Microparticles as autoantigens: Making immune complexes big

The paper by Nielsen et al in this issue of A&R makes a simple but important point: immune complexes can represent very big structures whose dimensions dwarf the small size expected for the usual pairings of antibody and antigen in solution (1). Indeed, as these studies show, immune complexes in the plasma of patients with systemic lupus erythematosus (SLE) include sub-cellular structures called microparticles (MPs) whose surface is studded with IgG, IgM and C1q, the hallmarks of antibody deposition and complement activation. Furthermore, these studies demonstrated that the number of MPs bearing IgG as well as the density of IgG on particles are increased in lupus even though the total number of MPs is not elevated. By focusing on the structural components of immune complexes in lupus, this study provides a new perspective for understanding the mechanisms driving autoantibody production in SLE and the role of immune complexes in immunopathogenesis.