H-2-restricted helper activity mediated by immunoglobulin-bearing lymphocytes

The genetic requirements for helper activity mediated by a unique, Ig-bearing lymphocyte population were studied. This Lyt-1+, I-A+, Thy-1- population, called BH, preferentially helps expression of NPb idiotypic plaque-forming cells when added to T cell-depleted responder cultures. Furthermore, the BH population can directly bind NPb idiotypic determinants. Using H-2 congenic mice, we show that BH helper activity can be expressed only when BH cells share I-A subregion alleles with responder B cell populations. This H-2 restriction is not a result of thymic influences, because the activity of BH cells from athymic mice are also H-2 restricted. Macrophages present in the BH population do not contribute to the H-2 restriction. Results are presented that definitively rule out the possible role for T lymphocytes in BH activity and demonstrate that a single helper population expresses both Lyt-1 and I-A determinants. These results indicate that Ig-bearing cells serve a regulatory as well as an effector role in immune responses and that, like other regulatory lymphoid subsets, their activity is regulated in part by MHC-encoded determinants.