Therapeutic PD-L1 blockade requires mTOR activation
PD-1 is markedly up-regulated on exhausted CTLs and can inhibit proximal TCR signaling and distal AKT, mTOR, S6, and ERK phosphorylation (
Francisco et al., 2009;
Parry et al., 2005;
Sheppard et al., 2004;
Yokosuka et al., 2012). Moreover, blocking PD-1
D-L1 interactions can restore TCR signaling (
Fife et al., 2009;
Zinselmeyer et al., 2013) and boost antiviral T cell responses and viral control during LCMV-Cl13 infection (
Barber et al., 2006). First, we confirmed the ability of PD-1 to suppress mTORC1 activity (based on p-S6 staining) in virus-specific CTLs following TCR stimulation (
Figure 3A). Next, to examine if PD-1 suppression of mTOR activity was relevant in antiviral CTLs
in vivo, LCMV-Cl13 infected mice were treated with a blocking α-PD-L1 mAb, either with or without the mTORC inhibitor rapamycin. Anti-PD-L1 mAb treatment augmented the amounts of p-S6235/236, CD98, CD71 and glucose uptake in CTLs (
Figures 3B-C) (
Finlay et al., 2009). The increase in mTOR activity was accompanied by a marked increase in the frequency and number of IFN-γ- and granzyme B (GzmB)-producing virus-specific CTLs and decrease in viral load (
Figures 3D-G). Importantly, rapamycin abrogated the beneficial effects of α-PD-L1 blockade on viral control and virus-specific T cell responses, including the aforementioned markers of anabolic metabolism (
Figures 3B-F). Taken together, these data demonstrate that PD-1 suppression of the mTOR pathway contributes to CTL exhaustion
in vivo and that recovery of mTOR activity is a part of the therapeutic effects of α-PD-L1 during chronic LCMV-Cl13 infection.