Jonathan D. Glass

Immunologic NO Synthase: Elevation in Severe AIDS Dementia and Induction by HIV-1 gp41

Indirect mechanisms are implicated in the pathogenesis of the dementia associated with human immunodeficiency virus-type 1 (HIV-1) infection. Proinflammatory molecules such as tumor necrosis factor α and eicosanoids are elevated in the central nervous system of patients with HIV-1-related dementia. Nitric oxide (NO) is a potential mediator of neuronal injury, because cytokines may activate the immunologic (type II) isoform of NO synthase (iNOS). The levels of iNOS in severe HIV-1-associated dementia coincided with increased expression of the HIV-1 coat protein gp41. Furthermore, gp41 induced iNOS in primary cultures of mixed rat neuronal and glial cells and killed neurons through a NO-dependent mechanism. Thus, gp41-induced NO formation may contribute to the severe cognitive dysfunction associated with HIV-1 infection.

Clinical‐neuropathologic correlation in HIV‐associated dementia

The structural abnormalities that correlate with the clinical manifestations of HIV-associated dementia (HIVD) are unclear. In a prospectively categorized group of patients with and without HIVD who were followed to autopsy, we correlated HIV-related neuropathologic changes with the presence and severity of HIVD. We also assessed the effect of antiretroviral therapy on the neuropathologic changes. Finally, using reverse transcriptase-poly-merase chain reaction on homogenized brain tissue, we correlated the relative expression of mRNA for tumor necrosis factor-α (TNF-α) with cognitive impairment and with the patterns of neuropathologic changes. The presence of multinucleated giant cells and diffuse myelin pallor were specific for HIVD, but these pathologic changes occurred in only 50% of patients with dementia. Patients treated with antiretroviral agents for >12 months were less likely to show multinucleated giant cells or diffuse myelin pallor. Levels of mRNA for TNF-α from frontal subcortical white matter were significantly greater in patients with HIVD than in AIDS patients without dementia or in seronegative controls. We conclude that routine histopathologic examination of the brain fails to detect multinucleated giant cells and diffuse myelin pallor in 50% of patients dying with HIVD. This suggests that more subtle neuropathologic correlates for the clinical manifestations of HIVD exist. Our observations of elevated levels of TNF-α mRNA in HIVD indicate that indirect mechanisms of brain dysfunction, such as abnormal cytokine expression, may contribute to the pathogenesis of HIVD.

Macrophage responses and myelin clearance during Wallerian degeneration: relevance to immune-mediated demyelination

Macrophages are important effector cells in immune-mediated demyelination. Current concepts regarding their entry and activation focus on the effects of T-cell-derived cytokines. This presentation describes the responses of macrophages and microglia to a non-inflammatory, non-immune injury, Wallerian degeneration. During Wallerian degeneration in the peripheral nervous system (PNS), macrophages are promptly and abundantly recruited from the circulation, and myelin clearance is prompt. In the central nervous system (CNS), the appearance of macrophages is markedly slower, and entry from the circulation is modest or absent. Myelin clearance is similarly delayed. The nature of the factors promoting macrophage entry and activation in Wallerian degeneration, and the bases for the differences between PNS and CNS, are relevant to current issues in immune-mediated demyelination.

Cellular localization of tumor necrosis factor mRNA in neurological tissue from HIV-infected patients by combined reverse transcriptase/polymerase chain reaction in situ hybridization and immunohistochemistry

HIV-induced neurological disease is postulated to be caused by indirect mechanisms. Tumor necrosis factor (TNF)alpha is increased in the brains in human immunodeficiency virus (HIV)-associated dementia and in the spinal cord in vacuolar myelopathy and may play a pathogenetic role in these diseases. Microglia, astrocytes and infiltrating macrophages can be induced to produce TNF alpha and each has been identified as a source of TNF alpha in neurological disease. Reverse transcriptase synthesis of cDNA and polymerase chain reaction amplification of the cDNA was combined with immunocytochemistry to identify the cellular source of TNF alpha in HIV-induced neurological disease. Cells positive for TNF alpha mRNA were more abundant in white matter than gray matter of the brain from demented individuals. TNF alpha mRNA-positive cells in brains and spinal cords were almost exclusively macrophage-lineage cells. Only rare TNF alpha mRNA-positive cells were astrocytes. We conclude that macrophage-lineage cells are the primary source of elevated central nervous system TNF alpha mRNA in providing further evidence that macrophage activation is an important element in the pathogenesis of HIV-associated neurological disease.

