Assessment of intracellular TAP-1 and TAP-2 in conjunction with surface MHC class I in plasma cells from patients with multiple myeloma.

Immunotherapy involving cytotoxic T lymphocytes (CTLs) is an attractive alternative for treatment of various malignancies, including multiple myeloma. For tumour cells to be recognized and killed by CTLs they must express cell surface major histocompatibility complex (MHC) class I molecules and the transporter associated with antigen processing (TAP). However, loss of MHC class I and the TAP protein are common among several types of solid tumours. This study assessed the expression of TAP protein (by intracellular flow cytometry) and cell surface MHC class I molecules in three human myeloma cell lines as well as the plasma cell population (CD38+ bright) in bone marrow specimens from 13 multiple myeloma patients. In all of the patients, 100% of the plasma cell population expressed both the TAP subunits and cell surface MHC class I molecules, but at varying intensities. Both TAP and MHC class I were also expressed in the three myeloma lines. Additionally, the function of the antigen transport machinery was evaluated by a peptide transporter assay in the three myeloma lines. TAP transporter activity was readily detectable in two out of three myeloma lines, whereas the diminished activity in the third cell line was completely restored by co-culturing with recombinant interferon-gamma (rIFN-gamma).