|
Pharmacolgical
and immunopathogenetic progress, as well as epidemiological aspects, have in
recent years transformed HIV infection into a chronic disease having
prevalently sexual transmission and pathogenetic expressions characterized by
an immunological complex interaction where both chronic activation and
immunodepression co-exist. In short, this infection has been changed to a
condition having better survival rates, which not only involves the risk
categories but also, with progressing incidence, "normal" populations
too. This metamorphosis has occurred thanks to an improved and faster
awareness of the role of the immune system and its complex networks.
This
has involved research on the thymus, the T-lymphocyte repertoire, and the
acknowledged importance of studying the lymphoid tissue and introduction of
new, combined therapies based on diverse associations of protease inhibitors,
nucleosidic analogues and not (Fig. 1). It is therefore now more appropriate
to describe HIV infection as a chronic disease in which the natural history
is determined by the dynamic balance between the virus' capacity for survival
and the possibility of the organism to neutralize it, not excluding the role
of genetic factors. In this scenario, although employing all its numerous
resources, the immune system is unable to eradicate the virus.
These
new concepts have produced a series of fairly clear pathogenetic prospects,
such as:
a.
There is no biological latency for the infection; even in a reduced quantity,
the virus continues to replicate itself, especially in the lymph nodes.
b.
Right from the earliest stages of the infection, the immune system is exposed
to chronic activation.
c.
The kind of immune response in the initial phases of the infection (the
expression of HLA molecules, deletion of the T-lymphocyte repertoire, the
type of chemokine receptors) can condition the course of the disease.
d.
A chronically depressed state of some immune functions (a lymphopenia of the
"naïve" lymphocytes, lymphocytes never coming into contact with the
antigen), co-existing with chronic activation and/or ensuing, determining a
very characteristic background where one sees the contemporaneous presence of
rises and falls in determined lymphocyte and cytokine populations.
e.
The problem of immune reconstitution following HAART therapy.
f.
The possibility of combining other treatments with the anti-retroviral one,
for better eradication of the virus from the "reservoir" of cells.
IMMUNOLOGICAL
ACTIVATION IN HIV INFECTION
The
primary infection from HIV causes massive viral replication that spreads the
virus to the lymphoid organs, characterizing a very vigorous immune response
that is both tumoral and cellular. However, this does not impede the virus
from continuing to replicate itself in inaccessible tissues to the immune
system, such as the CNS, the macrophages, and "resting" cells, i.e.
not in an active phase where the virus is integrated as a provirus DNA.
This
especially characterizes the infection's asymptomatic phase. Based on
mathematical models, it is calculated that at least 10 virions are produced
every day and more than 99% of this output originates from newly infected
cells, whose half-life is around 1-6 days. After about 6 months of infection,
the viremia may be a predictive criterion of the progression. It is
influenced both by factors correlated to the immune system (cytokines,
chemokines and their receptors, and HLA molecules) and to the virus
(replicating ability, mutation rate, and cytopathicity).
In
these initial phases, the immune system phenotypically expresses signs of its
activation via:
-
The presence of several cytotoxic CD8 T-lymphocytes specific for the virus
and activated .
-
The expression of activation markers specific for the T-lymphocytes (CD69
receptor; receptor for IL-2; receptors for transferrins) .
-
An abnormal output of Th1 cytokines, i.e. pro-inflammatory, progressively
replaced by Th2 cytokines, anti-inflammatory, that can be harmful to the aim
of an induction of effective cellular immunity.
Concerning
this, Autran et al recently defined the role of some lymphocytes equipped
with the CD7+ receptor, the CD4+CD7+ lymphocytes, which are associated with a
cytokine secretion profile of Th0/Th1 type, whereas the CD4 lymphocytes not
equipped with this receptor, the CD4+CD7- lymphocytes, would be associated
with a prevalently Th2-type profile. Our own immunology team (Perrella et al)
has also for some time been working on these populations, and has come up
with a series of preliminary data relating to their roles in HIV infection
(unpublished data).
In
fact, this equation, immunological activation = greater progression is
observed in African subjects where the co-infections of HIV with endemic
parasitic conditions determine a faster development of the infection. The
immune system's activation is unable to eradicate the infection and this is
harmful long-term, causing a collapse of the immune function.
It
is inferred from these premises that the distribution of the CD4+
lymphocytes, more then the pathogenetic damage effect, seems the
epiphenomenon of a powerful immunological activation and of its numerous
mediators including the CTL lymphocytes (in similarity to other major human
diseases, like chronic hepatitis B) and various mediators like the
cytokines.
