Excitotoxic neurodegeneration in mouse brain : roles of immune cells and cytokines

Abstract

Excitotoxic neurodegeneration is a common feature in many neurological diseases. However, the roles of immune cells and cytokines in this process are unclear. To address this issue, we used knockout (KO) mice deficient in various immune cells or cytokines in our studies. We first established a kainic acid (KA)-induced excitotoxic neurodegenerative model, by intranasal delivery of KA, in C57BL/6 mice, which are often used for transgenic and KO studies but are resistant to KA-induced brain injury through subcutaneous injection. Following intranasal administration of KA, the mice exhibited continuous seizure with an acceptable survival rate. The analysis of the brains of the surviving mice revealed pronounced neurodegeneration in the cornu ammonis 3 (CA3) area of the hippocampus. The surviving mice also showed increased cyclooxygenase-2 (COX-2) expression, astrogliosis, and elevated locomotor and rearing activities after KA treatment. By using the above model, we found that CD4/CD8-deficiency resulted in significant reduction of clinical symptoms and pathological changes in KA-treated young, but not in KA-treated middle-aged mice. Middle-aged wild-type mice as well as young and middle-aged CD4/CD8(-/-) mice had significantly slighter hippocampal changes compared with young wild-type mice following KA treatment. The results suggest that both T cell deficiency and increased age may be responsible for the relative resistance to KA-induced neurotoxicity. We then tried to further specify the effects of T cell subsets and B cells on the pathological changes. After KA treatment, CD4/CD8(-/-) mice exhibited the mildest seizures; the responses of CD8(-/-), Igh-6(-/-) and wild-type mice were intermediate, whereas CD4(-/-) mice displayed much more severe clinical signs and 100% early mortality. Histopathological analysis of the surviving mice revealed that CD4/CD8(-/-) mice had the slightest pathologic changes but Igh-6(-/-) mice showed more severe lesions in the hippocampus than CD8(-/-) and wild-type mice. The results suggest that T cell subsets and B cells may contribute differently to the pathogenesis. We applied CCR5 KO mice in our study but found no significant differences of clinical and pathological changes in vivo, or neuronal susceptibilities to KA insult in vitro, between KA-treated KO and wild-type mice. However, KA treatment stimulated mRNA expression of the monocyte chemoattractant protein-2 (MCP-2) in both the wild-type and KO mice. In addition, CCR2 and CCR3, which share the common ligand of MCP-2 with CCR5, were up-regulated in the KA-treated KO mice but not in the KA-treated wild-type mice. This indicates that increased CCR2 and CCR3 expression in CCR5 KO mice may have compensated for the functions of CCR5 in excitotoxic neurodegeneration. Finally, we used IL-12p35 KO mice in our model. After KA treatment, the hippocampal neurodegeneration was significantly slighter in the IL-12p35 KO than in the wild-type mice. Furthermore, by flow cytometric analysis of the expression of F4/80 and CD86 on the microglia isolated from adult mice, we found that microglial activation was significantly decreased in the KA-treated IL-12p35 KO mice compared with the KA-treated wild-type mice. The reduced microglial activation in the absence of IL-12p35 might at least partially account for the ameliorated pathological changes, indicating an important role of IL-12 in excitotoxic neurodegeneration. In conclusion, immune cells, such as T and B cells, and cytokines may play important roles in excitotoxic neurodegeneration. The control of the behaviors of immune cells and/or cytokines in brain may provide possible treatments for neurological diseases.