Detection of JC virus by anti-VP1 immunohistochemistry in brains with progressive multifocal leukoencephalopathy

We have assessed the diagnostic efficacy of a novel polyclonal rabbit antiserum directed to the recombinant major capsid protein VP1 of JC virus (JCV). Immunohistochemistry for VP1 was compared to non-radioactive JCV DNA in situ hybridization (ISH) in ten cases of progressive multifocal leukoencephalopathy (PML). Tissue sections from postmortem brains were studied from PML patients suffering from immunodeficient conditions of various causes: immunodeficiency syndrome (AIDS, n = 7), severe combined immune deficiency due to adenosine deaminase deficiency (n = 1), sarcoidosis (n = 1) and leukemia (n = 1). VP1 immunohistochemistry demonstrated the presence of JCV in lesional oligodendrocytes of all PML patients, whereas ISH was able to detect JCV in nine out of ten cases. We conclude that VP1 immunohistochemistry is a specific, sensitive and rapid method for confirming the diagnosis of PML. Received: 11 September 1996 / Revised, accepted: 12 March 1997     

Genotypes of JC virus,DNA of cytomegalovirus,and proviral DNA of human immunodeficiency virus in eyes of acquired immunodeficiency syndrome patients

JC virus (JCV) is a human polyomavirus that exists in at least eight different genotypes as a result of coevolution with different human populations all over the world. Well adapted to its host, it usually persists in the kidneys and possibly the brain. If the host becomes immunodeficient, JCV can cause the fatal demyelinating disease progressive multifocal leukoencephalopathy (PML). There is increasing evidence that JCV is transactivated by cytomegalovirus (CMV) and the human immunodeficiency virus (HIV). Both CMV and HIV can infect the retina of acquired immunodeficiency syndrome (AIDS) patients, causing severe necrosis in the case of CMV retinitis or a mild HIV-associated vasculopathy, with bleeding and cotton wool spots. The authors therefore investigated by polymerase chain reaction (PCR) whether DNA of these three viruses was detectable in paraffin-embedded eyes of AIDS patients with a clinical history of CMV retinitis. From a total of 65 eyes, JCV was detected in 21 (32%). Thirty-six (55%) were positive for CMV and 6 (9%) for proviral DNA of HIV. JCV and CMV were found in 13 eyes, JCV and HIV in 3 eyes, CMV and HIV in 1 eye, and DNA from all three viruses in 1 eye. The JCV genotypes were types 1A, 2A, 2E, 3, and 4. In 21 eyes of patients without AIDS, only one sample was JCV positive. In conclusion, JCV DNA can be detected in ocular tissue of AIDS patients at a significantly higher level than in eyes of nonimmunosuppressed patients. Further investigations will help to decide if JCV contributes to the retinopathy caused by CMV and HIV.     

Reappraisal of progressive multifocal leukoencephalopathy due to simian virus 40

Several cases of progressive multifocal leukoencephalopathy (PML) have been associated with simian virus 40 (SV40), rather than with JC virus (JCV), the polyomavirus originally isolated from PML tissue. PML has, therefore, been defined as a demyelinating syndrome with possible multiple viral etiologies. Tissues from three of the cases thought to be associated with SV40 were available for reexamination. Monoclonal antibodies specific for SV40 capsid antigen VP1, virus-specific biotinylated DNA probes for in situ hybridization, and virus-specific primers in the polymerase chain reaction (PCR) were used. Macaque PML brain served as a positive control tissue for SV40 brain infection. Monoclonal antibodies to SV40 VP1 failed to recognize viral antigen in lesions from all three human PML cases. The biotinylated DNA probe, which reacted with SV40 in macaque PML, failed to detect SV40 in human PML. However, JCV could be detected by in situ hybridization with a JCV-specific DNA probe. Moreover, JCV DNA sequences were amplified by PCR from the human PML tissues, whereas SV40 DNA sequences were amplified only from the macaque brain. Thus, we could not confirm the original reports that the demyelinating agent in these three cases of PML was SV40, rather than JCV. We conclude that SV40 infection of the central nervous system need not be ruled out in the differential diagnosis of PML. Received: 1 December 1997 / Revised, accepted: 23 February 1998     

Detection of JC virus DNA in peripheral lymphocytes from patients with and without progressive multifocal leukoencephalopathy.

