We show in this study that incubation of freshly isolated bone marrow cells with Mycobacterium tuberculosis (M. tb) secretory Ag (MTSA), in the absence of any growth or differentiation-inducing factor, differentiates them into dendritic cell (DC)-like APCs. These DCs expressed moderate to high levels of various markers typical of DCs. These included T cell costimulatory molecules CD80, CD86, CD40, and CD54 and high levels of surface MHC class I and II on CD11c+ cells. The levels and the kinetics of up-regulation of these molecules were comparable with those of GM-CSF-differentiated DCs. Furthermore, these DCs exhibited morphology characteristics to DCs like the presence of dendritic processes. These DCs were also potent stimulators of allogeneic T cells and preferentially induced the secretion of IFN-γ over IL-10 from the interacting T cells. Interestingly, the differentiation of bone marrow cells into DC-like APCs was obtained with many other M. tb Ags, including whole cell extract of M. tb. Further characterization of MTSA-differentiated DCs showed that they were immature in nature, as stimulation of these DCs with TNF-α, anti-CD40, or LPS further up-regulated the surface levels of various molecules together with an increase in their T cell stimulatory capacity. The Ag-specific T cell responses of MTSA-differentiated DCs were mainly contributed by the CD4+ subset, indicating that MTSA was largely MHC II restricted. Furthermore, stimulation of bone marrow cells with MTSA induced the nuclear translocation of the transcription factor NF-κB, thereby indicating its role during MTSA-induced differentiation of DCs.
Interactions of 10-kDa Mycobacterium tuberculosis secretory antigen (MTSA) with dendritic cells (DCs) were investigated to elucidate the role of secretory antigens in regulating immune responses to M. tuberculosis early in the course of infection. MTSA induced the maturation of different DC subsets. The cytokine profiles of these DCs were characteristic to each DC subset. Of interest, coculture of M. tuberculosis whole-cell extract (CE)-pulsed, MTSA-matured DCs with CE-specific T cells led to a marked reduction in interleukin (IL)-2 and interferon (IFN)-gamma production, thereby down-regulating proinflammatory responses to mycobacterial antigens. Attenuation of IL-2 and IFN-gamma levels of CE-specific T cells also was obtained when M. tuberculosis culture filtrate protein-activated DCs were employed as antigen-presenting cells, which suggests that MTSAs induce maturation of DCs at sites of infection, probably to down-regulate proinflammatory immune responses to mycobacteria that may subsequently be released from infected macrophages.
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