[A process of programmed cell death as a mechanisms of neuronal death in prion diseases]

Neuronal loss is a salient feature of prion diseases; however, its cause and mechanism, particularly its relationship with the accumulation of the pathogenic, protease resistant isoform PrPres of the cellular prion protein PrPc, are still unclear. A number of studies suggest that it could occur through a process of programmed cell death which is consistent with the lack of inflammation in these conditions. In this paper, we review the different techniques used to identify apoptosis of neurons, and analyse the studies demonstrating neuronal apoptosis in prion diseases, either experimentally, in animal or in human. Apoptosis of rat hippocampal neurons, in cultures exposed to a synthetic peptide homologous to the prion protein, has been identified on morphological criteria after staining by a fluorescent marker of DNA and by gel electrophoresis of neuronal DNA. Apoptosis of neurons has also been identified in vivo using in situ end labelling and electron microscopy in scrapie infected mice. In human, apoptotic neurons were identified by in situ end labelling in Creutzfeldt-Jakob Disease and in Fatal Familial Insomnia. Apoptotic neurons were mostly found in damaged regions and their presence and abundance seemed to correlate closely with neuronal loss. Neuronal apoptosis also correlated well with microglial activation as demonstrated by the expression of major histocompatibility complex class II, antigens, and with axonal damage as identified by beta-amyloid protein precursor immunostaining. In contrast, there was no clear correlation between the topography and severity of neuronal apoptosis and the type, topography and abundance of prion protein deposits as demonstrated by immunohistochemistry. Similarly, within the framework of comparable phenotypes, there was no difference in the abundance and distribution of apoptotic neurons according to the aetiology whether sporadic, familial, or iatrogenic, of the disease. The pathogenetic mechanism of neuronal apoptosis remains speculative and several hypothesis have been proposed. The lack of a direct association between neuronal damage and PrPres deposition may support models of neuropathogenesis based on "loss of function" of PrPc, such as withdrawal of defined activation signals inducing programmed cell death, rather than neurotoxicity. It is also possible that PrPres is neurotoxic and the dissociation between neuronal damage and the amount of protein only reflects variations in selective neuronal vulnerability. Finally, neuronal apoptosis might be an indirect consequence of PrPres deposition. PrPres-induced dendritic or axonal damage, perhaps enhanced by consequent microglial activation, might contribute to neuronal apoptosis either due to deafferentation or to retrograde neuronal degeneration.

Neuronal apoptosis does not correlate with dementia in HIV infection but is related to microglial activation and axonal damage

To characterize the distribution of apoptotic neurons and their relationships with the stage of disease, a history of HIV-dementia, and the degree of productive HIV infection, microglial activation and axonal damage, we examined the brains of 40 patients. Samples of frontal and temporal cortex, basal ganglia and brain stem were taken post-mortem from 20 patients with AIDS (including three with HIV-dementia, and eight with cognitive disorders that did not fulfil the criteria for HIV-dementia), 10 HIV-positive asymptomatic cases and 10 seronegative controls. Neuronal apoptosis was demonstrated by in situ end labelling in 18 AIDS cases and two pre-AIDS cases; a single apoptotic neuron was present in the temporal cortex of a control. Semiquantitative evaluation showed that the severity of neuronal apoptosis in the cerebral cortex correlated with the presence of cerebral atrophy, but not with a history of HIV dementia. There was no global quantitative correlation between neuronal apoptosis and HIV encephalitis or microglial activation. However, there was some topographical correlation between these changes. In the basal ganglia, apoptotic neurons were much more abundant in the vicinity of multinucleated giant cells and/or p24 expressing cells. Microglial activation was constantly present in these areas. Axonal damage was identified using beta-amyloid-precursor protein (betaAPP) immunostaining in 17 AIDS and eight pre-AIDS brains. Although no global quantitative correlation could be established between axonal damage and neuronal apoptosis there was an obvious topographic correlation supporting the view that axonal damage, either secondary to local microglial activation or due to the intervention of systemic factors, may also contribute to neuronal apoptosis.

