THE FATE OF SELF-REACTIVE B-CELLS DEPENDS PRIMARILY ON THE DEGREE OF ANTIGEN RECEPTOR ENGAGEMENT AND AVAILABILITY OF T-CELL HELP

Self-reactive B cells from tolerant double-transgenic (Dbl-Tg) mice co expressing hen egg lysozyme (HEL) and rearranged anti-HEL immunoglobul in genes have a relatively short life span when compared to normal B c ells, irrespective of whether they are exposed to antigen in multivale nt membrane-bound form (mHEL-Dbl-Tg mice) or soluble form (sHEL-Dbl-Tg mice). The factors responsible for determining the fate of these B ce lls after encounter with self-antigen were investigated using a cell-t racking technique in which anti-HEL Ig-Tg spleen cells were labeled wi th the intracellular dye 5-carboxyfluorescein diacetate-succinimidyl e ster (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 f or at least 5 d. On transfer into either mHEL-Tg or sHEL-Tg hosts, the y underwent activation and then rapidly disappeared from the blood and spleen over the next 3 d, consistent with the short life span reporte d previously. Immunohistology of spleens from sHEL-Tg recipients indic ated 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 P ALS depended on a critical threshold of Ig receptor binding correspond ing 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 p attern was observed after transfer into non-Tg recipients given exogen ous (foreign) HEL. Moreover, bone marrow-derived immature Ig-Tg B cell s 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 d id not influence the fate of self-reactive B cells in a tolerant envir onment. On the other hand, HEL-binding B cells transferred into sHEL-D bl-Tg recipients persisted over the 3-d period of study, apparently du e to insufficient availability of antigen, as indicated by the fact th at 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 t he form of preactivated CD4(+) T cells specific for major histocompati bility complex-peptide complexes on the B cell surface, HEL-binding B cells migrated through the outer PALS of the spleen to the follicle, w here they formed germinal centers, or to adjacent red pulp, where they formed proliferative foci and secreted significant amounts of anti-HE L antibody. Taken together, these results indicated that the outcome o f the interaction between self-antigen and B cells is largely determin ed by a combination of the degree of receptor engagement and availabil ity of T cell help.