The PD-1–PD-L1 pathway inhibits perforin-mediated killing of PD-L1+ vascular endothelial cells by CD8+ T cells, thereby limiting vascular damage during systemic LCMV infection.

Persistent viral infections are, by definition, associated with ineffective antiviral immunity, in particular those infections caused by viruses that are highly productive and replicative (including HIV, HBV and HCV). The reasons for ineffective antiviral immunity in these types of infections are complex and manifold, and only recently a more comprehensive picture of the parameters responsible for attenuation of immune function is emerging. One reason for poor viral control in these types of infections is the functional deterioration of antiviral T-cell responses and understanding the underlying mechanisms is of key importance. This review summarizes our current knowledge of cell-intrinsic and cellextrinsic parameters that contribute to T-cell exhaustion during chronic viral infections and discusses related implications for host survival, immunopathology, and control of infection. Key words: Persistent viral infection . T-cell dysfunction . T-cell immunity IntroductionPersistent viral infections, by definition, are not entirely controlled by the host's immune system and are often associated with relevant clinical disease outcomes. Persistent viral infections can be roughly divided into (i) highly productive and replicative infections (associated with continued high abundance of viral antigens) and (ii) latent/reactivating infections (associated with low and perhaps sporadic levels of viral antigens). Well-known examples of the former are HIV, HBV and HCV, which together infect more than 2 billion people worldwide (http://www. unaids.org/; http://www.epidemic.org/; http://www.hepb.org/), and lymphocytic choriomeningitis virus (LCMV) infection in the mouse. Examples of the latter are herpes virus infections, including EBV, CMV, and HSV, which affect together almost 100% of the population. Viruses falling into these two categories have evolved very diverse strategies for co-existence in an immunocompetent host. Highly productive and replicative viruses tend to induce impaired virus-specific adaptive immune responses, which are characterized by limited and dysfunctional T-cell responses, and mutational escape from immune (or drug) pressure are common features of these viruses. Viruses causing latent/reactivating infections have, as a result of their large coding capacity, evolved strategies to interfere with host immunity (immune evasion mechanisms) and have acquired genetic programs that permit a latent, non-replicative existence of the viral genetic information with the option of resuming viral replication in response to certain stimuli. These viral reactivation events, however, are generally well controlled by host immunity and only become clinically apparent in circumstances of immunosuppression.This review focuses on highly productive and replicative persistent viral infections (hereafter referred to as persistent viral infection) with specific emphasis on cell-intrinsic and -extrinsic mechanisms responsible for constraining the size of the antiviral T-cell response and for T-cell dysfunction (summarized in Fig...

Foxp3+ regulatory T (Treg) cells are essential for the maintenance of immune homeostasis and tolerance. During viral infections, Treg cells can limit the immunopathology resulting from excessive inflammation, yet potentially inhibit effective antiviral T cell responses and promote virus persistence. We report here that the fast-replicating LCMV strain Docile triggers a massive expansion of the Treg population that directly correlates with the size of the virus inoculum and its tendency to establish a chronic, persistent infection. This Treg cell proliferation was greatly enhanced in IL-21R−/− mice and depletion of Treg cells partially rescued defective CD8+ T cell cytokine responses and improved viral clearance in some but not all organs. Notably, IL-21 inhibited Treg cell expansion in a cell intrinsic manner. Moreover, experimental augmentation of Treg cells driven by injection of IL-2/anti-IL-2 immune complexes drastically impaired the functionality of the antiviral T cell response and impeded virus clearance. As a consequence, mice became highly susceptible to chronic infection following exposure to low virus doses. These findings reveal virus-driven Treg cell proliferation as potential evasion strategy that facilitates T cell exhaustion and virus persistence. Furthermore, they suggest that besides its primary function as a direct survival signal for antiviral CD8+ T cells during chronic infections, IL-21 may also indirectly promote CD8+ T cell poly-functionality by restricting the suppressive activity of infection-induced Treg cells.

KIR3DL1 is one of the best-characterised inhibitory NK cell receptors. Unusually, one common allele at the 3DL1 locus encodes an activating receptor known as 3DS1. There is genetic evidence for a protective role of 3DS1 in certain viral diseases, but there has been uncertainty about expression of the 3DS1 protein. Using transfection, we show that surface expression of 3DS1 is reliant on the adaptor protein DNAX-activating protein 12 (DAP12). KIR3DS1 was recognised by the antibody Z27, a reagent that also detects KIR3DL1 but no other killer immunoglobulin-like receptor (KIR) molecule. Z27 stained 3DS1 on the surface of fresh circulating NK cells from 3DS1/3DS1 homozygotes. By double-staining with Z27 and DX9, an antibody specific for 3DL1, we obtained evidence that in 3DS1/3DL1 heterozygous donors significant numbers of NK cells express 3DS1 without co-expressing 3DL1 and that NK cells expressing both alleles are difficult to detect.

The identification of T cell co-inhibition as a central mechanism in the regulation of adaptive immunity during infectious diseases provides new opportunities for immunotherapeutic interventions. However, the fact that T cell activity is frequently downregulated during pathogen-directed responses suggests a pivotal physiological role of co-inhibitory pathways during infectious disease. Reports of exacerbated immunopathology in conditions of impaired co-inhibition foster the view that downregulation of T cell activity is an essential negative feedback mechanism that protects from excessive pathogen-directed immunity. Thus, targeting co-inhibitory pathways can bear detrimental potential through the deregulation of physiological processes. Here, we summarize recent preclinical and clinical interventions that report immune-related adverse events after targeting co-inhibitory pathways.