Abstract
Sequel to the earlier works by Omenyi et al which established the role of surface
thermodynamics in various biological processes from the electrostatic repulsion and van der
Waals attraction mechanisms, HIV-blood interactions were modeled. This involved the use of
the Hamaker coefficient approach as a thermodynamic tool in determining the interaction
processes. It therefore became necessary to apply the Lifshitz derivation for van der Waals forces
as an alternative to the contact angle approach which has been widely used in other biological
systems. The methodology involved taking blood samples from twenty HIV-infected persons and
from twenty uninfected persons for absorbance measurement using Ultraviolet Visible
Spectrophotometer (Ultrospec3100pro). From the absorbance data various variables (e.g.
dielectric constant, etc) required for computations with the Lifshitz formula were derived. CD4
counts using the digital CD4 counter were also obtained. Due to the very la
INTRODUCTION
1.1 Rationale:
At the 2001 Special Session of the UN General Assembly on AIDS, 189 nations
agreed that AIDS was a national and international development issue of the highest
priority [1]. Between December 2005 and March 2006, UNAIDS compiled data from
reports obtained from 126 countries on HIV/AIDS prevalence. In sub-Saharan Africa
a mature epidemic continues to ravage beyond limits that many experts believed
impossible. Also, relatively new but rapidly growing epidemics in regions such as
Eastern Europe and South-East Asia that may come to rival that of sub-Saharan
Africa in scope, had erupted [2].
Over time diverse clinical approaches to the issue of HIV/AIDS have been employed
to seek to proffer possible solutions to the threat. Progress in this regard has been slow
and far in between but has given birth to some palliative measures which include the
introduction of the Highly Active Anti-retroviral Therapy (HAART). However, the
results have not actually shown an easy and comprehensive solution due to the rapid
mutative genetic nature of the virus [3].
Much research has been and is still on, on this subject with a cure not yet in view. The
choice to approach it via the vehicle of surface thermodynamics against the
conventional clinical methods is a novel one. The optimism stems from the great
successes recorded with this approach in related areas of biology and medicine. The
role of surface properties in various biological processes is now well established. In
particular, interfacial tensions have been shown to play an important, if not crucial
role in phenomena as diverse as the critical closing and opening of vessels in the
microcirculation, cell adhesion, protein adsorption, antigen-antibody interactions, and
phagocytosis [4].
1.2 Background to Study:
The HIV is assumed to be a particle which is dispersed in a liquid (the serum) and
attacks another particle (the lymphocytes). The virus attaches itself on the surface of
the blood cell before penetrating it to attack the RNA. If the surface of the blood cell
is such that it will repel the virus, access to the virus into the interior of the cell would
have been denied. Thus, the initial actions take place on the surfaces of the cell and of
the virus (assumed to be particles). This interaction which involves two surfaces
coming together in the first instance can be viewed as a surface effect.
It therefore stands to reason that, if it is possible to determine the surface properties of
the interacting particles, then one can predict the mechanisms of their interactions.
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When two particles make contact, they establish a common area of contact. Some
original area of the surface of each particle has been displaced, and the work done to
displace a unit area of the surface is referred to as the surface free energy. The actions
therefore that take place on the surfaces are termed surface thermodynamic effects.
These actions are assumed to occur slowly so that thermodynamic equilibrium is
assured. This concept will be employed in this research work to characterize the HIVblood interactions with the serum as the intervening medium.
The clinicians have analyzed the surfaces of blood cells on which the virus binds.
There are receptors and coreceptors on these cells and suggested types and their roles
in the attachment processes are given in tables 1.1 and 1.2. Figures 1.1 and 1.2 show
the nature and the interactions of these cells.Fig.1.1: Human Immunodeficiency Virus (HIV) Anatomy [5]
HIV infects immune cells by interacting with proteins on the cells’ surfaces. The
CCR5 is the preferred co-receptor for HIV in the human immune system. Immune
cells that express CCR5 respond to sites of injury or inflammation. In order to
respond effectively, they must go to the site of action. When a tissue experiences
trauma or inflammation, nearby cells secret signal molecules called chemokines. The
chemokines diffuse out from the site of the trauma through the blood stream where
they come in contact with cells expressing the appropriate receptor. Each cell
expresses several different receptors so they can respond to different immune signals.
