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].