Summary The host response to viruses includes multiple cell types that have regulatory function. Most information focuses on CD4+ regulatory T cells that express the transcription factor Foxp3+ (Tregs), which are the topic of this review. We explain how viruses through specific and non-specific means can trigger the response of thymus-derived natural Tregs as well as induce Tregs. The latter derive under appropriate stimulation conditions either from uncommitted precursors or from differentiated cells that convert to become Tregs. We describe instances where Tregs appear to limit the efficacy of antiviral protective immunity and other perhaps more common immune-mediated inflammatory conditions, where the Tregs function to limit the extent of tissue damage that occurs during a virus infection. We discuss the controversial roles that Tregs may play in the pathogenesis of human immunodeficiency and hepatitis C virus infections. The issue of plasticity is discussed, since this may result in Tregs losing their protective function when present in inflammatory environments. Finally, we mention approaches used to manipulate Treg numbers and function and assess their current value and likely future success to manage the outcome of virus infection, especially those that are responsible for chronic tissue damage.

Herpes Simplex Virus-1 (HSV) infection of the cornea leads to a blinding immuno-inflammatory lesion of the eye termed stromal keratitis (SK). Recently, IL-17 producing CD4+ T cells (Th17) were shown to play a prominent role in many autoimmune conditions, but the role of IL-17 and/or of Th17 cells in virus immunopathology is unclear. Here we show that, after HSV infection of the cornea, IL-17 is upregulated in a biphasic manner with an initial peak production around day 2 pi and a second wave starting from day 7 pi with a steady increase until day 21 pi, a time point when clinical lesions are fully evident. Further studies demonstrated that innate cells particularly, γδ T cells, were major producers of IL-17 early after HSV infection. However, during the clinical phase of SK, the predominant source of IL-17 was Th17 cells which infiltrated the cornea only after the entry of Th1 cells. By ex-vivo stimulation, the half fraction of IFN-γ producing CD4+ T (Th1) cells were HSV specific, whereas very few Th17 cells responded to HSV stimulation. The delayed influx of Th17 cells in the cornea was attributed to the local chemokine and cytokine milieu. Finally, HSV infection of IL-17 receptor knockout mice, as well as IL-17 neutralization in WT mice showed diminished SK severity. In conclusion, our results show that IL-17 and Th17 cells contribute to the pathogenesis of SK, the most common cause of infectious blindness in the western world.

Reactions to pathogens are usually tuned to effect immunity and limit tissue damage. Several host counterinflammatory mechanisms inhibit tissue damage but these may also act to constrain the effectiveness of immunity to acute infections, as we demonstrate in mice acutely infected with influenza A virus (IAV). We show that compared with wild type (WT), galectin-9 knockout (G9KO) mice mounted a more robust acute phase virus-specific CD8 T-cell response as well as higher and more rapid virus-specific serum IgM, IgG, and IgA responses and also cleared virus more rapidly than did WT mice. Blocking galectin-9 signals to Tim-3-expressing cells using a Tim-3 fusion protein resulted in improved immune responses in WT mice. When IAV immune mice were challenged with a heterologous IAV, the secondary IAV-specific CD8 T-cell responses were fourto fivefold higher in G9KO compared with WT mice. Our results indicate that manipulating galectin signals may represent a convenient approach to improve immune responses to some vaccines.T he host immune response to pathogens needs precise regulation to minimize tissue damage while still achieving defense (1, 2). Some bystander tissue damage usually happens because several host defenses can destroy cells or orchestrate inflammatory reactions. With chronic infections, for example, immune-mediated tissue damage would be more severe were it not for several cellular and chemical host components that inhibit inflammatory reactions (1). However, the activity of some of these counterinflammatory mechanisms could act to constrain the efficiency of protective immune components (3). For instance, regulatory T cells (Tregs) can inhibit inflammatory reactions associated with chronic virus infections (4), but the same Treg response can also limit the magnitude of protective immunity to a virus or induced by a vaccine (5, 6). Other host components may also function to limit and help resolve inflammatory reactions. These include some cytokines (7), groups of molecules derived from omega-3 polyunsaturated fatty acids (8), as well as some of the carbohydrate binding proteins of the galectin family (9). Galectin-9 (Gal-9), for example, upon binding to Tim-3 on T cells acts to limit the extent of immunopathological lesions in autoimmunity (10) as well as in some chronic infections (11-13). In the present study, we investigated whether the inhibitory effects of Gal-9 on Tim-3-expressing cells could influence the outcome of acute infection with influenza A virus (IAV). We demonstrate that animals lacking the regulatory effects of Gal-9/Tim-3 triggering mounted superior CD8 T-cell and humoral immune responses and they were more refractory to IAV. Moreover, IAV immune G9KO mice challenged with a heterologous IAV strain generated better virus-specific memory CD8 T-cell responses than WT animals. Our results indicate that manipulating galectin signaling may represent a convenient approach to improve responses to some vaccines. Results Virus-Specific CD8 T cells Up-Regulate Tim-3 Expression after IAVInfection. ...

