Progressive multifocal leukoencephalopathy (PML) in the acquired immunodeficiency syndrome (AIDS). A neuropathological autopsy study of 21 cases

Progressive multifocal leukoencephalopathy and immune reconstitution inflammatory syndrome (IRIS)

Progressive multifocal leukoencephalopathy is a viral encephalitis induced by the John Cunningham (JC) virus, an ubiquitous neurotropic papovavirus of the genus polyomavirus that in healthy people in latency resides in kidney and bone marrow cells. Activation and entry into the CNS were first seen in patients with malignancies of the hematopoietic system and an impaired immune system. During the 1980 and the 1990s with the appearance of human immunodeficiency virus infection in humans, PML was found to be the most important opportunistic infection of the central nervous system. As a result of highly efficient immunosuppressive and immunomodulatory treatments, in recent years, the number of PML cases again increased. PML is prevented by an intact cellular immune response and accordingly immune reconstitution can terminate established disease in the CNS. However, forced immune reconstitution can lead to massive destruction of virus-infected cells. This may result in clinical exacerbation associated with high morbidity and mortality and referred to as PML with immune reconstitution inflammatory syndrome (PML-IRIS). In the present review, we discuss virological properties and routes of infection in the CNS, but mostly focus on the pathology of PML and PML-IRIS and on the role of the immune system in these disorders. We show that PML and PML-IRIS result from predominant JC virus infection of oligodendrocytes and, to a lesser extent, of infected neurons. Inflammation in these encephalitides seems to be driven by a dominant cytotoxic T cell response which is massively exaggerated during IRIS.

Progressive multifocal leukoencephalopathy and promyelocytic leukemia nuclear bodies: a review of clinical, neuropathological, and virological aspects of JC virus-induced demyelinating disease

Progressive multifocal leukoencephalopathy is a fatal viral-induced demyelinating disease that was once rare but has become more prevalent today. Over the past decades, much has been learned about the disease from molecular study of the etiological agent of the disease, JC virus. Recently, promyelocytic leukemia nuclear bodies (PML-NBs), punctuate structures for important nuclear functions in eukaryotic cells, were identified as an intranuclear target of JC virus infection. Neuropathologically, JC virus-infected glial cells display diffuse amphophilic viral inclusions by hematoxylin–eosin staining (full inclusions), a diagnostic hallmark of this disease. Recent results using immunohistochemistry, however, revealed the presence of punctate viral inclusions preferentially located along the inner nuclear periphery (dot-shaped inclusions). Dot-shaped inclusions reflect the accumulation of viral progeny at PML-NBs, which may be disrupted after viral replication. Structural changes to PML-NBs have been reported for a variety of human diseases, including cancers and neurodegenerative disorders. Thus, PML-NBs may provide clues to the further pathogenesis of JC virus-induced demyelinating disease. Here, we review what we have learned since the disease entity establishment, including a look at recent progress in understanding the relationship between JC virus, etiology and PML-NBs.

[Demyelinating disorders]

Leukoencephalopathies in adults are frequent and exhibit highly variable aetiology, including multiple acquired causes such as inflammatory, vascular or toxic diseases and neoplasias. In contrast leukodystrophies are genetically determined, chronic progressive myelin disorders with a variable pathogenetic background and a great diversity of clinical and paraclinical findings. Some diseases, namely those with an additional inborn error of metabolism, are treatable. Genetic counselling appears to be of major importance for patients and their families. In the light of numerous acquired adulthood leukoencephalopathies a clear delineation of late-onset genetic leukodystrophies is necessary. Clinical symptoms and MRI patterns of some of the major leukodystrophies are reported, including possibilities of biochemical and genetic testing.

Progressive multifocal leukoencephalopathy showing extensive spinal cord involvement in a patient with lymphocytopenia

