Differential Activation of Microglia After Experimental Spinal Cord Injury

Abstract

This study sought to experimentally clarify time-dependent, differential microglial activation at various spinal cord locations in response to injury. The spinal cords of Wistar rats were either sharply transected at the Th 11 or subjected to compression at the same site. Immediately to 4 weeks after injury, each spinal cord was fixed and cut into longitudinal frozen sections, and was immunostained with OX42 for resident and activated microglia, OX-6 for activated microglia, GFAP for activated astrocytes, and biotinylated BS-I, a lectin for both resident and activated microglia. From three to 24 hours after injury, we observed a narrow belt around the transection site in which OX42 positive microglia were dramatically reduced in number, or often absent. BS-I labeling of the zone disclosed the rapid transformation of those microglia possessing typical antler-like processes to macrophage-like cells. At day 1 and thereafter, the zone of reduced OX42 immunoreactivity was gradually replaced by macrophage-like OX42-positive round cells, and the lesion itself was ultimately capped by fibrogliotic scar tissue. By 2-4 weeks postinjury, another phase of microglial activation was observed in those white matter tracts undergoing Wallerian degeneration. These microglia characterized by the presence of newly-expressed MHC class II antigens. We posit that the decreased OX42 immunoreactivity suggests that CR3 is quickly saturated by activated iC3b and internalized, but not down-regulated. The trigger for this transformation most likely occurs through signaling by iC3b-saturated CR3. In contrast, microglia activation along those degenerating tracts undergoing Wallerian degeneration does not appear to be CR3-related, as the CR3 is upregulated. These observations indicate microglia have at least two different spatial and temporal patterns of activation. One is rapid and most likely involves the blood-borne complement activating system. The other accompanies Wallerian degeneration and is independent of the blood-borne complement system.

Activated Microglia Contribute to the Maintenance of Chronic Pain after Spinal Cord Injury

“…For now, the most conservative conclusion is that peripheral and central injuries produce unique microglial and astroglial pathologies that contribute in different ways to pain phenomena. Furthermore, it may be that central injury results in an even more complex microglial response than peripheral injury; in spinal cord transection and compression models, there are time-and location-dependent variations in microglial activation (Watanabe et al, 1999).…”

CD40 Expression by Microglial Cells Is Required for Their Completion of a Two-Step Activation Process during Central Nervous System Autoimmune Inflammation

“…Many studies, especially those that examined microglial cells in vitro, define activated microglial cells as cells that up-regulated MHC class II and costimulatory molecules (9). Histology based studies, investigating the role of microglial cells in various CNS pathologies, defined activated microglial cells as MHC class II-positive cells or as cells that up-regulated macrophage markers such as CD11b (27,28), while others defined microglial cell activation as a sequence of morphological changes from ramified to amoeboid forms (29), or as cells that proliferate in the CNS (30). Only a limited number of in vitro studies have suggested that microglial cell activation could be a multistep process (9), although little is known about the process of microglial cell activation in vivo during CNS pathologies.…”

Post-traumatic inflammation following spinal cord injury

“…60 Microglia activation and invasion of macrophages Resting microglia occupy approximately 13% of the entire glial cell population and are distributed diffusely throughout the CNS. 64 Microglia respond rapidly to disturbances within the microenvironment by change in morphology, expression of specific cell surface molecules, and release of cytokines such as interleukin-1 (IL-1), tumour necrosis factor alpha (TNF-a) and chemokines such as leucotrienes and prostaglandins. The term 'activated microglia' is used to describe proliferating nonphagocytic cells that demonstrate changes in their immunophenotype and their morphology, but have not undergone transformation into brain phagocytic macrophages.…”

“…The morphological diversity of microglia is indicative of functional heterogeneity. 2,36,64,65 For example, the concomitant increase in expression of major histocompatibility complex (MHC) antigen class II is only seen in a subpopulation of microglia with increased OX42 immunoreactivity. 7 The OX42 antibody recognizes complement CD 11 (C3b receptor) expressed by resting and activated resident microglia as well as by peripheral macrophages.…”

“…7 In the first 24 h following transection of the spinal cord, a narrow belt containing a dramatic reduction in the number of OX42-positive microglia has been described around the injury site. 64 Post-traumatic activation of microglia is evident at day one. However, the number of activated microglia increases during the first 7 days and then plateaus, 2-4 weeks postinjury, depending on the animal strain.…”