Introduction
The intracellular protozoan parasite Trypanosoma cruzi causes Chagas' disease in humans
(Mengel and Rossi, 1992; Rassi et al., 2010). According to the latest studies, about
5–8 million people are infected by this parasite around the world, representing a
significant global economic burden (Rassi et al., 2010; Lee et al., 2013; Maguire,
2015). The infection may be divided in acute, indeterminate and chronic (Mengel and
Rossi, 1992; Andrade et al., 2014). The acute disease, much less frequent nowadays,
may be, in most patients, successfully treated with benznidazole, a drug that efficiently
kills the parasite (Rassi et al., 2010). On the other hand, the chronic disease is
by far the most important condition, concerning the total number of infected people.
The chronic disease is characterized by a persistent inflammatory reaction and destruction
of host cells, affecting mainly the peripheral autonomous nervous system in the gastrointestinal
tract, the heart muscle and intracardiac nerves in ~30–40% of the infected patients,
causing the development of megaesophagus and megacolon, and cardiomegaly associated
with progressive and untreatable heart failure, in addition to an increased frequency
of sudden death (Andrade et al., 2014). Yet, the majority (about 60–70%) of the patients
that progress to the chronic phase remain clinically asymptomatic (Umezawa et al.,
2001). Contrarily to most predictions, a large, multicentric, placebo-controlled,
double-blinded clinical trial has shown that treatment with benznidazole has failed
to modify the clinical outcome of chronic Chagas' disease, in spite of promoting a
significant parasite load reduction (Morillo et al., 2015). these results argue that
the treatment concept fails rather than the anti-parasite effect of the drug itself,
suggesting that the presence of the parasite in the chronic infection may not be the
only factor responsible for its clinical progression but perhaps a chronic infection
that is inappropriately dealt with by the host defenses would be a major question,
possibly involving a malfunction of regulatory immune mechanisms and autoimmune phenomena
(Mengel and Rossi, 1992; Cardillo et al., 2015). This theoretical framework also accommodates
most chronically T. cruzi-infected patients, where this malfunction of regulatory
immune mechanisms would not take place, a balanced immune response would be achieved,
and pathology would not prevail (Cardillo et al., 2015).
Regulatory t cells in chagas' disease
Regulatory T cells may curb inflammatory responses, allowing a partially effective
anti-parasite immune response to remain active. In fact, the role of CD4+CD25+Foxp3+
Treg cells during the chronic T. cruzi infection has been evaluated in mouse and humans.
The most recent studies have suggested an important role for this T cell subpopulation
in the chronic disease. For instance, recent findings in humans have shown an increased
percentage of Treg cells in chagasic subjects in the indeterminate chronic phase of
the infection (free of disease) when compared to patients with heart damage, suggesting
an important role for Tregs in Chagas' disease (de Araujo et al., 2011).
In addition, in vivo Treg blockade by using nondepleting monoclonal antibodies to
CD25, a marker for these cells, diminished the myocardial inflammation in the chronic
infection, a finding that was partially ascribed to consequent increased levels of
IL-2 secretion and expansion of CD4+CD25+ regulatory T cells (Nihei et al., 2014;
Bonney and Engman, 2015), providing an indication that the functional activity of
Treg cells might be of critical importance during the chronic phase of the infection.
It should be noted that not all CD4+CD25+foxp3+ T cells in humans are suppressive
and a fraction of them have other effector functions (Miyara et al., 2015). It was
recently described that the expression of CD15s (sialyl Lewis x) identifies the most
suppressive foxp3 high regulatory CD4+CD25+ T cells (Miyara et al., 2015), discriminating
suppressive from effector CD4+CD25+foxp3+ T cells in humans. Interestingly, a previously
published study has described that the expression of CD15s was decreased in peripheral
blood lymphocytes from patients with severe Chagas' disease (Laucella et al., 2001).
Therefore, the expression of CD15s in Tregs may be also relevant to predict progression
to pathology or treatment efficacy in chagasic patients and further studies concerning
this point are warranted.
