Positive Selection of Anti–Thy-1 Autoreactive B-1 Cells and Natural Serum Autoantibody Production Independent from Bone Marrow B Cell Development

A natural serum autoantibody specific for the Thy-1 glycoprotein (anti–Thy-1 autoantibody [ATA]) is produced by B-1 cells that are positively selected by self-antigen. Here, using ATAμκ transgenic mice we show that cells with this B cell receptor are negatively selected during bone marrow (BM) development. In a Thy-1 null environment, BM ATA B cells progress to a normal follicular stage in spleen. However, in a self-antigen–positive environment, development is arrested at an immature stage in the spleen, concomitant with induction of CD5. Such cells are tolerant and short-lived, different from B-1. Nonetheless, ATA-positive selection was evident by self-antigen–dependent high serum ATA production, comprising ∼90% of serum immunoglobulin M in ATAμκ mice. Splenectomy did not eliminate ATA production and transfer of tolerant splenic B cells did not induce it. These findings demonstrate that B-1 positive selection, resulting in the production of natural serum ATA, arises independently from the major pathway of BM B cell development and selection.