In summary, we find that in the Lyn-/- model of lupus, activated B-1 cells contribute to the accumulation of IgM secreting plasma cells, are an important source of a few IgM ANA specificities, and limit the production of IgG against non-nucleic acid containing autoantigens and the activation of CD8 T cells. Here, LYN is linked to systemic lupus erythematosus.