Progressive multifocal leukoencephalopathy (PML) results from lytic infection of oligodendrocytes by JC virus (JCV). Although JCV has been identified in mononuclear cells in bone marrow and hematogenous dissemination of the virus to the central nervous system has been suspected, JCV has never been clearly demonstrated in the peripheral circulation. Using polymerase chain reaction technology, we examined peripheral lymphocytes of 19 patients with brain biopsy-proven PML for the JCV genome. Two non-PML control groups, consisting of 26 patients seopositive for human immunodeficiency virus type 1 (HIV-1) and 30 immunocompetent patients with Parkinson's disease, were also examined for the presence of the JCV genome in lymphocytes. Cerebrospinal fluid from 10 patients with PML was examined for the presence of the JCV genome as well. The JCV genome was detected in the lymphocytes of 89% (17) of the patients with PML, 38% (10) of the HIV-1-seropositive patients without PML, and none of the patients with Parkinson's disease. Sequencing of the JCV regulatory region from the lymphocytes of three patients revealed the prototype MAD-1 strain of JCV in one patient with PML, a MAD-4 strain in a second patient with PML, and a slightly modified MAD-4 strain in an HIV-1-positive patient without PML. Only 3 of 10 patients with PML who had JCV detected in lymphocytes had the JCV genome in their cerebrospinal fluid. These results demonstrate that the JCV genome can be found in circulating lymphocytes from patients with PML and suggest that lymphocytes are an important vector for hematogenous dissemination of JCV to the central nervous system.(ABSTRACT TRUNCATED AT 250 WORDS)     

JCV detection in multiple sclerosis patients treated with natalizumab

Natalizumab therapy is associated with an increased risk of progressive multifocal leukoencephalopathy (PML). Because the prognosis of established PML is uniformly dismal, identification of highly susceptible patients to the disease may improve outcomes. We wanted to investigate whether serial plasma and cerebrospinal fluid (CSF) screening for polyomavirus would identify patients with laboratory evidence of viral infection prior to the development of clinical PML. Two hundred MS patients had pre-treatment CSF/plasma screening for JC virus (JCV) and BK virus (BKV) DNA, and thereafter every six treatments of natalizumab. In all positive patients treatment is stopped (due to potential risk of PML), they have follow-up clinical examinations and plasma/CSF JCV/BKV tests until all evaluations are normal. No patient developed clinical evidence of PML. Eight of the 200 patients had detectable JCV or BKV DNA. Five patients were positive for BKV DNA in the CSF and three patients were positive for JCV DNA (one in plasma, two in CSF). After cessation of natalizumab treatment, all patients converted to undetectable viral DNA. Screening for JCV in CSF in natalizumab-treated patients could help identify those at heightened risk for developing PML and discontinuing treatment in these patients may abort development of the clinical illness.     

Diagnosis of progressive multifocal leukoencephalopathy by brain biopsy with biotin labeled DNA:DNA in situ hybridization

DNA:DNA in situ hybridization using a cloned JC virus (JCV) DNA probe labeled with biotin confirmed the presence of JCV DNA in formalin-fixed, paraffin-embedded brain biopsies from four cases of progressive multifocal leukoencephalopathy (PML). Only small pieces of tissue were available in each case. Detection of the JC DNA:DNA hybrids was carried out by affinity cytochemistry. JCV DNA was identified predominantly in the nuclei of interfascicular oligodendrocytes in demyelinated areas of the biopsies. JC virus was isolated from one case, and the diagnosis of PML was substantiated in all cases by electron microscopic identification or immunocytochemical labeling of JC viral antigen. In situ hybridization using a biotin labeled JCV DNA probe is a specific, sensitive and convenient method for confirming the diagnosis of PML in suspected cases evaluated by brain biopsy.     

Relation of JC virus DNA in the cerebrospinal fluid to survival in acquired immunodeficiency syndrome patients with biopsy-proven progressive multifocal leukoencephalopathy.

The detection and semiquantitation of JC virus (JCV) DNA in cerebrospinal fluid (CSF) is prognostic of survival and is a marker of the course of progressive multifocal leukoencephalopathy (PML). CSF samples from 15 acquired immunodeficiency syndrome (AIDS) patients with biopsy-proven PML were analyzed by semiquantitative polymerase chain reaction (PCR). A low JCV burden was predictive of longer survival compared with a high JCV burden (median survival from entry, 24 [2-63] vs 7.6 [4-17] weeks). Further analyses indicated a possible threshold of 50 to 100 copies/microl separating high- and moderate-risk cases. Patients with a JCV load below this level survived longer than those with a JCV load above it.     

