Multiple juvenile idiopathic arthritis subtypes demonstrate proinflammatory IgG glycosylation.

Juvenile idiopathic arthritis (JIA) contains a heterogeneous mixture of arthritis phenotypes of unknown etiology. Human leukocyte antigen (HLA) associations have been observed for multiple subtypes, suggesting a role for adaptive immune mistargeting (1, 2). The participation of antibodies in disease pathogenesis is supported by several observations, including the prevalence of autoantibodies (such as ANA and anti-DEK), circulating immune complexes, and complement consumption in blood and joint fluid in some patients (3-6). One remarkable feature of the epidemiology of JIA is age of onset. The peak of JIA incidence occurs around 2-3 years of age (1, 7, 8). This peak is composed primarily of patients in the oligoarticular subgroup, but is evident also in seronegative polyarticular JIA, psoriatic JIA, and potentially systemic JIA (7, 9-11). By contrast, onset before age 1 year is unusual. The basis for this epidemiological pattern is unknown. One possibility is that it reflects the point of first contact between a genetically susceptible host and a specific environmental trigger. Supporting this concept, younger and older JIA patients exhibit different HLA associations, even within the same ILAR subtype (1, 2). A complementary possibility is that some feature of early childhood immunity may favor initiation of arthritis. Children and adults differ immunologically in multiple respects (12). For example, the proportion of circulating lymphocytes that are naive is initially high and falls gradually. Circulating IgG nadirs at approximately 3-4 months of age with the decay of maternal antibodies, rising to 60% of adult levels by 1 year and to adult levels by 10 years of age (13). Humoral immunity is also functionally immature in young children, evident most strikingly in poor responses to polysaccharide antigens in the first 18-24 months of life (14). Another potentially important difference between pediatric and adult immunity concerns IgG glycosylation. Approximately 3% of the mass of IgG is carbohydrate, representing principally 2 branched glycans that attach to a canonical asparagine (Asn297) in each heavy chain (Figure 1A) (15). These oligosaccharides reside within the Fc region, where they help to maintain its three-dimensional conformation (16). Polymorphisms in IgG glycans modulate its ability to bind Fc receptors and fix complement, and thus are highly determinative of antibody effector function (15, 17). In particular, IgG glycans lacking galactose (G0) bind mannose-binding lectin, thereby facilitating activation of complement (Figure 1B) (18-20). Interestingly, one analysis of IgG glycans in healthy individuals found that children, like older adults, exhibited an excess of pro-inflammatory G0 IgG glycoforms (21), though a smaller survey did not identify this pattern (22). Figure 1 Structure of IgG-associated N-glycans Hypogalactosylation of IgG may be particularly relevant for arthritis. Studies over the last 30 years have demonstrated that adult rheumatoid arthritis (RA) is associated with an increased prevalence of circulating G0 IgG (23-29). Further, elevation in G0 IgG can predate the diagnosis of RA by more than 3 years, varies directly with disease activity, and is particularly pronounced for antibodies directed against citrullinated peptides (28, 30). Finally, animal studies have suggested that antibodies rendered G0 by enzymatic techniques are especially arthritogenic (31). Together, these studies support a role for aberrant IgG glycosylation in the pathogenesis of RA. A similar mechanism may be operative in JIA. Several groups have studied IgG glycans in patients with arthritis beginning before the age of 16 years (24-26, 32, 33). Using diverse methodologies, these series have generally (though not invariably (32)) come to the conclusion that IgG hypogalactosylation is a feature of childhood arthritis. Yet the data have important limitations. First, the number of patients studied was relatively small (total n=100), divided among multiple phenotypic classes using diverse nomenclatures. Second, most patients were sampled while receiving treatment, known to alter IgG glycosylation (27, 29). Finally and most critically, patients were not always matched to controls by age. Indeed, only a very few normal samples have been characterized from children below the age of 6 years, the peak of JIA onset, rendering IgG glycan data from young JIA patients exceptionally difficult to interpret (21, 22). In the present study, we set out to characterize IgG hypogalactosylation in JIA, with a particular focus on early childhood. To achieve this goal, we collected samples from healthy children and patients with JIA naive to disease modifying anti-rheumatic drug (DMARD) treatment, enabling age-normalized analysis of IgG glycan variation by disease subtype, laboratory features and age of onset. Our results establish the association of JIA with aberrant IgG glycosylation, and demonstrate marked elevation in hypogalactosylated IgG in healthy young children, suggesting a possible mechanism favoring onset of JIA in early childhood.