THE ROLE OF CASEINOLYTIC PEPTIDASE B IN CYTOKINE-MEDIATED INFLAMMATION

Scarano F, De Lutiis MA. The role of Caseinolytic peptidase B in cytokine-mediated inflammation.  International Journal of Infection. 2025;9(2):63-66.

F. Scarano^1* and M.A. De Lutiis²

¹ Department of Clinical and Experimental Medicine, University of Foggia, Italy;
² Biology Department, University “G. D’Annunzio” of Chieti-Pescara, Chieti, Italy.

*Correspondence to:
Dr. Filippo Scarano,
Department of Clinical and Experimental Medicine,
University of Foggia, Via Rovelli 50,
71122 Foggia, Italy.
e-mail: filippo.scarano@unifg.it

Received: 24 June, 2025 Accepted: 01 August, 2025

ISSN 1972-6945 [online] Copyright 2025 © by Biolife-publisher This publication and/or article is for individual use only and may not be further reproduced without written permission from the copyright holder. Unauthorized reproduction may result in financial and other penalties. Disclosure: all authors report no conflicts of interest relevant to this article.

ABSTRACT

Caseinolytic peptidase B (ClpB) is a chaperone ATPase enzyme that mediates many biological processes, including the generation of inflammatory cytokines. The bacterial protein ClpB plays a crucial role in the cellular stress response, particularly heat stress. ClpB belongs to the Enterobacteriaceae family, which also includes Escherichia coli. ClpB is involved in bacterial survival and resistance by restructuring denatured proteins and it activates macrophages to produce the inflammatory cytokines TNF and IL-6. The immune response to microorganisms generates interferon-gamma (IFN-γ), which helps eliminate ΔClpB mutants. ClpB is highly immunogenic and could be a valuable diagnostic tool. During host stress, ClpB promotes protein recovery and adaptation and interacts with the host. It can be secreted or exposed to the surface, triggering the release of cytokines. It dysregulates proteins involved in specific functions, such as pigmentation, inflammation, energy homeostasis, and sexual function. The hypothalamic-pituitary-adrenal (HPA) axis pathway regulates the body’s response to stress and leads to the production of hormones such as cortisol. Microbiota modulate the HPA axis through immune, metabolic, and nervous system mechanisms. ClpB contributes to systemic inflammation by inducing autoantibodies and can alter cortisol release.

KEYWORDS: Caseinolytic peptidase B, ClpB, ATPase enzyme, cytokine, bacteria, immune response

INTRODUCTION

Caseinolytic Peptidase B (ClpB) is a chaperokine ATPase involved in the stimulation of host cytokines (1). It is a ring-shaped hexamer with two NBD domains (AAA+) in each monomer and a regulatory intermediate domain (2). ClpB is a bacterial protein that is implicated in host-pathogen interactions and modulates cytokine activity in the immune system.  ClpB is a heat shock chaperonin produced by some bacteria of the Enterobacteriaceae family, including Escherichia coli (3) that is linked to the intestinal microbiota and affects metabolic, neuropsychiatric, autoimmune, and inflammatory disorders (4).

ClpB is a Hsp100-type chaperone protein produced by commensal bacteria such as E. coli. The BClpB protein is involved in the survival of bacteria by helping them survive environmental stress by restructuring denatured proteins (5). For some bacteria, such as Mycobacterium tuberculosis, ClpB is essential for stress tolerance and latent survival (6). ClpB is purified from M. tuberculosis and also acts as a chaperokine, binding to macrophages and inducing the release of pro-inflammatory cytokines such as TNF and IL-6 (6).

DISCUSSION                                                                                                                                                                                          

ClpB promotes bacterial survival and directly modulates host immunity (7). In infections, ClpB promotes the survival of various pathogens under conditions of host-induced stress-heat, oxidative stress, and nutrient starvation, and its absence often leads to attenuated virulence in animal models (8).

For example, in Francisella tularensis, a small (≈0.2 × 0.7 µm) aerobic, facultatively intracellular Gram-negative bacillus, ΔClpB mutants are eliminated more rapidly and stimulate a protective immune response to interferon-gamma (IFN-γ) (9). F. tularensis is a bacterium belonging to the Francisellaceae family that is one of the most infectious microorganisms, and causes tularemia, an anthropozoonosis (10). It is a nonmotile, nonspore-forming, encapsulated coccobacillus, that is oxidase and catalase positive. F. tularensis is an intracellular parasite that infects various animals and can also be transmitted to humans via vectors such as cats, ticks, insects, and other parasites (11).

