VIRUSMYTH HOMEPAGE
AIDS DEMENTIA: MISPLACED BLAME?
By Robert Root-Bernstein
Science News 31 Aug. 1991
The notion that AIDS dementia is caused
by HIV infection of brain neurons ("AIDS dementia: Neurons nixed by
virus?" SN: 5/18/91, p.311) is unlikely for four reasons.
First. unlike other viruses that attack
the central nervous system. HIV has no known tropism for neurons. Second.
HIV generally requires the CD4 receptor of Thelper cells and macrophages
to enter a cell, but neither the CD4 protein nor any similar protein exists
on neurons. Third, viruses do not usually have access to the brain until
the bloodbrain barrier has been breached. Fourth. and most important,
the same sorts of dementias seen in AIDS patients also occur in other immunosuppressed
people, proving that HIV is not a necessary cause. Once the bloodbrain
barrier has been breached, any infection can-and in AIDS patients often
does - get in. Cytomegalovirus, herpes simplex viruses, cryptococcus and
toxoplasmosis are very frequently isolated from the central nervous systems
of AIDS patients.
For these reasons, I proposed a radically
different explanation of AIDS dementia last year. Animal models
of diseases that cause nervous system destruction have existed for
nearly a century and their pathology mimics many aspects of AIDS dementia,
including demyelinization and neuron death. To induce these experimental
diseases, an immunestimulating bacterial "adjuvant" and a protein
mimicking one found in the nervous system (such as myelin basic protein)
are combined and inoculated into an animal. Neither alone is sufficient
to cause the disease process, but in combination they produce an autoimmune
reaction that destroys the bloodbrain barrier and then destroys the
myelin sheaths and neurons.
Autopsy records of patients with dementias
involving demyelinization-a large proportion of dementia patients-almost
always reveal a combination of bacterial (usually mycobacterial) and herpesvirus
(usually cytomegalovirus) infections. People who do not develop such dementias
have one or neither infection. but almost never both.
The importance of these observations for
understanding AIDS dementia is fourfold. First, the known prerequisites
for producing nervous system destruction in animals exist in both AIDS
and nonAIDS dementia, but not in people who are spared this
problem. Second, there is no correlation between dementia and the presence
of HIV, since HIV is present in virtually every AIDS patient, with or without
dementia. Third, once the bloodbrain barrier has been breached by
the autoimmune process, HIV (and many other infectious agents) have access
to the central nervous system. explaining the frequent isolation of the
virus without implicating it directly in the dementia. And fourth, if my
explanation is correct, then antimycobacterial and antiherpesvirus treatments
should protect many HlVpositive patients from developing dementia.*
Robert S. RootBernstein
Associate Professor of Physiology Michigan State University East Lansing. Mich.
VIRUSMYTH HOMEPAGE