B lymphocyte activation upon exclusive recognition of major histocompatibility antigens by T helper cells.

This study has investigated whether exclusive recognition of I-A or I-E molecules on the B cell surface by T helper cells is sufficient to activate resting B cells. Lines and clones of long-term-cultured T helper cells with specificity for I-A or I-E antigens have been derived from mixed lymphocyte cultures between spleen cells from major histocompatibility complex (MHC)-congenic mouse strains. These cells were tested for helper activity and proved competent to induce resting B lymphocytes expressing the specific MHC antigens to polyclonal expansion and maturation to Ig secretion. B cell activation was shown to require direct recognition of I-A/E antigens by the helper cells on the responding B lymphocyte surface and it could not be achieved by soluble factors released by "third-party" helper cell activity ongoing in the same cultures. Since B lymphocyte activation occurs in the absence of antigen recognition by the responding B cells, these observations suggest that I-A and I-E molecules expressed on the B cell surface participate in the functional reception of T helper cell-derived induction signals.