Administration of nondepleting anti-CD25 mAb increases the production of IL-10 by
T cells during the chronic T. cruzi infection in mice (Nihei et al., 2014). It is
clear that IL-10 is a pro-survival cytokine during the acute infection, by controlling
immune hyperactivation and/or by increase immunity to the parasite (Hunter et al.,
1997; Roffe et al., 2012). In Chagas disease, IL-10 seems to be part of a regulated,
nonpathogenic immune response in patients without cardiomyopathy (Poveda et al., 2014).
These studies suggest that exogenous IL-10 supplementation may be helpful in the control
of myocarditis during Chagas' disease. Pegylated IL-10 shows an increased in vivo
half-life and helps in the activation of CD8 T cells. This activity would certainly
be important in the control of residual tissue infection (Oft, 2014). Additionally,
pegylated IL-10, through its potential antifibrotic activity (Mattos et al., 2012),
would be helpful in the control of heart fibrosis, along T. cruzi infection, making
exogenous IL-10 supplementation an extremely interesting approach to be tested in
future studies.
The interleukin 2 axis in immunity and autoimmunity
Interleukin-2 (IL-2) was first identified as a growth and differentiation factor produced
and utilized by T cells (Liao et al., 2013). IL-2 is mainly produced by CD4 T cells
and, upon activation, the production of IL-2 declines along with the T cell differentiation
process (Sallusto et al., 2004). Therefore, memory T cells produce less IL-2 than
naïve T cells. IL-2 is also produced to a lesser extent by CD8 and NK T cells, some
dendritic and mast cells (Liao et al., 2013) There are two types of receptors for
IL-2 (IL-2R), the high and the low affinity receptors (Liao et al., 2013). The low
affinity receptor is a molecular complex formed by the association of the beta and
the common gamma chains. The IL-2R beta chain (CD122) is expressed on T cells and
many other cell lineages (Liao et al., 2013). The high affinity receptor has the addition
of the IL-2R alpha chain (CD25) to the low affinity molecular complex (Liao et al.,
2013). CD25 is mainly expressed in some activated T and B cell subpopulations as well
as NK cells (Liao et al., 2013). IL-2 is of crucial importance for the T cell proliferation
and survival during activation and differentiation to short-lived memory cells (Liao
et al., 2013), being dispensable after the expression of high levels of the IL-7 receptor
or the expression of the IL-15 alpha chain receptor (Liao et al., 2013). Therefore,
it seems that IL-7 and/or IL-15 rather than IL-2 are the common gamma chain cytokines
that help in the proliferation and maintenance of long-lived memory T cells (Xu et
al., 2014). IL-2 is also a potent cytokine, helping CD8 T cell activation and differentiation
to effector cells, participating in the process of NK cell activation and development
of their lytic activity (Liao et al., 2013). On the other hand, IL-2 is involved in
the regulatory arm of the immune response, by augmenting the proliferation of regulatory
Foxp3+ T cells and the reinforcement of the FAS/FAS-L immunoregulatory pathway (Liao
et al., 2013; Kosmaczewska, 2014). These two opposing functional activities seem to
be related to the amount of the cytokine available in the microenvironment (Kosmaczewska,
2014). Therefore, low amounts of IL-2 would favor the immunoregulatory pathway, whereas
high amounts would be required for driving effector immune responses (Kosmaczewska,
2014). The role of IL-2 in the immunoregulatory axis of the immune system is evident
in mice and humans where the availability of IL-2 was reduced (Liao et al., 2013;
Kosmaczewska, 2014). For instance, IL-2R-beta or IL-2R–alpha chain knockout mice develop
autoimmune disease and blocking of IL-2 by in vivo treatment with monoclonal antibody
to IL-2 accelerates autoimmunity in mice (Setoguchi et al., 2005). In humans, several
polymorphisms in the IL-2 pathway have been linked to type I diabetes and loss of
function of Foxp3+ Treg biological activities. In addition, low levels of circulating
IL-2 have been associated with different autoimmune diseases (Yamanouchi et al., 2007).