Ocular infection with herpes simplex virus (HSV) causes corneal neovascularization (CV), an essential step in the pathogenesis of the blinding immuno-inflammatory lesion, stromal keratitis (SK). The infection results in IL-17A production which contributes to CV in ways that together serve to shift the balance between corneal concentrations of VEGF-A and the soluble receptor molecule sVEGFR-1 which binds to VEGF-A and blocks its function (a so-called VEGF trap). Accordingly, animals lacking responses to IL-17A signaling, either because of IL-17 receptor-A knockout or WT animals that received neutralizing mAb to I L-17A had diminished CV compared to controls. The procedures reduced VEGF-A protein levels, but had no effect on the levels of sVEGFR-1 present. Hence the VEGF trap was strengthened. IL-17A also caused increased CXCL1/KC synthesis, which attracts neutrophils to the inflammatory site. Neutrophils further influenced the extent of CV by acting as an additional source of VEGF-A as well as metalloproteinase (MMP) enzymes that degrade the soluble receptor inhibiting its VEGF blocking activity. Our results indicate that suppressing the expression of IL-17A, or increasing the activity of the VEGF trap, represent useful approaches to inhibit CV and the control of an ocular lesion that is an important cause of human blindness.

Herpes simplex virus (HSV) infection of adult humans occasionally results in life-threatening herpes simplex encephalitis (HSE) for reasons that remain to be defined. An animal system that could prove useful to model HSE could be miR-155 knockout mice (miR-155KO). Thus we observe that mice with a deficiency of miR-155 are highly susceptible to HSE with a majority of animals (75–80%) developing HSE after ocular infection with HSV-1. The lesions appeared to primarily represent the destructive consequences of viral replication and animals could be protected from HSE by acyclovir treatment provided 4 days after ocular infection. The miR-155KO animals were also more susceptible to develop zosteriform lesions, a reflection of viral replication and dissemination within the nervous system. One explanation for the heightened susceptibility to HSE and zosteriform lesions could be because miR-155KO animals develop diminished CD8 T cell responses when the numbers, functionality and homing capacity of effector CD8 T cell responses were compared. Indeed, adoptive transfer of HSV-immune CD8 T cells to infected miR-155KO mice at 24 hours post infection provided protection from HSE. Deficiencies in CD8 T cell numbers and function also explained the observation that miR-155KO animals were less able than control animals to maintain HSV latency. Our observations may be the first to link miR-155 expression with increased susceptibility of the nervous system to virus infection.

Ocular herpes simplex virus infection can cause a blinding CD4+ T cell orchestrated immuno-inflammatory lesion in the cornea called Stromal Keratitis (SK). A key to controlling the severity of SK lesions is to suppress the activity of T cells that orchestrate lesions and enhance the representation of regulatory cells that inhibit effector cell function. In this report we show that a single administration of TCDD (2, 3, 7, 8- Tetrachlorodibenzo-p-dioxin), a non-physiological ligand for the AhR receptor, was an effective means of reducing the severity of SK lesions. It acted by causing apoptosis of Foxp3- CD4+ T cells but had no effect on Foxp3+ CD4+ Tregs. TCDD also decreased the proliferation of Foxp3- CD4+ T cells. The consequence was an increase in the ratio of Tregs to T effectors which likely accounted for the reduced inflammatory responses. In addition, in vitro studies revealed that TCDD addition to anti-CD3/CD28 stimulated naïve CD4+ T cells caused a significant induction of Tregs, but inhibited the differentiation of Th1 and Th17 cells. Since a single TCDD administration given after the disease process had been initiated generated long lasting anti-inflammatory effects, the approach holds promise as a therapeutic means of controlling virus induced inflammatory lesions.

Ocular HSV-1 infection can result in SK, a blinding immunoinflammatory lesion that represents an immunopathological response to the infection. CD4+ T cells are the main orchestrators, and lesions are more severe if the regulatory T cell response is compromised from the onset of infection. Little is known about the role for Foxp3+ CD4+ Tregs during ongoing inflammatory reactions, which is the topic of this report. We used DEREG mice and depleted Treg at different times after infection. We show that lesions became more severe even when depletion was begun in the clinical phase of the disease. This outcome was explained both by Treg influencing the activity of inflammatory effector T cells at the lesion site, as well as an effect in lymphoid tissues that led to reduced numbers of effectors and less trafficking of T cells and neutrophils to the eye. Our results demonstrate that Treg can beneficially influence the impact of ongoing tissue damaging responses to a virus infection and imply that therapies that boost Treg function in the clinical phase hold promise as a modality to control a lesion that is an important cause of human blindness.

An approximately 5-month-old American Staffordshire terrier was presented with a history of recurrent peritoneal effusion. Abdominal radiographs and ultrasound showed a loculated effusion in the ventral abdomen with dorsal displacement of abdominal organs, hepatomegaly and rounding of liver and splenic margins. Computed tomography demonstrated centrally located gastrointestinal segments surrounded by a thin soft tissue band and a thickened peritoneal lining. At necropsy a fibrous membrane continuous with liver and splenic capsules encapsulated all abdominal organs. Microscopically the abdominal wall and fibrous capsule consisted of an irregular thick layer of hypocellular connective tissue. The final diagnosis was sclerosing encapsulating peritonitis.