Synergistic polyreactivity of antibodies—where distinct oligo- or polyreactive clones combine to enable broad-spectrum antigen binding—was recently demonstrated for human anti-galactose-α-1,3-galactose (αGal) antibodies. However, it remains unclear whether this phenomenon also occurs in other antigen-defined antibody populations. Here, we investigated synergistic polyreactivity in isohemagglutinins (naturally occurring antibodies against ABO blood group antigens) and its potential role in antimicrobial defense. Isohemagglutinins were affinity-purified from pooled plasma from groups of healthy donors sharing the same ABO blood group, then characterized by solid-phase immunoassays, SDS-PAGE, and Western blotting. Binding to formaldehyde-fixed Streptococcus pneumoniae representing 30 serotypes was assessed by flow cytometry. Specificity was further examined using inhibition assays with soluble pneumococcal polysaccharides. To evaluate clinical relevance, ABO blood group types (used as proxies for isohemagglutinin profiles) were linked to serotype-specific invasive pneumococcal disease (IPD) events using Danish nationwide registry data. The purified isohemagglutinins retained specificity for their cognate ABO antigens. Notably, they bound all tested pneumococcal strains, primarily through reactivity with the common cell wall polysaccharide. Adsorption studies further revealed serotype-specific capsule binding mediated by distinct isohemagglutinin clones, demonstrating synergistic polyreactivity. Despite this, population-level analyses showed only modest protection: most notably, individuals with blood types B or O (producers of anti-A and anti-A,B isohemagglutinins, respectively) had a 19% lower prevalence of serotype 9V IPD than blood type A or AB individuals. Although the overall protective effect of isohemagglutinins against IPD seems limited, the observed synergistic polyreactivity supports the concept that this is a general feature of antigen-defined antibodies, extending beyond anti-αGal antibodies.
Bernth Jensen JM, Dinh KM, Hindhede L, Erikstrup LT, Hansen AG, et al.
Medical microbiology and immunology •
IgG antibodies against terminal galactose-α-1,3-galactose (anti-αGal antibodies) are naturally occurring in humans, but their origins remain poorly understood. These antibodies target various microorganisms including Staphylococcus aureus, a common nasal commensal and the major cause of skin abscesses. This study investigates the impact of S. aureus colonization and abscess events on plasma anti-αGal antibody levels. We measured plasma anti-αGal antibody levels using a quantitative immunoassay in: (i) 101 pairs of healthy individuals with and without nasal S. aureus colonization, (ii) 106 healthy individuals before and after abscess formation, and (iii) 43 patients with recurrent skin abscesses compared with 75 patient controls and 60 healthy controls. We observed a 35% reduction (95%CI: 7-54%) in anti-αGal antibody levels in nasal S. aureus carriers. Conversely, we found a 30% increase (95%CI: 4-66%) in individuals within 187 days post-skin abscess, and patients with recurrent skin abscesses exhibited 81% higher (95%CI: 14-190%) levels than patient controls, and 110% higher (95%CI: 39-230%) than healthy controls. This study suggests that skin abscesses lead to elevated plasma anti-αGal antibody levels and that these antibodies might convey or correlate with mucosal immunity to S. aureus.
Bernth Jensen JM, Hansen AT, Söderström A, Jørgensen CS, Larsen CS, et al.
Clinical immunology (Orlando, Fla.) •
Functional antibody deficiency is clinically assessed from antibody responses to vaccination. However, diagnostic vaccination is complex and may fail in practice. We hypothesized that the levels of naturally occurring antibodies against galactose-α-1,3-galactose (αGal) may represent alternative markers of functional antibody capacity. We included data from 229 patients with suspected primary immunodeficiency in a retrospective study. Antibody levels against αGal and twelve pneumococcal serotypes were determined with solid-phase immunoassays. Pneumococcal vaccinations and treatment with normal human immunoglobulin were assessed from medical records. Anti-αGal antibody levels correlated positively with anti-pneumococcal antibody levels measured before and after pneumococcal vaccination. Contrary to the anti-pneumococcal antibody levels, the anti-αGal antibody level showed potential for predicting subsequent immunoglobulin treatment - a marker of disease severity. Naturally occurring antibodies may reflect the functional capacity tested by diagnostic vaccination but add more useful clinical data. The clinical utility of this easy test should be evaluated in prospective studies.
