Tissue hypoxia in autoimmune brain disease: A Columbian discovery?

In most patients with freshly diagnosed multiple sclerosis (MS), neurological deficiencies wax and wane. Later the remissions become less complete and finally, in secondary progressive MS, disability progresses unrelentingly. One theory explains fluctuation of episodes by waves of incoming inflammatory cells that attack the central nervous system, ultimately resulting in destruction of neuronal axons and surrounding myelin sheaths. Tissue repair and neuronal compensation have a major role in long-term fluctuation, but they may not easily account for the brusque changes often noted in early MS. This is clearly the case in experimental models of brain autoimmunity. For example, in experimental autoimmune encephalomyelitis (EAE) of the Lewis rat, a T-cell– mediated, myelin-directed autoimmune disease, severe paralysis can develop within hours and remit completely after a few days. If the disease bouts are too short to allow substantial tissue regeneration, what are then the mechanisms of the ephemeral deficiencies? Inflammatory mediators, such as tumor necrosis factor a, nitric oxide, and free oxygen radicals, have been evoked commonly as short-term, transient factors curtailing neuronal function. Now, Davies et al present studies pointing to another mechanism, transient hypoxia. Davies et al used the dark agouti (DA) rat model of EAE, a good choice. DA rats immunized against myelin oligodendrocyte glycoprotein develop disease, which by its relapsing course and its demyelinating lesions recapitulates human MS more faithfully than other EAE variants. In this model, the hypoxic status of affected tissues, essentially the lower spinal cord, was monitored by a graded set of assays. Most directly, the investigators documented hypoxia by needle electrodes inserted into tissue milieus to sample O2 pressure. Then they used pimonidazole, a derivative of an antibiotic toxic to anaerobic microbes, which now is an accepted marker of tissue hypoxia often applied to probe tumor microenvironments. The compound enters cells, where, dependent on low O2 pressure, it is reduced by cellular nitrogen reductases and then firmly binds to intracellular structures. As a third, complementary marker, the study chose hypoxia-inducible factor 1a (HIF-1a), a transcription factor acting as a key metabolic sensor. It accumulates in cells following hypoxia, or upon other signals including inflammatory mediators such as interleukin 6. The data provide undisputable evidence of hypoxic changes in the spinal cord white matter, and, importantly, pimonidazole staining correlates with disease severity. This may not be overly surprising, as it is well known that hypoxialike changes may be produced by inflammation-related effects involving diverse factors including proinflammatory mediators, or edema. In MS, inflammation creates microenvironmental changes, reminiscent of hypoxia. This is impressively borne out in the present studies, which show that markers of hypoxia are enhanced within and directly around inflammatory spinal cord infiltrates. However, there is another striking finding. Although hypoxia is substantial in the inflamed white matter, it is even higher in the gray matter of spinal cord cross-sections. Immunocytochemistry shows large-scale inflammatory activation of microglia/macrophages throughout the entire white matter tissue, infiltrated or not. However, staining for pimonidazole is strikingly complementary (Fig. 1). Whereas the white matter of the noninfiltrated tissues appears grossly unaffected, the adjacent gray matter parts of the same segments stain evenly positive. Although just sparsely infiltrated, they stain heavily with pimonidazole, suffer from reduced O2 pressure (confirmed by oxygen probes inserted into the gray matter), and contain hypoxic neuronal bodies with increased HIF-1a content. There is a second amazing observation. Reasoning that if hypoxia contributes to acute neurological deficits, overexposure to an O2 atmosphere should reverse disease, the researchers exposed animals with EAE to normobaric oxygen. The results were compelling. Even short-term exposure to raised O2 significantly improved clinical EAE. This effect was efficient and ultrafast, acting within an hour. This observation strengthens the hypoxia hypotheses, but it also recalls old, mostly ill-fated attempts to treat MS in pressure chambers. Should these now be resumed? Davies and colleagues withhold a clear recommendation. The authors have focused on spinal cord white matter as the site of action. This is quite reasonable; their experimental model is primarily a myelin-targeted autoimmune disease with inflammatory lesions spread mostly throughout the spinal cord white matter, and also in MS