B cell tolerance to epidermal ribonuclear‐associated neo‐autoantigen in vivo

Defining how self‐antigens are perceived by the immune system is pivotal to understand how tolerance is maintained under homeostatic conditions. Clinically relevant, natural autoantigens targeted by autoantibodies, in e.g. systemic lupus erythematosus (SLE), commonly have an intrinsic ability to engage not only the B cell receptor (BCR), but also a co‐stimulatory pathway in B cells, such as the Toll‐like receptor (TLR)‐7 pathway. Here we developed a novel mouse model displaying inducible expression of a fluorescent epidermal neo‐autoantigen carrying an OT‐II T cell epitope, B cell antigen and associated ribonucleic acids capable of stimulating TLR‐7. The neo‐autoantigen was expressed in skin, but did not drain in intact form into draining lymph nodes, even after ultraviolet B (UVB)‐stimulated induction of apoptosis in the basal layer. Adoptively transferred autoreactive B cells were excluded follicularly and perished at the T–B border in the spleen, preventing their recirculation and encounter with antigen peripherally. This transitional check‐point was bypassed by crossing the reporter to a BCR knock‐in line on a C4‐deficient background. Adoptively transferred OT‐II T cells homed rapidly into cutaneous lymph nodes and up‐regulated CD69. Surprisingly, however, tolerance was not broken, as the T cells subsequently down‐regulated activation markers and contracted. Our results highlight how sequestration of intracellular and peripheral antigen, the transitional B cell tolerance check‐point and T cell regulation co‐operate to maintain immunological tolerance in vivo.