Naturally occurring CD4⁺CD25⁺ regulatory T cells appear important to prevent activation of autoreactive T cells. This article demonstrates that the magnitude of a CD8⁺ T cell–mediated immune response to an acute viral infection is also subject to control by CD4⁺CD25⁺ T regulatory cells (T_reg). Accordingly, if natural T_reg were depleted with specific anti-CD25 antibody before infection with HSV, the resultant CD8⁺ T cell response to the immunodominant peptide SSIEFARL was significantly enhanced. This was shown by several in vitro measures of CD8⁺ T cell reactivity and by assays that directly determine CD8⁺ T cell function, such as proliferation and cytotoxicity in vivo. The enhanced responsiveness in CD25-depleted animals was between three- and fourfold with the effect evident both in the acute and memory phases of the immune response. Surprisingly, HSV infection resulted in enhanced T_reg function with such cells able to suppress CD8⁺ T cell responses to both viral and unrelated antigens. Our results are discussed both in term of how viral infection might temporarily diminish immunity to other infectious agents and their application to vaccines. Thus, controlling suppressor effects at the time of vaccination could result in more effective immunity.

CD4⁺CD25⁺ regulatory T cells (T_reg) can inhibit a variety of autoimmune and inflammatory diseases, but their involvement in regulating virus-induced immunopathology is not known. We have evaluated the role of T_reg in viral immunopathological lesion stromal keratitis. This frequent cause of human blindness results from a T cell-mediated immunoinflammatory response to HSV in the corneal stroma. The results show that lesions were significantly more severe if mice were depleted of T_reg before infection. The T_reg was also shown to modulate lesion expression induced by adoptive transfer of pathogenic CD4⁺ T cells in infected SCID recipients. The mechanism of T_reg control of stromal keratitis involved suppressed antiviral immunity and impaired expression of the molecule required for T cell migration to lesion sites. Interestingly, T_reg isolated from ocular lesions in nondepleted mice showed in vitro inhibitory effects involving IL-10, but were not very effective in established lesions. Our results decipher the in vivo role of T_reg in a virus-induced immunopathology and imply that manipulation of regulatory cell function represents a useful approach to control viral-induced immunoinflammatory disease.

This report analyzes the role of vascular endothelial growth factor (VEGF)-induced angiogenesis in the immunoinflammatory lesion stromal keratitis induced by ocular infection with herpes simplex virus (HSV). Our results show that infection with replication-competent, but not mutant, viruses results in the expression of VEGF mRNA and protein in the cornea. This a rapid event, with VEGF mRNA detectable by 12 h postinfection (p.i.) and proteins detectable by 24 h p.i. VEGF production occurred both in the virus-infected corneal epithelium and in the underlying stroma, in which viral antigens were undetectable. In the stroma, VEGF was produced by inflammatory cells; these initially were predominantly polymorphonuclear leukocytes (PMN), but at later time points both PMN and macrophage-like cells were VEGF producers. In the epithelium, the major site of VEGF-expressing cells in early infection, the infected cells themselves were usually negative for VEGF. Similarly, in vitro infection studies indicated that the cells which produced VEGF were not those which expressed virus. Attesting to the possible role of VEGF-induced angiogenesis in the pathogenesis of herpetic stromal keratitis were experiments showing that VEGF inhibition with mFlt(1–3)-immunoglobulin G diminished angiogenesis and the severity of lesions after HSV infection. These observations are the first to evaluate VEGF-induced angiogenesis in the pathogenesis of stromal keratitis. Our results indicate that the control of angiogenesis represents a useful adjunct to therapy of herpetic ocular disease, an important cause of human blindness.

In this communication, we demonstrate that galectin (Gal)-9 acts to constrain CD8⁺ T cell immunity to Herpes Simplex Virus (HSV) infection. In support of this, we show that animals unable to produce Gal-9, because of gene knockout, develop acute and memory responses to HSV that are of greater magnitude and better quality than those that occur in normal infected animals. Interestingly, infusion of normal infected mice with α-lactose, the sugar that binds to the carbohydrate-binding domain of Gal-9 limiting its engagement of T cell immunoglobulin and mucin (TIM-3) receptors, also caused a more elevated and higher quality CD8⁺ T cell response to HSV particularly in the acute phase. Such sugar treated infected mice also had expanded populations of effector as well as memory CD8⁺ T cells. The increased effector T cell responses led to significantly more efficient virus control. The mechanisms responsible for the outcome of the Gal-9/TIM-3 interaction in normal infected mice involved direct inhibitory effects on TIM-3⁺ CD8⁺ T effector cells as well as the promotion of Foxp3⁺ regulatory T cell activity. Our results indicate that manipulating galectin signals, as can be achieved using appropriate sugars, may represent a convenient and inexpensive approach to enhance acute and memory responses to a virus infection. Adaptive immune responses to foreign antigens require precise regulation, otherwise excessive bystander damage to host tissues may occur and responses to other antigens could be compromised. Some galectin proteins binding to receptors on cells of the immune system form part of the regulatory system, although this topic has received scant attention as it relates to antiviral control. In this report, we evaluate the role of the galectin-9/TIM-3 receptor pathway at regulating acute and memory CD8⁺ T cell responses to herpes simplex virus (HSV) infection. We demonstrate that CD8⁺ T cell responses to HSV were significantly increased in magnitude and improved in quality in mice unable to produce galectin-9 because of gene knockout compared to wild type controls. Furthermore, inhibiting the galectin-9 mediated response to TIM-3 using the sugar alpha-lactose that binds to Gal-9 led to a similar response pattern. The influence of galectin-9 to constrain CD8⁺ T cell responses involved direct inhibitory effects on the T effectors as well as the promotion of regulatory T cell responses. Our results indicate that manipulating the interaction of galectins with receptors using simple sugars may represent a convenient and inexpensive approach to enhance T cell responses to virus infections, and could prove useful to increase the efficacy of some vaccines.