A 64-year-old Japanese man who was diagnosed as having cerebral infarcts at an early clinical stage was found to have progressive multifocal leukoencephalopathy (PML). A decrease of leukocytes and lymphocytes had been detected in the previous year. During a total clinical course of 11 months, he showed marked depletion of lymphocytes ranging from 264/microL to 459/microL. Autopsy disclosed no underlying diseases such as malignancies or tuberculosis. Extensive PML lesions were seen in the cerebral white matter. Small perivascular cuffs comprising many CD8+ T lymphocytes and a few CD4+ T cells were scattered in the PML lesions. CD20+ B cells were rarely evident. The subsets of the infiltrating lymphocytes differed from those of primary or spontaneous PML. Similar extensive PML lesions were observed not only in the cerebellum and brainstem but also in the spinal cord. All 26 segments of the spinal cord, especially the cervical, lumbar and sacral cord, showed extensive lesions involving the lateral and anterior columns. To our knowledge, only three cases of PML with such extensive spinal cord lesions have been reported previously. These three cases, and the present one, may represent a group of PML that shows extensive lesions in the spinal cord as well as the cerebrum, cerebellum and brainstem. The underlying disease in the present case was unclear. Because lymphocytopenia is not observed in primary or spontaneous PML, and the immunohistochemical findings of the infiltrating lymphocytes in the present case are different from primary or spontaneous PML, the decrease in his total blood lymphocytes may have played a significant role in his immunosuppressed condition as the underlying disease.

Diagnosis of inflammatory demyelination in biopsy specimens: a practical approach

Multiple sclerosis is the most frequent demyelinating disease in adults. It is characterized by demyelination, inflammation, gliosis and a variable loss of axons. Clinically and histologically, it shares features with other demyelinating and/or inflammatory CNS diseases. Diagnosis of an inflammatory demyelinating disease can be challenging, especially in small biopsy specimens. Here, we summarize the histological hallmarks and most important neuropathological differential diagnoses of early MS, and provide practical guidelines for the diagnosis of inflammatory demyelinating diseases.

[New trends in progressive multifocal leukoencephalopathy]

Infection by Polyomavirus JC is a model of chronic active viral infection, closely controlled by the immune system. Progressive multifocal leucoencephalopathy (PML) is a deadly demyelinating disease of the central nervous system, consecutive to the lytic infection of oligodendrocytes by JC virus. Reactivation of JC virus occurs only in the setting of severe cellular immune deficiency. During the last 25 years, the incidence of PML has significantly increased related to the AIDS pandemic and, more recently, to the growing use of immunosuppressive drugs. There is no specific antiviral treatment for PML. Nevertheless, the availability of highly active antiretroviral therapy has changed the clinical course of PML in HIV-infected individuals. One-year mortality has decreased from 90 percent to approximately 50 percent as a result of reconstitution of the immune system. Recent advances in JC virus biology give new perspectives to the pathogenesis of PML. New trends in the understanding of the cellular immune response against the JC virus have direct implications for patient management and may lead to develop future strategy of immunotherapies for PML.

Real-time PCR in clinical microbiology: applications for routine laboratory testing

Real-time PCR has revolutionized the way clinical microbiology laboratories diagnose many human microbial infections. This testing method combines PCR chemistry with fluorescent probe detection of amplified product in the same reaction vessel. In general, both PCR and amplified product detection are completed in an hour or less, which is considerably faster than conventional PCR detection methods. Real-time PCR assays provide sensitivity and specificity equivalent to that of conventional PCR combined with Southern blot analysis, and since amplification and detection steps are performed in the same closed vessel, the risk of releasing amplified nucleic acids into the environment is negligible. The combination of excellent sensitivity and specificity, low contamination risk, and speed has made real-time PCR technology an appealing alternative to culture- or immunoassay-based testing methods for diagnosing many infectious diseases. This review focuses on the application of real-time PCR in the clinical microbiology laboratory.

AIDS-related progressive multifocal leukoencephalopathy in the era of HAART: report of two cases and review of the literature

Progressive multifocal leukoencephalopathy (PML) is a demyelinating disease of the central nervous system. It is caused by the JC virus (JCV), a human polyomavirus replicating in human glial cells. PML is the result of the reactivation of latent JCV infection that usually occurs in the setting of cellular immunodeficiencies such as HIV-1 infection. Epidemiologic data suggest that the impact of highly active antiretroviral therapy (HAART) on the incidence of PML is less profound than seen with other opportunistic infections. Given the lack of an effective and specific therapy for PML, HAART remains the only therapeutic option in patients with PML. However, a significant number of cases appear unresponsive to antiretroviral therapy. Moreover, there is growing data on unexpected inflammatory cases of PML after initiation of HAART. Thus, PML will remain a relevant cause of morbidity and mortality in HIV- 1-infected patients. Here we report two cases of PML, along with a concise review of the literature on this important disease.