High sensitivity detection of JC-virus DNA in postmortem brain tissue by in situ PCR

Opportunistic infection of the central nervous system by human polyomavirus JC can cause a devastating disease, progressive multifocal leukoencephalopathy (PML). To gain new neuropathological insights into JC-virus (JCV) infection patterns in PML at the light microscopic level, the highly sensitive indirect in situ polymerase chain reaction (in situ PCR) was employed in up to 15-year old formalin-fixed and paraffin-embedded postmortem brain tissue derived from nine AIDS patients with PML. In situ PCR, in which target DNA is amplified intracellularly and detected by a specific labelled probe in morphologically intact tissue, was compared with conventional in situ hybridization (ISH). Validity was ensured by the inclusion of 13 controls. JCV detection with in situ PCR proved to be highly sensitive since in all nine brain samples the number of positive cells exceeded the ISH results by 2-3-fold. Whereas by routine staining the brain tissue of each individual patient showed regions with severe, mild or no involvement by PML, improved detection of JCV DNA by in situ PCR allowed a regrading into five different degrees of JCV infection. Significant myelin staining was observed, suggesting that cell-to-cell contact may not be the only means of virus spread but that new cells could also be infected by virus released after cell lysis. Furthermore, using in situ PCR hitherto unreported intracellular distribution patterns of JCV DNA in oligodendro- and astrocytes were observed by light microscopy.     

An siRNA against JC virus (JCV) agnoprotein inhibits JCV infection in JCV‐producing cells inoculated in nude mice

JC virus (JCV) is the etiological agent of the demyelinating disease progressive multifocal leukoencephalopathy (PML). Because JCV has a very narrow host range, it has been difficult to develop an animal model of JCV infection; as a result, no effective therapy for PML has been established. In this study, we have tried to create an animal model that replaces an in vivo JCV infection. As a result, we have obtained a stable persistence of JCV‐infected human cells in the mouse brain by inoculating the virus‐infected cells into the nude mice brains. In this model, the JCV‐infected cells were well preserved in the nude mouse brains for 2 weeks. We then treated JCV‐injected brains with an siRNA against the JCV agnoprotein that is known to be an effective inhibitor of JCV infection in vitro. A highly purified type I collagen, atelocollagen, was used as a carrier for the siRNA. The siRNA inhibited the expression of JCV protein in inoculated JCV‐infected cells in the mouse brain, compared to the medium containing only atelocollagen used as a placebo. Thus, the combination of siRNA and atelocollagen might be a candidate therapeutic agent for the treatment of JCV infection.     

Analysis of cerebrospinal fluid and cerebrospinal fluid cells from patients with multiple sclerosis for detection of JC virus DNA

OBJECTIVE: 1) To determine whether JC virus (JCV) DNA was present in the cerebrospinal fluid (CSF) and blood from patients with multiple sclerosis (MS) in comparison with controls and 2) to find out if our clinical material, based on presence of JCV DNA, included any patient at risk for progressive multifocal leukoencephalopathy (PML). METHODS: The prevalence of JCV DNA was analyzed in CSF and plasma from 217 patients with MS, 86 patients with clinically isolated syndrome (CIS), and 212 patients with other neurological diseases (OND). In addition, we analyzed CSF cells, the first report of JCV DNA in CSF cells in a single sample, and peripheral blood cells in a subgroup of MS (n = 49), CIS (n = 14) and OND (n = 53). RESULTS: A low copy number of JCV DNA was detected in one MS cell free CSF sample and in one MS CSF cell samples. None of these had any signs of PML or developed this disease during follow-up. In addition, two OND plasma samples were JCV DNA positive, whereas all the other samples had no detectable virus. CONCLUSION: A low copy number of JCV DNA may occasionally be observed both in MS and other diseases and may occur as part of the normal biology of JC virus in humans. This study does not support the hypothesis that patients with MS would be at increased risk to develop PML, and consequently screening of CSF as a measurable risk for PML is not useful.     

