Epilepsy is a chronic brain disease characterized by repetitive, paroxysmal and transient central nervous system dysfunction caused by excessive discharge of brain neurons (Thijs et al., 2019). It has a high incidence rate, which is next only to cerebrovascular diseases in neurological diseases (Perucca et al., 2018). Epilepsy affects more than 70 million people worldwide, and 60% of patients whose onset of epilepsy occurs in childhood (Vezzani et al., 2016; Trinka et al., 2019). Epidemiological data have revealed the incidence of chinese children is about 5%, and there are about 6 million children with epilepsy (Ding et al., 2021). As a chronic disease, epilepsy requires long-term treatment, which seriously affects the life and study of children, and brings heavy psychological and economic burdens to individuals, families and society (Gao et al., 2015).

Due to the complex pathogenesis of epilepsy, different types of epilepsy have different symptoms and characteristics. Studies have shown that the accumulation of immune inflammatory damage may be the core mechanism of epilepsy (Vezzani et al., 2015; Bazhanova et al., 2021). Immune system dysfunction and inflammatory response are the basic mechanisms that cause excessive excitement of brain nerves (Marchi et al., 2014; Rana and Musto, 2018). In the process of epilepsy caused by different etiologies, inflammatory response may play an important role in all stages, not only as a direct cause, but also as inducing factors and aggravating factors (de Vries et al., 2016).

Immunoglobulin G (IgG), which constitutes 10–20% of plasma proteins, plays an important role in the immune system to recognize and clear foreign antigens and pathogens (Meng et al., 2020). There is a conservative N-glycosylation modification site on Asn297 in the constant region of IgG (Hou et al., 2019). N-glycosylation is a ubiquitous co- and post-translational modification that enriches protein structure and function (Wang et al., 2019). The state of IgG glycosylation modification is closely related to its physical and chemical properties and biological functions (Li et al., 2019). Changes in blood IgG N-glycosylation have been observed in Amyotrophic lateral sclerosis, Multiple sclerosis, Alzheimer’s, and other neurological diseases (Grigorian et al., 2012; Gonçalves et al., 2015; Wuhrer et al., 2015; Costa et al., 2019; Fang et al., 2020; Zhang et al., 2021). Although the mechanisms of IgG glycosylation are well studied, the association of IgG glycosylation with epilepsy is particularly rare. As the abnormal activation of key inflammation is involved in the occurrence of epilepsy, the composition change of IgG glycome regulates the balance of pro-inflammatory and anti-inflammatory. We hypothesized that the abnormal modification of IgG glycome might be one of the influencing factors of epilepsy in children. Therefore, this study aims to profile the IgG N-glycome in Chinese epilepsy children and to provide a new idea for the mechanism research and treatment of neurological diseases such as epilepsy.

Whereas studies on glycosylation of human serum IgG as well as nervous system diseases such as Alzheimer’s disease are abundant, IgG glycosylation of epilepsy has been less studied. Particularly in epilepsy, to our knowledge, this is the first study that presents the N-glycan profile and investigated the disparate points of the IgG glycome in identifying individuals with epilepsy children using a case–control epidemiological approach.

This study aimed to identify changes in the peripheral serum IgG glycome of children with epilepsy that could represent an interphenotype of the disorder: a link between the genome, the environment, and the disease phenotype. Indeed, this study found that children with epilepsy had a different IgG glycosylation profile than controls.

Observing our classification model, individuals with epilepsy had a reduced relative abundance of GP7, GP8, GP9, GP10, GP12, GP14, GP15, GP16, and GP21, and an increased relative abundance of GP1, GP2, GP3, GP4, GP5, GP11, and GP13. Specifically, the model demonstrated that the presence of epilepsy pathology was predominantly explained with a reduced relative abundance of a galactose N-glycan structure and a core fucosylated N-glycan structure, as well as an increased relative abundance of a core fucosylated N-glycan structure, a bisected GlcNAc N-glycan structure and a high-mannitose N-glycan structure (GP5).

Epilepsy, as the third common chronic brain disease, is characterized by long-term susceptibility to seizures (Manford, 2017). Numerous evidences showed that immune system dysfunction and inflammatory signals may play a role in various stages of epilepsy development (Matin et al., 2015; Cordero-Arreola et al., 2017; Husari and Dubey, 2019). Pro-inflammatory molecules can change the excitability of neurons and affect the physiological function of glia through paracrine or autocrine, thus disrupting the communication of neurons (Saghazadeh et al., 2014). Interleukin-6 (IL-6) not only regulates immune function, but also affects neuron function, and promotes glial formation and neurogenesis (Erta et al., 2012). For example, Kalueff et al. (2004) found in the mouse model that the increase of IL-6 level in the mouse brain can promote convulsion;Virta et al. (2002) showed that the levels of IL-6 in human plasma and cerebrospinal fluid are significantly increased within 24 h after general compulsive spasm and febrile convulsion.