THE GENETIC
ROLE IN THE COURSE OF THE INFECTION
It's
known that humankind is characterized by a high degree of individual
variability, and that the greater or lesser ability to resist infections is
conditioned by different genic dispositions and, in particular, of the HLA
haplotypes.
Analysis
of the immuno-virological events associated with the primary infection has
suggested that a different expansion of the V? regions on the CD8+
lymphocytes
causes a different expression of the cytotoxic HIV-specific response,
more
or less favorable on the condition's development.
The
recent discovery that some genes conferred an almost total immunity to HIV
infection to 1%
of
Caucasian and that high levels of chemokines have a suppressive power on the
virus' replication, has reinforced the concept of genetic conditioning on the
infection's expression and prognosis. These findings have supplied new
elements for the construction of a more correct pathogenetic model that
provides for the presence of co-receptors (for the chemokines) for the
penetration of the virus into the target cell. These co-receptors are
utilized, normally, by the chemokines. There are appreciable doubts over the
fact that high chemokine levels would be protective against HIV infection,
whereas there are no doubts that a state of homozigosis (rare) due to a
deficit of the CCR5 gene does confer protection against the HIV virus.
THE
THYMUS AND HIV INFECTION
The
function of the thymus progressively declines with age, as bioptic and
radiological studies confirm. In HIVAb+ subjects the residual function of the
thymus is rapidly annulled, as is also observed in infants with this
infection.
The
economy of the thymus in the elaboration of the immune response is essential
because:
-
It generates the naïve and CD8+ lymphocytes.
-
It ensures a broad heterogeneity in the repertoire of receptors for the
antigen (TCR).
In the
event of a continuous destruction of the naïve lymphocytes without any new
generation, for the maintenance of lymphocytic homeostasis, the organism may
supply them via the peripheral expansion of already existing cells. In short,
in order to keep the lymphocyte pool steady, a division of the existing ones
is witnessed; but this involves a diminution in the degrees of heterogeneity
in the repertoire of T-lymphocytes, i.e. of the function of recognizing many
antigens.
In
HIVAb+ subjects, the functioning of the thymus can be controlled through the
study of TREC levels (the rearrangement of excision circles) that identify
the cells recently emigrated from the thymus among the circulating T-cells.
Over a lifespan, these levels diminish with age. In patients having HIV
infection, the TREC levels are lower than those found in similarly aged
healthy controls.
Besides
suggesting that thymic function does continue into adulthood, this provides a
significant contribution to the possible role of TREC markers. This is also
suggested in studies by McCune et al, where treatment with anti-retroviral
drugs induced an augmented TREC, though this line of evidence does not appear
to be confirmed by other studies.
These
hypotheses confirm that the body responds to the destruction of the CD4+
lymphocytes with various mechanisms in the attempt to maintain the circulating
lymphocyte pool constant. Until a short time ago, the most followed theory
about the pathogenesis of CD4+ lymphopenia sustained that the level of CD4+
lymphocytes was the outcome of a balancing between destruction and production
and that, long-term, the reproductive capacities become exhausted, causing
the serious depletion of CD4+ lymphocytes. This hypothesis has been
contradicted by a series of experimental evidence (Hellerstein et al) that
has substantially suggested that the decline in CD4+ lymphocytes was caused
by shorter survival rates and reduced reproductive capacity.
In
fact, in experimental trials that make use of a non-radioactive marking
technique, it was also demonstrated by Pantaleo et al that:
-
The quantity of destroyed CD4+ lymphocytes did not correlate with the new
lymphocytic production.
-
The production of CD4+ cells in HIV Ab+ subjects is not significantly
different, in the early stages, from that of HIV-negative subjects, the
turnover being 2-3 times higher.
In
the light of these recent studies, the equation destruction = reconstitution
no longer seems acceptable, emerging that the limited capacity to renew the
cells is one of the possible mechanisms of the progressive destruction of the
immune system.
HAART
THERAPY AND IMMUNOLOGICAL RECONSTITUTION
Many
studies have shown that combined anti-retroviral treatments exercise
an
important role in the pathogenesis of HIV infection, improving lymphocytic
homeostasis and cutting down on viral replication. In particular, the pilot
study by Autran demonstrated that HAART therapy produced an early rise in
memory cells (CD4+CD45RO+) and a much delayed rise in the naïve lymphocytes
(CD4+CD45RA+), as well as a decline in the activated CD8+ lymphocytes, i.e.
CD8+CD38+, in subjects in an advanced stage of the disease.
