Whither the Anergic B-Cell?

Abstract

The kinetics of potentially autoreactive B-cells were investigated in a double-transgenic model of self-tolerance. Two types of transgenic mice were created for this purpose. In the first the great majority of B-cells expressed the rearranged heavy and light chain genes encoding a high affinity receptor for hen egg lysozyme (HEL), while the second type expressed HEL in either soluble (sHEL) or membrane-bound (mHEL) form. Double-transgenic (Dbl-Tg) mice were produced either by mating the two types of founders or by transferring bone marrow cells from Ig-transgenic (Ig-Tg) donors into irradiated HEL-Tg recipients. The lifespan of B-cells from the Dbl-Tg mice was measured by oral loading with the thymidine analogue, 5-bromo-2′-deoxyuridine (BrdU) for periods varying from three days to six weeks.

Experiments in various combinations of Dbl-Tg mice revealed a three-tiered hierarchy of B-cell unresponsiveness, each level of which was characterised by a different B-cell lifespan. Exposure to mHEL led to rapid deletion of B-cclls at an immature stage in their development with a median lifespan of approximately 15 hours. B-cells exposed to sHEL above a critical tolerogenic threshold were not deleted in the bone marrow but migrated to the spleen in an anergic state where they died within three days. If the receptor occupancy of sHEL was below the tolerogenic threshold, B-cells were neither deleted nor rendered anergic (ie were “indifferent”) and had a normal median lifespan of four to five weeks.

A number of conclusions were derived from these studies

(i) there is a correlation between receptor occupancy by self antigen and B-cell lifespan;

(ii) anergy and deletion in self-reactive B-cells appear to be part of a spectrum of unresponsiveness rather than being totally discrete molecular entities;

(iii) a lack of T-ccll help in addition to receptor occupancy is a crucial factor in determining the relatively short lifespan of anergic B-cells; and

(iv) B-cells from young mice have a higher turnover rate than in adult mice suggesting selection of newly-generated B-cells into the long-lived repertoire; the short lifespan of tolerant self-reactive cells further implies a major role for negative selection in this process.

Key Words:

• B-cell
• lifespan
• anergy
• tolerance
• autoimmunity