821

CANX

calnexin;CANX;calnexin

major histocompatibility complex class I antigen-binding protein p88

5q35

protein-coding

Defects in human leukocyte antigen class I antigen processing machinery (APM) component expression can have a negative impact on the clinical course of tumors and the response to T cell-based immunotherapy. Since brain metastases of breast cancer are of increasing clinical significance, the APM component expression levels and CD8(+) T cell infiltration patterns were analyzed in primary breast and metastatic brain lesions of breast cancer by immunohistochemistry. Comparison of unpaired 50 primary and 33 brain metastases showed lower expression of beta2-microglobulin, transporter associated with antigen processing (TAP) 1, TAP2 and calnexin in the brain lesions. Although no significant differences were found in APM component scores between primary breast and brain lesions in 15 paired cases, primary breast lesions of which patients eventually developed brain metastases showed lower levels of beta2-microglobulin, TAP1 and calnexin compared with breast lesions without known brain metastases. The extent of CD8(+) T cell infiltration was significantly higher in the lesions without metastasis compared with the ones with brain metastases, and was positively associated with the expression of TAP1 and calnexin. Furthermore, mouse tumor cells stably transfected with silencing hairpin (sh)RNA for TAP1 demonstrated a decreased susceptibility to cytotoxic T lymphocytes in vitro and enhanced spontaneous brain metastasis in vivo. These data support the functional significance of TAP1 expression in tumor cells. Taken together, our data suggest that patients with low or defective TAP1 or calnexin in primary breast cancers may be at higher risks for developing brain metastasis due to the defects in T cell-based immunosurveillance.

Characterization of the molecular basis of tumor recognition by T cells has shown that major histocompatibility complex (MHC) class I molecules play a crucial role in presenting antigenic peptide epitopes to cytotoxic T lymphocytes. MHC class Ia downregulation has been repeatedly described on melanoma cells and is thought to be involved in the failure of the immune system to control tumor progression. Proper assembly of MHC class I molecules is dependent on several cofactors, e.g. the chaperones calnexin and calreticulin residing in the endoplasmic reticulum. Alterations in the expression of these chaperones may have important implications for MHC class I assembly, peptide loading, and presentation on the tumor cell surface and thus may contribute to the immune escape phenotype of tumor cells. In the present study, we compared melanoma lesions representing different stages of tumor progression with regard to the expression of calnexin and calreticulin in tumor cells by means of immunohistochemistry. metastatic melanoma lesions exhibited significant downregulation of calnexin as compared to primary melanoma lesions. In contrast, chaperone calreticulin was expressed in melanoma cells of primary as well as of metastatic lesions. Our data suggest that chaperone-downregulation, particularly calnexin-downregulation, may contribute to the metastatic phenotype of melanoma cells in vivo. Consistently, conserved chaperone expression in metastatic melanoma lesions may be a useful criterion for selection of patients for treatment with T cell-based immunotherapies.