Elevated NKG2D ligand expression in experimental autoimmune encephalomyelitis

Abstract

T cells remain unclear. Expressing myelin-reactive T cell receptor (TCR) is not sufficient to make a T cell encephalitogenic. In fact, the frequencies of myelin-reactive T cells are comparable between MS patients and healthy individuals, but the ones in MS patients have activated/memory phenotypes. In the animal model of MS, experimental autoimmune encephalomyelitis (EAE), myelin-specific T cells activated with antigen presenting cells (APCs) plus myelin peptide are encephalitogenic, whereas T cells activated with anti-CD3/CD28 antibodies are not. This suggests that APCs provide critical cytokines beyond T cell receptor activation and co-stimulation, contributing to encephalitogenicity. To identify which cytokines are critical for generating an encephalitogenic T cell, naive myelin-specific CD4 T cells were activated with anti-CD3/CD28 in the presence of various cytokines, which recapitulated the signals normally provided by APCs. The activated T cells were then adoptively transferred to wildtype recipients, and the degree of encephalitogenicity of T cells was examined by EAE. We showed that no single cytokine was sufficient, but the combinations of IL-6/IL-23 or IL-12/IL-23 generated highly encephalitogenic T cells. Previous studies have revealed an essential role of IL-23 in EAE pathogenesis. However, naive T cells differentiated with IL-23 alone could not transfer disease, due to limited expression of its receptor. IL-6 and IL-12 induced the initial expression of IL-23R on naive T cells, and IL-23 further enhanced the expression of its own receptor. IL-23R signaling induced downstream STAT3 phosphorylation in both IL-6/IL-23 and IL-12/ IL-23-generated T cells. Neutralizing IL-6 or IL-12 with antibodies significantly reduced the severity of EAE, which was induced by APCgenerated T cells, indicating the crucial roles of IL-6 and IL-12 during the initial T cell activation. We have identified that IL-6/IL-23 or IL-12/ IL-23 combination in addition to TCR activation and co-stimulation were the minimal signals necessary to generate encephalitogenic T cells. This provides potential targets that can be manipulated therapeutically, resulting in innovative treatments for MS.