The Role of Astrocytes in the Neuropathogenesis of African Trypanosomiasis

Gichuki, Charity Wangui (1994) The Role of Astrocytes in the Neuropathogenesis of African Trypanosomiasis. PhD thesis, University of Glasgow.

Abstract

This thesis concerns the role of astrocytes in the neuropathogenesis of trypanosomiasis caused by Trypanosoma brucei brucei in mice, a model of the human disease caused by T.b. rhodesiense and T.b. gambiense. Chapter 1 of this thesis includes a literature review of the published work on human African trypanosomiasis (HAT) with a bias on the pathology and possible pathogenic mechanisms, particularly as they relate to the central nervous system (CNS). It also reviews the role of astrocytes in the normal and diseased CNS, and their possible role in the genesis of CNS pathological changes during trypanosome infections. The biological activities of cytokines reported to be produced in the CNS of trypanosome-infected animal models are considered. The chronic trypanosomiasis mouse model, used in this study, is described highlighting the similarities of the disease syndromes in this model to HAT. Chapter 2 describes the experimental techniques used in this study. Chapter 3 demonstrates the occurrence of astrocyte activation, as judged by an increase in the intensity of glial fibrillary acidic protein (GFAP) staining and morphological changes, within the CNS of mice chronically infected with T.b. brucei. It was shown that astrocyte activation first occurred 21 days after infection around the ventricles and the choroid plexus, along the choroid fissure, including, the hippocampus and the base of the cerebellum. It was also demonstrated that astrocyte activation preceded the infiltration of the choroid fissure, the perivascular spaces and the meninges, with inflammatory cells. It would appear that this activation involves production of mediators of inflammation which initiate extravasation of inflammatory cells, and possibly induce further astrocyte activation. From these initial sites, astrocyte activation spreads first to the white matter, the corpus callosum and the cerebellar lobes, and then to the grey matter, of both the cerebrum and the cerebellum. The infiltration of inflammatory cells seemed to enhance the spread and the degree of astrocyte activation. It is known that these areas where initial astrocyte activation occurred, have an incomplete blood-brain barrier and, they have been reported as the first sites of trypanosome invasion, at around day 14 of infection. This suggested that the astrocyte activation observed in these areas, was the first CNS response to invasion by trypanosomes. Subcurative therapy with Berenil, seemed to facilitate inflammatory cell infiltration and astrocyte activation and to hasten the progress of the CNS pathological changes. Treatment of relapsed infections with a second subcurative dose of Berenil increased the inflammatory cell infiltration and the astrocyte activation, resulting in a post-treatment reactive encephalitis (PTRE), similar to that observed in human reactive arsenical encephalopathy (RAE). Curative trypanocidal therapy rapidly removed the infiltrating cells followed by a slow regression of astrocyte activation. It was concluded that the CNS pathological changes are the host's response to invasion by the parasite and that astrocytes played an important role in initiating these changes, possibly by producing inflammatory mediators such as cytokines. Chapter 4 demonstrates the response of cultured astrocytes, as judged by cytokine transcript expression, to in vitro stimulation with trypanosomes, whole trypanosome lysate and variable surface glycoprotein (VSG). Using reverse transcription, cDNA amplification by polymerase chain reaction, and gel electrophoresis, it was found that astrocytes responded by expressing transcripts for the cytokines IL-1alpha and -beta, IL-6, TNFalpha, MIP-1 and GM-CSF. Trypanosomes and whole trypanosome lysate were more potent stimuli than VSG in inducing expression of cytokine transcripts. To investigate translation of the gene transcripts into cytokine protein, immunocytochemistry using antimouse cytokine antibodies, was performed on astrocytes stimulated with trypanosome lysate in vitro. Cytokine protein for IL-1alpha, TNFalpha and IL-6, was detectable by 2 hours after exposure to trypanosome lysate. The cytokines shown to be expressed, in this study, are known to act in tandem and synergy to perform important inflammatory functions, including, leukocyte extravasation, migration, proliferation, adhesion-molecule and MHC-antigen expression, and cytokine production by inflammatory cells, microglia and endothelial cells. In addition, the same cytokines activate astrocytes in an autocrine manner, causing proliferation, MHC-antigen and adhesion-molecule expression. It is proposed that astrocytes respond to trypanosome invasion of the CNS by producing cytokines which recruit inflammatory cells into the subarachnoid and perivascular spaces, and the choroid fissure. The recruited inflammatory cells respond to these cytokines by proliferating, differentiating to antibody-producing plasma cells or to actively phagocytic cells, in the case of macrophages, as well as by producing cytokines. (Abstract shortened by ProQuest.).

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Adviser: Max Murray
• Keywords: Neurosciences, Pathology, Parasitology
• Date of Award: 1994
• Depositing User: Enlighten Team
• Unique ID: glathesis:1994-76298
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 19 Nov 2019 16:09
• Last Modified: 19 Nov 2019 16:09
• URI: https://theses.gla.ac.uk/id/eprint/76298