In the preceding report (Eur. J. Immunol. 1989. 19: 1677) we have demonstrated that normal B cells, including small B cells, are capable of presenting Ig kappa-1b allotypic determinants of their endogeneously synthesized Ig+ to Ig kappa-1b-immune major histocompatibility complex (MHC) class II-restricted T cells. A panel of Ig kappa-1b allotype-specific T cell clones from August rats has been developed to study further the presentation of self surface Ig by B cells from Ig kappa-1-congeneic August.1b rats. All the clones studied were of the T helper/inducer phenotype (W3/25+,OX8-) and restricted by the RT-1Bc molecule. These clones responded both to the serum IgG(Ig kappa-1b) in the presence of irradiated spleen cells (SC) from August rats and to the Ig kappa-1b-bearing irradiated B cells from August.1b rats. SC presentation of secreted IgG was much less effective than B cell presentation of membrane Ig. Using CNBr cleavage of isolated C kappa (Ig kappa-1b) domain followed by high-performance liquid chromatography fractionation of the derived antigenic peptides, the kappa chain sequence between amino acids 176 and 214 has been identified as the T cell epitope recognized by all T cell clones in association with RT-1Bc. The fragment 176-214 of the Ig kappa-1b allotype differs from that of Ig kappa-1a allotype by three amino acid substitutions at positions 184, 185, 188. T cell recognition of pL kappa-1b(176-214) required no additional processing by the antigen-presenting cell: the efficient presentation of the peptide but not of intact IgG(Ig kappa-1b) by the paraformaldehyde-fixed SC was observed. These data provide clear-cut evidence for an absolute requirement of the processing of Ig molecules for T cell recognition to occur in our experimental system. Although the fixation of B cells from August.1b rats diminished their Ig kappa-1b-presenting ability, fixed Ig kappa-1b+ B cells were still able to induce Ig kappa-1b-specific T cell clone responses. Our results suggest that B cells can express the processed form of self-synthesized surface Ig in addition to intact surface Ig molecules. The former can be recognized by MHC-restricted T cell.
