CD40 Ligand Expression in Mycobacterium bovis BCG Infection and Its Regulation by Cytokines : A Direct Role of Interleukin 12

Abstract

Background Activation and clonal expansion of T cells require not only the recognition of processed antigen on the surface of the antigen-presenting cell (APC) by T-cell receptor (TCR), but also involve co-stimulatory signals that are provided by the simultaneous engagement of cell surface molecules expressed by both the APC and the T cell. Interaction between CD40 and its ligand (CD40L) is known to mediate host immune response and T-cell-mediated effector functions in mycobacterial infections in mice. In this work, we investigated the capacity of Mycobacterium bovis ( M. bovis) BCG to induce the expression of CD40L on human T cells. Methods Human cells were obtained from healthy adults by centrifugation using Ficoll/Hypaque. Cells (1 × 10 6) were incubated in RPMI medium with BCG. After incubation at 37°C in 5% CO 2 atmosphere for 40 h, cells were collected and double-stained with anti-CD40L-PE and anti-CD4-FITC or anti-CD8-FITC mAb. The quantification of positively stained population was based on samples stained with isotype control antibodies analyzed on a FACScan. Results M. bovis BCG stimulation induced a significant amount of CD40L expression on CD4+ T cells, while CD40L was not significantly detected on human CD8+ T cells. The highest CD40L expression on BCG-activated T cells in synergism with interleukin-12 endogenous occurred after a 40-h cell culture with a dose of 10 μg/mL of BCG (84.86 ± 11.77; mean ± standard deviation [SD]). This CD40L expression on BCG-activated human T cells was significantly inhibited by anti-IL-12 mAb (5.08 ± 1.7; mean ± SD). In contrast, anti-IFN-γ and anti-IL-2 mAb did not have an important role in this expression. Conclusions These results indicate that the capacity of BCG to induce CD40L expression on human cells represents a novel mechanism underlying the regulation of cellular responses against tuberculosis. Furthermore, the results showed a direct role of IL-12 to enhance the expression of CD40L on BCG-activated human cells.