Acute, relapsing brain oedema with diffuse blood-brain barrier alteration and axonal damage in the acquired immunodeficiency syndrome

A 38-year-old homosexual male with AIDS suffered four neurological episodes including headaches, confusion, visual impairment, memory disturbances, and dysarthria which resolved spontaneously in a few days. He was admitted to hospital during a fifth episode. Neurological examination revealed a cerebellar syndrome. General examination was normal. CD4 count was 90. CSF contained two WBCs/mm(3) and 12.30 mg/dL protein. MRI revealed diffuse ill defined increased signal on T2-weighted images in the white matter. His condition worsened rapidly with vomiting and he died 1 month after admission. Neuropathological examination revealed diffuse brain oedema with ventricular compression, central diencephalic herniation and bilateral tonsilar herniation in the absence of a focal lesion. Microscopical examination revealed predominant involvement of the white matter with diffuse myelin pallor and massive perivascular dilatation containing an exudate expressing serum proteins and occasional macrophages. The same exudate was also diffuse in the leptomeninges. Parenchymal damage predominated around the perivascular spaces and included loosening of tissue, axonal damage with spheroids and reactive astrocytosis. There was no evidence of productive HIV encephalitis, no multinucleated giant cells; p24 immunostaining and RT-PCR for HIV genome were negative. There was neither significant inflammation nor microglial activation. In this illustrative case, the relapsing course of the neurological signs, the diffuse topography of the blood-brain barrier breakdown and the absence of local cause make it likely that the diffuse leak and axonal damage could be related to circulating factors.

[Central nervous system lesions in the early stages of HIV infection]

Early HIV-1 invasion of the central nervous system has been demonstrated by many cerebrospinal fluid studies; however, most HIV-1 carriers remain neurologically unimpaired during the so-called "asymptomatic" period lasting from seroconversion to symptomatic AIDS. Therefore, very few neuropathological studies have been conducted in the early pre-AIDS stages, and the natural history of central nervous system changes in HIV-1 infection remains poorly understood. Examination of brains of asymptomatic HIV-1 positive individuals who died accidentally and of rare cases with acute fatal encephalopathy revealing HIV infection, and comparison with experimental simian immunodeficiency virus and feline immunodeficiency virus infections suggest that, invasion of the CNS by HIV-1 occurs at the time of primary infection and induces an immunological process in the central nervous system. This includes an inflammatory T-cell reaction with vasculitis and leptomeningitis, and immune activation of brain parenchyma with increased number of microglial cells, upregulation of major histocompatibility complex class II antigens and local production of cytokines. Myelin pallor and gliosis of the white matter are usually found and are likely to be the consequence of opening of the blood-brain barrier due to vasculitis; direct damage to oligodendrocytes by cytokines may also be involved. These white matter changes may explain, at least partly, the early cerebral atrophy observed, by magnetic resonance imaging, in asymptomatic HIV-1 carriers. In contrast, cortical damage seems to be a late event in the course of HIV-1 infection. There is no significant neuronal loss at the early stages of the disease, no accompanying increase in glial fibrillary acid protein staining in the cortex, and only exceptional neuronal apoptosis. Although HIV-1 proviral DNA may be demonstrated in a number of brains, viral replication remains very low during the asymptomatic stage of HIV-1 infection. This makes it likely that, although opening of the blood brain barrier may facilitate viral entry into the brain, specific immune responses including both neutralising antibodies and cytotoxic T-lymphocytes, continuously inhibit viral replication at this stage.

[Neuronal apoptosis in the central and peripheral nervous system in HIV infection]

Apart from the unique changes characteristic of "HIV encephalitis", the productive infection of central nervous system by HIV, which predominantly involves the white matter and basal ganglia, evidence is accumulating that the cerebral cortex may also be affected in AIDS patients. Neuronal loss, suspected at microscopic examination, has been demonstrated by a number of morphometric studies. However, the cause and mechanism of neuronal damage in HIV infection, are still unclear. In an attempt to look for an apoptotic process at the origin of neuronal loss in AIDS, we examined samples of frontal cortex, temporal cortex and basal ganglia from 12 patients who died from AIDS and 4 asymptomatic HIV-positive cases using in situ end labelling to demonstrate characteristic DNA fragmentation. These were compared with 5 asymptomatic seronegative controls, and 2 seronegative patients with Alzheimer's disease. We demonstrated neuronal apoptosis in all AIDS cases and in the Alzheimer's cases. Positive in situ end labelling was usually associated with morphological changes suggestive of neuronal apoptosis. Semiquantitative assessment of the density of apoptotic neurons showed that neuronal apoptosis was more severe in atrophic brains. In contrast, no correlation was found between the density of apoptotic neurons and the presence of HIV-encephalitis or a history of cognitive disorder. Only occasional apoptotic neurons were found in one asymptomatic, HIV-positive case. Apoptosis was never observed in asymptomatic seronegative cases. We also looked for apoptotic neurons in spinal ganglia of 20 AIDS cases, 5 of whom had a terminal sensory distal neuropathy, and 10 seronegative controls devoid of neuropathy. Apoptotic neurons were found in 6 of the AIDS patients and in none of the seronegative controls. However, no correlation was found between the severity of neuronal apoptosis in the spinal root ganglia and the presence of absence of a terminal distal sensory neuropathy. Experimental studies tend to support our in vivo findings. HIV-infection of primary cultures of human embryonic central nervous system induced frequent apoptosis of neurons. No apoptotic cell was identified in non infected control cultures.

