Bovine dendritic cells are more permissive for Mycobacterium bovis replication than macrophages, but release more IL-12 and induce better immune T-cell proliferation.

Bovine dendritic cells (DCs) were obtained by incubating blood monocytes with interleukin-4 (IL-4) and granulocyte-macrophage colony-stimulating factor (GM-CSF). The ability of DCs to phagocytose and allow the replication of virulent Mycobacterium bovis in vitro was studied, and compared with bovine blood monocyte-derived macrophages. In addition, the release of cytokines by M. bovis-infected DCs was assessed. DCs were shown to phagocytose M. bovis efficiently, and allowed a more substantial replication of M. bovis when compared to macrophages, as assessed by the metabolic activity of intracellular bacteria. During the course of M. bovis infection, it was found that macrophages released substantial amounts of pro-inflammatory factors such as tumour necrosis factor-alpha (TNF-alpha), nitric oxide (NO) and interleukin-1 beta (IL-1 beta). M. bovis-infected DCs released much smaller quantities of NO, IL-1 beta and TNF-alpha (5- to 10-fold lower amounts), when compared to macrophages. Treating cells with interferon-gamma (IFN-gamma) before and during the in vitro infection process was shown to increase the release of NO, TNF-alpha and IL-1 beta by M. bovis-infected macrophages, but not by M. bovis-infected DCs. M. bovis-infected macrophages released more interleukin-10 (IL-10) than infected DCs. Treating cells with IFN-gamma/LPS was shown to reduce M. bovis metabolic activity in infected macrophages, but had no such impact on M. bovis metabolic activity in infected DCs. A variety of T-cell-derived cytokines (IFN-gamma, GM-CSF, IL-4) had no impact on the replication of M. bovis in infected DCs. On the other hand, DCs infected with M. bovis sustained a more efficient replication of autologous sensitized T lymphocytes compared to M. bovis-infected macrophages. M. bovis-infected DCs released more substantial amounts of interleukin-12 (IL-12) than similarly infected macrophages. These data suggest a complementary role for DCs and macrophages with regard to bacteriostatic activity and induction of an efficient immune response against M. bovis.