JC virus granule cell neuronopathy: A novel clinical syndrome distinct from progressive multifocal leukoencephalopathy

Progressive multifocal leukoencephalopathy (PML) typically affects the CNS white matter of the central nervous system. We present an human immunodeficiency virus-infected patient with polyomavirus JC infection restricted to granule cell neurons of the cerebellum and with corresponding neurological symptomatology. Magnetic resonance imaging demonstrated cerebellar atrophy without white matter lesions and stereotactic biopsy showed selective infection of the cerebellar granular cell layer, with preservation of Purkinje cells and absence of classic progressive multifocal leukoencephalopathy histopathology in underlying white matter. Evolution over 8 years was marked by symptomatic improvement corresponding to highly active antiretroviral therapy (HAART), with modest increase in CD4(+) T-cell counts. We propose to call this novel syndrome JCV granule cell neuronopathy (JCV GCN).

Predicted amino acid sequences for 100 JCV strains

DNA sequence variation between JCV genotypes is confined largely to noncoding intergenic regions and introns. Nevertheless, evidence suggests that the amino acid sequence variations among the 8 genotypes of JCV can influence the potential for neurovirulence of the virus. In the current study, the amino acid sequences for 100 JCV genomes were translated and grouped into genotype families. Subtype consensus sequences were determined and the type-specific amino acid sequence variants were identified.

Progressive Multifocal Leukoencephalopathy

Progressive multifocal leukoencephalopathy is a subacute demyelinating disease that occurs in patients with defects in cell-mediated immunity, including those with AIDS and lymphoproliferative disorders. It is caused by reactivation of JC virus (JCV), which infects 70% to 90% of the population by adulthood, but remains latent in normal hosts. Once reactivated, JCV infects oligodendrocytes and astrocytes, with resultant cell lysis, leading to focal areas of demyelination and necrosis in cerebral white matter causing focal neurologic deficits and characteristic findings on MRI. Polymerase chain reaction for the detection of JCV is a sensitive and specific test, replacing brain biopsy as the initial diagnostic test in the appropriate clinical setting. Historically, the prognosis of progressive multifocal leukoencephalopathy is poor, with most patients dying within 6 months of diagnosis. Antiviral medications targeted against JCV have shown little success. However, with the use of highly active antiretroviral therapy, survival of AIDS patients with progressive multifocal leukoencephalopathy has improved.

Molecular mimicry in infectious encephalitis and neuritis: binding of antibodies against infectious agents on Western blots of human nervous tissue

OBJECTIVES

To study the occurrence of cross-reactivities of antibodies against infectious agents with human nervous tissue.

METHODS

Binding of 25 antibodies against 17 neurotropic pathogens comprising Borrelia burgdorferi, Toxoplasma gondii, and various DNA and RNA viruses to Western blots of human cortex and myelin from central and peripheral nervous system was investigated.

RESULTS

Fourteen of the 25 antibodies tested showed binding to Western blots of human nervous tissue, suggesting the presence of shared epitopes. Binding of 11 antibodies against 10 pathogens to cortex and/or myelin correlated with the tissue targeted by neuropathological lesions. Three antibodies did not show such correlation; 11 antibodies did not bind at all.

CONCLUSIONS

Our results suggest that shared epitopes between infectious agents and human nervous tissues are more common than previously expected. Thus, molecular mimicry should be considered more frequently as a possible pathogenetic mechanism, among others, inducing tissue damage in encephalitis and neuritis caused by various pathogens.

Central Nervous System Opportunistic Infections in HIV-1 Infection

Neurologic disease is commonly encountered in the population infected with human immunodeficiency virus type 1 (HIV-1). Although HIV-1 is responsible for many of these neurologic complications, other organisms will affect the nervous system as the immune deficiency state progresses. With the wide use of potent antiretroviral therapy, the mortality from and incidence of opportunistic infections (OIs) among persons with advanced HIV-1 infection has decreased. Nevertheless, these diseases are still seen frequently, especially among those with limited access to new antiretroviral therapies. Therefore, it remains important to recognize the most common OIs of the central nervous system (CNS) as well as primary CNS lymphoma, which will be the focus of this review.

Reduced concentrations of HIV-RNA and TNF-alpha coexist in CSF of AIDS patients with progressive multifocal leukoencephalopathy

OBJECTIVES

To confirm reduced human immunodeficiency virus type-1 (HIV-1) burden in the CSF of patients with progressive multifocal leukoencephalopathy (PML) and to verify whether this viral load coincides with the absence of inflammatory changes in the CSF.