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Figure 15b
The
Fig.1.2: Interaction of a Dendritic Cell (right) having HIV bound to its surface
(arrow) with a Lymphocyte (left) [6]
CCR5 is a seven trans-membrane protein or 7TM which means that it crosses the
plasma membrane of the cell seven times. 7TM proteins are sensors for the cell. They
communicate what happens outside the cell to the inside of the cell through a process
called alosterism. The Chemokines bind to CCR5 which causes the CCR5 to change
shape both outside and inside of the cell. The altered shape of CCR5 changes the
interactions with G-proteins inside the cell initiating a signal transduction cascade that
activates the cell to go to the site of injury.
The redundancy inherent in the immune system allows many Chemokines to signal
for multiple coreceptors. CCR5 binds the Chemokine’s RANTES, MIP-1? and
MIP-1?. It is important to note that these Chemokines also bind to other receptors.
Both RANTES and MIP-1? can bind to CCR1 and RANTES can also bind to CCR3.
This is an example of redundancy which is common in the immune system. In this
way, if one pathway is blocked, the immune response can be achieved through
another. Thus, as these receptors interact with the stream of Chemokines they direct
the cell to the site of injury or inflammation. In summary, CCR5 plays an important
role in the movement of immune cells to the site of action. The key points include;
? CCR5 is a censor protein on certain immune cells.
? CCR5 binds to selected Chemokines like MIP-1?, MIP-1? and RANTES.
? CCR5 transduces signals inside the immune cell.
? These signals result in chemotaxis or movement of the cell to the site of
injury.
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Table 1.1: Cell Surface Receptors Implicated in Binding HIV Virions: Receptors
other than CD4 or Coreceptors that attach HIV Virions to Cell Surfaces [7]
Receptor Affinity
(Kd)
Expression Role in attachment and
infection
Reference
Gal-C High
(11.6 nM)
Neuronal and
glial cells
Confers inefficient
infection presumably by
aiding attachment
Harouse et al. (1991)
Sulphatide (sulphate
derivative of Gal-C)
Colorectal
epithelial cells
and primary
macrophages
Confers efficient CD4-
independent infection by
NDK, a TCLA HIV-1
strain Requires CXCR4
coreceptor
Fantini et al. (1993);
Seddiki et al. (1994);
Delezay et al. (1997)
Placental
membrane-binding
protein
High
(1.3–0.6
nM)
Cloned from a
placental
cDNA library
Binds virus particles to
the cell surface and thus
enhances infectivity via
CD4 and coreceptors.
May trap HIV in the
periphery and carry to
T-cells in lymph nodes
Curtis et al. (1992);
Geijtenbeek et al.
(2000)
DC-SIGN On dendritic
cells
DC-SIGNR Endothelial
cells, such as
liver,sinusoidal
and lymph
node sinus
endothelial
cells
Acts in the same way as
DC-SIGN
Pohlmann et al.
(2001)
Mannose-specific Macrophages Binds gp120 Larkin et al. (1989)
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macrophage
endocytosis receptor
Heparans Many cell
types
Attaches virus particles
to cell surfaces via an
interaction with the V3
loop thus enhancing
infectivity via CD4 and
coreceptors. Acts
predominantly for
CXCR4-using viruses
Mondor et al. (1998)
LFA-1/ICAM-1 LFA-1 is
expressed on
haematopoietic
cells, ICAM-1
is on a wide
variety of cell
types
ICAM-1 encorporated
onto virions enhances
attachment and infection
of LFA-1
+
cells
Fortin et al. (1999);
Paquette et al.
