In vaccine development, a major objective is to induce strong, specific T cell responses. This might be obtained by targeting antigen to cell surface molecules that efficiently channel the antigen into endocytic compartments for loading of MHC molecules. Antibodies have been used to deliver antigen; however, it is important to define optimal targets on antigen-presenting cells (APC) for efficient delivery. For this purpose, we have established a T cell readout that can be used to screen large numbers of different mAb for their ability to load MHC class II molecules. The novel human CD4+ T cell clone is specific for mouse Ig C kappa (40-48) and restricted by HLA-DR4 (DRA1,B1*0401). DR4 apparently presents both mouse and human C kappa 40-48, but there is no cross-reaction at the T cell level. B cells from DR4 transgenic mice spontaneously process and present the mouse C kappa peptide. The mouse C kappa -specific T cell readout was used to demonstrate that mouse mAb specific for human dendritic cell (DC)-specific ICAM-grabbing non-integrin (DC-SIGN), a novel DC-specific molecule, were 10- to 1000-fold more potent at inducing kappa-specific human CD4+ T cell proliferation compared to control mAb. Consistent with this finding, DC-SIGN-specific mAb were rapidly internalized upon binding and found in intracellular vesicles. These results strongly argue that DC-SIGN-specific mAb are channeled into the MHC class II presentation pathway. Thus, DC-SIGN could be an efficient target for antibody-mediated delivery of T cell epitopes in vaccine development.