The evolutionary arms race between NK cells and viruses: Who gets the short end of the stick?

NK cells are innate lymphocytes that play a key role in the control of various viral infections. Recent studies indicate that NK cells may acquire some features of adaptive immune cells, including the formation of long-lived memory cells. A large and growing body of data indicates that NK cells regulate the adaptive immune response as well. The function and the activation status of NK cells are tightly regulated by signals induced by a broad range of inhibitory and activating cell surface receptors and cytokines released by other immune cells. Here, we review the function of mouse NK-cell receptors involved in virus control and in the regulation of the adaptive immune response. In addition, we discuss viral strategies used to evade NK-cell-mediated control during infection. Finally, the role of several activating Ly49 receptors specific for mouse cytomegalovirus (MCMV), as well as some controversial issues in the field, will be discussed

Cardiotoxicity of immune checkpoint inhibitors

Cardiac toxicity after conventional antineoplastic drugs (eg, anthracyclines) has historically been a relevant issue. In addition, targeted therapies and biological molecules can also induce cardiotoxicity. Immune checkpoint inhibitors are a novel class of anticancer drugs, distinct from targeted or tumour type-specific therapies. Cancer immunotherapy with immune checkpoint blockers (ie, monoclonal antibodies targeting cytotoxic T lymphocyte-associated antigen 4 (CTLA-4), programmed cell death 1 (PD-1) and its ligand (PD-L1)) has revolutionised the management of a wide variety of malignancies endowed with poor prognosis. These inhibitors unleash antitumour immunity, mediate cancer regression and improve the survival in a percentage of patients with different types of malignancies, but can also produce a wide spectrum of immune-related adverse events. Interestingly, PD-1 and PD-L1 are expressed in rodent and human cardiomyocytes, and early animal studies have demonstrated that CTLA-4 and PD-1 deletion can cause autoimmune myocarditis. Cardiac toxicity has largely been underestimated in recent reviews of toxicity of checkpoint inhibitors, but during the last years several cases of myocarditis and fatal heart failure have been reported in patients treated with checkpoint inhibitors alone and in combination. Here we describe the mechanisms of the most prominent checkpoint inhibitors, specifically ipilimumab (anti-CTLA-4, the godfather of checkpoint inhibitors) patient and monoclonal antibodies targeting PD-1 (eg, nivolumab, pembrolizumab) and PD-L1 (eg, atezolizumab). We also discuss what is known and what needs to be done about cardiotoxicity of checkpoint inhibitors in patients with cancer. Severe cardiovascular effects associated with checkpoint blockade introduce important issues for oncologists, cardiologists and immunologists

Cytotoxicity of CD56bright NK Cells towards Autologous Activated CD4+ T Cells Is Mediated through NKG2D, LFA-1 and TRAIL and Dampened via CD94/NKG2A

In mouse models of chronic inflammatory diseases, Natural Killer (NK) cells can play an immunoregulatory role by eliminating chronically activated leukocytes. Indirect evidence suggests that NK cells may also be immunoregulatory in humans. Two subsets of human NK cells can be phenotypically distinguished as CD16+CD56dim and CD16dim/−CD56bright. An expansion in the CD56bright NK cell subset has been associated with clinical responses to therapy in various autoimmune diseases, suggesting an immunoregulatory role for this subset in vivo. Here we compared the regulation of activated human CD4+ T cells by CD56dim and CD56bright autologous NK cells in vitro. Both subsets efficiently killed activated, but not resting, CD4+ T cells. The activating receptor NKG2D, as well as the integrin LFA-1 and the TRAIL pathway, played important roles in this process. Degranulation by NK cells towards activated CD4+ T cells was enhanced by IL-2, IL-15, IL-12+IL-18 and IFN-α. Interestingly, IL-7 and IL-21 stimulated degranulation by CD56bright NK cells but not by CD56dim NK cells. NK cell killing of activated CD4+ T cells was suppressed by HLA-E on CD4+ T cells, as blocking the interaction between HLA-E and the inhibitory CD94/NKG2A NK cell receptor enhanced NK cell degranulation. This study provides new insight into CD56dim and CD56bright NK cell-mediated elimination of activated autologous CD4+ T cells, which potentially may provide an opportunity for therapeutic treatment of chronic inflammation

Modulation of natural killer cell activity by viruses

Since their discovery, our understanding of NK cells has evolved from branding them marginal innate immunity cells to key players in anti-viral and anti-tumor immunity. Importance of NK cells in control of various viral infections is perhaps best illustrated by the existence of plethora of viral mechanisms aimed to modulate their function. These mechanisms include not only virally encoded immunoevasion proteins but also viral miRNA. Moreover, the evidence has been accumulated supporting the role of viral immunoevasion of NK cells in viral pathogenesis in vivo

Mouse TIGIT

The activity of natural killer (NK) cells is controlled by a balance of signals derived from inhibitory and activating receptors. TIGIT is a novel inhibitory receptor, recently shown in humans to interact with two ligands: PVR and Nectin2 and to inhibit human NK-cell cytotoxicity. Whether mouse TIGIT (mTIGIT) inhibits mouse NK-cell cytotoxicity is unknown. Here we show that mTIGIT is expressed by mouse NK cells and interacts with mouse PVR (mPVR). Using mouse and human Ig fusion proteins we show that while the human TIGIT (hTIGIT) cross-reacts with mPVR, the binding of mTIGIT is restricted to mPVR. We further demonstrate using surface plasmon resonance (SPR) and staining with Ig fusion proteins that mTIGIT binds to mPVR with higher affinity than the co-stimulatory PVR-binding receptor mDNAM1. Functionally, we show that triggering of mTIGIT leads to the inhibition of NK-cell cytotoxicity, that IFN-γ secretion is enhanced when mTIGIT is blocked and that the TIGIT-mediated inhibition is dominant over the signals delivered by the PVR-binding co-stimulatory receptors. Additionally, we identify the inhibitory motif responsible for mTIGIT inhibition. In conclusion we show that TIGIT is a powerful inhibitory receptor for mouse NK cells