ClpB possesses immunogenicity and diagnostic potential that is similar to that found in leptospirosis, where the ClpB gene from Leptospira interrogans (ClpBLi) is expressed during infection and is highly immunogenic (12). In this case, animals produce specific antibodies, suggesting its potential as a diagnostic antigen. Leptospira, which is “leptos” in Greek (meaning “thin”), is classified into L. interrogans and L. biflexa, which comprise 60 serotypes grouped into 28 serogroups, including the saprophytic, non-pathogenic Leptospira that live freely in water (13). The species L. interrogans comprises more than 250 different serotypes or serovars that are divided into 24 serogroups, all of which are considered pathogenic (14). The disease caused by this bacterium is known as leptospirosis, which is a major zoonosis spread worldwide.

Humans express a mitochondrial homologue of ClpB that is also part of the AAA⁺ family, but it acts intracellularly and is not secreted as a cytokine stimulator (15).  ClpB plays a dual role in infection; at the intrabacterial level, it promotes protein recovery and adaptation during host stress, and it also interacts with the host and can be secreted or surface-exposed, triggering cytokine release and immune responses (16).

Studies demonstrate that ClpB is a virulence factor and a potential target for new therapies or vaccines (17). This protein from intestinal bacteria, such as Enterobacteriaceae, structurally mimics the host neuropeptide α-melanocyte-stimulating hormone (α-MSH), which plays a role in homeostasis and inflammatory responses by regulating cytokine production via melanocortin (MC) receptors (18).  The molecular mechanisms of α-MSH mimicry is involved in IL-6 and IL-10 dysregulation through the MC signaling pathway (19). MC receptors are a family of G-protein-coupled receptors (GPCRs) that are distinguished into five subtypes: MC1R-MC5R, each with specific functions that affect pigmentation, inflammation, energy homeostasis, and sexual function, amongst others (20,21). Mutations in the MC4R gene are the most common cause of obesity and are found in approximately 1-6% of severe obesity cases (22). MC4R is found primarily in the paraventricular nucleus of the hypothalamus, the brainstem, and sympathetic motor neurons. It contributes to the regulation of appetite, energy metabolism, cardiovascular function, reproduction, pain, mood, and sexual function (23).

Gut-brain immune signaling involves the cytokines IL-1β and IL-6, through the hypothalamic-pituitary-adrenal (HPA) axis pathway and vagus nerve (24). HPA is a neuroendocrine system that regulates the body’s response to stress that is made up of three main components: the hypothalamus, the pituitary gland, and the adrenal glands (25). The HPA produces hormones which help the body respond to environmental stressors, such as cortisol (26). The microbiota modulates the HPA axis through immune, metabolic, and nervous system mechanisms (e.g., SCFAs, LPS, peptidoglycans, vagus nerve) (27). By inducing autoantibodies, ClpB contributes to systemic inflammation and can alter cortisol release (28). ClpB has been used as a biomarker for microbiota-related disorders with immune components (4).

ClpB is an important chaperone present in bacteria and in mitochondrial homology in eukaryote cells (such as Skd3/CLPB in humans) (29). ClpB causes resistance in microorganisms surviving in hostile conditions generated by the inflammatory response, including high temperature, variable pH, and oxidative and nitrosative stress (30).  In M. tuberculosis, cytosolic ClpB is released and interacts with macrophages, modulating the inflammatory response and contributing to the stability of tuberculosis granulomas (31,32). The release of ClpB from the bacterial cytoplasm can also activate macrophage cells in infections caused by Mycoplasma pneumoniae, L. interrogans, and F. tularensis (33).

CONCLUSIONS

ClpB is a chaperone in the Enterobacteriaceae family and mediates numerous biological processes. It plays a crucial role in bacterial responses to cellular stress and in the immune response, contributing to the elimination of ΔClpB mutants. In innate immunity, ClpB activates macrophages to produce inflammatory cytokines. ClpB may be a valuable marker in microbiota-related immune diseases.

Conflict of interest

The authors declare that they have no conflict of interest.

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