Cell cycle-dependent expression of TAP1, TAP2, and HLA-B27 messenger RNAs in a human breast cancer cell line

Tumor cells are generally poorly responsive to immunotherapy. The results presented here suggest that antigen presentation of somatic tumor cells may be diminished greatly in quiescence and may he determined in part by growth regulation. Peptides produced by proteasomes are transported into the endoplasmic reticulum by transporter proteins TAP-1 and TAP-2, where they bind and stabilize MHC class I molecules required for antigenic presentation on the cell surface. TAP-1 and TAP-2 mRNAs were undetectable in quiescent, serum-deprived human breast cancer cells (21PT). They appeared lit h after serum induction, near the G₁-S boundary. In contrast, HLA-B27 mRNA was biphasically up-regulated. These mRNAs were significantly down-regulated in must tissues that contain mainly terminally differentiated, nonproliferating cells. All of the investigated breast cancer cell lines showed lower expression levels of these mRNAs than did the corresponding normal cells.