METHODS

Paired CSF and plasma samples from 17 patients with PML, 26 with non-PML cerebral opportunistic infections, nine with HIV-1 leukoencephalopathy (HIVE), and 12 neurologically asymptomatic AIDS patients were subjected to HIV-RNA titration. Tumour necrosis factor (TNF)-alpha was also measured and the CSF albumin: serum albumin ratio (Q(Alb)) was calculated.

RESULTS

The CSF HIV-1 burden of patients with PML did not differ from that of neurologically asymptomatic patients (p=0.21), but was significantly lower than CSF burden of the remaining patients (non-PML opportunistic infections, p<0.001; HIVE, p<0.001). Q(Alb) was normal for all neurologically asymptomatic patients, for 86.6% patients with PML, and 62.5% patients with HIVE (p=0.09). Q(Alb) was altered in 91.6% patients with non-PML opportunistic infections. TNF-alpha in CSF was higher in patients with non-PML opportunistic infections (p<0.001) and those with HIVE (p<0.001) than in patients with PML who consistently had TNF-alpha concentrations<10 pg/ml.

CONCLUSIONS

These results, while indicating a reduced HIV replication in CSF of patients with PML which might serve as a disease marker, emphasise the increased CSF HIV-RNA concentration in patients with HIVE and patients with non-PML opportunistic infections. Low concentrations of HIV-RNA in CSF coincide with reduced TNF-alpha concentrations, possibly due to particular features of PML compared with other opportunistic infections as it develops without detectable inflammatory changes in the CSF.

Progressive multifocal leukoencephalopathy and JC virus genotypes in West African patients with acquired immunodeficiency syndrome: a pathologic and DNA sequence analysis of 4 cases

OBJECTIVE

Progressive multifocal leukoencephalopathy is caused by polyomavirus JC in immunosuppressed patients. JC virus genotypes are identified by sequence analysis of the viral genome. Despite the prevalence of acquired immunodeficiency syndrome in sub-Saharan Africa, few cases of progressive multifocal leukoencephalopathy have been reported from this region. Here we describe 4 African cases and provide an analysis of viral genotypes.

METHODS

Immunohistochemical staining by labeled streptavidin-biotin for capsid protein antigen was performed on all cases. Polymerase chain reaction amplification of viral genomic DNA was followed by direct cycle sequencing.

RESULTS

JC virus type 3 was identified in 2 cases, and type 6 was isolated in 1 case. The viral regulatory region from 1 case showed an uncommon rearrangement pattern.

CONCLUSIONS

Progressive multifocal leukoencephalopathy in West African patients with acquired immunodeficiency syndrome is caused by African genotypes of JC virus (types 3 and 6). The prevalence of disease in this autopsy series from sub-Saharan Africa (1.5%) was less than has been reported from Europe and the United States (4% to 10%) and may be partly due to biological differences in JC virus genotypes. Further studies will be needed to confirm this observation.

Predictive factors for prolonged survival in acquired immunodeficiency syndrome-associated progressive multifocal leukoencephalopathy

Progressive multifocal leukoencephalopathy (PML) complicating the acquired immunodeficiency syndrome (AIDS) is typically inexorably progressive with death usually occurring within 6 months of symptom onset. Occasional patients have been observed to survive longer than 1 year, often with remission of clinical features. In this study, we identify predictive factors for prolonged survival in patients with biopsy proven, AIDS-associated PML, by comparing 7 patients with survival exceeding 12 months from symptom onset with 45 patients with shorter survivals. PML was the presenting manifestation of AIDS in 5 (71.4%) of 7 long-term survivors compared with 8 (17.8%) of 45 short-term survivors. CD4 T-lymphocyte counts were substantially higher in the long-term survivors, with 3 (42.9%) of 7 having counts exceeding 300 cells/mm3 in comparison with only 1 (4.3%) of 23 short-term survivors. Contrast enhancement on radiographic imaging was observed in 3 (50%) of 6 long-term survivors in comparison with 4 (8.9%) of 45 short-term survivors. Neurological recovery and radiographic improvement were not observed in any short-term survivors but were seen in 5 (71.4%) long-term survivors. There was no association between treatment modalities and survival. Predictors of long-term survival in AIDS patients with PML include PML as the heralding manifestation of AIDS, high CD4 T-lymphocyte count at disease onset, lesion enhancement on computed tomographic scan or magnetic resonance imaging, and evidence of recovery of neurological function.

