Autoreactive B-cell elimination by pathogenic IgG specific for the same antigen: Implications for peripheral tolerance

Harmful pathogenic IgG auto-antibodies are produced against desmoglein 3 (Dsg3) in pemphigus vulgaris, an autoimmune blistering disease. Dsg3 is a cadherin-type cell adhesion molecule expressed in desmosomes of the skin and mucous membranes. In AK7-transgenic mice expressing non-pathogenic AK7 IgM against Dsg3, autoreactive transgenic B cells escape from the deletion or inactivation and exist in the periphery. However, when a pathogenic anti-Dsg3 IgG1 mAb (AK23) capable of inducing blisters was injected into AK7-transgenic mice, AK7 B cells were eliminated from the bone marrow (BM) and spleen only when Dsg3 was expressed in the periphery. In contrast, non-pathogenic IgG mAbs (AK7, AK9) failed to eliminate AK7 B cells. Interestingly, the AK23-mediated elimination of mature AK7 B cells in the spleen was significantly diminished in AK7-transgenic mice on a Rag2^-/- background while BM B cells were still eliminated, suggesting the presence of T-cell-dependent and -independent mechanisms. T cell transfer studies into AK7- Rag2^-/- mice revealed that autoreactive B-cell elimination in the periphery requires CD4⁺ T cells from wild-type mice but not from gld (FasL mutant) mice. The B-cell elimination was impaired in both BM and periphery when Bcl2 was over-expressed in AK7 B cells. These findings suggest that autoreactive B cells exist unless they are harmful, but once harmful or dangerous events such as tissue destruction are sensed, the mature autoreactive B cells in the periphery are eliminated via a Fas-mediated process in a CD4⁺ T cell-dependent manner.