The HIV reservoir is largely composed of resting memory CD4+ T-cells, a large fraction of which express the co-inhibitory receptors PD-1 and CTLA-4 that limit T-cell activation. Furthermore, PD-1 expression contributes to CD8+ T-cell dysfunction during HIV infection. We hypothesized that a dual PD-1 and CTLA-4 immune checkpoint blockade (ICB) will facilitate T-cell activation, revert latency, and restore SIV-specific T-cell responses, thus enhancing clearance of the viral reservoir.
33 rhesus macaques (RMs) were i.v. infected with SIVmac239 and initiated ART (TDF/FTC/DTG) at day 60, which was maintained for 1 year. RMs received 4, weekly ICB infusions: control antibody (n=6); anti-PD-1 mAb (n=6); anti-CTLA-4 mAb (n=6); anti-CTLA-4 plus anti-PD-1 mAb (n=7); and bi-specific anti-CTLA-4/PD-1 (n=8). All RMs underwent analytic ART interruption (ATI). Peripheral blood (PB), lymph node (LN), and rectal biopsy (RB) were collected longitudinally.
ICB treatment was well tolerated and demonstrated PD-1 receptor occupancy in PB and LN, including in T follicular helper cells. Dual PD-1/CTLA-4 ICB induced a significant increase in the number of cycling (Ki-67+) memory CD4+ and CD8+ T-cells in LN (1.96- and 1.96-fold, respectively), with significant expansion in the number of effector memory T-cells in PB (4.61- and 2.20-fold, respectively; p<0.01 for all these measures). Enhanced T-cell cycling was dependent on CTLA-4 inhibition, highlighting the synergy of the dual blockade. In addition, dual ICB increased the frequency of activated (CD38+HLA-DR+) and T-bet+ cells in LN. Notably, increased CD4+ T-cell cycling induced by dual ICB resulted in higher levels of plasma SIV RNA (indicative of viral reactivation) relative to monotherapy. Both the frequency (viremia >100 copies/mL measured in 4 out of 7 RMs and at multiple time points) and magnitude (average of 913 SIV-RNA copies/mL) of plasma SIV RNA was increased in dual vs monotherapy. Following ATI, only anti-PD-1 monotherapy produced a significant (p=0.01), one-week delay in SIV rebound. The impact of ICB on the size of the reservoir during ART is under investigation.
PD-1/CTLA-4 dual blockade was well tolerated in ART-suppressed, SIV-infected RMs, and was pharmacologically active as demonstrated by the expansion of effector T-cells in PB. This effect was coupled to a synergistic and potent activity in increasing SIV RNA in plasma, suggesting checkpoint blockade may facilitate viral induction and improve T-cell function.