Apoptotic-Cell Accumulation and Mertk Expression Are Linked to Diminished Antiviral CD8 ⁺ T Cell Immunity in Chronic Infections

Exhaustion of antiviral immunity driven by inhibitory signals is one hallmark of persistent viral infection. Notably, PD-1 and IL-10 are two major contributors to CD8+ T cell dysfunction. How these molecules are specifically induced during chronic viral infection remains mainly unknown. Using the lymphocytic choriomeningitis virus model, we show that the apoptotic-cell accumulation and expression of tyrosine kinase Mertk are linked to the outcome of chronic viral infection. Early CD8+ T cell activation correlated with increased dead-cell deposition, rapid induction of IL-10 and TGF-β in macrophages and dendritic cells (DCs), and a pronounced upregulation of PD-1 on CD8+ T cells and its ligand PD-L1. In TCR-β-deficient mice lacking CD8+ T cells, dead-cell generation and expression of IL-10, TGF-β, PD-1, and PD-L1 were markedly restricted. Our findings suggest that CD8+ T cell-mediated killing of infected targets generates large quantities of apoptotic cells, which activate the phosphatidylserine-binding kinase Mertk on macrophages and DCs. This signalling cascade subsequently promotes expression of IL-10, TGF-β, PD-1, and partially PD-L1. Consistent with this model, loss of Mertk in Mertk−/− mice reduced inhibitory cytokines and PD-1 expression, accelerated antiviral CD8+ T cell responses, and improved viral control. Collectively, our study provides important insight into cellular basis of T cell regulation identifying apoptotic cells and Mertk activation as key mechanisms initiating the suppression of CD8+ T cell immunity during chronic viral infection.