Absence of reuptake of serotonin influences susceptibility to clinical autoimmune disease and neuroantigen-specific interferon-gamma production in mouse EAE

Abstract

Serotonin (5-hydroxytryptamine, 5-HT) is one of the most extensively studied neurotransmitters of the central nervous system. It also has been identified in constituents of the immune system. Therefore serotonin has been suggested to serve as a mediator of bidirectional interactions between the nervous system and the immune system. We investigated this interaction in experimental autoimmune encephalomyelitis (EAE), a well-defined animal model of autoimmune disease of the central nervous system (CNS) mimicking features of the human disease multiple sclerosis. EAE was induced by immunization with the autoantigens myelin basic protein (MBP) or the immunodominant peptide of myelin oligodendrocyte glycoprotein (MOG) spanning amino acids 35-55 (MOGp 35-55). We studied EAE in knockout (KO) mice lacking the 5-HT transporter (5-HTT) on a C57.BL/6 background, in comparison with wild-type C57.BL/6 animals. After immunization with MOGp 35-55, or with rat MBP, the disease courses of the 5-HTT knockout mice were attenuated as compared to wildtype control mice. This difference was more pronounced in female animals. To dissect potential immune mechanisms underlying this phenomenon, histological studies of the CNS and cytokine measurements in mononuclear cells from the spleens of 5-HTT KO mice and wild-type controls were performed. We found a reduction of the inflammatory infiltrate in the CNS and of the neuroantigen-specific production of IFN-gamma in splenocytes, again accompanied by a gender difference. These findings suggest a potential role of extracellular 5-HT homeostasis in the fine-tuning of neuroantigen-specific immune responses.

Fig. 1

Clinical disease courses of male and female 5-HTT KO mice and wild type control mice after immunization with MOGp 35–55 in CFA. All animals were included in this graph, also the mice which did not develop clinical symptoms of EAE. 5-HTT KO mice display an attenuated clinical EAE course with a more pronounced difference in female than in male mice. The graphs shown summarize the data of 11–18 mice for each of the four different groups tested in altogether five independent experiments. Error bars indicate standard error of the mean (SEM) for the female mice.

Fig. 2

Histological analysis of infiltration (a) and of CD3-positive cells (b) by immunohistochemistry in the CNS during acute EAE at day 14 after immunization with MOGp 35–55 in CFA. Female 5-HTT KO mice show a tendency towards reduction of general infiltration and presence of CD3 positive cells. The difference in CD3 staining is significant at P < 0·001. Shown are the cumulative histological data of 3–4 individual animals per group. Error bars indicate the standard error of the mean. (c) shows a representative CD3 staining of EAE spinal cord from a female 5-HTT KO mouse (left) in comparison with a female WT mouse (right).Bar represents 50 µm.