[Sequences of human herpes virus type 8 and Epstein-Barr virus in AIDS-related primary central nervous system non-Hodgkin's lymphoma]

Presence of human herpes virus type 8 (HHV8), detected by nested PCR, and expression of Epstein-Barr virus (EBV), as assessed by immunochemistry and in situ hybridization, were evaluated in 20 primary non-Hodgkin immunoblastic lymphomas (NHL) of the central nervous system (CNS) from patients who died from AIDS, and in 10 samples of cerebral tissues from patients who died from AIDS, without cerebral lymphoma or Kaposi's sarcoma, as controls. Six lymphomas (30%) contained HHV8 sequences, and 19 (95%) expressed EBV; detection of HHV8 was more frequent in patients with Kaposi's sarcoma, than in other subjects (4/6 versus 2/14). Three (30%) controls contained HHV8 sequences, whereas none expressed EBV. AIDS-related CNS NHL is therefore clearly associated with EBV expression, while the presence of HHV8 appears occasional, probably associated with a low tissue viral load. The high frequency of HHV8 in AIDS-related primary CNS NHL patients with Kaposi's sarcoma suggests that this virus could play a role in the pathogenesis of some cerebral lymphomas.

[Neuronal apoptosis in the course of human immunodeficiency virus infection]

Apart from the unique changes characteristic of "HIV encephalitis", the productive infection of central nervous system by HIV, which involves predominantly the white matter and basal ganglia, evidence is accumulating that the cerebral cortex may also be affected in AIDS patients. Neuronal loss, suspected at microscopical examination, has been demonstrated by a number of morphometric studies. However, the cause and mechanism of neuronal damage in HIV infection, are still unclear. In an attempt to look for an apoptotic process at the origin of neuronal loss in AIDS, we examined samples of frontal cortex, temporal cortex and basal ganglia from 12 patients who died from AIDS and 4 HIV-positive asymptomatic cases using in situ end labelling to demonstrate characteristic DNA fragmentation. These were compared with 5 seronegative asymptomatic controls, and 2 seronegative patients with Alzheimer's disease. We demonstrated neuronal apoptosis in all the AIDS cases and in the Alzheimer's cases. Positive in situ end labelling was usually associated with morphological changes suggestive of neuronal apoptosis. Semiquantitative appreciation of the density of apoptotic neurons showed that neuronal apoptosis was more severe in atrophic brains. In contrast, no correlation was found between the density of apoptotic neurons and the presence of HIV encephalitis or a history of cognitive disorder. Only occasional apoptotic neurons were found in one asymptomatic, HIV-positive case. Apoptosis was never observed in asymptomatic seronegative cases. Experimental studies tend to support our in vivo findings. Infection by HIV of primary cultures of human embryonic central nervous system induced frequent apoptosis of neurons. No apoptotic cell was identified in non infected control cultures.

