Blood Immunopathology of Tuberculosis Patients Disrupts Monocyte‐Dependent T‐Cell Activation and Cytokine Expression

Pulmonary tuberculosis in humans is characterised by features of immunopathology, which influence both antimycobacterial therapy and the long-term prognosis. In the blood of tuberculosis patients, immunopathology manifests itself in reduced immune responses to mitogenic substances. Previous studies have demonstrated the influence of tuberculosis serum on T-cell and monocyte function, but the exact mechanisms remain unclear. Here, we performed a case/control study to analyse the influence of tuberculosis serum milieu changes on (i) T-cell stimulation (using Staphylococcal Enterotoxin B), (ii) monocyte stimulation (using the Toll-like receptor agonist Pam3CSK4), (iii) T-cell/monocyte interaction characterised by the response against the lectin phytohemagglutinin, by using a novel peripheral blood mononuclear cell in vitro assay. Cell-specific activation marker and cytokine expression were determined by multicolor flow cytometry. Staphylococcal Enterotoxin B mainly induced cytokine expression by T cells, while Pam3CSK4 stimulated monocytes to secrete distinct cytokine signatures. Phytohemagglutinin induced activation and cytokine expression in both T cells and monocytes. Notably, tuberculosis patient serum samples affected exclusively phytohemagglutinin stimulated T-cell responses and particularly activation marker as well as CD40L/IL-2 positive CD4⁺ T-cell subsets were decreased as compared to serum from healthy contacts. Neither Staphylococcal Enterotoxin B-mediated T-cell stimulation nor phytohemagglutinin or Pam3CSK4 induced monocyte cytokines (i.e., Interleukin-6, Interleukin-8, Tumour Necrosis Factor-α) were affected by the tuberculosis patients' serum samples. These results highlight the immunosuppressive influence of the tuberculosis serum milieu, which specifically reduced T-cell responses to phytohemagglutinin, probably through impaired function of the accessory monocytes required for stimulation.