Immune responses to human cancer stem-like cells/cancer-initiating cells.

Cancer stem-like cells (CSC)/cancer-initiating cells (CIC) are defined as minor subpopulations of cancer cells that are endowed with properties of higher tumor-initiating ability, self-renewal ability and differentiation ability. Accumulating results of recent studies have revealed that CSC/CIC are resistant to standard cancer therapies, including chemotherapy, radiotherapy and molecular targeting therapy, and eradiation of CSC/CIC is, thus, critical to cure cancer. Cancer immunotherapy is expected to become the "fourth" cancer therapy. Cytotoxic T lymphocytes (CTL) play an essential role in immune responses to cancers, and CTL can recognize CSC/CIC in an antigen-specific manner. CSC/CIC express several tumor-associated antigens (TAA), and cancer testis (CT) antigens are reasonable sources for CSC/CIC-targeting immunotherapy. In this review article, we discuss CSC/CIC recognition by CTL, regulation of immune systems by CSC/CIC, TAA expression in CSC/CIC, and the advantages of CSC/CIC-targeting immunotherapy.