(1998)
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Table 1.2: Human Polymorphisms in Chemokine and Coreceptor Receptor
Genes that influence HIV Infection and Disease Progression [7]
Genotype
Frequency
Effect
CCR5 32/wild-type Up to 18 % in
Caucasians
Slows disease progression
CCR5 32/ 32 Up to 1 % in
Caucasians
Protects against infection
CCR5 m303 leads to premature stop
codon and CCR5 truncated in E1
3/209 healthy
donors
In combination with a 32 CCR5 allele confers
T-cells with resistance to R5 viruses
CCR5 P1 allele, characterized by a pattern
of 10 specific bases at different sites,
including A at –2459
43–68 % Accelerates disease progression
CCR5 A/G at –2459 43–68 % Slows disease progression
CCR2 V64I is linked to a point mutation in
the promoter region of CCR5
10–15 % in
Caucasians and
US Africans
Slows disease progression
SDF-1 in 3´ untranslated region of mRNA.
In SDF-1 but not SDF-1 mRNA
16–25 % Homozygotes have slower disease progression,
even slower if 32/wild-type CCR5 or V64I
CCR2 also
RANTES promoter AC, GC and AG at
sites –471, –96 (sites equivalent to –403
and –28 as described by Liu et al., 1999)
Variable
depending on
population
Faster/slower disease progression depending on
genotype and population (Gonzalez et al., 2001).
Some protection from transmission if –471A
present
MIP-1 intron +113, +459 Variable
depending on
population
Faster/slower disease progression depending on
genotype and population (Gonzalez et al., 2001)
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1.3 Statement of Problem:
The discovery and application of highly active anti-retroviral therapy (HAART) to
suppress HIV has revolutionized the clinical management of HIV/AIDS cases. The
HIV however, has the capacity to develop resistance to the antiretroviral drugs and
this phenomenon has turned out to be a significant cause of failure of HAART. HIV,
being an RNA-based rapidly mutating virus, (unlike the DNA-based counterparts)
lacks the ability to check for and correct genetic mutations that can occur during
replication. In chronic HIV cases, about ten billion new viral species can be generated
daily. This rapid genetic variation has made it rather very difficult to proffer a clinical
solution to the problem [3] and the worldwide picture is one of increasing rates of
infection [8].
It is against this backdrop that this study explores a novel and rare approach using
surface thermodynamics to seek a way forward in the research on the topic of
HIV-blood interactions. The successes recorded in the use of this approach in finding
solutions that have brought about many scientific applications cannot be
overemphasized [4].
1.4 Objective of the Study:
This research work is aimed at employing the concept of surface thermodynamics to
study the interaction between the virus and the blood cells with a view to proffering a
solution to the HIV/AIDS pandemic. The following tasks therefore, must be kept in
view;
(i) Determine the mechanism of interaction of HIV with white blood cells.
(ii) Seek a thermodynamic interpretation of such interactions through van
der Waals attraction mechanism.
(iii) Quantify such interactions through actual measurements.
(iv) Recommend possible approach to eliminating the HIV-blood
interactions.
Thus, the main thrust of this research work is the use of surface thermodynamics in
explaining the HIV/AIDS jinx.
1.5 Scope and Limits of the Study:
The scope of this research is limited to specifying the relevance of van der Waals
forces to the fusion of the HIV with the receptor cells and how such fusion process
could be quantified and prevented. This is intended to be achieved by the application
of surface thermodynamics using the concept of Hamaker coefficients derived from
absorbance data required for the computation of the Lifshitz formula. The sign of the
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combined Hamaker coefficient will suggest whether there is attraction or repulsion
between the virus and the blood cells.
The next approach would be to suggest a formulation that would aid in preventing
contact between the virus and the blood cell, and hence prevent their interactions.
This entails the development a model that would render the absolute combined
Hamaker coefficient, A132abs negative thus causing the virus and the lymphocytes to
repel each other [9].
Sourcing for additives to achieve this aim is beyond the scope of this work. Other
approach for the determination of Hamaker coefficients, e.g. by contact angle data,
will not be considered. These will serve as suggested areas for further research.