T cell responses to S-glutathionylated And heteroclitic viral epitopes and CCl2-mediated immune dysregulation in mice infected with a neurotropic coronavirus

Mice infected with neurotropic variants of the murine coronavirus, mouse hepatitis virus, (strains JHMV or J2.2–V–1) develop acute and chronic CNS infections, and provide a model system to study the pathogenesis of virus–induced neuroinflammation, mechanisms of virus persistence, and anti–viral immune responses in the CNS.

Using the J2.2–V–1 model of CNS infection, we addressed the role of sustained CCL2 production during viral infection using mice in which CCL2 was expressed transgenically in oligodendrocytes. Tonic CCL2 expression in the CNS resulted in delayed kinetics of virus clearance, and converted what is typically a mild, nonlethal disease to acutely lethal encephalitis, with the majority of mice succumbing to the infection. CCL2 induced a rapid and dysregulated inflammatory response that was no longer protective and was unable to efficiently clear virus from the CNS. Infected CCL2 Tg mice had increased numbers of Foxp3–expressing CD4 T cells (Tregs) and of macrophages and microglia expressing elevated levels of YM–1, a marker for alternatively activated macrophages, and nitric oxide. Our results showed that CCL2 has effects beyond serving as a chemoattractant for leukocytes, and has effects on the composition and function of inflammatory cells at sites of infection.

In a separate set of experiments, I identified and characterized two additional heteroclitic variants of the JHMV epitope S598 that induced CD8 T cells with greater antigen sensitivity to the native S598 determinant relative to the cells primed by the native epitope. One of these heteroclitic epitopes elicited a T cell response with nearly complete cross–reactivity towards the native peptide. The structural data show that these heteroclitic epitopes induced modest conformational changes in the local environment of the peptide–MHCI complex. I also provide data to support the notion that heteroclitic determinants augment functional avidity by increasing surface epitope density. Collectively, these data will help guide the design of heteroclitic epitopes in the setting of vaccine development.

Lastly, I examined the consequences of oxidative stress induced by viral infection on antigen presentation. The brains of JHMV–infected mice were found to have signs of oxidative stress, with significantly decreased ratios of reduced (GSH) to oxidized (GSSG) glutathione, suggesting that there is an environment that is conducive for cysteine modification with oxidized glutathione. We found that virus–induced oxidative stress resulted in the presentation of both native and S–glutathionylated forms of the JHMV epitope S510 by infected cells. A subset of the S510–specific CD8 T cells failed to recognize the modified form of the epitope, suggesting that GSH–modification of a cysteine–containing viral epitope might interfere with T cell recognition. Further, GSH-modified peptides were identified in stressed human cells, including herpes virus–transformed B cells, suggesting that the modification is not limited to mouse cells. Collectively these findings have implications for both anti–viral immunity and anti–tumor immunity, where oxidative stress has been shown to play a role during infection and tumorgenesis.