Human dendritic cells very efficiently present a heterologous antigen expressed on the surface of recombinant gram-positive bacteria to CD4(+) T lymphocytes

Recombinant Streptococcus gordonii expressing on the surface the C-fragment of tetanus toxin was tested as an Ag delivery system for human monocyte-de rived dendritic cells (DCs), DCs incubated with recombinant S, gordonii wer e much more efficient than DCs pulsed with soluble C-fragment of tetanus to xin at stimulating specific CD4(+) T cells as determined by cell proliferat ion and IFN-gamma release. Compared with DCs treated with soluble Ag, DCs f ed with recombinant bacteria required 10(2)- to 10(3)-fold less Ag and were at least 10(2) times more effective on a per-cell basis for activating spe cific T cells. S, gordonii was internalized in DCs by conventional phagocyt osis, and cytochalasin D inhibited presentation of bacteria-associated Ag, but not of soluble Ag, suggesting that phagocytosis was required for proper delivery of recombinant Ag, Bacteria were also very potent inducers of DC maturation, although they enhanced the capacity of DCs to activate specific CD4(+) T cells at concentrations that did not stimulate DC maturation. In particular, S, gordonii dose-dependently up-regulated expression of membran e molecules (MHC I and II, CD80, CD86, CD54, CD40, CD83) and reduced both p hagocytic and endocytic activities. Furthermore, bacteria promoted in a dos e-dependent manner DC release of cytokines (IL-6, TNF-alpha, IL-1 beta, IL- 12, TGF-beta, and IL-10) and of the chemokines IL-8, RANTES, IFN-gamma-indu cible protein-10, and monokine induced by IFN-gamma, Thus, recombinant Gram -positive bacteria appear a powerful tool for vaccine design due to their e xtremely high capacity to deliver Ags into DCs, as well as induce DC matura tion and secretion of T cell chemoattractans.