Human dendritic cells are superior to B cells at presenting a major histocompatibility complex class II-restricted heterologous antigen expressed on recombinant Streptococcus gordonii

Bacteria are being actively investigated as vaccine carriers for inducing o r boosting protective immune responses. In this study, human monocyte-deriv ed dendritic cells (DCs) and normal B cells were compared for their capacit y to present the C fragment of tetanus toxin (TTFC), expressed on the surfa ce of recombinant Streptococcus gordonii, to specific CD4(+) T lymphocytes. DCs were more efficient than B cells at presenting soluble TTFC and remark ably more capable of presenting bacterium-associated TTFC both in terms of the amount of antigen required to obtain a given T-cell response and on a p er-cell basis. This difference was associated with a much lower capacity of B cells to endocytose soluble TTFC and phagocytose recombinant S. gordonii . In addition, S. gordonii induced the phenotypic maturation of DCs but not of B cells. The results thus indicate that DCs but not B cells play a cruc ial role in the amplification of class Ii-restricted immune responses induc ed by immunization with recombinant gram-positive bacteria.