T cells discriminate between Ia antigens expressed on allogeneic accessory cells and B cells: a potential function for carbohydrate side chains on Ia molecules.

Previous studies have shown that the peptides obtained from accessory cell and B-cell Ia molecules are identical but that the alpha chains of B-cell Ia molecules are more extensively sialylated than those of accessory cells. The present studies were designed to determine whether this glycosylation difference can account for the functional difference in the capacity of the two cell types to activate alloreactive T cells. The experimental data show that normal resting B cells lack the capacity to induce DNA synthesis or differentiation in alloreactive T cells. T cells do recognize polymorphisms in B-cell Ia molecules, however, because they can be specifically primed for a subsequent proliferative stimulus of the same haplotype. The mitogenic signal for T cells is delivered by either allogeneic accessory cells or neuraminidase-treated B cells. Therefore, the T-cell receptor(s) may contain a site specific for the nonpolymorphic asialocarbohydrate moiety on the alpha chains of accessory cell Ia molecules.