpected observation, human OCEL1 failed to stably express in Escherichia coli, while mouse Ocel1 did. Through sequence alignment and truncation analyses, the researchers identified a unique N-terminal 30-amino-acid segment in human OCEL1, containing the proline-rich Proline-Proline-Glutamate (PPE) motif (-PPGPGPP-). The researchers restored stable expression by either mutating PPE’s 81st/82nd prolines or deleting the element. Furthermore, fusing the PPE motif to stable proteins caused them to destabilize, confirming PPE as a bacterial target.

Subsequently, mass spectrometry identified bacterial FK506-binding protein (FKBP) family isomerases as PPE binders. Overexpressing these FKBPs reduced wild-type OCEL1 levels via K48-linked ubiquitination, a process notably blocked by the inhibitor MG132. Because this degradation did not affect PPE-mutated OCEL1, the results validate that OCEL1 breakdown is specifically FKBP-mediated.

Further, cell experiments showed OCEL1 overexpression suppressed Lipopolysaccharide / Pseudomonas aeruginosa strain PAO1 (LPS/PAO1)-induced proinflammatory cytokines (IL-1β, IL-6, TNF), while its depletion amplified responses. Animal studies revealed OCEL1-deficient mice had severe hyperinflammation. Humanized mice with wild-type OCEL1 showed stronger inflammation than PPE-mutated ones during PAO1 infection (no difference with FKBP-deficient PAO1). Mechanistically, OCEL1 binds NF-κB Essential Modulator, Leucine Zipper domain (NEMO’s LZ domain), thereby inhibiting TNF Receptor Associated Factor 6-mediated Lysine 63 (TRAF6-mediated K63) ubiquitination. This interaction effectively suppresses NF-κB activation during resting states.

By uncovering a novel "enzyme activity-sensing" pathogen recognition mode, this study reveals a strategy distinct from traditional receptors, expanding the understanding of host-pathogen interactions. The research clarifies a new virulence function for bacterial FKBP isomerases: the targeted degradation of OCEL1. Consequently, this study reveals the OCEL1-NEMO axis as a fine-tuned NF-κB regulator.

Translationally, these findings provide therapeutic targets. PPE-FKBP inhibitors enhance anti-infection responses, while stabilized OCEL1 mitigates inflammatory damage (e.g., sepsis). Additionally, OCEL1’s link to lung cancer prognosis offers fresh insights for cancer therapy, highlighting broad scientific and clinical value.

(Editor: MA Yun)