ABSTRACT: Expression data from mouse tumor-specific CD4+ T cells The central role of tumor-specific Th1 cells in fighting cancer is becoming increasingly appreciated. However, little is known about how these cells are generated in vivo. Here, we used flow cytometry and gene expression microarrays to characterize the primary activation and Th1 differentiation of naïve tumor-specific CD4+ T cells in a mouse model for cancer immunosurveillance. We took advantage of T cell receptor-transgenic mice where CD4+ T cells recognize a tumor-specific antigen secreted by the major histocompatibility complex (MHC) class II-negative MOPC315 myeloma. Cancer cells were injected subcutaneously and T cell activation was analyzed in draining lymph nodes and at the incipient tumor site at day +8. After activation and migration to the incipient tumor site, tumor-specific CD4+ T cells had 29 up-regulated molecules (CD2, CD5, CD11a, CD18, CD25, CD28, CD44, CD45, CD49d, CD51, CD54, CD69, CD71, CD83, CD86, CD90, CD95, CD102, CD122, CD153, CD166, CD200, CD249, CD254, CD274, CD279, Ly6C, MHC class I, and CCR7) and 5 down-regulated molecules (CD27, CD31, CD45RB, CD62L, and CD126) on the surface. The activated CD4+ T cells produced interferon , a cytokine consistent with Th1 polarization, and also interleukin 2 (IL-2), IL-3, IL-10, and tumor necrosis factor . Activation of naïve tumor-specific CD4+ T cells in draining lymph node resulted in the up-regulation of 609 genes and down-regulation of 284 genes. Bioinformatics analysis of genes differentially expressed identified functional pathways related to tumor-specific Th1 cell activation. This study may represent a useful resource to guide the development of Th1-based immunotherapy for cancer.