Other
authors have not subsequently agreed with this data. Our own research team
too, of the Ospedale D. Cotugno (Perrella,Guarnaccia,
Atripaldi,
D'Antonio, Canonico and De Luca) in co-operation with the
Immunology
team at the Università Federico II (Racioppi, Matarese), has shown
(with
some preliminary data presented on the occasion of the "International
Conference
on the Emergence of Infectious Diseases", Naples 1998) that the increase
of naïve lymphocytes was not significant after 12 months of therapy, while
the rise in memory lymphocytes was confirmed from the 4th week of
treatment.
Our
study (currently submitted to "AIDS") (Tab. I ) was carried out on
both peripheral blood samples and on cell suspensions obtained by lymph node
biopsy and related to the analysis of memory and naïve cells,
CD8+CD38+
lymphocytes, and a new immunophenotype, CD4+CD7+ lymphocytes, which is
correlated to a cytokine profile of mainly Th1-type.
This
study highlighted:
- A
significant reduction in CD8+CD38+ lymphocytes in the peripheral blood
compared to the lymph nodes.
-
The naïve lymphocytes did not differ in the two regions.
-
The CD4+CD7+ lymphocytes were diminished in the two regions.
Summing
up this preliminary data, it is possible to derive that:
-
At least in the early months, the increase in therapy is correlatable with a
redistribution of the T-lymphocytes from the lymphoid tissue to the
peripheral circulation.
-
The T-lymphocytic activation is only diminished in the peripheral circulation
but persists in the lymph nodes.
-
In our patients, after 12 months' therapy, there was no recovery of the naïve
population detected.
This
consideration was also corroborated by other research by our team, where
analysis of the T-lymphocyte repertoire displayed a better heterogeneity in
the lymph nodes compared to the peripheral blood.
In
other words, this redistribution would ensure an improved functional quality
for the T-lymphocytes versus a greater number of antigens, determining
the
reduced incidence rate of opportunistic infectious conditions.
Another
important aspect of HAART therapy has to do with its duration. It is known
that a suspension can rapidly determine a rebound of the viremia. Indeed,
according to different studies, even small quantities of provirus DNA
Integrated
into the cell's genome, can induce massive viral replication.
Moreover,
a further limitation of this therapy is determined by its ineffectiveness
regarding the infected cell reservoir, i.e. of those cells that are in a
phase of dormancy and escape the action of drugs, and finally, by the
emergence of resistant strains.
These
limitations leave room for possible alternative therapies, such as:
1.
Genetic manipulations on the CCR5 receptor gene.
2.
The use of cytokines in the restoration of the immune system.
THERAPEUTIC
PROSPECTS
The
duration and cost of the anti-retroviral drugs represent one of the
unresolved problems having a major impact both in terms of prognosis and
Public Health.
It
is one of the tasks that the regional health authority is called upon to
respond to, which depends on the interaction between the Health Assessor, the
Hon. Ettore Liguori, the Health Service Area, co-ordinated by Dr. Roberto
Pepe (see photo), and CERIFARC, scientifically co-ordinated by the
undersigned.
One
of the next objectives is the formulation, by our team, of a combined therapy
protocol of anti-retroviral drugs, or the association of IL-2 with the HAART
therapy.
The
objectives of the study are essentially centered on:
1.
Evaluation of the combination in terms of immunological restoration.
2.
The impact on reservoir levels of infected naïve cells.
3.
Improving the long-term prognosis.
4.
Cutting the total duration of the HAART therapy.
The
diagnostic methodology will be conducted via the monitored
determination
Of
the following variables:
a.
analysis of the circulating T-lymphocyte immunophenotypes (naïve, memory,
CD4+, CD8+);
b.
assessment of the state of activation of the CD8+ lymphocytes;
c.
analysis of the T-lymphocyte repertoire using the method of
"Spectratyping"
based
on the determination of TCR and of its variable regions;
d.
measuring the plasma levels of Th1 and Th2 cytokines and of Leptine;
e.
TREC levels through real-time PCR;
f.
quantitative analysis of the circulating HIV RNA via RT-PCR.
CONCLUSION
In
spite of the progress made, the immunopathogenesis and a definitive therapy
for Aids are still to be clarified.
It
appears evident that the immune system does not just carry out a passive role
of targeting the infection but also plays an active part expressed by the
stimulation on viral amplification via the mediation of cytokines and
chemokines.
Where
there is contemporaneous activation and suppression of the immune system, it
is clear that the immune system is not just carrying out a passive role of
targeting the infection but also an active role expressed by the stimulation
on viral amplification through the mediation of cytokines and chemokines.
In
the scenario of a contemporaneous presence of activation and depression of
the immune system, one cannot exclude the concomitance of genetic factors and
the role of the viral factors expressed by very heterogeneous
quasi-species.
|
|
Mensile di scienza medica e
attualita`