Functionally Anergic lpr and gld B220⁺ T Cell Receptor (TCR)-α/β⁺ Double-Negative T Cells Express CD28 and Respond to Costimulation with Phorbol Myristate Acetate and Antibodies to CD28 and the TCR

Mice homozygous for lpr and gld develop lymphadenopathy characterized by the progressive accumulation of an unusual population of CD4^-, CD8^-, CD2^-, IL-2R^- double-negative (DN) T cells that express reduced levels of TCR-α/β, high levels of CD45 (B220) and Ly-6C and variable levels of CD69. These cells are refractory to most stimuli, including staphylococcal entertoxins and cross-linking of the TCR, Ly-6C, and CD69. For normal T cells, the binding of ligand to the TCR alone is insufficient to induce a proliferative response and can result in the induction of a state of prolonged anergy. Efficient stimulation is dependent on the delivery of a second or costimulatory signal. Recently it was reported that CD28 can provide costimulatory signals to T cells and, that these signals can prevent anergy induction in T cell clones. We investigated the possibility that lpr and gld DN T cells are unresponsive because they fail to transduce signals via CD28. These studies showed that highly purified 6220⁺ TCR-α/β⁺ DN T cells expressed high levels of CD28, responded weakly to stimulation with PMA and anti-CD28 mAb and quite strongly to PMA, anti-CD28 antibody and high concentrations of immobilized anti-TCR-α/β antibodies. The latter stimulus also induced low levels of expression of CD2 and IL-2R and secretion of modest amounts of IL-2. Although DN T cells proliferated and secreted IL-2, these responses differed qualitatively and quantitatively from those of +/+ and lprB220^- T cells. Consistent with its effects on normal T cells, cyclosporin A partially inhibited the response of DN T cells to TCR cross-linking and CD28 ligation. Studies of synergism between CD28-, Ly-6C-, and CD69-mediated signals revealed that ligation of CD28 enhanced the proliferative response induced by cross-linking of Ly-6C or CD69 on +/+, lpr and gld B220^- T cells but had no effect on the unresponsiveness of DN T cells to these stimuli. Ligation of CD28 did not reverse the unresponsiveness of DN T cells to SEB and had only a weak synergistic effect on the response of B220^- T cells. Together, these observations suggest that the mechanisms leading to immunosuppression of DN T cells are complex and appear to involve abnormalities in signal transduction via the TCR and CD28 and possibly via Ly-6C and CD69 as well.