Defective Human Leukocyte Antigen Class I-associated Antigen Presentation Caused by a Novel β2-Microglobulin Loss-of-function in Melanoma Cells

Abstract

The major histocompatibility complex class I molecules consist of three subunits, the 45-kDa heavy chain, the 12-kDa ␤ 2 -microglobulin (␤ 2 m), and an ϳ8 -9-residue antigenic peptide. Without ␤ 2 m, the major histocompatibility complex class I molecules cannot assemble, thereby abolishing their transport to the cell membrane and the subsequent recognition by antigen-specific T cells. The human leukocyte antigen (HLA) 4 class I molecules, encoded by the genes located in the major histocompatibility complex, are composed of three subunits including a 45-kDa HLA class I heavy chain (HC), a 12-kDa ␤ 2 -microglobulin (␤ 2 m), and an ϳ8 -9-residue peptide (1). Expression of these molecules on the cell surface requires the stepwise assembly of HCs, ␤ 2 m, and peptides in the endoplasmic reticulum (ER) followed by the transport of the trimeric molecule to the plasma membrane. These processes are dependent on a functional antigen processing machinery (APM), which includes the proteasome subunits, the peptide transporters TAP1 and TAP2, and a number of ER-resident chaperons such as calnexin, calreticulin, ERp57, and tapasin (2, 3). ␤ 2 m plays an integral part in the assembly and transport of HLA class I molecules because it stabilizes the HC-␤ 2 m heterodimer through noncovalent protein-protein interactions, thereby allowing binding of endogenous antigenic peptides with the help of TAP and tapasin (4). As a result, the assembled HC-␤ 2 m-peptide trimeric complexes can travel to the cell surface, where they are recognized by HLA class I-restricted, antigen-specific cytotoxic T lymphocytes (CTLs).The lack of HLA class I molecule expression on the cell surface usually reflects defects in ␤ 2 m protein synthesis caused by mutations in the ␤ 2 m gene, as has been found mostly in malignant cells thus far (5). This abnormality renders tumor cells resistant to tumor antigenspecific CTLs and may have a negative impact on the elimination of tumor cells by host CTLs. The defects underlying ␤ 2 m loss have thus far been found to be structural in nature, involving lack of translation because of small deletions or point mutations in most cases and lack of transcription because of large deletions in a few cases (5). Because of a lack of ␤ 2 m expression, the resulting HLA class I loss cannot be corrected by interferon (IFN-␥), a cytokine that up-regulates the expression of most of the molecules participating in the assembly and transport of HLA class I molecules. On the other hand, a low level of HLA class I expression on cells usually reflects nonstructural defects in the APM components because this abnormality can be corrected by .In the present study, we have elucidated the mechanism underlying HLA class I down-regulation identified in a melanoma cell line and in the metastasis from which the cell line was derived (7). This HLA class I down-regulation phenotype cannot be corrected by IFN-␥ and was unexpectedly found to be caused by an abnormal full-length ␤ 2 m protein that can neither form stable complexes with HCs nor assist in...

Multiple Structural and Epigenetic Defects in the Human Leukocyte Antigen Class I Antigen Presentation Pathway in a Recurrent Metastatic Melanoma Following Immunotherapy

“…In this regard, Lehmann et al (51) demonstrated that the TAs recognized by the CTLs isolated from recurrent melanoma metastases were different from those recognized by the CTLs isolated from autologous primary lesions. In addition, some of us (14) have shown a shifting of CTL specificity from HLA-A2-MART-1 [27][28][29][30][31][32][33][34][35] complexes to HLA-A2-Tyr 369 -377 complexes in two sequential melanoma metastases with distinct HLA class I abnormalities caused by a mutant ␤ 2 m protein (32). These results are consistent with the possibility that the specificity of a patient's immune response can change in response to changes in the expression of targets by melanoma cells, which have developed immunoescape mechanisms.…”

“…Cells were allowed to expand for 3 weeks and then harvested when necessary. The specificity of CTLs was assessed by their cytotoxicity of HLA-A3-transfected HEK293 cells (4 ϫ 10 6 cells/ml) pulsed with gp100 [17][18][19][20][21][22][23][24][25] peptide (400 g/ml) versus those pulsed with irrelevant HLA-A3-binding CSPG4 495-503 peptide (TLLDVVNRK, 400 g/ml) using a standard 4-h 51 Cr-release assay (32). These CTLs were then assayed for their ability to recognize and lyse mock-and tapasin-transfected COPA-159 cells treated with IFN-␥, 5AdC, or their combinations.…”

Histone deacetylase inhibitors induce TAP, LMP, Tapasin genes and MHC class I antigen presentation by melanoma cells

“…In addition, restoration of TAP activity by transfection of tumor cells enhances class I mediated antigen presentation and induces susceptibility to CTL killing, both in vitro and in vivo [1,23]. In contrast, IFN-γ and TNF-α treatment failed to elicit class I expression in some class I deficient tumors which are associated with a defective β 2 m gene [5] or DNA hypermethylation [40]. However, mutation of β 2 m pis not a common mechanism of class I deficiency in tumors [9,15].…”

Thymosin Apha 1–A Peptide Immune Modulator with a Broad Range of Clinical Applications

“…Immune escape by virally-infected and tumor cells has been correlated with down-regulation of antigen-presenting molecules [66,67].…”