Linked help from bacterial proteins drives autoantibody production in small vessel vasculitis.

The small vessel vasculitides granulomatosis with polyangiitis (GPA) and microscopic polyangiitis are associated with autoantibodies to neutrophil cytoplasm antigens (ANCA), principally proteinase-3 (PR3) and myeloperoxidase (MPO). There is an association between GPA and nasal carriage of Staphylococcus aureus. The recent finding that S. aureus produces proteins that bind tightly to and block the function of both PR3 and MPO suggests a mechanism for ANCA formation. The bacterial protein-autoantigen conjugate is recognised by B cells with ANCA specificity, internalised, and the bacterial protein processed and presented to T cells with specificity for bacterial peptides. The T cell can then provide help to the B cell, allowing class switching, affinity maturation and the production of pathogenic ANCA. This mechanism predicts that T cells with this specificity will be found in patients, and that the bacterial protein-autoantigen conjugate will be particularly efficient at eliciting ANCA production.