Highly active antiretroviral therapy significantly improves the prognosis of patients with HIV-associated progressive multifocal leukoencephalopathy

OBJECTIVE

To evaluate the impact of different antiretroviral therapies on the prognosis of AIDS patients affected by progressive multifocal leukoencephalopathy (PML).

METHODS

A retrospective analysis of all HIV-infected patients admitted to hospital between 1988 and 1996 found 29 patients (25 men) with histologically or PCR-confirmed PML. Their mean age was 39.3 years. The median CD4 cell count was 40 x 10(6)/l (mean, 106 x 10(6)/l). Six patients had CD4 cell counts > 200 x 10(6)/l. Fourteen patients never received or stopped antiretroviral therapy following diagnosis (group A), 10 patients were treated with nucleoside analogues alone (group B), and five patients started highly active antiretroviral therapy (HAART) including protease inhibitors (group C).

RESULTS

The median survival following the onset of symptoms was 131 days, but differed significantly between the three groups: group A, 127 days; group B, 123 days; group C, > 500 days (P < 0.0002 for the difference between group C versus group A and B, stratified log-rank test). As of July 1997, four out of five patients on HAART were still alive 391, 500, 543, and 589 days after diagnosis of PML and have either experienced a resolution of the symptoms (three patients) or had progressed very slowly (one patient). A multivariate analysis using Cox regression found younger age at diagnosis to be the only other variable associated with improved survival (P < 0.02). CD4 cell count, gender, prior AIDS diagnosis, mode of HIV transmission, and therapy with foscarnet, cytarabine, or interferon-alpha did not affect survival in this cohort (P > 0.1).

CONCLUSION

This study of a large cohort of patients with confirmed PML indicates that AIDS patients with PML may benefit significantly from HAART. All patients with PML should be offered optimal antiretroviral combination therapy.

Detection of JC virus DNA in cerebrospinal fluid from multiple sclerosis patients

JC virus (JCV), the causative agent of progressive multifocal leukoencephalopathy (PML), has been proposed as a possible aetiopathogenic factor in multiple sclerosis (MS). We performed a study to search the LT region of JCV genome by nested PCR in cerebrospinal fluid (CSF), peripheral blood mononuclear cell (PBMC) and urine samples collected from 121 MS patients, 24 patients with other neurological disorders (OND), 30 non neurological patients (NND) and in PBMCs and urine of 40 healthy subjects. JCV DNA has been found in the CSF of 11 MS patients (9%) while all the CSFs from the 24 OND and the 30 NND cases were negative. No significant differences have been observed as regard to the frequency of JCV DNA detection in PBMCs and urine between the MS patients and the control groups. Nucleotide sequences analysis of seven JCV CSF isolates showed that five strains were identical the prototypal strain, while the other two had a base mutation (T-->C) in 4286 nucleotide (nt). The finding of JCV DNA in the CSF of MS patients suggest that JCV could play a role in the triggering and/or in the maintenance of MS aetiopathogenic process, and therefore it should be taken in consideration when monitoring this disease.

Comprehensive investigation of the presence of JC virus in AIDS patients with and without progressive multifocal leukoencephalopathy

Progressive multifocal leukoencephalopathy (PML), a viral-induced demyelinating disease, is becoming relatively common, while many diagnostic and pathogenetic aspects remain to be clarified. A study was therefore undertaken in 64 AIDS patients suffering from various neurological disorders, including PML (12 subjects), with the specific objective of searching for JC virus (JCV) DNA by nested PCR (n-PCR) in cerebrospinal fluid (CSF), peripheral blood mononuclear cells (PBMCs), and urine collected from all patients. CSF examination, CD4 and CD8 counts, neurological examinations, and neuroradiological investigations were undertaken. JCV DNA was detected in 92% of CSF specimens in 75% of the PBMCs and urine samples from the PML patients, whereas among the non-PML patients JCV DNA was not detected in any CSF samples, but was found in 10% of PBMCs and in 39% of the urine specimens. BKV and JCV DNA viruria was observed simultaneously in 6% of the AIDS patients without PML. The routine CSF tests including IgG oligoclonal bands, the Link, and Tourtellotte IgG indexes, did not show a typical pattern in PML cases. The data obtained clearly indicate that the detection of JCV DNA in CSF constitutes an efficient marker for PML diagnosis. The simultaneous presence of JCV DNA in the CSF, PBMCs, and urine samples from the PML patients, who did not differ from controls with regard to their immunosuppressive status, suggests that JCV could be carried into the central nervous system (CNS) by infected PBMCs.