The Fc segment of Ig G is modified by glycosylation to mediate complement activation, antibody-dependent cell-mediated cytotoxicity (ADCC), etc., thereby regulating inflammation, affecting the body’s pro-inflammatory or anti-inflammatory effects, and then affecting diseases (Russell et al., 2018; Štambuk et al., 2020). The core fucosylation of IgG has been the focus of research because of its role in ADCC (Pucic et al., 2010; Adua et al., 2021). In our latest study, it was shown that the level of IgG fucosylation in COVID-19 patients decreased, which up-regulated ADCC in acute immune response (Hou et al., 2021). In this current study, the IgG core fucosylation level of epileptic children is lower than that of healthy children of the gender-age matched, which indicates that abnormal modification of core fucosylation may be an important biomarker of epilepsy.

The deletion or reduction of galactosylated glycan at the end of IgG makes Ig G exposed at the end of sugar chain to acetylglucosamine, which is not easy to be cleared by phagocytes, leading to the decrease of binding force between antibody and FcγRIIB, activating complement and promoting the occurrence and development of inflammation (Adua et al., 2021). We found that the level of agalactosylation in children with epilepsy is higher, and the levels of monogalactose and digalactose are lower compared to normal healthy children (age-gender matched), which just proves above point. This is not consistent with the results found in the cerebrospinal fluid of patients with amyotrophic lateral sclerosis (Costa et al., 2019).

Sialylation of IgG can inhibit the combination of C1q and galactosylated glycans, thus limiting the pro-inflammatory effect (Zhang et al., 2021). Numerous clinical evidences showed that intravenous immunoglobulin had remarkable curative effect on epilepsy, and the anti-inflammatory activity of sialylation had been found (Dubey et al., 2020; González-Castillo et al., 2020). Although this study discovered that individual sialylation levels in children with epilepsy were lower than those in the control group, there was no significant difference in total sialylation levels, which could be attributed to differences in individual disease states, genetic predispositions, or temporary physiological conditions. Epilepsy has long been considered to be related to some autoimmune diseases (Matin et al., 2015; Cordero-Arreola et al., 2017), and our previous studies found that abnormal modification of IgG N-glycans was related to autoimmune diseases such as systemic lupus erythematosus and rheumatoid arthritis (Li et al., 2021).

Higher levels of bisecting GlcNAc on IgG enhance ADCC via increased FcγRIII binding and elevate the proinflammatory function of IgG (Hou et al., 2021). Although there is no significant difference in the total bisected GlcNAc glycans between the two groups. Further analysis showed that the bisected N-GlcNAc glycans without core fucosed glycan were higher than that of the control group, which indicated that core fucosed glycans and bisected acetylglucosamine have an antagonistic effect. The lack of core fucosed glycans may lead to the weakening of the binding ability of the Fc segment and FcγRIII, enhancing the ADCC effect and triggering pro-inflammatory effects (Hou et al., 2021). At present, however, it is still uncertain whether the enhancement of ADCC is directly caused by the change of bisected N-GlcNAc glycans level or caused by the change of core fucosed glycan level (Gao et al., 2017).

These findings provide new insights into the pathogenesis of childhood epilepsy. However, some limitations should be considered: First, as this was a cross-sectional study, our results cannot fully address the causal relationship between IgG glycome and childhood epilepsy. Second, the moderate sample size limits the generalization of conclusions. Third, some relapsed patients may affect the concentration of IgG glycosylation due to the side effects of long-term use of antiepileptic drugs. Despite these limitations, we analyzed the IgG N-glycans group in Chinese children with epilepsy, which helps to understand the immune response to childhood epilepsy. This finding, of course, should be confirmed on larger sample size and different stages of epilepsy.

In conclusion, we presented the N-glycosylation profile of IgG from epilepsy patients’ plasma and demonstrated that levels of core fucosylation and galactosylation were lower in epilepsy patients than in controls. Meanwhile, the Gal-ratio (the proportion of nogalactosylated glycan in the total) was higher than in healthy children; lack of nogalactosylation can activate the complement system and promote the occurrence of inflammatory response, reduce IgG anti-inflammatory response, and contribute to the pathogenesis of epilepsy. In summary, this study provides new insights into the pathogenesis of epilepsy in children, although further studies are needed.

The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found in the article/Supplementary Material.

The studies involving human participants were reviewed and approved by the Ethics Committee of the Shandong First Medical University & Shandong Academy of Medical Sciences, Taian, China. The ethics approval was given in compliance with the Declaration of Helsinki. Written informed consent to participate in this study was provided by the participants’ legal guardian/next of kin.

JL and GW contributed to the study concept and design. FS, HJ, MW, PL, and LJ contributed to the acquisition of subjects and/or data. YL, XC, and HZ contributed to the analysis and interpretation of data. JL, YL, LJ, and XC contributed to the preparation of the manuscript. All authors read and approved the final manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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