Variable progression of HIV‐associated dementia

A consecutive series of 71 patients diagnosed with HIV-associated dementia (HAD) (1984–1994) were studied to characterize the clinical course of HAD, and to identify predictive markers of rapid neurologic progression. Neurologic progression rate was determined from the change in the Memorial Sloan Kettering (MSK) dementia severity score from diagnosis to death. Those with the most rapid progression in neurologic disability were compared with those with slow or no progression. Autopsy material was immunostained for macrophage activation markers and gp41 in 30 individuals. Median survival was 3.3 months and 6.1 months for rapid-progression and no-progression patients, respectively. Rapid progression was associated with injection drug use but not with race, gender, or age. CD4+ cell counts were lower at diagnosis among rapid-progression than no-progression patients but no differences in AIDS-defining illnesses or patterns of antiretroviral therapy were found. At presentation, rapid-progression patients had more prominent symptoms of mental slowing than those with no progression; however, no other clinical features, CSF, or imaging features distinguished the groups. Less abundant macrophage activation in both basal ganglia and midfrontal gyrus regions, as judged by HAM56 immunostaining, was noted in 9 no-progression patients, compared with 12 rapid-progression patients. Neurologic progression and survival with HAD is highly variable. A significant proportion of individuals with dementia have prolonged survival of more than 12 months and remain cognitively stable. A history of injection drug use and presentation with prominent psychomotor slowing is associated with more rapid neurologic progression, and these patients tend to show more abundant macrophage activation within the CNS.

Prolonged survival of transected nerve fibres in C57BL/Ola mice is an intrinsic characteristic of the axon

SummaryTransected axons in C57BL/Ola mice survive for extraordinary lengths of time as compared to those of normal rodents. The biological difference in the substrain that confers the phenotype of prolonged axonal survival is unknown. Previous studies suggest the ‘defect’ to be a property of the nervous system itself, rather than one of haematogenous cells. Neuronal or non-neuronal elements could be responsible for this phenotype. This study was undertaken to determine whether Schwann cells, the most numerous of the non-neuronal cells intrinsic to the peripheral nerve, are responsible for delayed degeneration of transected axons. We created sciatic nerve chimeras by transplanting nerve segments between standard C57BL/6 and C57BL/Ola mice, allowing regeneration of host axons through the grafts containing donor Schwann cells. These nerves were then transected and the time course of axonal degeneration was observed. The results show that fast or slow degeneration is a property conferred by the host, and therefore cannot be ascribed to the Schwann cells. Similarly, transected C57BL/Ola axons in explanted dorsal root ganglia cultures survived longer than transected axons from standard mice. Taken together these results indicate that the responsible abnormality is intrinsic to the C57BL/Ola axon.

Cytomegalovirus encephalitis in acquired immunodeficiency syndrome (AIDS)

Cytomegalovirus encephalitis (CMVE) is frequently diagnosed only at postmortem because its specific clinical features have not been fully identified. We have described the clinical, radiologic, and laboratory features of CMVE in a retrospective review of 14 autopsy-confirmed cases of CMVE and compared them with a control group of demented acquired immunodeficiency syndrome (AIDS) patients without CMVE. CMVE was more common among homosexual men, and a subacute onset was more typical (mean duration of presenting symptoms was 3.5 weeks versus 18 weeks in demented controls). Median survival times were 4.6 weeks for CMVE and 28 weeks for controls. CMVE was accompanied by prominent systemic CMV infection at autopsy, including CMV adrenalitis (92%), CMV pneumonitis (42%), systemic Mycobacterium avium intracellulare (MAI; 58%), and CMV retinitis (58%). Hyponatremia and MAI bacteremia were found in 58% of CMVE cases. Polymerase chain reaction (PCR) of CSF samples identified CMV genome in 33% of CMVE cases. CMVE was associated with periventricular enhancement on CTs and periventricular lesions with meningeal enhancement on MRI scans. CMVE should be particularly suspected in homosexual men presenting with subacute encephalopathy who have had AIDS for more than 1 year and have a history of systemic CMV infection. Other features supporting the diagnosis of CMVE include periventricular lesions, hyponatremia, and identification of CMV genome in CSF by PCR.

