Cytokine‐induced transient monocyte IL‐7Ra expression and the serum milieu in tuberculosis

Abstract

Bacterial components and cytokines induce IL‐7 receptor (IL‐7Rα) expression in monocytes. Aberrant low IL‐7Rα expression of monocytes has been identified as a feature of tuberculosis immunopathology. Here, we investigated the mechanisms underlying IL‐7Rα regulation of monocytes and tuberculosis serum effects on IL‐7Rα expression. Serum samples from tuberculosis patients and healthy controls, cytokine candidates, and mycobacterial components were analyzed for in vitro effects on IL‐7Rα expression of primary monocytes, monocyte‐derived macrophages (MDM), and monocyte cell lines. IL‐7Rα regulation during culture and the role of FoxO1 were characterized. In vitro activation‐induced IL‐7Rα expression in human monocytes and serum samples from tuberculosis patients boosted IL‐7Rα expression. Although pathognomonic tuberculosis cytokines were not associated with serum effects, we identified cytokines (i.e., GM‐CSF, IL‐1β, TNF‐α, IFN‐γ) that induced IL‐7Rα expression in monocytes and/or MDM comparable to mycobacterial components. Blocking of cytokine subsets (i.e., IL‐1β/TNF‐α in monocytes, GM‐CSF in MDM) largely diminished IL‐7Rα expression induced by mycobacterial components. Finally, we showed that in vitro‐induced IL‐7Rα expression was transient and dependent on constitutive FoxO1 expression in primary monocytes and monocyte cell lines. This study demonstrated the crucial roles of cytokines and constitutive FoxO1 expression for transient IL‐7Rα expression in monocytes.

Monocyte pathology in human tuberculosis is due to plasma milieu changes and aberrant STAT signalling

“…Higher expression was also detected for HLA‐DR and CD14 in the presence of tuberculosis patients' plasma (both p < 0.0001). This confirmed previous results for plasma‐induced HLA‐DR expression [23], and indicated that plasma effects on reference monocytes may not completely resemble monocyte phenotype changes in acute tuberculosis.…”

“…This finding suggested that different plasma cytokines are involved in tuberculosis immunopathology. In accordance, a previous study failed to identify individual plasma cytokines from tuberculosis patients associated with induced IL‐7Rα expression in monocytes [23]. Distinct association pattern seen for pSTAT3, which correlated positively with CD33 expression, and pSTAT5, which correlated with CD40 and CD64 in the present study, narrowed down potential causative factors.…”

“…[22]). Recently, we established an in vitro assay based on tuberculosis patients' plasma samples, which were added to the culture medium of reference monocytes and compared to matched control plasma samples [23]. This assay confirmed tuberculosis plasma specific effects on monocyte HLA‐DR/IL‐7 receptor expression and strengthened the hypothesis that the plasma milieu in tuberculosis is responsible for immunopathology seen for T cells and monocytes [23].…”

“…Using our previously established monocyte in vitro assay [23], we characterized the effect of plasma milieu on monocyte phenotypes and cytokine signalling. A schematic depiction of the assay is shown in Figure 3.…”

“…A schematic depiction of the assay is shown in Figure 3. Monocytes were enriched (using magnetic cell sorting (EasySept Monocyte negative selection kit; Stemcell Technology) as described before [23]) from peripheral blood mononuclear cells (PBMCs) purified from peripheral blood of healthy individuals by density gradient centrifugation (Histopaque-1077; Sigma-Aldrich) according to manufacturers' guidelines. Enriched monocytes (5 Â 10 4 per well) were cultured in RPMI 1640 medium (Thermo Fisher Scientific) supplemented with L-Glutamine (2 mM, Sigma Aldrich), Hepes (10 mM, Thermo Fisher Scientific) and 10% of heterologous plasma either from a patient with tuberculosis or a control.…”