Resistance and susceptibility of macrophages to mycobacteria are under the control of the Bcg/Nramp1 gene, which also controls the NO- production in response to macrophage activators. There is recent evidence indicating that mycobacteria induces apoptosis in infected macrophages. Using murine macrophage lines, congenic at the Bcg/Nramp1 gene, this report shows that B10R are more prone than B10S macrophages to undergo apoptosis after exposure to live virulent Mycobacterium tuberculosis H37Rv (Mtb) or PPD, as determined by cell viability, DNA fragmentation, hypoploidy, and the terminal deoxynucleotide transferase dUTP-biotin nick-end labeling assay. Induction of apoptosis correlated with NO- production. Aminoguanidine and anti-TNF-alpha inhibited NO- production and apoptosis. B10R and B10S macrophages were equally affected by sodium nitroprusside, a donor of NO-, but its effect, mainly in B10R cells, was enhanced by the presence of Mtb. Nonvirulent mycobacteria induced lower levels of NO- and did not cause cell death. Killed Mtb, mannose-capped lipoarabinomannan (ManLAM), and LPS rescued macrophages from apoptosis albeit induce NO-. These findings suggest the existence of opposite pathways: metabolically active mycobacteria promotes apoptosis whereas their structural components inhibit it. Apoptosis may be a critical mechanism by which Nramp1 gene controls the macrophage infection with virulent mycobacteria.

Household contacts (HHCs) of pulmonary tuberculosis patients are at high risk of Mycobacterium tuberculosis infection and early disease development. Identification of individuals at risk of tuberculosis disease is a desirable goal for tuberculosis control. Interferon-gamma release assays (IGRAs) using specific M. tuberculosis antigens provide an alternative to tuberculin skin testing (TST) for infection detection. Additionally, the levels of IFNγ produced in response to these antigens may have prognostic value. We estimated the prevalence of M. tuberculosis infection by IGRA and TST in HHCs and their source population (SP), and assessed whether IFNγ levels in HHCs correlate with tuberculosis development. A cohort of 2060 HHCs was followed for 2–3 years after exposure to a tuberculosis case. Besides TST, IFNγ responses to mycobacterial antigens: CFP, CFP-10, HspX and Ag85A were assessed in 7-days whole blood cultures and compared to 766 individuals from the SP in Medellín, Colombia. Isoniazid prophylaxis was not offered to child contacts because Colombian tuberculosis regulations consider it only in children under 5 years, TST positive without BCG vaccination. Using TST 65.9% of HHCs and 42.7% subjects from the SP were positive (OR 2.60, p<0.0001). IFNγ response to CFP-10, a biomarker of M. tuberculosis infection, tested positive in 66.3% HHCs and 24.3% from the SP (OR = 6.07, p<0.0001). Tuberculosis incidence rate was 7.0/1000 person years. Children <5 years accounted for 21.6% of incident cases. No significant difference was found between positive and negative IFNγ responders to CFP-10 (HR 1.82 95% CI 0.79–4.20 p = 0.16). However, a significant trend for tuberculosis development amongst high HHC IFNγ producers was observed (trend Log rank p = 0.007). CFP-10-induced IFNγ production is useful to establish tuberculosis infection prevalence amongst HHC and identify those at highest risk of disease. The high tuberculosis incidence amongst children supports administration of chemoprohylaxis to child contacts regardless of BCG vaccination.