Heidi Elsaesser

Blockade of chronic type I interferon signaling to control persistent LCMV infection.

INTERFER(ON)ing Persistence During persistent viral infections, a dysregulated immune response fails to control the infection. Wilson et al. (p. 202) and Teijaro et al. (p. 207; see the Perspective by Odorizzi and Wherry) show this occurs because type I interferons (IFN I), critical for early responses to viral infection, contribute to the altered immunity seen during persistent infection. Antibody blockade of IFN I signaling during chronic lymphocytic choriomeningitis virus (LCMV) in mice resulted in reduced viral titers at later stages of infection, reduced expression of inhibitory immune molecules and prevented the disruptions to secondary lymphoid organs typically observed during persistent infection with LCMV. Whether type I IFNs are also detrimental to persistent viral infection humans, such as HIV and hepatitis C virus, remains to be determined. Blockade of type I interferons leads to better control of persistent lymphocytic choriomeningitis virus infection. [Also see Perspective by Odorizzi and Wherry] Type I interferons (IFN-I) are critical for antiviral immunity; however, chronic IFN-I signaling is associated with hyperimmune activation and disease progression in persistent infections. We demonstrated in mice that blockade of IFN-I signaling diminished chronic immune activation and immune suppression, restored lymphoid tissue architecture, and increased immune parameters associated with control of virus replication, ultimately facilitating clearance of the persistent infection. The accelerated control of persistent infection induced by blocking IFN-I signaling required CD4 T cells and was associated with enhanced IFN-γ production. Thus, we demonstrated that interfering with chronic IFN-I signaling during persistent infection redirects the immune environment to enable control of infection.

IL-21 Is Required to Control Chronic Viral Infection

Controlling Chronic Viral Infections Chronic viral infections such as HIV and hepatitis B and C viruses are major public health concerns. T cell—mediated immune responses are critical for controlling viral infections. In contrast to acute infections, chronic viral infections are characterized by “exhausted” cytotoxic CD8+ T cells, cells which exhibit reduced proliferative capacity, cytokine secretion, and cytotoxicity. Treatments that reverse exhaustion result in increased viral control. Despite their exhaustion, these CD8+ T cells eventually help to control chronic infections by killing virally infected cells, and require CD4+ T cell help to do so. How do CD4+ T cells provide help to CD8+ T cells during chronic infection (see the Perspective by Johnson and Jameson)? Elsaesser et al. (p. 1569, published online 7 May), Yi et al. (p. 1572, published online 14 May), and Fröhlich et al. (p. 1576, published online 28 May) now show that the cytokine, interleukin-21 (IL-21), known to be critical for the differentiation of certain CD4+ T cell effector subsets, is an essential factor produced by CD4+ T cells that helps CD8+ T cells to control chronic lymphocytic choriomeningitis virus infection in mice. Acute and chronic infections resulted in differing amounts of IL-21 production by virus-specific CD4+ T cells. CD8+ T cells required IL-21 directly, and when CD8+ T cells were unable to signal through IL-21 or IL-21 was not available, they were reduced in number, exhibited a more exhausted phenotype, and were not able to control the virus. In contrast, the absence of IL-21–dependent signaling did not affect primary CD8+ T cell responses to acute infection or responses to a viral rechallenge, suggesting that differentiation of memory CD8+ T cells is independent of IL-21. Interleukin-21 produced by CD4+ T cells helps CD8+ T cells control viral infection in a mouse model. CD4+ and CD8+ T cell functions are rapidly aborted during chronic infection, preventing viral clearance. CD4+ T cell help is required throughout chronic infection so as to sustain CD8+ T cell responses; however, the necessary factor(s) provided by CD4+ T cells are currently unknown. Using a mouse model of chronic viral infection, we demonstrated that interleukin-21 (IL-21) is an essential component of CD4+ T cell help. In the absence of IL-21 signaling, despite elevated CD4+ T cell responses, CD8+ T cell responses are severely impaired. CD8+ T cells directly require IL-21 to avoid deletion, maintain immunity, and resolve persistent infection. Thus, IL-21 specifically sustains CD8+ T cell effector activity and provides a mechanism of CD4+ T cell help during chronic viral infection.

Viral persistence redirects CD4 T cell differentiation toward T follicular helper cells.

Persistent virus infection drives follicular T helper cell differentiation.