AIDS-associated progressive multifocal leukoencephalopathy revealed by new-onset seizures

PURPOSE

To describe the clinical features of new-onset seizures in HIV-1-infected persons with progressive multifocal leukoencephalopathy (PML), and to discuss potential mechanisms.

PATIENTS AND METHODS

Forty-nine consecutive HIV-1-infected patients with PML attended our institutions between January 1988 and September 1993. We retrospectively analyzed cases with seizures as the presenting symptom of PML.

RESULTS

Twenty percent of the HIV-1-infected patients with PML presented with new-onset seizures of various types, generalized or partial. None of them met the criteria of the AIDS dementia complex or had a concomitant opportunistic infection. Their mean CD4 cell count was < 60/mm3. Brain magnetic resonance imaging showed areas of increased signal intensity on T2-weighted images in 9 cases, and atrophy in only 1 case. Lesions most often involved subcortical white matter in parieto-occipital or frontal lobes, but 2 patients had posterior fossa lesions. Image-guided stereotactic brain biopsies in 8 cases and postmortem examination in 2 confirmed the diagnosis of PML. Typical histological lesions were observed in all cases, and positive immunolabelling of oligodendroglial nuclei was obtained in all cases with the polyclonal antibody directed against late SV40 antigens. Putative causative factors for the seizures include demyelinated lesions adjacent to the cerebral cortex acting as irritative foci, axonal conduction abnormalities, or disturbances of the neuron-glia balance.

CONCLUSION

These cases illustrate that PML should be considered as a possible cause of new-onset seizures in patients with HIV-1 infection.

Early pathological changes in progressive multifocal leukoencephalopathy: a report of two asymptomatic cases occurring prior to the AIDS epidemic

Serial sections of formalin-fixed, paraffin-embedded blocks from two asymptomatic, non-AIDS cases of progressive multifocal leukoencephalopathy (PML) were stained with a double-label immunocytochemical method for detection of glial fibrillary acidic protein and JC virus (JCV) capsid proteins and with luxol fast blue/hematoxylin-eosin. In case 1 small, rounded lesions of about 1-mm diameter were seen within a restricted area in the posterior part of the superior frontal gyrus of both cerebral hemispheres, suggesting an early manifestation of the disease. Fully developed demyelinated lesions of the classical type with JCV-infected oligodendrocytes appeared in the white matter and along its border with the cortex. Less-well-developed lesions, believed to be precursors to the fully developed ones, were seen in the gray and white matter. Of special interest were areas which contained small collections of enlarged, glial fibrillary acidic protein (GFAP)-positive astrocytes without capsid antigen and which seemed to lack destruction of myelin as judged from the appearance of matching serial sections stained for myelin. Large lesions in the brain of case 2 showed the well-known features of advanced PML. The close relation between some astrocytes and oligodendrocytes with viral antigen raises the possibility of early intercellular passage of virus. Vacuolation, seen within or near lesions in both cases, has previously been noted in the CNS infected by HIV, but not in PML. It is suggested that PML, a disease of both oligodendrocytes and astrocytes, may actually begin in astroglial cells which, under the influence of a restricted JCV infection, become reactive, express GFAP and pass on virus to the more highly susceptible oligodendrocytes with which they are in contact.

Cellular changes in the cerebellar granular layer in AIDS-associated PML

Six cases of AIDS-associated progressive multifocal leukoencephalopathy (PML) exhibited peculiar cellular changes in the cerebellar granular layer. These cells without discernible cytoplasm showed hypochromatic nuclei about twice as large as those of normal granule cells. They were restricted exclusively to the granular layer and always surrounded PML foci. An astrocytic, leukocytic or macrophage/microglial nature was largely excluded by immunocytochemistry. Human immunodeficiency virus (HIV) antigen p 24 could not be found in these cells and there was no unequivocal detection of JC virus (JCV) DNA and no ultrastructural evidence of papovavirus particles in them. They possibly represent altered cerebellar granule cells abortively or latently infected with JCV.