The Relationship Between the Human Immunodeficiency Virus and the Acquired Immunodeficiency Syndrome: Executive Summary (Draft)

DRAFT

10. HIV mutates in a patient's body, and disease progression is associated
with more virulent strains.
In HIV-infected individuals, variants of the virus emerge that kill cells more
efficiently and replicate in a wider range of cells than the initial strains with
which the patients were infected. The emergence of these variants occurs prior
to disease progression.
11. HIV damages the body's sources of CD4+ T cells and centers of immune
activity.
HIV destroys precursor cells and the structures in the bone marrow and thymus
that are needed for the development of mature immune cells. This damage
may help explain why the immune systems of people with AIDS do not
spontaneously regenerate. The virus also progressively destroys the lymph
nodes, the centers of immune activity in the body. Significantly, in the
approximately 5 percent of HIV-infected patients whose disease does not
progress, the lymph node architecture appears to remain intact.
In severe combined immunodeficiency (SCID) mice with human thymus/liver
implants, molecular clones of HIV produce the same pattems of cell killing and
pathogenesis as seen with virus isolated from humans.
12. HIV is similar in genetic structure and morphology to other lentiviruses
that often cause immunodeficiency in their animal hosts in addition to
slow, progressive wasting disorders, neurodegeneration and death.
Like HIV in humans, animal viruses such as visna virus in sheep and simian
immunodeficiency virus (SIV) in macaques primarily infect cells of the immune
system such as T cells and macrophages. SIV, for example, infects several
subspecies of macaques causing diarrhea, wasting, CD4+ T cell depletion,
opportunistic infections and death.
13. The specific immunologic profile that typifies AIDS - a persistently low
CD4+ T cell count - is extraordinarily rare in the absence of HIV infection
or other known causes of immunosuppression.
In a CDC survey, only 47 (.02 percent) of 230,179 individuals diagnosed with
AIDS had CD4+ T cell counts lower than 300/mm3 of blood in the absence of a
positive HIV antibody test and other conditions or therapies associated with
depressed levels of these cells.
In the NIAID-supported Multicenter AIDS Cohort Study (MACS), 22,643 CD4+ T
cell determinations in 2,713 HIV-seronegative homosexual men revealed only
one individual with a CD4+ T cell count persistently lower than 300 cells/mm3,
and this individual was receiving immunosuppressive therapy.