Transforming growth factor-β negatively modulates T-cell responses in sepsis

Abstract

Sepsis is associated with depressed T-cell functions and increased circulating levels of immunosuppressive agents. TGF-β is a potential anti-inflammatory cytokine that can modify T-cell growth and differentiation. The up-regulation of TGF-β and the mechanism of its action on the T-cells during septic injury have not been resolved. We hypothesized that in sepsis TGF-β produced by macrophages acts on T-cells in a paracrine manner to suppress interleukin (IL)-2 production and proliferation. In this study, we examined the circulating TGF-β levels in a rat model of Gram-negative bacterial sepsis, and compared the abilities of adherent and non-adherent splenocytes to produce TGF-β. Additionally, we investigated the causal relationships of hrTGF-β to concanavalin A (ConA)-induced T-cell responses and the intracellular mechanism of the generation of these responses in normal splenic rat T-cells. Sepsis was induced in rats by intra-abdominally implanting fecal pellets containing Escherichia coli (150 CFU) and Bacteroides fragilis (10 000 CFU). Adherent and non-adherent splenocytes were isolated by differential adherence using Ficoll gradient centrifugation. T-cells were purified by use of Nylon wool columns. We observed a 3–6-fold increase in the circulating levels of TGF-β in sepsis. Western blots and ELISA determinations revealed a 2.5–3-fold increase in cell-associated TGF-β protein levels in adherent splenic cells. Northern analyses also showed a marked increase in TGF-β mRNA expression in adherent cells during sepsis. On the other hand, a significant change was not observed in the TGF-β protein and mRNA expression in non-adherent splenocytes. Pretreatment of control rat T-cells with hrTGF-β decreased both ConA-induced proliferation (by 35–40%) and IL-2 mRNA expression (by >50%). Further, whereas incubation of control rat T-cells with either ConA or TGF-β for 24 h resulted in a 10–15-fold increase in cAMP generation, the addition of hrTGF-β along with ConA resulted in a 50–60-fold increase in cAMP. These results suggest that in sepsis, TGF-β produced by splenic macrophages can act in a paracrine manner on T-cells to depress their IL-2 mRNA expression, IL-2 production and proliferation after up-regulation of cAMP which can interfere with T-cell signaling for proliferation.