X., T. Furthermore, we assessed the expression levels of specific antibodies against ovalbumin and found they Ibrutinib-biotin were downregulated inFut8+/mice, with potential recovery observed with L-fucose administration. These findings confirm that core fucosylation plays a vital role in regulating IgG levels in serum, which may provide insights into a novel mechanism in adaptive immune regulation. Keywords:immunoglobulins G, Fut8, core fucosylation, L-fucose, Fc receptor Immunoglobulin G (IgG), the primary molecule of the adaptive immune response, is the most abundant immunoglobulin isotype in the plasma. IgG consists of four subclasses in humans (IgG1, IgG2, IgG3, and IgG4) (1) and mice (IgG1, IgG2a, IgG2b, and IgG3) (2), each characterized by unique structures and functions. The typical structure of IgG is composed of two heavy (H) chains and two light (L) Ibrutinib-biotin chains linked together by disulfide bonds (3). Each heavy chain comprises a variable domain at the N-terminal and three constant domains (CH1-3). The light chains also possess a variable part at the N-terminal and a constant domain IQGAP1 name (CL) (3). The fragment antigen binding (Fab) domains, known as antigen acknowledgement domains, contain the complementarity determining regions. These regions are located in the N-terminal part of CHs and CLs, Ibrutinib-biotin responsible for determining the antigen-specificity (4). The fragment crystallizable (Fc) regions of the C terminal are made up of two CHs (CH2 and CH3), which bind to the immune effector molecules such as Fc receptors (FcR) (5). Different IgG subclasses have varying affinities for FcR Ibrutinib-biotin (6). FcRs generally exhibit function and structure homology between humans and mice but may also exhibit differences (7). Among IgG subclasses, hIgG1 is the most abundant and dominant subclass of IgG in therapeutics and immune responses (8). Similarly, mIgG2, as orthologs and functional homologs of hIgG1, which shows a preference for Fc-gamma receptor (FcR) (9,10,11), holds great importance in protective and pathogenic properties in mice, both in innate and adaptive immunity (12). N-glycosylation is the most prevalent modification of IgG.N-glycans attached to Fab fragments can influence the antibodys reactivity. Approximately 25% of Fabs are altered by numerous glycan types, impacting their structural formation (13,14), antigen-binding specificity (15,16), and half-life (17). Notably, theN-glycosylation at asparagine 297 (Asn297, N297) in the Fc fragment is usually well-known for playing an essential role in the immune response (18,19,20). This site has one of the potentially 30 glycan species (21). Among these, core fucosylation, catalyzed by 1,6-fucosyltransferase (Fut8), which transfers L-fucose from GDP-fucose to the innermost GlcNAc, is one of the most pivotal modifications, with more than 94% of IgGs being modified by core fucosylation (22). The reason why IgGs are highly altered by core fucosylation remains unclear. However, the extent of core fucosylation of IgGs affects the strength of the immune response. A deficiency of core fucosylation on IgG can significantly increase the binding ability between IgG and FcRIII in humans (23) and lead to enhanced complement-dependent cytotoxicity, antibody-dependent cellular cytotoxicity (ADCC) (24,25). On the other hand, the lower levels of core fucosylated IgG are associated with the severity of some diseases. For instance, in the fetuses mediating fetal or neonatal alloimmune thrombocytopenia, the alloantibodies IgG1 against human platelet antigens during pregnancy contain lower core fucosylation, correlating with disease severity (26). Similarly, recent studies have also shown that afucosylated (noncore fucosylated) IgG1 is usually closely associated with disease severity in dengue fever (18) and COVID-19 (27). Very recently, we found that the level of afucosylated IgG was increased in the sera of the patients with lung malignancy, chronic obstructive pulmonary disease, and interstitial pneumonia compared to healthy subjects (28). Therefore, the degree of core fucosylation of IgG modulates its binding to the Fc receptors, with significant implications Ibrutinib-biotin for the efficacy of antibody-based therapies, vaccine development, and immune therapy. In the current study, we investigated the significance of core fucosylation in regulating IgG levels and the underlying mechanisms for altering IgG inFut8heterozygous knockout (Fut8+/) mice. We observed a reduction in IgG levels inFut8+/mice compared to the WT (Fut8+/+) mice, which could be restored by the administration of L-fucose, thereby enhancing GDP-fucose levels through the salvage pathway (29). This phenomenon was further confirmed by producing specific antibodies against ovalbumin (OVA). Additionally, we found that FcR, a mouse.