Page 22 - AIDSBK23C

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OTHER HUMAN RETROVIRUSES

HIV-2:-- The numerous strains of HIV-1 isolated from various geographic regions of the
world are all immunologically similar and differ only slightly in their DNA sequences. A second
retrovirus designated HIV-2 has been isolated from a number of patients with AIDS, first in
West African countries and subsequently in Western Europe, the United States, and elsewhere.
Most cases have appeared in West Africa and have appeared only sporadically in other parts of
the world, such as India.[101] HIV-2 is believed to have been present in Africa as early as the
1940’s. HIV-2, which has greater homology to simian immunodeficiency virus (SIV) than to
HIV-1, appears to have become established in human populations as a zoonotic infection from
the primate reservoir of sooty mangabeys (Cercocebus atys).[24,41]
HIV-2 infection is mainly found in West African nations, including Guinea-Bissau, The
Gambia, Senegal, Cape Verde, Cote d'Ivoire, Mali, Sierra Leone, and Nigeria. HIV-2 is spread
in a manner similar to HIV-1, though the high-risk groups are commercial sex workers and
persons with other sexually transmitted diseases. The peak age of persons infected with HIV-2
appears to be higher than that of HIV-1, but there appears to be no sex difference in rates of
infection. The overall prevalence of HIV-2 in endemic areas appears to have declined in the
st
early part of the 21 century. This decline may be due to a low rate of vertical transmission
(about 4%) and to low infectivity from few viral particles shed into genital secretions. The lower
infectivity of HIV-2 is likely related to lower RNA levels.[101,102]
HIV-2 appears to utilize the same cellular mechanisms for infection as HIV-1, including
the use of CD4 receptors and chemokine coreceptors. Persons infected with HIV-2 infection
have a longer asymptomatic phase, higher CD4 cell counts, lower viral RNA levels, and slower
progression to AIDS than HIV-1 infection.[45] There is a well-preserved and functionally
heterogeneous HIV-specific memory CD4+ T cell response that is associated with delayed
disease progression in the majority of people infected with HIV-2.[103] Broadly neutralizing
antibodies, rare in HIV-1 infection, are present with HIV-2 infection and their presence is
equivalent to a vaccine response.[101]
Just as HIV-1 has distinct subtypes, so does HIV-2. There are eight distinct subtypes of
HIV-2 but only A and B have become endemic. There is up to a 25% difference in genetic
homology among these subtypes. All five subtypes can be detected by enzyme immunoassay
(EIA) and Western blot assays for HIV-2 similar to those for HIV-1. The reverse transcriptase
enzyme is similar in structure and function in both HIV-1 and HIV-2. Infection with HIV-2
eventually leads to AIDS. Persons can be coinfected with HIV-1 and HIV-2.[101,102,104]
The genetic sequences of HIV-1 and HIV-2 are only partially homologous. HIV-2, or
other as yet uncharacterized members of the HIV-group of viruses, will not necessarily be
detected by using the various laboratory tests for HIV-1 antibody, including enzyme
immunoassay (EIA) and Western blot (WB) tests, in general use for HIV-1. Instead, separate
EIA and WB assays are employed for diagnosis of HIV-2. HIV-2 is genetically more closely
related to simian immunodeficiency virus (SIV) than HIV-1.[105]
This potential problem of genetic variation with HIV was illustrated in 1994 with the
detection of a strain of HIV-1 (designated MVP-5180, or subtype O), a new HIV variant
originating in the region of West-Central Africa, which showed only slightly more homology
with other HIV-1 strains than with HIV-2. This variant is detectable with many testing methods