Differential induction of interleukin-12, interleukin-18, and interleukin-1β converting enzyme mRNA in experimental autoimmune encephalomyelitis of the Lewis rat

Abstract

Experimental autoimmune encephalomyelitis (EAE) is a model of autoimmune central nervous system (CNS) disease that is mediated by autoreactive Th1 cells secreting the proinflammatory cytokine interferon (IFN)-γ. Interleukin (IL)-12 in its heterodimeric p35/p40 isoform and the recently described cytokine IL-18 potently induce T cell production of IFN-γ. Interleukin-1β converting enzyme (ICE) is required to convert IL-18 precursor protein into its biologically active mature form. In this study, we used semiquantitative reverse transciptase-polymerase chain reaction to determine steady state levels of IL-12, IL-18, and ICE mRNA in the spinal cord of Lewis rats at different stages of EAE. In control rats, we found significant IL-18, ICE, and IL-12p35, but not IL-12p40 mRNA expression. IL-18 mRNA increased during the acute stage of EAE together with a marked induction of ICE mRNA. IL-12p35 mRNA levels did not change significantly throughout the course of EAE. Surprisingly, the peak expression of IL-12p40 mRNA was delayed by several days relative to the peak of T cell infiltration and IFN-γ mRNA synthesis. Our data implicate the IL-18/ICE pathway in the amplification of Th1-mediated immune responses in the CNS but suggest a different, so far undefined role of endogenous IL-12 in the late effector phase of EAE.

Introduction

Experimental autoimmune encephalomyelitis (EAE) is a widely used model of autoimmune central nervous system (CNS) disease that mimics several aspects of human multiple sclerosis (Raine, 1984; Wekerle, 1993). EAE is mediated by autoreactive Th1 cells secreting interferon (IFN)-γ and interleukin (IL)-2. For the generation and efficient activation of Th1 cells several costimulatory signals have to be delivered during initial stages of a cellular immune response (Abbas et al., 1996). IL-12 is a heterodimeric cytokine that is composed of two different covalently linked subunits, p35 and p40. Its production by accessory cells during the initiation of an immune response polarizes naive T cells toward a Th1 pattern of cytokine production (Trinchieri, 1993). IL-18 was discovered only recently as a `IFN-γ-inducing-factor' in a mouse model of endotoxic shock (Okamura et al., 1995). IL-18 is synthesized as an inactive precursor protein that exhibits structural homology to IL-1 (Bazan et al., 1996) and accordingly has to be processed by the caspase IL-1β converting enzyme (ICE) to get functionally active (Ghayur et al., 1997; Gu et al., 1997). Similar to IL-12, IL-18 enhances the production of IFN-γ by Th1 cells although both cytokines apparently use different intracellular signaling pathways (Jacobson et al., 1995; Thierfelder et al., 1996; Kohno et al., 1997; Matsumoto et al., 1997; Robinson et al., 1997).

In line with a Th1-mediated immunopathogenesis, IL12p40 mRNA has been detected in active multiple sclerosis lesions (Windhagen et al., 1995) and disease-enhancing effects of IL-12 have been demonstrated in EAE (Leonard et al., 1995; Segal and Shevach, 1996; Smith et al., 1997; Segal et al., 1998). IL-18 mRNA has been found in active stages of murine autoimmune diabetes (Rothe et al., 1997) but its expression in CNS autoimmune diseases has not been studied so far. Accordingly, the relative contribution of both cytokines to the regulation of Th1-mediated immune responses in the CNS is currently unknown. In the present study we used a semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR) approach to analyze the time course of IL-12 p35/p40, IL-18, and ICE mRNA expression in the spinal cord of Lewis rats at different stages of EAE actively induced with the encephalitogenic 68–86 peptide of guinea pig myelin basic protein.