PKC-ѳ is dispensable for OX40L-induced TCR-independent Treg proliferation but contributes by enabling IL-2 production from effector T-cells.

We have previously shown that OX40L/OX40 interaction is critical for TCR-independent selective proliferation of Foxp3 ⁺ Tregs, but not Foxp3 ^- effector T-cells (Teff), when CD4 ⁺ T-cells are co-cultured with GM-CSF derived bone marrow dendritic cells (G-BMDCs). Events downstream of OX40L/OX40 interaction in Tregs responsible for this novel mechanism are not understood. Earlier, OX40L/OX40 interaction has been shown to stimulate CD4 ⁺ T-cells through the formation of a signalosome involving TRAF2/PKC-Ѳ leading to NF-kB activation. In this study, using CD4 ⁺ T-cells from WT and OX40 ^-/- mice we first established that OX40 mediated activation of NF-kB was critical for this Treg proliferation. Although CD4 ⁺ T-cells from PKC-Ѳ ^-/- mice were also defective in G-BMDC induced Treg proliferation ex vivo, this defect could be readily corrected by adding exogenous IL-2 to the co-cultures. Furthermore, by treating WT, OX40 ^-/- , and PKC-Ѳ ^-/- mice with soluble OX40L we established that OX40L/OX40 interaction was required and sufficient to induce Treg proliferation in vivo independent of PKC-Ѳ status. Although PKC-Ѳ is dispensable for TCR-independent Treg proliferation per se, it is essential for optimum IL-2 production by Teff cells. Finally, our findings suggest that OX40L binding to OX40 likely results in recruitment of TRAF1 for downstream signalling.