Effects of streptococcus mutans on dendritic cell activation and function

OBJECTIVES: Pathogenic and non-pathogenic microorganisms can modify the immune response by subverting dendritic cell (DC) activation phenotype, antigen uptake, processing and presentation. This study characterises the expression of DC activation markers CD40, CD80, CD86 and MHC class II molecules following co-culture of DC's with Streptococcus mutans. Using a known peptide antigen ‘Ea', we elucidate the characteristics of antigen presentation and demonstrate subsequent antigen (Ea) specific CD4 T cell activation in response to DC's activated by S. mutans.
METHODS: Dendritic cells were generated from C57BL/6 mice by culturing bone marrow cells in complete RPMI supplemented with 10% mouse granulocyte-macrophage colony stimulating factor (GM-CSF). Dendritic cells were co-cultured in vitro with S. mutans. DC surface expression of CD40, CD80, CD86 and MHCII, was assessed by flow cytometry. Characteristics of antigen presentation were determined by culturing S. mutans with EaGFP, with DCs. Ea peptide expression in the context of MHC Class II was assessed by staining with a monoclonal antibody designated Y-AE. Proliferation of Ea antigen specific T cells co-cultured with DCs was analysed at 96 h by flow-cytometric staining for EdU (5-ethynyl-2'deoxyuridine) incorporation.

RESULTS: Dendritic cells were activated in response to both live and heat killed S. mutans, exhibiting up-regulation of CD40, CD80, CD86 and MHCII compared to un-stimulated DC populations. Antigen specific T cell proliferation in response to DC's activated with live S. mutans was reduced markedly compared to those activated with heat killed S. mutans. CONCLUSIONS: Although similar activation phenotypes are induced by live and heat killed S. mutans, the ability of activated DC's to thereafter drive an antigen specific CD4 T cell response is reduced following co-culture with live bacteria rather than heat killed bacteria. This work was funded by Medical Research Scotland.