Breakdown of tolerance to a neo-self antigen in double transgenic mice in which B cells present the antigen

Transgenic (Tg) mice expressing a foreign Ag, hen egg lysozyme (HEL), under control of the alpha A-crystallin promoter ("HEL-Tg" mice) develop immunot olerance to HEL attributed to the expression of HEL in their thymus, In thi s paper we analyzed the immune response in double (Dbl)-Tg mice generated b y mating the HEL-Tg mice with Tg mice that express HEL Abs on their B cells ("Ig-Tg" mice). The B cell compartment of the Dbl-Tg mice was unaffected b y the HEL presence and was essentially identical to that of the Ig-Tg mice, A partial breakdown of tolerance was seen in the T cell response to HEL of the Dbl-Tg mice, i.e., their lymphocyte proliferative response against HEL was remarkably higher than that of the HEL-Tg mice. T-lymphocytes of both Dbl-Tg and Ig-Tg mice responded to HEL at concentrations drastically lower than those found stimulatory to lymphocytes of the wild-type controls. Cell mixing experiments demonstrated that 1) the lymphocyte response against lo w concentrations of HEL is due to the exceedingly efficient Ag presenting c apacity of the Ab expressing B cells and 2) breakdown of tolerance in Dbl-T g mice can also be attributed to the APC capacity of B cells, that sensitiz e in vivo and stimulate in vitro populations of T cells with low affinity t oward HEL, assumed to be escapees of thymic deletion. These results thus in dicate that T cell tolerance can be partially overcome by the highly potent Ag presenting capacity of Ab expressing B cells.