Potent anti-tumor responses to immunization with dendritic cells loaded with tumor tissue and an NKT cell ligand.

Cancer immunotherapy is well tolerated and specific, but its efficacy remains variable. To enhance anti-tumor CD8(+) T-cell responses induced by immunization with antigen-loaded dendritic cells (DCs), we explored the impact of eliciting a potent source of T-cell help from activated invariant natural killer (NK)-like T cells (iNKT cells) using the specific glycolipid ligand alpha-galactosylceramide (alpha-GalCer). As cytokines released by iNKT cells may drive proliferation of CD4(+)CD25(+) regulatory T cells (Tregs), we assessed this immunization strategy in animals treated with anti-CD25 antibody to inactivate Treg function. Combining DC immunization with iNKT cell activation was found to significantly enhance anti-tumor activity, which was improved further by the prior inactivation of Tregs. The improved anti-tumor activity with Treg inactivation was associated with a prolonged proliferative burst of responding CD8(+) T cells. We could find no evidence that inclusion of alpha-GalCer in the vaccine enhanced Treg numbers, or that the 'helper' function of iNKT cells was improved in the absence of Treg activity. Rather, the two activities appeared to act independently to improve the tumor-specific T-cell response. Inactivating regulatory T cells and eliciting iNKT cell activation are therefore two useful strategies that can be used in combination to improve anti-tumor immunization with antigen-loaded DCs.