Arthritogenic Monoclonal Antibodies from K/BxN Mice

Arthritis in the K/BxN mouse model is provoked by pathogenic antibodies (Abs) directed against a ubiquitously expressed protein, glucose-6-phosphate isomerase (GPI). To begin dissecting the repertoire of arthritogenic immunoglobulins (Igs) in the K/BxN model, and to provide a basis for comparison with RA patientswe have generated anti-GPI monoclonal Abs (mAbs) from spontaneously activated B cells in the lymphoid organs of arthritic mice. B cell clones with anti-GPI specificities were present at extraordinarily high frequencies in the spleen, and less frequently in other lymphoid organs and in the synovial fluid. None of the anti-GPI mAbs induced arthritis when injected individually into healthy recipients, but most were effective when combined in pairs or larger pools. Arthritogenic combinations depended on mAbs of the IgG1 isotype, which bound to GPI with Kd in the 10−9 M range, with no indication of cooperative binding between complementing pairs. Pathogenicity was not associated with recognition of a particular epitope, but the ability to form mAb/GPI multimers by simultaneous recognition of different epitopes was clearly required, consistent with the known role of complement and FcRs in this model. Sequence analysis revealed structural similarities amongst the mAbs, indicating that a particular subset of B cells may evade tolerance in K/BxN mice, and that affinity maturation by somatic mutation likely takes place. These results confirm that GPI itself, rather than a cross-reactive molecule, is the target of pathogenic Igs

Differential physiological role of BIN1 isoforms in skeletal muscle development, function and regeneration

International audienceSkeletal muscle developmentand regeneration are tightly regulated processes. How the intracellular organization of muscle fibers is achieved during these steps is unclear. Here we focus on the cellular and physiological roles of amphiphysin 2(BIN1), a membrane remodeling protein mutated in both congenital and adult centronuclear myopathies(CNM),that is ubiquitously expressed and hasskeletal muscle-specific isoforms. We created and characterized constitutive, muscle-specific and inducible Bin1homozygous and heterozygous knockout mice targeting either ubiquitousor muscle-specific isoforms.Constitutive Bin1-deficient mice diedat birth from lack of feeding due to a skeletal muscle defect.T-tubules and other organelles weremisplaced and altered, supporting a general early role of BIN1 on intracellular organization in addition to membrane remodeling.Whereasrestricted deletion of Bin1in unchallenged adult muscles had no impact, the forced switch from the muscle-specificisoformsto the ubiquitousisoformsthrough deletion of the in-frame muscle–specific exon delayed muscle regeneration.Thus, BIN1 ubiquitous function is necessary for muscle development and function while its muscle-specific isoformsfine-tune muscle regenerationinadulthood, supporting that BIN1 centronuclear myopathy with congenital onset are due to developmental defects while later onsetmay be due to regeneration defects