Sterol regulatory element–binding proteins are essential for the metabolic programming of effector T cells and adaptive immunity

Newly activated CD8+ T cells reprogram their metabolism to meet the extraordinary biosynthetic demands of clonal expansion; however, the signals that mediate metabolic reprogramming remain poorly defined. Here we demonstrate an essential role for sterol regulatory element–binding proteins (SREBPs) in the acquisition of effector-cell metabolism. Without SREBP signaling, CD8+ T cells were unable to blast, which resulted in attenuated clonal expansion during viral infection. Mechanistic studies indicated that SREBPs were essential for meeting the heightened lipid requirements of membrane synthesis during blastogenesis. SREBPs were dispensable for homeostatic proliferation, which indicated a context-specific requirement for SREBPs in effector responses. Our studies provide insights into the molecular signals that underlie the metabolic reprogramming of CD8+ T cells during the transition from quiescence to activation.

IL-10 and PD-L1 operate through distinct pathways to suppress T-cell activity during persistent viral infection

Suppression of T-cell responses by host-derived regulatory factors is a key event leading to viral persistence. Antibody blockade of either IL-10 or programmed death-ligand 1 (PD-L1) during viral persistence enhances T-cell function and reduces viral titers. Because blockade of these immunoregulatory networks represents a powerful approach to establish immune control during persistent infection, it is important to determine whether these immunoinhibitory factors act independently or jointly and if combined blockade of these factors further enhances T-cell immunity and viral clearance. Herein, we demonstrate that the IL-10 and PD-L1 immunosuppressive pathways are mechanistically distinct. As a result, simultaneous blockade of IL-10 and PD-L1 was significantly more effective in restoring antiviral T-cell responses than blockade of either alone, and led to substantially enhanced control of an established persistent viral infection. Thus, combinatorial blockade of multiple immune-regulatory molecules may ultimately restore the T-cell responses required to tip the balance from viral persistence to immune-mediated control or elimination of persistent infection.

IL-10 blockade facilitates DNA vaccine-induced T cell responses and enhances clearance of persistent virus infection

Therapeutic vaccination is a potentially powerful strategy to establish immune control and eradicate persistent viral infections. Large and multifunctional antiviral T cell responses are associated with control of viral persistence; however, for reasons that were mostly unclear, current therapeutic vaccination approaches to restore T cell immunity and control viral infection have been ineffective. Herein, we confirmed that neutralization of the immunosuppressive factor interleukin (IL)-10 stimulated T cell responses and improved control of established persistent lymphocytic choriomeningitis virus (LCMV) infection. Importantly, blockade of IL-10 also allowed an otherwise ineffective therapeutic DNA vaccine to further stimulate antiviral immunity, thereby increasing T cell responses and enhancing clearance of persistent LCMV replication. We therefore propose that a reason that current therapeutic vaccination strategies fail to resurrect/sustain T cell responses is because they do not alleviate the immunosuppressive environment. Consequently, blocking key suppressive factors could render ineffective vaccines more efficient at improving T cell immunity, and thereby allow immune-mediated control of persistent viral infection.

IL-10 directly suppresses CD4 but not CD8 T cell effector and memory responses following acute viral infection

Mounting effective T cell responses is critical for eliciting long-lasting immunity following viral infection and vaccination. A multitude of inhibitory and stimulatory factors are induced following infection, and it is the compilation of these signals that quantitatively and qualitatively program the ensuing effector and memory T cell response. In response to lymphocytic choriomeningitis virus (LCMV) infection, the immunosuppressive cytokine IL-10 is rapidly up-regulated; however, how IL-10 is regulating what is often considered an “optimal” immune response is unclear. We demonstrate that IL-10 directly inhibits effector and memory CD4 T cell responses following an acutely resolved viral infection. Blockade of IL-10 enhanced the magnitude and the functional capacity of effector CD4 T cells that translated into increased and more effective memory responses. On the other hand, lack of IL-10 signaling did not impact memory CD8 T cell development. We propose that blockade of IL-10 may be an effective adjuvant to specifically enhance CD4 T cell immunity and protection following vaccination.

