Induction of a helper T (TH)-cell response is a critical element in the generation of anti-tumor immunity. The majority of immunotherapeutic approaches have so far been concerned with the generation of cytotoxic T lymphocytes (CTLs). This also accounts for gp100, a melanoma-associated protein which induces a potent CTL response. Because of the high immunogenicity of gp100, we considered it of special interest to explore the feasibility of generating gp100-specific TH cells. Human dendritic cells (DCs) were loaded with recombinant gp100 protein, and the response of autologous TH cells was evaluated in vitro and in vivo. We have observed that gp100 peptides can be presented by DCs of certain MHC class II haplotypes, which led to proliferation and cytokine production of TH-1 cells in vitro. Furthermore, transfer of gp100 protein-loaded human DCs into SCID mice also induced proliferation of autologous, unprimed peripheral blood leukocytes (PBLs) and selective expansion of TH cells. When human T cells from the spleen of SCID mice were recovered and restimulated in vitro, they strongly proliferated in response to gp100-loaded DCs, while showing minimal proliferative activity in response to DCs loaded with a control antigen. Thus, it is possible to induce an efficient MHC class IIrestricted TH response by in vitro stimulation or in vivo vaccination with DCs which have been loaded with a purified tumor-associated antigen. Int.