The Transcription Factor FoxO1 Sustains Expression of the Inhibitory Receptor PD-1 and Survival of Antiviral CD8+ T Cells during Chronic Infection

• TCR activation of AKT and mTOR is impaired in exhausted CTLs
• Therapeutic blockade of PD-1 signaling is mTOR dependent
• FoxO1 is necessary to sustain CTL responses and control chronic viral infection
• FoxO1 regulates the expression and differentiation of PD-1hi exhausted CTLs

SummaryProtein kinase B (also known as AKT) and the mechanistic target of rapamycin (mTOR) are central regulators of T cell differentiation, proliferation, metabolism, and survival. Here, we show that during chronic murine lymphocytic choriomeningitis virus infection, activation of AKT and mTOR are impaired in antiviral cytotoxic T lymphocytes (CTLs), resulting in enhanced activity of the transcription factor FoxO1. Blockade of inhibitory receptor programmed cell death protein 1 (PD-1) in vivo increased mTOR activity in virus-specific CTLs, and its therapeutic effects were abrogated by the mTOR inhibitor rapamycin. FoxO1 functioned as a transcriptional activator of PD-1 that promoted the differentiation of terminally exhausted CTLs. Importantly, FoxO1-null CTLs failed to persist and control chronic viral infection. Collectively, this study shows that CTLs adapt to persistent infection through a positive feedback pathway (PD-1→FoxO1→PD-1) that functions to both desensitize virus-specific CTLs to antigen and support their survival during chronic viral infection.

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