Immune regulation within the central nervous system and its role in demyelinating diseases

Abstract

Multiple sclerosis (MS) and its animal model, experimental allergic encephalomyelitis (EAE), are diseases of the central nervous system (CNS) characterized by perivascular mononuclear inflammation and oligodendrocyte loss with demyelination. However, it is still not clear whether the pathology in MS and EAE is due to neuroantigen-specific CD4+ T cell-mediated autoimmune responses and/or bystander myelin damage resulting from an inflammatory response.

We demonstrate that immunological tolerance to CNS myelin antigens is either absent or incomplete. Levels of autoreactive T cells recognising myelin antigens (such as MBP, PLP, MAG and MOG) are increased in blood and cerebrospinal fluid (CSF) from patients with MS. The frequency of MOG-reactive T cells was about 16-fold higher in MS CSF than in blood. Anti-MOG IgG antibody and antibody secreting cells were detected in CSF from 7 of 30 (23%) and 8 of 10 (80%) patients with MS, respectively. Although autoreactive T cells initiate the chain of pathogenetic events that leads to the development of inflammatory CNS lesions in MS and EAE, the CNS has specialized cells that execute immunological effector functions. Nitric oxide (NO) might be involved in myelin and oligodendrocyte injury. The CSF from 13/38 (34%) patients with MS stimulated glial cells to produce NO. Overproduction of NO was confirmed in EAE. The major source of NO overproduction was glial cells, but not infiltrating mononuclear cells. The peak of NO production in EAE was observed at day 7 post-immunization (p.i.) and then gradually declined at day 14, 21 and 28 in parallel with clinical improvement.

Cytokines are engaged in the communication between Iymphocytes in the immune system and glial cells within the CNS. TGF-beta1 inhibited microglia and astrocyte proliferation and down-regulated the expression of ICAM-1 in microglia. In contrast, IFN-y upregulated ICAM-1 expression, but inhibited proliferative response of glial cells. By combining TGF-beta1 and IFN-y, we show that TGF-beta1-mediated inhibition of proliferation and downregulation of ICAM-1 is prevented by IFN-y To explore the mechanism of TGF-beta1 in inhibiting glial cell reactions, we studied glial cells for susceptibility to apoptosis induced by TGF-beta1. TGF-beta1 selectively induced apoptosis of microglia, but not of astrocytes or oligodendrocytes. To further study the apoptotic mechanism, bcl-2 oncoprotein in microglia astrocytes and oligodendrocytes was measured. Bcl-2 was mainly expressed in microglia.

The relative expression of bcl-2 in microglia was not related to frequency of microglial apoptosis, suggesting that TGF-beta1-mediated microglial apoptosis was regulated by a bcl-2-independent mechanism A striking feature of EAE is the loss of infiltrating T cells and macrophages from the CNS during clinical remission. Local tissue factors, particularly tissue cytokines and mediators within the local microenvironment, play a key role in eliminating infiltrating T cells and macrophages. Under natural culture conditions, microglia expressed the immunosuppressive TGF-beta1 and IL-10 to a higher degree than the proinflammatory IL-Ibeta, IL-6, IL-12, IFN-y and TNF-a. High TGF-t31 amd IL- 10 levels could reflect one mechanism for immune privilege within the CNS under physiological conditions.

Stimulation of microglia with LPS or IFN-y resulted in shift from anti- to proinflammatory cytokine profiles in microglia. In order to elucidate the basis for oligodendrocyte loss and apoptosis of infiltrating T cells and macrophages within the CNS, expression of cytolysin mRNA in glial cells was examined. The expression of cytolysin mRNA under natural culture conditions was undetectable or very low. IFN-y-mediated upregulation of cytolysin mRNA in oligodendrocytes was markedly more prominent than in microglia and astrocytes, indicating that there may exist a glial cell-dependent cytotoxic pathway within the CNS. Cross talk between glial cells and Iymphoid cells in response to inflammation within the CNS has received increasing attention. We isolated infiltrating mononuclear cells (inMNC) amd Iymph node MNC (InMNC) from Lewis rats with EAE. MBP-induced proliferation and MBP-specific IgG secreting cells were lower in inMNC compared to InMNC, while MBP-reactive IFN-y secreting cells in inMNC were higher. Using an in vitro culture system, astrocytes contribute to the inactivation of MBP-reactive Iymphoid cells through soluble factor(s) pathway. Astrocytes did not induce apoptosis of InMNC. On the other hand, activated T cells can trigger astrocytes to produce IFN-y by cell-cell contact. Taken together, these data suggest that autosensitization of T cells to myelin antigens may be a common trigger for the initiation of autoimmune demyelinating diseases. The brain constitutes an environment that is specifically designed to accommodate, regulate and shape immune responses.