Tuberculous pleuritis is a good model for the study of specific cells at the site of active Mycobacterium tuberculosis (Mtb) infection. We investigated the frequency and phenotype of NK cells in paired samples of peripheral blood and pleural fluid (PF) from patients with tuberculosis (TB) or parapneumonic infection. We demonstrated for the first time a reduction of NK cells in PF from TB with an enrichment in the CD56brightCD16− subset. In agreement, in PF NK cells we observed an increased expression of CD94, NKG2A, CD62L, and CCR7 molecules and lower expression of Bcl-2 and perforin. The activation markers CD69 and HLA-DR were also increased. The enrichment in the CD56bright subset was due to an increased susceptibility to apoptosis of CD56+CD16+ NK cells mediated by heat-labile and stable soluble factors present in tuberculous effusions and not in PF from other etiologies. Furthermore, in TB patients, Mtb-induced IFN-γ production by PF NK cells was not dependent on the presence of CD3+, CD19+, and CD14+ cells, suggesting a direct interaction of CD56bright cells with Mtb and/or the involvement of other accessory cells present at the site of Mtb infection.

The role of CD16^– and CD16⁺ Mo subsets in human TB remains unknown. Our aim was to characterize Mo subsets from TB patients and to assess whether the inflammatory milieu from TB pleurisy modulate their phenotype and recruitment. We found an expansion of peripheral CD16⁺ Mo that correlated with disease severity and with TNF-α plasma levels. Circulating Mo from TB patients are activated, showing a higher CD14, CD16, and CD11b expression and Mtb binding than HS. Both subsets coexpressed CCR2/CCR5, showing a potential ability to migrate to the inflammatory site. In tuberculous PF, the CD16⁺ subset was the main Mo/MΦ population, accumulation that can be favored by the induction of CD16 expression in CD16^– Mo triggered by soluble factors found in this inflammatory milieu. CD16⁺ Mo in PF were characterized by a high density of receptors for Mtb recognition (DC-SIGN, MR, CD11b) and for lipid-antigens presentation (CD1b), allowing them to induce a successful, specific T cell proliferation response. Hence, in tuberculous PF, CD16⁺ Mo constitute the main APC population; whereas in PB, their predominance is associated with the severity of pulmonary TB, suggesting a paradoxical role of the CD16⁺ Mo subset that depends on the cellular localization.

Tuberculous pleurisy allows the study of human cells at the site of activeMycobacterium tuberculosisinfection. In this study, we found that among pleural fluid (PF) lymphocytes, natural killer (NK) cells are a major source of early gamma interferon (IFN-γ) uponM. tuberculosisstimulation, leading us to investigate the mechanisms and molecules involved in this process. We show that the whole bacterium is the best inducer of IFN-γ, although a high-molecular-weight fraction of culture filtrate proteins fromM. tuberculosisH37Rv and the whole-cell lysate also induce its expression. The mannose receptor seems to mediate the inhibitory effect of mannosylated lipoarabinomannan, and Toll-like receptor 2 and 4 agonists activate NK cells but do not induce IFN-γ likeM. tuberculosisdoes. Antigen-presenting cells (APC) and NK cells bindM. tuberculosis, and although interleukin-12 is required, it is not sufficient to induce IFN-γ expression, indicating that NK cell-APC contact takes place. Indeed, major histocompatibility complex class I, adhesion, and costimulatory molecules as well as NK receptors regulate IFN-γ induction. The signaling pathway is partially inhibited by dexamethasone and sensitive to Ca²⁺flux and cyclosporine. Inhibition of p38 and extracellular-regulated kinase mitogen-activated protein kinase pathways reduces the number of IFN-γ⁺NK cells. Phosphorylated p38 (p-p38) is detected in ex vivo PF-NK cells, andM. tuberculosistriggers p-p38 in PF-NK cells at the same time that binding between NK andM. tuberculosisreaches its maximum value. Thus, interplay betweenM. tuberculosisand NK cells/APC triggering IFN-γ would be expected to play a beneficial role in tuberculous pleurisy by helping to maintain a type 1 profile.

In Argentina, multidrug-resistant tuberculosis (MDR-TB) outbreaks emerged among hospitalized patients with AIDS in the early 1990s and thereafter disseminated to the immunocompetent community. Epidemiological, bacteriological, and genotyping data allowed the identification of certain MDRMycobacterium tuberculosisoutbreak strains, such as the so-called strain M of the Haarlem lineage and strain Ra of the Latin America and Mediterranean lineage. In the current study, we evaluated the immune responses induced by strains M and Ra in peripheral blood mononuclear cells from patients with active MDR-TB or fully drug-susceptible tuberculosis (S-TB) and in purified protein derivative-positive healthy controls (group N). Our results demonstrated that strain M was a weaker gamma interferon (IFN-γ) inducer than H37Rv for group N. Strain M induced the highest interleukin-4 expression in CD4⁺and CD8⁺T cells from MDR- and S-TB patients, along with the lowest cytotoxic T-lymphocyte (CTL) activity in patients and controls. Hence, impairment of CTL activity is a hallmark of strain M and could be an evasion mechanism employed by this strain to avoid the killing of macrophages by M-specific CTL effectors. In addition, MDR-TB patients had an increased proportion of circulating regulatory T cells (Treg cells), and these cells were further expanded upon in vitroM. tuberculosisstimulation. Experimental Treg cell depletion increased IFN-γ expression and CTL activity in TB patients, with M- and Ra-induced CTL responses remaining low in MDR-TB patients. Altogether, these results suggest that immunity to MDR strains might depend upon a balance between the individual host response and the ability of differentM. tuberculosisgenotypes to drive Th1 or Th2 profiles.