Sensitivity of HIV-1 subtype C viruses to Griffithsin, cyanovirin-N and scytovirin: potential HIV-1 microbicides
Date
2013-05-07
Authors
Alexandre, Kabamba Bankoledi
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Abstract
The majority of HIV-1 infections around the world occur via sexual intercourse and
women, especially in developing countries, are disproportionately affected. Recently a
number of strategies have been proposed to control the spread of HIV, among these the
use of microbicides to prevent the sexual transmission of the virus. A clinical trial of 1%
tenofovir gel that conferred up to 39% protection provided a proof-of-concept that an
anti-HIV microbicide is feasible. Various other compounds, acting at different stages of
HIV-1 life cycle, are also being investigated as potential microbicides. These include the
lectins Griffithsin (GRFT), cyanovirin-N (CV-N) and scytovirin (SVN). GRFT was
isolated from the red algae griffithsia sp. while CV-N and SVN were isolated from the
blue green alga Nostoc ellipsosporum and the cyanobacterium Scytonema varium,
respectively. These lectins bind mannose-rich glycans found on the surface of HIV-1
envelope and act as entry inhibitors. Although HIV-1 subtype C is the main cause of
infections around the world, almost all studies conducted with GRFT, CV-N and SVN
are based on subtype B viruses. The Chapter Two sought to establish the neutralization
sensitivity of HIV-1 subtype C viruses to the three lectins, using both a cell line and
primary cells, and compared this sensitivity to subtype B. This Chapter also examined
mannose-rich glycans on HIV-1 that are involved in GRFT, CV-N and SVN binding. The
conclusion from this study was that the neutralization of subtype C viruses by these
lectins is similar to subtype B and that the 234 and 295 mannose-rich glycans were
involved in their interaction with the virus. In general these data supported further studies
on the use of GRFT, CV-N and SVN for prevention of HIV-1 subtype C sexual
transmission. In Chapter Three, the ability of GRFT to expose the CD4 binding site
(CD4bs) on HIV-1 gp120 is explored. I found that this exposure resulted in the
enhancement of HIV-1 binding to plates coated with anti-CD4bs antibodies b12 and b6
or the CD4 receptor mimetic CD4-IgG2. This lectin also synergized with b12 and HIVpositive
plasma containing antibodies to the CD4bs to neutralize the virus. Furthermore,
the glycan at position 386, which shields the CD4bs, was shown to be involved in both
GRFT enhancement of HIV-1 binding to b12 and b6 and in the synergistic interaction
between the lectin and these antibodies. The importance of this study is that it
investigated in details the effect of GRFT binding on HIV-1 envelope and also suggests
this lectin can be used in combination with anti-HIV-1 antibodies to synergistically
enhance the anti-viral activity. In Chapter Four I investigated GRFT, CV-N and SVN
inhibition of the virus binding to the DC-SIGN receptor and their inhibition of the DCSIGN
transfer of HIV-1 to target cells. These lectins only moderately inhibited the virus
binding to the receptor while they potently inhibited its transfer to target cells. However,
the inhibition of transfer was stronger when the virus bound the lectins after binding the
DC-SIGN receptor compared to when it bound the lectins prior to binding the receptor.
These three lectins can, therefore, inhibit the sexual transmission of HIV-1 since the DCSIGN-
mediated transfer of the virus to susceptible cells is pivotal to this mode of
transmission. Chapter Five is an investigation of the ability of HIV-1 subtype C to escape
GRFT, CV-N and SVN, which involved growing the virus under escalating
concentrations of these compounds. This was to know how this virus behaves under
conditions of continuous exposure to the lectins. I found that HIV-1 subtype C became
increasingly resistant to the lectins and viral envelope sequence analysis showed that this
was associated with the deletion of mannose-rich glycans on gp120. Furthermore, of the
11 potential mannose-rich glycosylation sites on gp120 seven (230, 234, 241, 289, 339,
392 and 448) were involved in GRFT, CV-N and SVN resistance. Thus, the conclusion
was that although these three lectins are potent inhibitors of HIV-1 infection, the virus is
also able to escape their neutralization by deleting mannose-rich glycans on its envelope.
However, the fact that escape to these lectins involved multiple deglycosylation and was
only partial suggests that HIV-1 subtype C escape from GRFT, CV-N and SVN in a
microbicide formulation may not be an easy process. We discuss the implications of these
findings in Chapter Six and suggest future studies that could complement data presented
in this thesis. Overall our data show that GRFT, CV-N and SVN can prevent the sexual
transmission of both free and DC-SIGN associated HIV-1 particles and supports further
development of these lectins as microbicides against HIV-1.
Description
Thesis (Ph.D. (Virology))--University of the Witwatersrand, Faculty of Health Sciences, 2012