Immunoglobulin‐specific T‐B cell interaction

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 Igχ;‐1b allotypic determinants of their endogeneously synthesized Ig+ to Igχ;‐1b‐immune major histocompatibility complex (MHC) class II‐restricted T cells. A panel of Igχ;‐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 Igχ;‐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(Igχ;‐1b) in the presence of irradiated spleen cells (SC) from August rats and to the Igχ;‐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 Cχ;(Igχ;‐1b) domain followed by high‐performance liquid chromatography fractionation of the derived antigenic peptides, the χ; 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 Igχ;‐1b allotype differs from that of Igχ;‐1a allotype by three amino acid substitutions at positions 184, 185, 188. T cell recognition of pLx‐1b(176‐214) required no additional processing by the antigen‐presenting cell: the efficient presentation of the peptide but not of intact IgG(Igχ;‐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 Igχ;‐1b‐presenting ability, fixed Igχ;‐1b+ B cells were still able to induce Igχ;‐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.