This study aims to investigate whether changes in brain structure are associated with CRS. Our study demonstrates the causal relationship between the brain and CRS using two-sample MR. In our clinical data analysis of cortical TH, we found a difference in the average cortical TH of the orbitofrontal cortex (OFC) between the CRS group and the HC group, with the former having greater TH. Consequently, we explored the potential involvement of the OFC in the pathogenesis of chronic rhinosinusitis.
Neuro-immune dysfunction involving neuropeptides and ion channels has been proposed as a subtype of non-allergic rhinitis and may also play a role in the pathogenesis of CRS[15]. Research suggests that neuropeptides and the parasympathetic nervous system in the nasal mucosa may contribute to the formation of nasal polyps. Consequently, capsaicin, a topically applicable drug, has been considered for preventing the recurrence of nasal polyps[28]. Capsaicin acts on C fibers, and studies indicate that it may regulate nasal hyperreactivity through neural modulation, potentially preventing the recurrence of nasal polyps[29]. The brain interacts with the immune system via the neuroendocrine and autonomic nervous systems. Alterations in emotional stress or psychological state can affect immune function through these pathways, thereby potentially influencing the occurrence and progression of inflammation[30]. An acute exacerbation of CRS is defined as a worsening of symptoms, typically reverting to baseline CRS symptom levels after intervention with corticosteroids and/or antibiotics. While bacterial infection is generally believed to exacerbate nasal and sinus symptoms, the reality may involve more complex factors such as exacerbated allergic rhinitis, acute viral respiratory infections, asthma exacerbations, or other stressors, including depression[10]. In conclusion, neurological function impairment is closely associated with infections and the immune system.
So, we are curious about the association between the OFC structure and the occurrence of CRS. In a study on elevated C-reactive protein (CRP) levels and their correlation with increased cortical surface area and decreased cortical TH in youth with bipolar disorder, it was demonstrated that in ROI analyses, higher CRP was associated with higher whole-brain SA (β = 0.16; P = 0.03) and lower whole-brain (β = −0.31; P = 0.03) and OFC cortical TH (β = −0.29; P = 0.04) within the BD group, and was associated with higher OFC SA (β = 0.17; P = 0.03) within the CG[31]. Higher NGAL levels may be associated with reduced lateral orbitofrontal cortex TH, and this association might be specific to HIV-positive individuals. In our Mendelian analysis, we identified that the SA of the OFC is a risk factor for CRS[32]. Our cortical TH analysis also confirmed differences in the average cortical TH of the OFC between the CRS and HC groups. Based on these findings, we believe that structural changes in the OFC may be associated with changes in the immune status of the nasal cavity.
In recent years, the potential impact of brain structural changes on immune status has garnered increasing attention. The bidirectional signaling between the brain and the immune system plays a crucial role in emotion, motivation, and various psychological and physiological health issues[33, 34]. Notably, brain systems involved in processing rewards and promoting goal-directed behavior, such as the OFC, are of particular interest. The OFC is believed to play a critical role in assigning value to social and achievement-related rewards and assessing the likelihood of obtaining these rewards. Literature has highlighted that high reward reactivity is associated with elevated levels of inflammatory markers. For example, individuals with bipolar disorder often exhibit high levels of reward sensitivity and increased inflammatory markers. Conversely, low reward sensitivity is also linked to high levels of inflammatory markers. This phenomenon suggests that alterations in reward-related brain functions may be associated with increased inflammation, regardless of reward sensitivity levels. Data from the literature indicate a significant correlation between OFC activation during reward anticipation and higher composite inflammation scores in individuals with a high goal pursuit tendency[35]. Negative emotional states (such as depression, anger, and fatigue) and unhealthy behaviors (such as high-fat/high-sugar diets and substance use) can further increase inflammation by affecting the function of the autonomic nervous system and the hypothalamic-pituitary-adrenal (HPA) axis[34, 36, 37]. According to the literature on " Nerve Stimulation: Immunomodulation and Control of Inflammation", the role of the nervous system in regulating immune responses and inflammation is considered a key mechanism[38]. Neurostimulation (such as electroacupuncture) can regulate the function of immune cells by activating specific neural networks, thereby controlling inflammation and preventing organ damage[39, 40]. Specifically, neurostimulation can trigger the release of neurotransmitters and cytokines, which can communicate bidirectionally with the immune system[41]. For example, activation of the sympathetic nervous system can achieve localized inflammation control by releasing norepinephrine, thereby avoiding side effects on non-target tissues[42–44]. Additionally, Vagus nerve stimulation has been shown to alleviate inflammatory responses by reducing the release of pro-inflammatory cytokines[45]. These studies suggest that changes in brain structure may influence immune status by altering the function of the nervous system. This mechanism shows potential clinical application value in treating inflammatory and infectious diseases (such as sepsis). Future mechanistic studies will help develop new non-invasive neurostimulation techniques to achieve more effective control over immune and organ function.
Zhang et al. (2020) in their study published in Nature suggested that the brain can directly influence the immune system through descending neural circuits[46]. In T-dependent immune responses, activation within the paraventricular nucleus (PVN) and central nucleus of the amygdala (CeA) enhances the formation of splenic B plasma cells. The CeA and PVN consist of neurons that produce corticotropin-releasing hormone (CRH), which drives the activation of the HPA axis during stress responses, leading to the release of corticosterone from the adrenal glands. This implies that neural activity in the OFC might also regulate immune responses by affecting descending neural pathways. The OFC plays a crucial role in regulating stress responses. Stress activates the HPA axis, resulting in the release of a series of neuroendocrine mediators such as CRH, adrenocorticotropic hormone (ACTH), and cortisol. These mediators can influence systemic immune responses, including those in the nasal immune system, through the bloodstream. Acute short-term stress typically enhances immune responses, whereas chronic stress may lead to immune dysfunction. Changes in the OFC may affect the release of neurotransmitters such as acetylcholine (ACh) and norepinephrine (NE), which play significant roles in local immune regulation. For example, ChAT+ T cells, which synthesize acetylcholine, have been found to promote B cell plasma cell formation, thereby enhancing immune responses. Chronic stress-induced high levels of cortisol might inhibit B cell activation and antibody production, weakening nasal immune defenses. In summary, we hypothesize that changes in the OFC could lead to local or systemic inflammatory responses, which in turn could affect nasal immune cells through various mechanisms. However, evidence for direct neuronal control of the immune system by the brain remains limited. Elucidating the neural-immune circuits will require the development of a set of molecular tools that can specifically target the autonomic nervous system.
Strengths and limitations
The strengths of this study include the use of a large GWAS dataset and the exclusion of relevant confounding factors. Our research provides detailed information on structural changes in specific brain regions, examining the effects of brain structure on CRS from two dimensions of the cerebral cortex (SA and TH; with global weighting). This increases the likelihood of finding significant estimates. Our findings offer suggestive evidence that alterations in brain structure are involved in the occurrence of CRS. In our analysis of cortical TH alterations, we identified differences between CRS patients and healthy controls, but the specific neural functions and underlying mechanisms require further investigation.
It is important to note that our SNPs did not meet the conventional GWAS significance threshold (P < 5×10^8), which may affect the interpretability of the results. Additionally, since we used genetic data from individuals of European ancestry, it remains to be seen whether these conclusions can be generalized to other populations. Finally, our study did not analyze the correlation with clinical indicators, such as inflammatory cytokines, nasal endoscopy scores, and CT scores, which will be a focus of our future research.