Brains of 19 AIDS patients with HIV encephalitis were examined by immunohistochemistry and in situhybridization using antisense HIV DNA and RNA probes. Double immunohistochemical labeling, using antibodies against viral and cell-type specific antigens, was utilized to study lesions in some brains. Other combined studies included use of in situhybridization and immunohistochemical labeling of the same section, using antibodies against either viral or cell-type specific antigens. Hybridization signals were abundant and were concentrated mainly in the white matter. Heavy labeling was found in the subcortical white matter, the corpus callosum, the internal capsule, and white matter regions of the brainstem and cerebellum. Deeper cortical layers often contained cells with hybridized probe when the subcortical white matter was intensely labeled. HIV nucleic acid sequences were found almost exclusively in macrophages. Counts showed that 16–25% of macrophages contained viral antigens and exhibited hybridized HIV probe. Almost all of these macrophages contained proviral DNA, viral RNA and viral proteins; i.e. they were actively replicating HIV. We also examined brains from three AIDS cases without clinical or pathological evidence of HIV encephalitis; no HIV sequences or immunoreactive proteins were detected.

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