Platts-Mills TA, Hilger C, Jappe U, van Hage M, Gadermaier G, et al.
Allergy •
Until recently, glycan epitopes have not been documented by the WHO/IUIS Allergen Nomenclature Sub-Committee. This was in part due to scarce or incomplete information on these oligosaccharides, but also due to the widely held opinion that IgE to these epitopes had little or no relevance to allergic symptoms. Most IgE-binding glycans recognized up to 2008 were considered to be "classical" cross-reactive carbohydrate determinants (CCD) that occur in insects, some helminths and throughout the plant kingdom. Since 2008, the prevailing opinion on lack of clinical relevance of IgE-binding glycans has been subject to a reevaluation. This was because IgE specific for the mammalian disaccharide galactose-alpha-1,3-galactose (alpha-gal) was identified as a cause of delayed anaphylaxis to mammalian meat in the United States, an observation that has been confirmed by allergists in many parts of the world. Several experimental studies have shown that oligosaccharides with one or more terminal alpha-gal epitopes can be attached as a hapten to many different mammalian proteins or lipids. The classical CCDs also behave like haptens since they can be expressed on proteins from multiple species. This is the explanation for extensive in vitro cross-reactivity related to CCDs. Because of these developments, the Allergen Nomenclature Sub-Committee recently decided to include glycans as potentially allergenic epitopes in an adjunct section of its website (www.allergen.org). In this article, the features of the main glycan groups known to be involved in IgE recognition are revisited, and their characteristic structural, functional, and clinical features are discussed.
Naturally occurring antibodies are abundant in human plasma, but their importance in the defence against bacterial pathogens is unclear. We studied the role of the most abundant of such antibodies, the antibody against terminal galactose-α-1,3-galactose (anti-αGal), in the protection against pneumococcal infections (Streptococcus pneumonia). All known pneumococcal capsular polysaccharides lack terminal galactose-α-1,3-galactose, yet highly purified human anti-αGal antibody of the IgG class reacted with 48 of 91 pneumococcal serotypes. Anti-αGal was found to contain multiple antibody subsets that possess distinct specificities beyond their general reactivity with terminal galactose-α-1,3-galactose. These subsets in concert targeted a wide range of microbial polysaccharides. We found that anti-αGal constituted up to 40% of the total antibody reactivity to pneumococci in normal human plasma, that anti-αGal drives phagocytosis of pneumococci by human neutrophils and that the anti-αGal level was twofold lower in patients prone to pneumococcal infections compared with controls. Moreover, during a 48-year period in Denmark, the 48 anti-αGal-reactive serotypes caused fewer invasive pneumococcal infections (n = 10 927) than the 43 non-reactive serotypes (n = 18 107), supporting protection on the population level. Our findings explain the broad-spectrum pathogen reactivity of anti-αGal and support that these naturally occurring polyreactive antibodies contribute significantly to human protective immunity.
Some human antibodies may paradoxically inhibit complement activation on bacteria and enhance pathogen survival in humans. This property was also claimed for IgG antibodies reacting with terminal galactose-α-1,3-galactose (Galα3Gal; IgG anti-αGal), a naturally occurring and abundant antibody in human plasma that targets numerous different pathogens. To reinvestigate these effects, we used IgG anti-αGal affinity isolated from a pool of normal human IgG and human hypogammaglobulinaemia serum as a complement source. Flow cytometry was performed to examine antibody binding and complement deposition on pig erythrocytes, Escherichia coli O86 and Streptococcus pneumoniae serotype 9V. Specific nanobodies were used to block the effect of single complement factors and to delineate the complement pathways involved. IgG anti-αGal was capable of activating the classical complement pathway on all the tested target cells. The degree of activation was exponentially related to the density of bound antibody on E. coli O86 and pig erythrocytes, but more linearly on S. pneumoniae 9V. The alternative pathway of complement amplified complement deposition. Deposited C3 fragments covered the activating IgG anti-αGal, obstructing its detection and highlighting this as a likely general caveat in studies of antibody density and complement deposition. The inherent capacity for complement activation by the purified carbohydrate reactive IgG anti-αGal was similar to that of normal human IgG. We propose that the previously reported complement inhibition by IgG anti-αGal relates to suboptimal assay configurations, in contrast to the complement activating property of the antibodies demonstrated in this paper.
