Elsevier

Journal of Neuroimmunology

Volume 101, Issue 1, 1 November 1999, Pages 87-97
Journal of Neuroimmunology

Non-MHC gene regulation of nerve root injury induced spinal cord inflammation and neuron death

https://doi.org/10.1016/S0165-5728(99)00136-8Get rights and content

Abstract

Spinal ventral root avulsion leads to an inflammatory response around lesioned motoneurons and the subsequent degeneration of a large proportion of the neurons. We demonstrate here differences in the regulation of cytokine mRNAs, microglia/macrophage activation, MHC expression and nerve cell survival in the two inbred rat strains DA and ACI. These strains have similar major MHC haplotypes, but differ in their non-MHC background genes. T cells were rare in the lesioned segments and depletion of T cells did not affect the response. Thus, non-MHC gene(s) regulate the inflammation and neuron death after nerve trauma by mechanisms not involving antigen-specific immune responses.

Introduction

It is well established that neurantigen-induced EAE, a model of mainly T cell-dependent neuroinflammation, is strongly genetically regulated by both MHC and non-MHC genes (Williams and Moore, 1973; Sundvall et al., 1995). The MHC gene influence in rat EAE may largely be due to polymorphisms within MHC class I and II molecules, which determine the ability to present particular autoantigenic peptides (Mustafa et al., 1994; Issazadeh et al., 1997; Weissert et al., 1998), whereas some non-MHC background genes are likely to regulate non-antigen specific features of the inflammation (Lorentzen et al., 1997; Dahlman et al., 1998; Weissert et al., 1998). In both mice (Baker et al., 1994; Sundvall et al., 1995; Encinas et al., 1996) and rats (Dahlman et al., 1998) non-MHC gene regions influencing disease susceptibility has been mapped, and ongoing positional cloning will be important to exactly delineate the regulating molecular pathways. Neuroinflammatory responses can be elicited also by mechanical nerve injuries, such as peripheral nerve lesions. This type of inflammation is characterized by the proliferation and activation of microglia and, to some extent, also astrocytes around the axotomized nerve cell bodies (Lieberman, 1971; Kreutzberg et al., 1989). In contrast to EAE, the genetic regulation of such trauma-induced “non-immune” inflammatory responses in the CNS has been poorly explored, although strain-dependent differences have been described in a previous study showing differences in some inflammatory parameters in one inbred (LEW) and one outbred (SD) rat strain after spinal cord contusion (Popovich et al., 1996). If the nerve root avulsion-induced response is indeed genetically regulated, there are prospects for a genetic dissection of inflammatory neurodegeneration, as in EAE, in order to pinpoint critical regulatory mechanisms.

The functional significance of axotomy-induced inflammation is not known, though some evidence suggest that it may facilitate regeneration (David et al., 1990; Lu and Richardson, 1991). However, since activated glia produce compounds with potentially neurotoxic effects, excessive activation may also be detrimental (Giulian et al., 1993; Blight, 1994). Nerve avulsion is a useful experimental model in which such effects can be explored, since this procedure, in contrast to peripheral axotomy, results in neuron death (Koliatsos et al., 1994). Non-exclusively, circumstantial evidence suggest a series of mechanisms for the neuron death after nerve avulsion; such as lack of neurotrophic factors (Kishino et al., 1997; Novikov et al., 1997), glutamate toxicity (Piehl et al., 1995) and nitric oxide (Wu, 1996). Several of these mechanisms may in turn be the result of the local microglial reaction. More classical inflammatory mediators may also take part in this local response, since MHC molecules are upregulated after axotomy (Maehlen et al., 1988; Olsson et al., 1989; Streit et al., 1989) and cytokines such as IFN-γ and TNF-α are well known inducers of MHC expression (Fierz et al., 1985; Suzumura et al., 1987; Vidovic et al., 1990).

In order to dissect the genetic regulation of these events, we have examined the inflammatory response and neuron death after nerve root avulsion in two different inbred rat strains, i.e., DA and ACI. These particular strains were selected for two reasons: Firstly, the DA strain is highly susceptible to EAE (Lorentzen et al., 1995; Issazadeh et al., 1996), whereas the ACI strain is relatively resistant to this disease (Lorentzen et al., 1997; Weissert et al., 1998); secondly, the two strains have similar MHC haplotypes (Hedrich, 1990; Remmers et al., 1995), thus allowing any strain differences to be mapped to non-MHC genes.

The extensive strain-dependent differences in the magnitude of the inflammatory response and the kinetics of neuronal death reported in this study add very valuable information to the concept of neuroinflammation and may enable future positional cloning of gene(s) critically regulating these events.

Section snippets

Lesions

A total of 109 DA or ACI rats (body weight approximately 225 g) were subjected to unilateral avulsion of lumbar ventral roots. In addition, six rats were sham-operated and used as controls for the specificity of the lesion-induced effects. Rats were anesthetized with medetomidin 0.2 mg/kg (Orion Pharma, Sollentuna, Sweden) and ketamine 30 mg/kg (Parke-Davis Scandinavia, Solna, Sweden). A half-sided laminectomy was performed on the L3–L4 vertebrae and a longitudinal incision was made in the

Glial reaction and cellular inflammatory response

Three days after unilateral avulsion of the L4–L5 ventral roots, the corresponding nerve cell bodies in the spinal cord were swollen. Markers for microglia (GS-I B4 and Ox×42) revealed intense activation in the gray substance of the ipsilateral ventral horn. However, almost no MHC class II expression on activated microglia was detectable at this stage. One week after the lesion, activated MHC class II positive microglia had appeared along lesioned axons passing through the ventral funiculus

Discussion

We here demonstrate a strong genetic impact on the inflammatory response and motoneuron degeneration after ventral root avulsions. The DA as compared to the ACI strain displayed a much stronger activation of glia, including expression of proinflammatory cytokines and MHC class I and II, during the most intense inflammatory phase. Since DA and ACI rats have the same or very similar MHC genes and we were unable to demonstrate any differences in the response between the DA and DA.1H haplotypes,

Acknowledgements

This work was supported by the Swedish Medical Research Council, Åke Wibergs stiftelse, Tore Nilsons stiftelse, Magn. Bergvalls stiftelse, NHR, David och Astrid Hageléns stiftelse and Hjärnfonden.

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