Background/Purpose:

Systemic lupus erythematosus (SLE) is an autoimmune disorder characterized by the propagation of autoreactive B cell populations leading to the production of pathogenic antibodies directed towards self antigens such as:  nuclear antigens, cytokines, and dsDNA.  However, this autoreactive B cell expansion within SLE patients is not uniform suggesting specific antigen selection, and skews the antibody repertoire.  One such expansion is in antibodies encoded by the VH4-34 gene.  This antibody family has been shown to enrich for binding to autoantigens such as dsDNA, and can be uniquely examined due to the expression of the idiotope specific for the 9G4 idiotypic antibody.  9G4+ antibodies are unique in that the VH region expresses a hydrophobic patch (HP)(which acts as the 9G4 idiotope) within the framework 1 region of the heavy chain, which allows for antigen binding independent of the CDR3.  This HP is what dictates binding to erythrocytes through interactions with N-acteyllactosamine glycans.  Additionally, a secreted glycan binding protein family, the galectins, is also increased within SLE patients and has been shown to bind to many of the same antigenic targets as 9G4+ antibodies.   Interestingly, SLE patients exhibit autoantibodies (autoAbs) against galectins, strongly suggesting an importance for galectin autoreactivity and a possible interaction between VH4-34 autoAbs and galectins.  The purpose of this study is to understand this role between VH4-34 encoded Ig and galectin binding and/or competition in context of the pathogenesis of SLE. 

Methods:

ELISA was utilized to assess total IgG and 9G4-specific anti-galectin autoAb levels of varying galectin family members within the sera of 16 SLE patients and healthy controls (HC).  Single-cell sorts of 9G4+ IgG B cell populations followed by monoclonal antibody (mAb) expression and site directed mutagenesis (SDM) of the VH4-34 HP generated pairs of antibodies that could be independently tested for galectin binding either dependent or independent of the HP. 

Results:

The VH4-34 gene segment encoded a significant portion of the total galectin-3 autoAbs directed against recombinant human galectins (50% of SLE patients were anti-Galectin-3 positive).  Galectin-1 and galecin-9 autoAb titers showed no such relationship however there was 9G4 autoreactivity in 6.25% and 18.75% of SLE patients respectively, and no 9G4+ anti-galectin reactivity within the HC group.  There was no correlation of anti-galectin autoAbs and SLEDAI.    Two mAb derived from a pool of 20 (6.67%) 9G4+ mAbs and their SDM mutants tested, were cross-reactive between both galectin-1 and galectin-9.  This interaction was also determined to be a result of the CDR3 and not the HP.

Conclusion:

These data determine a novel antigen specific interaction between 9G4+ Ig and certain galectins.  Understanding how these interactions interfere with galectin biology will be useful in determining the physiologic cost of this relationship within SLE.  The derivation of anti-galectin 9G4+ mAb, which binds through the CDR3, suggests galectins to be potent autoantigens.  Work supported by NIH-NIAID 5R37AI049660-12

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/autoantibodies-utilizing-the-immunoglobulin-heavy-chain-variable-region-gene-4-34-vh4-34-exhibit-autoreactivity-towards-and-potential-competition-with-galectins-within-systemic-lupus-erythematosus/