Type I interferon suppresses de novo virus-specific CD4 Th1 immunity during an established persistent viral infection

Significance Potent antiviral CD4 Th1 responses generated at the onset of persistent infection are lost as infection progresses. However, it is unknown how CD4 T cell responses are mounted in the midst of an established persistent infection to restore the diminishing Th1 response. We report that an established persistent virus infection suppresses the induction and distribution of new virus-specific CD4 Th1 cells. The failure to generate new Th1 responses is mediated by chronic type I interferon (IFN-I) signaling, and its blockade effectively restored de novo Th1 development. Our study identifies a mechanism of immunosuppression and a method to restore Th1 generation during persistent infection. CD4 T cells are central to orchestrate, sustain, and potentially regenerate antiviral immunity throughout persistent viral infections. Although the evolving immune environment during persistent infection reshapes established CD4 T-cell responses, the fate of naïve CD4 T cells primed in the midst of persistent infection is unclear. We demonstrate that, in marked contrast to the onset of infection, virus-specific CD4 T cells primed during an established persistent infection have diminished ability to develop Th1 responses, to efficiently accumulate in peripheral tissues, and almost exclusively differentiate into T follicular helper cells. Consistent with suppressed Th1 and heightened Tfh differentiation, virus-specific CD4 T cells primed during the established persistent infection provide help to B cells, but only limited help to CD8 T cells. The suppression of de novo Th1 generation and tissue distribution was mediated by chronic type I IFN (IFN-I) production and was effectively restored by blocking IFN-I signaling during CD4 T-cell priming. Thus, we establish a suppressive function of chronic IFN-I signaling and mechanism of immunoregulation during an established persistent virus infection.

Selective inhibitor of endosomal trafficking pathways exploited by multiple toxins and viruses

Significance Bacterial and viral infections are a significant public health burden. To corrupt normal host cellular functions, many bacterial toxins and all viruses must gain entry to host cells, a process that exploits the host’s own cellular machinery. In this study, we use high-throughput technologies to screen for chemical inhibitors of bacterial toxin and viral entry. We report the discovery of a small molecule that inhibits several viruses and bacterial toxins. In addition to the therapeutic potential, this compound represents a powerful probe for dissecting the mechanisms of mammalian membrane trafficking processes. Pathogenic microorganisms and toxins have evolved a variety of mechanisms to gain access to the host-cell cytosol and thereby exert virulent effects upon the host. One common mechanism of cellular entry requires trafficking to an acidified endosome, which promotes translocation across the host membrane. To identify small-molecule inhibitors that block this process, a library of 30,000 small molecules was screened for inhibitors of anthrax lethal toxin. Here we report that 4-bromobenzaldehyde N-(2,6-dimethylphenyl)semicarbazone, the most active compound identified in the screen, inhibits intoxication by lethal toxin and blocks the entry of multiple other acid-dependent bacterial toxins and viruses into mammalian cells. This compound, which we named EGA, also delays lysosomal targeting and degradation of the EGF receptor, indicating that it targets host-membrane trafficking. In contrast, EGA does not block endosomal recycling of transferrin, retrograde trafficking of ricin, phagolysosomal trafficking, or phagosome permeabilization by Franciscella tularensis. Furthermore, EGA does not neutralize acidic organelles, demonstrating that its mechanism of action is distinct from pH-raising agents such as ammonium chloride and bafilomycin A1. EGA is a powerful tool for the study of membrane trafficking and represents a class of host-targeted compounds for therapeutic development to treat infectious disease.

Suppression of Fcγ-Receptor-Mediated Antibody Effector Function during Persistent Viral Infection

Understanding how viruses subvert host immunity and persist is essential for developing strategies to eliminate infection. T cell exhaustion during chronic viral infection is well described, but effects on antibody-mediated effector activity are unclear. Herein, we show that increased amounts of immune complexes generated in mice persistently infected with lymphocytic choriomeningitis virus (LCMV) suppressed multiple Fcγ-receptor (FcγR) functions. The high amounts of immune complexes suppressed antibody-mediated cell depletion, therapeutic antibody-killing of LCMV infected cells and human CD20-expressing tumors, as well as reduced immune complex-mediated cross-presentation to T cells. Suppression of FcγR activity was not due to inhibitory FcγRs or high concentrations of free antibody, and proper FcγR functions were restored when persistently infected mice specifically lacked immune complexes. Thus, we identify a mechanism of immunosuppression during viral persistence with implications for understanding effective antibody activity aimed at pathogen control.