Small B Cells as Antigen Presenting Cells in the Induction of Tolerance to Soluble Protein Antigens: A Dissertation

Abstract

This thesis proposes a mechanism for the induction of peripheral tolerance to protein antigens. I have investigated the mechanism of tolerance induction to soluble protein antigens by targeting an antigen to small, resting B cells. For this purpose I have used a rabbit antibody directed at the IgD molecule found on the surface of most small, resting B cells but missing or lowered on activated B cells. Intravenous injection of normal mice with 100 μg of an ultracentrifuged Fab fragment of rabbit anti-mouse IgD (Fab anti-δ) makes these mice profoundly tolerant to challenge with nonimmune rabbit Fab (Fab NRG) fragments. This tolerance is antigen specific since treated mice make normal responses to an irrelevant antigen, chicken immunoglobulin (Ig). Fab fragments of rabbit Ig (rabbit Fab) not targeted to B cells do not induce tolerance as well as Fab anti-δ. Evidence suggests that the B cells must remain in a resting state for tolerance to be induced, since injection of F(ab)'2 anti-δ does not induce tolerance. Investigation of the mechanisms of the tolerance, by adoptive transfer, have shown that rabbit Fab specific B cell function has been impaired. The major effect however is in helper T cell function, as shown by adoptive transfer and lack of help for a hapten response. In vitro proliferation experiments show that the T cell response has not been shifted toward activation of different T cell subsets which do not help Ig production, nor is there any change in the Ig isotypes produced. Suppression does not appear to be the major cause of the helper T cell defect as shown by cell mixing experiments. This work shows that an antigen targeted to small B cells can induce tolerance to a soluble protein antigen, and suggests a role for small B cells in tolerance to self-proteins not presented in the thymus.