Relevance of antigen‐induced IL‐6 and mitogen‐induced or spontaneous IFN‐γ secretions in whole blood cultures for detection of Mycobacterium tuberculosis infection and disease

For an effective control of tuberculosis (TB), there is a persistent need for biomarkers that can report true estimates of TB infection (TBI) and predict its progression towards active TB disease. We investigated whether the cell‐mediated immune responses to Mycobacterium tuberculosis (Mtb) antigens could provide such biomarkers. The study subjects (n = 174) comprised a cohort of smear‐positive, drug‐sensitive, HIV‐negative pulmonary TB patients (n = 54) and their household contacts (HC, n = 120). Whole blood cultures, in the presence or absence of Mtb antigens‐ membrane (MtM), purified protein derivative (PPD) and alpha‐crystallin (Acr), or the mitogen PHA were subjected to determinations, by flow cytometry, for T cell proliferative and, by ELISA, for IFN‐γ, TNF‐α, and IL‐6 cytokine responses. Additionally, serum levels of the three cytokines were also estimated. The strongest cell‐proliferative and cytokine responses were induced by MtM and IL‐6 was the most abundantly produced cytokine. While none of the responses induced by Mtb antigens or the serum cytokines levels could discriminate between TB and HC, the ex vivo cytokine responses induced by PHA or ‘spontaneously’ could apparently do so. The concentrations of IFN‐γ induced by PHA in TB blood cultures were significantly lower than in HC cultures (AUC = 0.72). Conversely, the spontaneous IFN‐γ or TNF‐α secretions in TB cultures were significantly higher than in HC cultures (AUC = 0.66). Our results suggest that IL‐6 responses to MtM could be a sensitive indicator of TBI, and low levels of PHA‐induced or high levels of spontaneous IFN‐γ secretions in HC blood cultures may indicate a progressive infection.