Phagocytosis of bacille Calmette-Guerin-infected necrotic macrophages induces a maturation phenotype and evokes antigen-presentation functions in dendritic cells

The interaction of pathogens with dendritic cells (DCs) seems to play a critical role in the initiation of the immune response. Tissue damage and induction of an inflammatory reaction are events frequently associated with the progression of the infection. Although DCs are very efficient at phagocytosing pathogens, the capacity of these cells to uptake microbes from a necrotic environment has not yet been proven. Here we have investigated the ability of murine bone marrow-derived DCs to maturate and acquire antigen-presentation functions when cocultured with bacille Calmette-Guerin (BCG)-infected necrotic macrophages. Immature DCs exhibited a prominent capacity to ingest necrotic material as demonstrated by flow cytometry analysis and confocal microscopy. Furthermore, after exposure to BCG-infected necrotic macrophages, DCs underwent phenotypic changes, including the up-regulation of maturation specific markers (major histocompatibility complex class II, CD40, CD80, and CD86) and the capacity to stimulate antigen-specific CD4(+) T cells with higher efficiency than when they were directly infected with a similar number of bacteria. Antigen presentation following phagocytosis of BCG-infected necrotic macrophages was demonstrated by their ability to stimulate in vitro proliferation and interferon-gamma production of antigen-specific CD4(+) T cells. These results suggest that the functional changes occurring in DCs after interaction with a pathogen can be favoured when the encounter takes place in a necrotic environment and it may constitute an important mechanism for the amplification of class II-restricted immune responses induced during infection.