Clinicopathologic correlations of HIV‐1‐associated vacuolar myelopathy An autopsy‐based case‐control study

To determine the clinical correlates of HIV-1-associated vacuolar myelopathy (VM), we designed a case-control study based on 215 AIDS autopsies in which we examined the spinal cord. We defined a case as an individual dying with AIDS and with VM present at autopsy; we defined a control as an individual dying with AIDS without VM. VM was found in 100 of 215 (46.5%) autopsies, with no apparent temporal trends. A higher number of AIDS-defining illnesses was strongly associated with the likelihood of VM (trend chi-square = 26.52, p < 0.001). Systemic infection with Mycobacterium avium-intracellulare and Pneumocystis carinii pneumonia were each associated with the pathologic findings of VM in both univariate and multivariate models. In the brain, multinucleated giant cells were detected in more cases than in controls (odds ratio = 3.68, 95% CI = 1.73 to 7.47, p < 0.001). The clinical features of HIV-1 dementia were not associated with VM; in contrast, predominantly sensory neuropathy was more common in VM cases than in controls (odds ratio = 5.00, 95% CI = 1.35 to 18.5, p < 0.05). Fifty-six cases with VM had detailed neurologic evaluations, but only 15 (26.8%) had signs and symptoms of myelopathy. The presence of symptomatic myelopathy was related to the pathologic severity: none of 17 cases with grade 1, five of 26 with grade 2, and 10 of 13 with grade 3 had clinical features of myelopathy (trend chi-square = 21.16, p < 0.005). VM is a common neuropathology finding that is frequently unrecognized during life. The association with the number of systemic illnesses, M avium-intracellulare infection, and P carinii pneumonia suggests that the development of VM is related to the severity of immunosuppression.

Neuronal density in the superior frontal and temporal gyri does not correlate with the degree of human immunodeficiency virus-associated dementia

Human immune deficiency virus (HIV) disease may be associated, neuropathologically, with significant neuronal loss and clinically with a severe dementia. However, the significance of neuronal loss in the development of dementia has not been established. In this study we have undertaken a stereological determination of the neuronal numerical density and neuronal volumes in post mortem tissue from the superior frontal and superior temporal gyri in 32 patients who died of acquired immune deficiency syndrome (AIDS). All were prospectively clinically characterized, with dementia identified or excluded, and antiretroviral medication documented. This study combines morphometric techniques with prospective clinical assessment of dementia. As previously demonstrated, all patients dying with AIDS showed neuronal loss, but this was not related to the presence of HIV-associated dementia.

Cytokine expression of macrophages in HIV‐1‐associated vacuolar myelopathy

Macrophages are frequently present within the periaxonal and intramyelinic vacuoles that are located primarily in the posterior and lateral funiculi of the thoracic spinal cord in HIV-associated vacuolar myelopathy. But the role of these macrophages in the formation of the vacuoles is unclear. One hypothesis is that cytokines, such as interleukin-1 (IL-1) and tumor necrosis factor (TNF)-α, are produced locally by macrophages and have toxic effects on myelin or oligodendrocytes. The resulting myelin damage eventually culminates in the removal of myelin by macrophages and vacuole formation. We studied thoracic spinal cord specimens taken at autopsy from HIV-positive (+) and HIV-negative individuals. The predominant mononuclear cells present in HIV+ spinal cords are macrophages. They are located primarily in the posterior and lateral funiculi regardless of the presence or absence of vacuolar myelopathy. Macrophages and microglia are more frequent in HIV+ than HIV-negative individuals and these cells frequently stain for class I and class II antigens, IL-1, and TNF-α. Activated macrophages positive for IL-1 and TNF-α are greatly increased in the posterior and lateral funiculi of HIV+ individuals with and without vacuolar myelopathy, suggesting they are present prior to the development of vacuoles. Cytokines, such as TNF-α, may be toxic for myelin or oligodendrocytes, leading to myelin damage and removal by macrophages and vacuole formation.