Rarity of JC virus DNA sequences and early proteins in human gliomas and medulloblastomas: the controversial role of JC virus in human neurooncogenesis

JC virus (JCV), the agent of progressive multifocal leucoencephalopathy (PML), exerts an oncogenic effect in several laboratory animal models. Moreover, JCV genomic DNA and early viral protein T-antigen have been detected in various types of human central nervous system (CNS) neoplasms. To further explore this association we have studied paraffin-embedded brain biopsy tissue from 60 neoplasms (55 gliomas and five medulloblastomas) and 15 reactive gliosis cases for the presence of JCV DNA sequences and proteins. Four post mortem cases of HIV-associated PML were used as positive controls. Samples were assessed by polymerase chain reaction (PCR) amplification of early (large T antigen) and late (virion protein 3) sequences and immunohistochemistry (IHC) with both PAb 2024 and anti-SV40 large T antigen monoclonal antibodies. Five cases (three neoplasms and two reactive gliosis instances) showed low viral DNA levels when PCR-tested for VP3 or large T, while no case was immunoreactive for any of the two antibodies used. The four PML cases yielded positive results with both PCR and IHC. Additionally, IHC with both antibodies was applied to a tissue micro-array including 109 CNS tumours and 21 reactive gliosis samples. No immunoreactivity was detected in any of these tissue micro-array samples. The rarity of JCV DNA sequences and early proteins in our brain tumours enriches the controversy over the role of JCV in human neurooncogenesis, whose clarification is in need of further molecular and epidemiologic studies.     

Progressive multifocal leukoencephalopathy (PML) in AIDS and in the pre-AIDS era

Summary Twenty-five brains with definite, and three brains with possible, progressive multifocal leukoencephalopathy (PML), including six brains of AIDS patients, were studied with special regard to the detection of papovaviruses. Formalin-fixed serial paraffin sections were immunostained with monospecific anti-JC virus (JCV) and genus-specific anti-simian virus (SV) 40 antisera, and hybridized in situ with DNA probes for JCV and SV 40, respectively. Immunocytochemistry (ICC) and in situ hybridization (ISH) were similarly sensitive in detecting virus in classical PML lesions. In all but one definite PML cases at least one method detected virus (96%). Possible PML tissue was never labeled. Labeling patterns were generally similar in ICC and ISH: mainly oligodendroglia and, less frequently, astroglia harbored virus, whereas labeling of neurons and endothelia was absent. Bizarre giant astrocytes were occasionally labeled by ICC and ISH. Burnt-out lesions harbored JCV DNA but not virus antigens. SV 40 DNA was never detectable. PML morphology in AIDS cases did not usually differ from the disease process seen in the pre-AIDS era. However, two AIDS brains presented extremely extended and, in one case, unusually necrotizing PML damage; in the latter case, PML lesions contained large amounts not only of JCV, but also of human immunodeficiency virus (HIV) antigens. We conclude that ICC and ISH are methods of comparable sensitivity for detection of papovavirus in flourishing PML lesions. In burnt-out PML lesions only ISH may detect virus. The possibility of an exceptional non-JCV (e.g., SV 40) etiology of PML could be neither confirmed for disproved. In AIDS, massive coinfection by HIV of PML lesions may increase damage to tissue, resulting in unusually extended and necrotizing PML.Supported by the Lord Mayor's Medical-Scientific Fund Vienna. Major parts of this study were presented at the Scientific Winter Meeting in Kitzbühel, Austria, March 15–18, 1989     

A study of mefloquine treatment for progressive multifocal leukoencephalopathy: results and exploration of predictors of PML outcomes

Immune reconstitution has improved outcomes for progressive multifocal leukoencephalopathy (PML), a potentially lethal brain disease caused by JC virus (JCV). However, an antiviral treatment to control JCV is needed when immune reconstitution is delayed or not possible. On the basis of in vitro efficacy, this study evaluated the effect of mefloquine on PML and factors that may predict PML outcomes. This 38-week, open-label, randomized, parallel-group, proof-of-concept study compared patients with PML who received standard of care (SOC) with those who received SOC plus mefloquine (250 mg for 3 days, then 250 mg weekly). Patients randomized to SOC could add mefloquine treatment at week 4. The primary endpoint was change from baseline to weeks 4 and 8 in JCV DNA copy number (load) in cerebrospinal fluid (CSF). Exploratory analyses evaluated factors that might correlate with clinical outcome. The majority of enrolled patients were HIV positive. Preplanned interim data analyses suggested that the study was unlikely to successfully demonstrate a significant difference between groups; therefore, the study was terminated prematurely. There was no significant difference between groups in CSF JCV DNA loads or clinical/MRI findings. Decrease in CSF JCV DNA load from baseline to week 4 was associated with a better clinical outcome at 16 weeks, as measured by Karnofsky scores. This study found no evidence of anti-JCV activity by mefloquine. An early decrease of CSF JCV DNA load appears to be associated with a better clinical outcome.     

