OX40 controls functionally different T cell subsets and their resistance to depletion therapy

T cell depletion is a widely used approach in clinical transplantation. However, not all T cells are equally sensitive to depletion therapies and a significant fraction of T cells persists even after aggressive treatment. The functional attributes of such T cells and the mechanisms responsible for their resistance to depletion are poorly studied. In the present study, we showed that CD4⁺ T cells that are resistant to polyclonal anti-lymphocyte serum (ALS) mediated depletion exhibit phenotypic features of memory cells and uniformly express OX40 on the cell surface. Studies using the foxp3gfp knockin mice revealed that the remaining CD4⁺OX40⁺ cells consist of Foxp3⁺ Tregs and Foxp3^- T effector/memory cells. The ALS-resistant CD4⁺OX40⁺ cells failed to mediate skin allograft rejection upon adoptive transferring into congenic Rag^-/- mice, but removal of Foxp3⁺ Tregs from the OX40⁺ cells resulted in prompt skin allograft rejection. Importantly, OX40 is critical to survival of both Foxp3⁺ Tregs and T effector/memory cells. However, OX40 exhibits opposing effects on the functional status of Foxp3⁺ Tregs and T effector/memory cells, as stimulation of OX40 on T effector cells induced amplified cell proliferation but stimulation of OX40 on the Foxp3 ⁺ Tregs impaired their suppressor functions. Our study demonstrates that OX40 is a critical molecule in regulating survival and functions of depletionresistant T cells; and these findings may have important clinical implications.