The fate of self-reactive B cells depends primarily on the degree of antigen receptor engagement and availability of T cell help

Abstract

Self-reactive B cells from tolerant double-transgenic (Dbl-Tg) mice coexpressing hen egg lysozyme (HEL) and rearranged anti-HEL immunoglobulin genes have a relatively short life span when compared to normal B cells, irrespective of whether they are exposed to antigen in multivalent membrane-bound form (mHEL-Dbl-Tg mice) or soluble form (sHEL-Dbl-Tg mice). The factors responsible for determining the fate of these B cells after encounter with self-antigen were investigated using a cell-tracking technique in which anti-HEL Ig-Tg spleen cells were labeled with the intracellular dye 5-carboxyfluorescein diacetate-succinimidyl ester (CFSE) and injected either into non-Tg recipients or a variety of HEL-Tg hosts. In non-Tg recipients, HEL-binding B cells persisted in the circulation and could be detected in the follicles of the spleen for at least 5 d. On transfer into either mHEL-Tg or sHEL-Tg hosts, they underwent activation and then rapidly disappeared from the blood and spleen over the next 3 d, consistent with the short life span reported previously. Immunohistology of spleens from sHEL-Tg recipients indicated that the transferred B cells had migrated to the outer margins of the periarteriolar lymphoid sheath (PALS), where they were detectable for 24 h before being lost. The positioning of B cells in the outer PALS depended on a critical threshold of Ig receptor binding corresponding to a serum HEL concentration between 0.5 and 15 ng/ml, but was not restricted to endogenously expressed HEL in that the same migratory pattern was observed after transfer into non-Tg recipients given exogenous (foreign) HEL. Moreover, bone marrow-derived immature Ig-Tg B cells homed to the outer PALS of sHEL-Tg mice and then disappeared at the same rate as mature B cells, indicating that the stage of maturation did not influence the fate of self-reactive B cells in a tolerant environment. On the other hand, HEL-binding B cells transferred into sHEL-Dbl-Tg recipients persisted over the 3-d period of study, apparently due to insufficient availability of antigen, as indicated by the fact that the degree of Ig receptor downregulation on the transferred B cells was much less than in sHEL-Tg recipients. If T cell help was provided to Ig-Tg B cells at the time of transfer into sHEL-Tg recipients in the form of preactivated CD4+ T cells specific for major histocompatibility complex-peptide complexes on the B cell surface, HEL-binding B cells migrated through the outer PALS of the spleen to the follicle, where they formed germinal centers, or to adjacent red pulp, where they formed proliferative foci and secreted significant amounts of anti-HEL antibody. Taken together, these results indicated that the outcome of the interaction between self-antigen and B cells is largely determined by a combination of the degree of receptor engagement and availability of T cell help.