CD4 + helper T cells are critical for protective immune responses and yet suboptimally primed in response to tumors. Cell-based vaccination strategies are under evaluation in clinical trials but limited by the need to derive antigenpresenting cells (APC) from patients or compatible healthy donors. To overcome these limitations, we developed CD4 + T cell-targeted synthetic microbead-based artificial APC (aAPC) and used them to activate CD4 + T lymphocytes specific for a tumor-associated model antigen (Ag) directly from the naive repertoire. In vitro, aAPC specifically primed Ag-specific CD4 + T cells that were activated to express high levels of CD44, produced mainly interleukin 2, and could differentiate into Th1-ike or Th2-like cells in combination with polarizing cytokines. I.v. administration of aAPC led to Ag-specific CD4 + T-cell activation and proliferation in secondary lymphoid organs, conferred partial protection against subcutaneous tumors, and prevented the establishment of lung metastasis. Taken together, our data support the use of cell-free, synthetic aAPC as a specific and versatile alternative to expand peptide-specific CD4 + T cells in adoptive and active immunotherapy. ©2008 American Association for Cancer Research.