Neurotrophic and immune regulatory factors in neuroinflammation

Abstract

Inflammatory reactions in the nervous system act as a double-edged sword. On the one hand, neuroinflammation can have very devastating effects on the nervous system, leading to neurological dysfunction. Conversely, there is now convincing evidence demonstrating that neuroinflammatory reactions can also serve as a physiological response to central nervous system (CNS) disease or injury by providing protection and repair of the vulnerable and mostly nonreplaceable nervous tissue. There are indications that the production of neurotrophins by infiltrating lymphoid cells at the site of damage in the CNS may be responsible for this beneficial effect. However, knowledge of the cellular distribution and regulation of neurotrophin expression in CNS autoimmune T cells is currently very limited. These issues have been addressed in this thesis using both rat and human materials.

Neurotrophin expression in the rat was studied in myelin basic protein (MBP)-induced experimental autoimmune encephalomyelitis (EAE) in the LEW strain, which is a well-characterized model in which encephalitogenicity is conveyed by T cells bearing the TCRBV8S2 chain. Neurotrophin expression in the spinal cord was strongly upregulated during EAE. FACS-sorting of CNS-infiltrating cells into encephalitogenic TCRBV8S2+ and nonencephalitogenic TCRBV8S2 cells demonstrated higher inflammatory cytokine mRNA levels in the encephalitogenic cells, whereas at peak of disease the levels of brain-derived neurotrophic factor (BDNF) and neurotrophin (NT)-3 were higher in the nonencephalitogenic cells. This suggests that a potentially neuroprotective facet of CNS inflammation dominantly prevails within non-encephalitogenic lymphoid cells and that there are independent regulatory mechanisms for neurotrophin and inflammatory cytokine expression during EAE.

In order to study the regulation of neurotrophin expression in lymphoid cells, in vitro cell cultures of lymph node cells were performed. In contrast to IFN-gamma expression, stimulation with whole spinal cord homogenate (WSCH) or myelin resulted in a strong induction of neurotrophin expression in B cells and TCRBV8S2- cells, as well as in unsorted lymph node cells. Restimulation with antigen or mitogen did not lead to induction of neurotrophin expression in unsorted cells. However, in nonencephalitogenic T cells restimulation resulted in moderately increased BDNF mRNA levels. These results demonstrate a very distinct regulation of neurotrophin expression in rat immune cells.

In humans, mRNA for BDNF, but not NT-3 or NGF, was readily detected in peripheral blood mononuclear cells (PBMC) and levels were significantly increased in multiple sclerosis (MS) patients compared to controls. FACSsorting of PBMC from NIS patients demonstrated BDNF mRNA expression in several immune populations. These data suggest that a neuroprotective aspect of autoimmune inflammation is present in MS. A family of T cell Ig- and mucin-domaincontaining molecules (TIM) has recently been discovered. In mice, TIM-1 and TIM-3 are reciprocally expressed by Th2 and Th1 cells, respectively. It was of interest to study whether this is the same in humans and rats. If so, this may provide a very versatile tool for the characterization of Th1/Th2-biased immune reactions. In rat EAE, TIM3 but not TIM-1 was upregulated in the spinal cord and antigen restimulation of the encephalitogenic TCRBV8S2 population resulted in a robust upregulation of TIM-3 mRNA levels, demonstrating that this molecule can also be used as a Th1 -differentiation marker also in the rat. Interestingly, TIM-3 expression was also detected in resident cells of the nervous system, a finding which may have implications for immune regulation in the CNS. No similarities in expression patterns between the TIM molecules and neurotrophin mRNAs were observed.

To study TIM-1 and -3 expression in human autoimmune neuroinflammation, a collection of PBMC and cerebrospinal fluid (CSF) samples from NIS patients were analysed. TIM-1 but not TIM-3 mRNA was significantly upregulated in CSF cells of MS patients compared to controls and levels correlated with clinical disease activity. In contrast, CSF expression of TIM-3 correlated with increased expression of TNF-alpha, IFN-gamma and IL-10. No association between TIM and neurotrophin expression was observed. These findings support a role for TIM in human autoimmune neuroinflammation and provide evidence that the TIM-1 and TIM-3 can also be used as T helper differentiation markers in human disease. In conclusion, these results are important for understanding the interaction between the nervous and immune systems. In addition, further exploration of the role of the TIM molecules during autoimmunity, and of the neuroprotective potential of autoimmunity, may yield new therapeutic strategies for inflammatory neurodegenerative diseases.