Anti-GITR Agonist Therapy Intrinsically Enhances CD8 T Cell Responses to Chronic Lymphocytic Choriomeningitis Virus (LCMV), Thereby Circumventing LCMV-Induced Downregulation of Costimulatory GITR Ligand on APC

The costimulatory TNFR family member GITR can provide important survival signals for CD8 T cells. However, little is known about the regulation of this pathway during a chronic infection. In this study, we show that GITR ligand (GITRL) is maximally induced on APCs at day 2 post-lymphocytic choriomeningitis virus (LCMV) clone 13 infection, but is downregulated to below baseline levels by day 8 postinfection (p.i.), and remains so at the chronic stage of infection. At its peak, GITRL expression is highest on macrophages, with lower expression on conventional and plasmacytoid dendritic cells. GITR expression was highest on T regulatory cells but was also detected on Th1 and LCMV-specific CD8 T cells at day 8 p.i. and was maintained at low, but above baseline levels at the chronic stage of LCMV infection. As GITRL was limiting at the chronic stage of infection, we investigated the potential of therapeutic stimulation of GITR at this stage using agonistic anti-GITR Ab. Anti-GITR treatment at day 21 p.i. increased the frequency and number of LCMV-specific CD8 T cells, resulting in increased in vivo CTL activity and a concomitant decrease in viral load, despite the persistence of PD-1 expression. These effects of anti-GITR were CD8 T cell intrinsic, with no detectable effects on Th1 or T regulatory cells. In contrast to other TNFR agonists, such as anti-4-1BB, which can cause immune pathology, a single therapeutic dose of anti-GITR did not induce splenomegaly or increase serum alanine transaminase. These studies identify GITR as a promising therapeutic target for chronic infection.