Detection and typing of JC virus in autopsy brains and extraneural organs of AIDS patients and non-immunocompromised individuals.

The distribution of JC virus (JCV) variants in the brain, lung, liver, kidney, spleen and lymph nodes collected at autopsy from AIDS patients with (Group A: 10 Ss) and without (Group B: 5 Ss) progressive multifocal leukoencephalopathy (PML) and from HIV-negative patients (Group C: 5 Ss), was examined by amplifying the JCV large T antigen (LT), the regulatory (R) and the VP1 regions. Among the samples from the PML patients, JCV DNA was detected in all of the demyelinating areas, in 60% of the lesion-free brain tissues, in 60% of the lung tissues and in 40% of the spleen and kidney tissues, whereas all liver and lymph node sections were negative. JCV DNA was also found in two of the five brain specimens, in two of the five kidney specimens, in one of the five lung specimens from the HIV-positive patients without PML and in the brain specimens from two of the five HIV-negative subjects. Nucleotide sequence analysis indicated that all of the R region amplified from extraneural tissues had rearrangements similar to those of the Mad-4 strain and that VP1-region amplified products were similar to the Mad-1 strain. In the brain specimens from two PML patients, we found a unique rearranged R region, along with a VP1 region of JCV type 2. In addition, an almost unique variant with multiple rearrangements in the R region and unusual base mutations in the VP1 region was detected in the brain sample from another PML patient. The data indicate that diffuse visceral involvement of JCV is particularly frequent in AIDS patients with PML. Moreover, the presence of rearrangements and mutations, involving different regions of the viral genome, observed in PML-affected brain tissues, could represent a risk factor for the development of PML in immunosuppressed individuals.     

Progressive multifocal leukoencephalopathy: JC virus detection by in situ hybridization compared with immunohistochemistry

In four cases of progressive multifocal leukoencephalopathy (PML), we compared biotin-labeled DNA:DNA in situ hybridization with peroxidase immunohistochemistry for the detection of JC virus (JCV). The localization of JCV DNA and JCV capsid protein was compared in formalin-fixed, paraffin-embedded brain tissues. Infected oligodendrocytes showed both JCV DNA and JCV protein. However, bizarre astrocytes demonstrated JCV capsid protein less often than JCV DNA. In situ hybridization with a biotinylated probe was as sensitive and specific as immunohistochemistry for diagnosis on formalin-fixed tissue. The presence of both JCV DNA and viral capsid protein in bizarre astrocytes suggests that these cells are neither truly transformed nor permissively infected, but are distinctively altered by JCV.     

AIDS-associated progressive multifocal leukoencephalopathy (PML): comparison to non-AIDS PML with in situ hybridization and immunohistochemistry

We studied brain sections from 10 patients with the acquired immunodeficiency syndrome (AIDS) and progressive multifocal leukoencephalopathy (PML) by in situ hybridization with a biotin-labeled JC virus (JCV) DNA probe and by immunohistochemistry using antibody against the JCV capsid antigen. We compared the results with brain sections studied in the same fashion from 10 PML patients without AIDS. The pathology of JCV infection in AIDS was similar to non-AIDS PML except for minor differences in degree. AIDS-associated pathologic material showed a greater tendency toward necrosis and a higher density of JCV-infected cells. Replication of JCV was restricted to glial cells in all tissue studied. Bizarre astrocytes were less frequent in the AIDS patients, and perivascular inflammatory cells were more frequent. We could not demonstrate JCV in macrophages or microglial cells known to harbor HIV infection. In situ hybridization with nonradioactive probes serves as a useful technique for the confirmation of PML in AIDS.     

