OX40 signaling Induces Canonical Antigen Presentation-independent Proliferation of Human and Murine Thymic Regulatory T-cells

Abstract

Abstract Regulatory T-cells (Tregs) play a pivotal role in immune-tolerance, and loss of their function can lead to autoimmunity. Natural Tregs generated in the thymus contribute substantially to the Treg pool in the periphery where they suppress self-reactive Teff responses. Recently, we showed that OX40L (TNFSF4) can drive selective proliferation of peripheral Tregs independent of canonical antigen presentation (CAP-independent) in the presence of low dose IL-2. Therefore, we reasoned that OX40 signaling might be integral to the TCR-independent phase of murine and human thymic Treg (tTreg) development. Thymic Treg development is a two-step process: Strong T-cell receptor (TCR) signals combined with co-signals from the TNF receptor Super Family (TNFRSF) members facilitate tTreg precursor selection, followed by a TCR-independent phase of tTreg development when their maturation is driven by IL-2. Therefore, we investigated whether OX40 signaling can also play a critical role in the TCR-independent phase of tTreg development. OX40−/− mice had significantly reduced numbers of CD25−Foxp3low tTreg precursors and CD25+Foxp3+ matured tTregs, while OX40L treatment of WT mice induced significant proliferation of these cell subsets. Relative to tTeffs cells, OX40 was expressed at higher levels in both murine and human tTreg precursors and matured tTregs. In ex vivo cultures, OX40L increased tTreg maturation and induced CAP-independent proliferation of both murine and human tTregs which was mediated through the activation of AKT-mTOR signaling. These novel findings show an evolutionarily conserved role for OX40 signaling in tTreg development.