Is tumor expression of the major histocompatibility complex antigen required for T cell immune surveillance?

Tumor expression of major histocompatibility complex antigen (MHC) class I and class II is not essential for the induction of memory T cells. However, induction of MHC class I-restricted effector cytotoxic T cells (CTL) appears dependent on MHC class I expression on tumors. Moreover, the effector function of tumor-specific CTL requires direct recognition of the tumor. In contrast, both the inductive and the effector phases of MHC class II-restricted T cells are independent of MHC class II expression on tumors.