Progressive multifocal leukoencephalopathy and other forms of JC virus disease

Progressive multifocal leukoencephalopathy (PML) is a demyelinating disease of the brain caused by the JC virus (JCV). PML usually occurs via reactivation of JCV when an immune system becomes compromised. A diagnosis of PML is normally made on the basis of distinguishing neurological features at presentation, characteristic brain MRI changes and the presence of JCV DNA in cerebrospinal fluid. PML has a 3 month mortality rate of 20-50%, so prompt intervention is essential. Currently, reconstitution of the immune system affords the best prognosis for this condition. When PML is first suspected, and where possible, immunosuppressant or immunomodulatory therapy should be suspended or reduced. If PML is associated with a protein therapy that has a long half-life the use of plasma exchange to accelerate the removal of the drug from the circulation may aid the restoration of immune system function. Rapid improvements in immune function, however, might lead to transient worsening of the disease. In this Review, we critically appraise the controversies surrounding JCV infection, and provide practical management guidelines for PML.     

Coinfection of the central nervous system by cytomegalovirus and herpes simplex virus type 1 or 2 in AIDS patients: autopsy study on 82 cases by immunohistochemistry and polymerase chain reaction

We evaluated the frequency and histopathological features of concomitant infections of the central nervous system (CNS) with cytomegalovirus (CMV) and herpes simplex viruses type 1 or 2 (HSV1/2) in a large series of patients who had died from AIDS. Eighty-two autopsy cases with a histological diagnosis of CMV necrotizing encephalitis were examined retrospectively. CMV and HSV1/2 were detected by immunohistochemistry (IHC) with poly- and monoclonal antibodies and by nested polymerase chain reaction (PCR) for HSV 1 and 2 on DNA extracted from paraffin blocks. PCR for a β-globin genomic sequence was performed in all IHC-positive cases to verify the integrity of extracted DNA. Concomitant CMV/HSV infections were demonstrated by IHC in 13 cases (16%); using monoclonal antibodies, HSV1 was found in 9 cases and HSV2 in 4 cases. In half of the cases, HSV1- or HSV2-positive cells represented more than 25% of immunopositive CMV cells. In all 13 cases, double immunochemical staining showed cells containing both CMV and HSV antigens. PCR for HSV1 and 2 was positive in only 7 of 13 cases (5 HSV1 and 2 HSV2). In the remaining 6 negative cases PCR for β-globin was also repeatedly negative. HSV1 or 2 infection can be demonstrated by IHC in a significant proportion of AIDS cases with necrotizing CMV encephalitis. Nested PCR for HSV1 and 2 on DNA extracted from formalin-fixed and paraffin-embedded autopsy tissues was positive in only slighty above 50% of IHC-positive cases. Received: 28 November 1995 / Revised, accepted: 26 February 1996     

Virus production,cell proliferation and cell death in progressive multifocal leukoencephalopathy

Formalin fixed, paraffin embedded, progressive multifocal leukoencephalopathy (PML) brain lesions from eight stereotaxic needle biopsies and eight autopsies were immunocytochemically examined for tumor suppressor gene products (p53, retinoblastoma gene product, pRb), cell proliferation antigens (Ki-67, proliferation nuclear antigen (PCNA)) and bcl-2. The load of JC virus (JCV) was examined by immunocytochemistry for papovavirus common antigen and in situ hybridization for JCV-DNA. DNA fragmentation was investigated with in situ endlabeling (ISEL). The virus markers, tumor suppressor gene products, and cell proliferation antigens were intensively expressed in infected oligodendrocytes (PML cells) and in bizarre astrocytes. The PML cells were labelled uniformly with virus markers, anti-tumor suppressor gene products and anti-cell proliferation antigens; however, the variance of the reactivity of the bizarre astrocytes was quite broad. The bcl-2 was strongly expressed in infiltrating lymphocytes and reactive astrocytes, but not in oligodendroglia. Only few apoptotic bodies or nuclear fragments were labelled with ISEL in seven of eight cases. In seven of eight cases, some PML cells were labelled with ISEL. In two of eight cases, a few bizarre astrocytes were faintly labelled. Histologically normal tissues surrounding the PML lesions and the negative control cases showed no unequivocal labeling with all markers. We confirmed the results of previous studies suggesting overexpression and/or stabilization of p53 following overexpression of PCNA in glial cells in the PML brain. We conclude that apoptosis does not seem to be an important mechanism for glial cell elimination in the PML brain. In addition, uniform expression of the virus antigen, tumor suppressor gene products and proliferation markers was associated with little variance in the morphology of PML cells and variable expression of these markers was associated with variable morphology of bizarre astrocytes in the PML brain.