Rabies encephalitis in a patient with AIDS: a clinicopathological study

A 46-year-old man was bitten by a dog in Mali; anti-rabies vaccination was incomplete. Three months later he was admitted to hospital with fever and diarrhea. Human immunodeficiency virus (HIV) serology was positive and CD4 count was 70/mm3. His status worsened rapidly with confusion hydrophobia and hypersialorrhea. Despite anti-rabies serotherapy and vaccination, he died suddenly 12 days after admission. Immunofluorescence on cerebral tissue samples established rabies encephalitis. Neuropathology showed mild encephalitis with occasional Babès nodules and rare perivascular mononuclear cuffs. Intraneuronal Negri inclusion bodies were remarkably diffuse and abundant. They were clearly demonstrated by immunocytochemistry and electron microscopy. Apoptotic neurons were identified in the brain stem and hippocampus in the vicinity of inflammatory foci. In contrast, apoptosis could not be demonstrated in non-inflammatory areas, even where Negri bodies were numerous. There was no associated HIV encephalitis or opportunistic infection. The occurrence of rabies encephalitis in AIDS represents a random association, but is probably not exceptional as rabies is endemic in many countries and the AIDS epidemic is spreading worldwide. In this case, although the incubation duration and clinical presentation were comparable to those in classical rabies the T-cell-mediated immunosuppression may account for the weak inflammatory reaction and unusually abundant viral multiplication. This observation confirms that all those at risk for rabies, particularly immunocompromised patients, should receive complete anti-rabies treatment including vaccines and specific immunoglobulins, as soon as possible after infection.

Herpes simplex virus type 1 encephalitis in acquired immunodeficiency syndrome

Herpes simplex (HSV) infection of the central nervous system is uncommon in AIDS and usually has an atypical topography. This review is centred around the case of a 49-year-old homosexual patient with AIDS who died from diffuse encephalopathy. Neuropathological examination revealed necrotic and haemorrhagic changes involving both temporal lobes, insulae and cingulate gyri. Cowdry type A intranuclear inclusion bodies were abundant but inflammation was minimal. Electron microscopy confirmed characteristic herpes virus particles. Immunocyto-chemistry was positive for HSV type 1 and 2. In situ hybridization and PCR, however, were positive for HSV type 1 but excluded HSV type 2. There was associated cytomegalovirus ventriculitis but clearly separated from HSV encephalitis. There were no histological features of HIV encephalitis and HIV could not be demonstrated by immunocytochemistry or by PCR to demonstrate proviral DNA. Apoptotic neurons were numerous in areas with a severe macrophage reaction. Only two pathological cases with characteristic limbic distribution and necrotic haemorrhagic histologic have been reported previously. The rarity of these reports suggests that in advanced AIDS, the immune reaction causing a typical necrotizing encephalitis cannot be mounted. Distinction between HSV type 1 and 2 infection may be difficult by immunocytochemistry and usually requires in situ hybridization, tissue culture or PCR. In AIDS patients, HSV-1 has been identified as responsible for encephalitis whereas HSV-2 has been more responsible for myelitis. Associated productive HIV infection of the CNS was found in none of the cases. In contrast, cytomegalovirus encephalitis was found in nine of 11 cases of AIDS-associated HSV encephalitis.

Neuropathology of early HIV-1 infection

Early HIV-1 invasion of the central nervous system has been demonstrated by many cerebrospinal fluid studies; however, most HIV-1 carriers remain neurologically unimpaired during the so called "asymptomatic" period lasting from seroconversion to symptomatic AIDS. Therefore, neuropathological studies in the early pre-AIDS stages are very few, and the natural history of central nervous system changes in HIV-1 infection remains poorly understood. Examination of brains of asymptomatic HIV-1 positive individuals who died accidentally and of rare cases with acute fatal encephalopathy revealing HIV infection, and comparison with experimental simian immunodeficiency virus and feline immunodeficiency virus infections suggest that, invasion of the CNS by HIV-1 occurs at the time of primary infection and induces an immunological process in the central nervous system. This includes an inflammatory T-cell reaction with vasculitis and leptomeningitis, and immune activation of brain parenchyma with increased number of microglial cells, upregulation of major histocompatibility complex class II antigens and local production of cytokines. Myelin pallor and gliosis of the white matter are usually found and are likely to be the consequence of opening of the blood brain barrier due to vasculitis; direct damage to oligodendrocytes by cytokines may also interfere. These white matter changes may explain, at least partly, the early cerebral atrophy observed, by magnetic resonance imaging, in asymptomatic HIV-1 carriers. In contrast, cortical damage seems to be a late event in the course of HIV-1 infection. There is no significant neuronal loss at the early stages of the disease, no accompanying increase in glial fibrillary acid protein staining in the cortex, and only exceptional neuronal apoptosis. Although HIV-1 proviral DNA may be demonstrated in a number of brains, viral replication remains very low during the asymptomatic stage of HIV-1 infection. This makes it likely that, although opening of the blood brain barrier may facilitate viral entry into the brain, specific immune responses including both neutralising antibodies and cytotoxic T-lymphocytes, continuously inhibits viral replication at that stage.