Chagasic patients with cardiomyopathy show decreased production of IL-2 by peripheral
blood leucocytes (Briceno and Mosca, 1996). However, serum IL-2 levels have been reported
to increase in chagasic patients with cardiomyopathy (Poveda et al., 2014). The reasons
for the discrepancy are not clear, but IL-2 may be produced by cells trapped in other
tissues and the presence of serum neutralizing factors, such as soluble serum CD25RA
or anti-cytokine antibodies generated by a dysregulated immune response may reduce
IL-2-bioactivity (Campen et al., 1988; Tsybikov et al., 2015). Soluble serum CD25RA
and autoantibodies to many different self-antigens and structures have been described
in Chagas disease (Moretti et al., 2002; Vicco et al., 2013). Autoimmune diseases
usually proceed with lymphopenia and signs of T cell senescence (Stockinger et al.,
2004; Thewissen et al., 2007). These characteristics may reflect a dysfunctional thymus
and lower T cell output (Thewissen et al., 2005; Vieira et al., 2008). A similar picture
has emerged for Chagas' disease. CD4 and CD8 T cells from chagasic patients carry
markers of immunosenescence and present an exhausted functional phenotype with diminished
production of IL-2 (Albareda et al., 2009, 2015). Increased levels of immunosenescence
correlated with more severe forms of the disease (Cardillo et al., 1993; Albareda
et al., 2009, 2015). Recent studies have also shown that the thymus is damaged during
T. cruzi infection and therefore, lower numbers of Tregs and conventional thymic T
cell emigrants would reach peripheral organs. This diminishes T cell renew along the
infection similarly to what has been previously described in other classical autoimmune
diseases (Gonzalez et al., 2016). Therefore, immunesenescence and autoimmunity would
prevail in about 30% of infected people due to the low numbers and poor regulatory
activity of Tregs that lack IL-2 produced by the pool of effector conventional T cells.
Toward novel strategies to treat chronic chagas' disease
The study of acute and chronic phases of infection with intracellular pathogens, such
as T. cruzi, allows the elucidation of the mechanisms and conditions that may be targeted
to reprogram the host immune system by using tools that interfere with components
of the regulatory arm of the immune system machinery. This knowledge would certainly
result in a better understanding of the necessary balance to achieve or reestablish
the health of the host during T. cruzi infection, thus providing new strategies to
treat Chagas' disease. In this regard, the in vivo biological activity of nondepleting
antibodies to CD25 seems to reinforce rather than inhibit the function of regulatory
T cells either in mice or humans (Nihei et al., 2014; Huss et al., 2015). In addition,
the in vivo administration of IL-2 or complexes of IL-2/anti-IL-2 to increase the
numbers and functional activity of regulatory T cells would also be a valuable approach
to be used in Chagas' disease (Letourneau et al., 2010). Low doses of IL-2 have already
been used successfully in many clinical trials (Matsuoka et al., 2013; Kosmaczewska,
2014; Humrich et al., 2015). However, a consensus for the low-dose regimen of IL-2
administration has not been established and serious concerns about which dose of IL-2
to use in different clinical conditions remains open to debate (Perol et al., 2014).
The notion concerning the manipulation of Treg cells either by antibodies to CD25,
IL-2, and/or IL-10 might open up a new avenue for therapeutic strategies in Chagas'
disease. It should be noted that more pre-clinical studies, using different strains
of T. cruzi in combination with distinct strains of mice should be performed in order
to better establish these conceptual clinical interventions, perhaps in addition to
more acquiescent schemes of benznidazole treatment (Bustamante et al., 2014).
Author contributions
JM Design the paper hypothesis, wrote the first draft. FC Design the paper hypothesis,
wrote the first draft. LP Design the paper hypothesis, wrote the first draft.
Conflict of interest statement
The authors declare that the research was conducted in the absence of any commercial
or financial relationships that could be construed as a potential conflict of interest.