Asthma is known as a chronic inflammatory disease and typically characterized by airway inflammation, airway hyperresponsiveness and airway remodeling, where inflammatory cells and various cellular components are involved [20–22]. Previous research has shown that IL-17A recruits a series of cytokines and chemokines by activating downstream cells, thereby acting on neutrophils [23, 24], therefor, blocking or suppressing IL-17A-neutrophil axis may alleviate airway inflammation in neutrophilic asthma.
Nowadays, antibodies are widely used in cell signal pathway’s blocking. Monoclonal antibodies and soluble receptors have high specificity and efficacy, however, high cost, frequent injections and side effects such as allergic risk and infusion reaction often along with it [25, 26]. To overcome these disadvantages, a new strategy was emerged. Vaccine was established by reconstructing the peptide of targeted self-cytokine’s immunodominant epitope region with foreign Th epitopes region into a carrier to induce specific autoantibody [27, 28]. Under this theory, a recombinant Mycobacterium smegmatis inducing IL-17A autoantibody was designed in previous study . Experimental results showed a persistent effect of generating IL-17A autoantibody after immunisation with rMS and confirmed that these autoantibodies had high activity to neutralize IL-17A.
Since it has been reported that DO11.10 mice challenged by OVA exhibits an increase in neutrophil infiltration instead of eosinophil , rMS was vaccinated into DO11.10 mice to study the effects on neutrophilic asthmatic airway. Airway inflammation is a prominent hallmark in pathogenesis of asthma, and IL-17A is a pivotal mediator in neutrophilic inflammation, as it recruits neutrophil from peripheral blood into local bronchial tube via C-X-C chemokine [30–32]. Neutrophil itself has potential to release a bunch of cytokines and chemokines such as IL-6, TNF-α, CXCL-1 and CXCL-2 to participate in airway inflammation [33, 34]. IL-6 and IL-23 are downstream cytokines of IL-17A, which can maintain and enhance IL-17A production [35–37]. Moreover, TNF-α not only can stabilize IL-17 mRNA expression, but also have a chemoattractant effect on neutrophils [38, 39]. Thus, IL-17A-neutrophil axis is amplified by a loop feedback. In our experiment, data showed that neutrophils and macrophages were elevated in BALF of OVA-challenged mice, and which were significantly reduced by rMS. Histopathological examination also revealed an obvious decrease of inflammatory cells around bronchia in rMS vaccinated mice compared to asthmatic ones. Meanwhile, cytokines and chemokines related to IL-17A-neutrophil axis were all notably decreased in rMS group contrast to asthma group. These results indicate that rMS suppresses inflammatory cells and mediators in non-eosinophilic asthma through dampening IL-17A-neutrophil axis. Besides inflammatory mediators released from neutrophil, neutrophil enzyme may be another main cause of airway injury. MPO generates reactive oxygen species, which directly contribute to airway epithelial damage, and neutrophil elastase also plays important roles in airway remolding, as it can degrade structural proteins, increase vascular permeability and metaplasia of bronchial mucus glands [40–42]. Experimental data showed the activity of MPO in both BALF and lung tissues were obviously elevated in asthma mice compared to control subjects, and these increases were reversed by rMS. Further, PAS staining showed rMS effectively inhibited goblet cell hyperplasia and mucus secretion. These results indicate that vaccination with rMS could suppress neutrophil enzyme activity to alleviate airway inflammation and impairment.
In conclusion, vaccination of recombinant Mycobacterium smegmatis breaks IL-17A-neutrophil axis via inducing IL-17A autoantibody followed by reduction of inflammatory cells and mediators and suppression of the activity of neutrophil enzymes and mucus secretion, thereby attenuating airway inflammation in mice with neutrophilic asthma. Thus, recombinant Mycobacterium smegmatis may have protective effects on airway insults of asthma.