Tuberculosis
M. tuberculosis (Mtb) the causative agent of TB is well-adapted
to the human host, such that it can lie dormant for several years
(latent TB), sometimes a life time, without causing disease. Only 5-10%
of subjects infected with Mtb progress to disease during their life.
Activation of latent TB can be due to several reasons among which HIV
co-infection is a major pre-disposing factor [68]. Although, Mtb is
spread through aerosols and replicates in lung epithelial cells, it can
also replicate in lymph nodes, bones, stomach, kidneys and other organs
causing extra-pulmonary TB. In extreme cases Mtb can be
systemically disseminated precipitating a potentially fatal condition
known as miliary TB. Upon entering the host through aerosol, Mtb bacilli
are taken up by alveolar macrophages by phagocytosis facilitated by cell
surface receptors, e.g. toll like receptors (TLR), C-type lectin
receptors (CLR), scavenger receptors (SR), complement receptors (CR) and
Fc receptors [reviewed in 69] and replicate in macrophages in the
lung parenchyma. Primed DCs traffic to the lymph node and trigger
activation of adaptive immune cells, which are recruited to the lung and
gradually an organized structure, the granuloma, begins to form, which
comprises a caseous, necrotic core with replicating bacilli, surrounded
by an inner ring of epithelioid interlocked macrophages, neutrophils and
foam cells and an outer ring of T cells, B cells and NK cells [70].
The resolution of infection within the granuloma relies on host immune
responses, which can potentially be impacted by Treg cell function.
Indeed, the role of Tregs has been studied in the context of early acute
stage of Mtb infection and the chronic phase of infection with evidence
from mouse, primate and human studies, as summarized below.
Tregs in TB : Acute phase of infection: an
analysis of animal model studies suggests early expansion to be
detrimental. Mice models highlight the impact of Tregs on TB to be
phase specific with Treg frequencies inadvertently high in the acute
phase, which is detrimental for infection control [71-76]. Aerosol
infection of mice with mycobacteria leads to activation of
CD4+ Teff cells by infected DCs in the pulmonary lymph
node at approximately day 11 and subsequent expansion and accumulation
of CD4+ (effector and regulatory) T cells in the lungs
by day 14-21 [71]. Significant disease associated lung pathology and
cfu (colony forming units) burden can be observed at day 14-21 and this
period can be classified as the early phase of infection in mice [71,
74, 75]. Time points subsequent to this, e.g. 4-7 wks post infection
can be classified as late stages of infection [71, 74, 75]. Whilst
timelines for early and late phases can vary with multiplicity of
infection, in general 50-200 Mtb cfu results in increased Treg
frequencies in lung and pulmonary lymph node at 10-21 days which is
maintained till 60-127 days post infection [73,76]. This early
expansion was found to be deleterious to emerging protective anti-TB Th
responses [72-74, 76, 77]. Depletion of Tregs in C57BL/6 mice by
systemic administration of anti-CD25 3 days prior to infection with BCG
resulted in enhanced culture filtrate protein (CFP) specific
IFNγ+ and IL-2+CD4+ cells in lungs and spleen of BCG-infected mice 14
days post infection suggesting that presence of Treg cells hinders
appearance of protective Th1 responses [74]. Also, adoptive transfer
of CD25+ Treg into Mtb infected mice leads to reduced
frequencies of Mtb-specific Teff cells in the lungs at 14-17 days post
aerosol infection [72]. Importantly, absence of protective Th1
responses due to expansion of Tregs leads to increased bacterial burden
in the acute phase [73]. However, this dampening effect of Tregs on
protective immune responses is transient and not evident in later stages
of infection [71, 74, 75]. Depletion of CD25+ Treg
had no effect on cfu burden or lung pathology in BCG or Mtb Erdman
infected mice at days 21 and 44 post infection [74]. Similar results
were reported in another study, where Treg depletion in Mtb Erdman or
Kurono infected DBA/2 mice reduced cfu at 2 weeks post infection but had
no effect on bacterial burden or pathology subsequently, at 3 and 5
weeks [75]. It has now been demonstrated in mice that Mtb-specific
Tregs are culled via IL-12 driven expression of T-bet by 32 days post
infection; T-bet being known for its pro-apoptotic effects [71]. How
Mtb infection drives this early expansion of Mtb-specific Tregs, which
is beneficial to the pathogen remains to be elucidated.
Chronic phase: animal model studies show loss in Treg
frequency or failure to recruit Tregs to site of infection can be
detrimental. In contrast to the detrimental role of Tregs in the early
/ acute stage of infection in murine models, several studies in
mouse and primate models highlight a potentially beneficial role for
Tregs in the chronic phase of infection. Comparison of TB disease
progression and pathology in TB resistant and TB susceptible mouse
strains showed TB resistant mouse strains to have higher Treg
frequencies and consequently less TB induced lung pathology in the
chronic phase of the disease [78, 79] compared to TB sensitive mice,
which recruit significantly fewer Tregs to the lung [79].
Interestingly infecting TB sensitive C3HeN/FeJ mice with M.
maserensis (environmental mycobacterium) resulted in a boost in Treg
frequencies with a reduction in lung pathology and improved survival
[78]. These observations have been corroborated in non-human primate
models of TB infection, where cynomolgus macaques infected with 25 cfu
of Mtb Erdman can either develop active TB or establish latency
[80]. In this experimental system it was observed that macaques that
developed latent TB had higher basal pre-infection Treg frequencies
compared to animals that develop active disease [80]. In a separate
study, IL-2 administered either pre- or post-Mtb infection in macaques
resulted in Treg frequency expansion, which in turn led to reduced
bacterial burden and TB induced pathology, suggesting that expansion of
Treg cells in later stage of chronic TB infection can help control
excessive TB induced inflammation [81].
Human studies: In contrast to animal model studies where
changes in circulating Treg frequency can impact infection levels,
reports of Treg frequencies in human TB are varied. Some studies show an
increase in peripheral Treg frequencies in TB [55-58]. However, our
study [61] and others [59, 60] found no differences in
peripheral Treg frequencies between pulmonary TB patients and healthy
controls. This disparity may be linked to differences in markers used
for Treg delineation, which vary and can include, CD4 and CD25 [55,
56]; CD4, CD25 and FoxP3 [57, 59], a combination of CD4,
CD45RA/CD45RO, CD127, CD25 and FoxP3 to identify memory Tregs [60,
61] or
CD4+CD127loCD25+FoxP3+CD45RO+Ki67+to identify activated Treg cells [60]. Beyond variation in markers
used for definition, a further limitation of only tracking Treg
frequency to define Treg function in a disease like TB, is the impact of
trafficking; thus Treg frequencies have been shown to be higher at the
site of infection in the broncheoalveolar lavage compared to that in the
peripheral blood of pulmonary TB subjects [55, 82].