CD8A and cancer: Given the very low precursor frequencies of these naive cancer-specific CD8+ T cells and difficulties in expanding very few naive antigen-specific CD8+ T cells in vitro, further investigations into the immunogenicity of WT1A versus WT1B and which cell populations were being recruited into the response (that is, cross-reactive versus non-cross-reactive) were conducted by immunizing HLA-A2.1 transgenic mice with both peptides.