CD137 is a costimulatory molecule that is expressed on the surface of T cells upon antigen-specific and TCR-dependent stimulation. Interestingly, CD137 was elevated on CD4+ T cells that had been primed in vivo by irradiated tumor cells and Surek together with HB304 (Figure 5C). These CD137+ cells were exclusively identified as CD44+ memory cells (data not shown). By contrast, neither an expansion of the memory compartment nor a significant upregulation of CD137 was seen in the CD8+ T-cell population (Figure 5D).