Melanomas with concordant loss of multiple melanocytic differentiation proteins: immune escape that may be overcome by targeting unique or undefined antigens

Melanoma-reactive HLA-A*0201-restricted cytotoxic T lymphocyte (CTL) lines generated in vitro lyse autologous and HLA-matched allogeneic melanoma cell s and recognize multiple shared peptide antigens from tyrosinase, MART-1, a nd Pmel17/gp100. However, a subset of melanomas fail to be lysed by these T cells. In the present report, four different HLA-A*0201(+) melanoma cell l ines not lysed by melanoma-reactive allogeneic CTL have been evaluated in d etail. All four are deficient in expression of the melanocytic differentiat ion proteins (MDP) tyrosinase, Pmel17/gp100, gp75/ trp-1, and MART-1/Melan- A. This concordant loss of multiple MDP explains their resistance to lysis by melanoma-reactive allogeneic CTL and confirms that a subset of melanomas may be resistant to tumor vaccines directed against multiple MDP-derived e pitopes. All four melanoma lines expressed normal levels of HLA-A*0201, and all were susceptible to lysis by xenoreactive-peptide-dependent HLA-A*0201 -specific CTL clones, indicating that none had identifiable defects in anti gen-processing pathways. Despite the lack of shared MDP-derived antigens, o ne of these MDP-negative melanomas, DM331, stimulated an effective autologo us CTL response in vitro, which was restricted to autologous tumor reactivi ty. MHC-associated peptides isolated by immunoaffinity chromatography from HLA-AI and HLA-AZ molecules of DM331 tumor cells included at least three pe ptide epitopes recognized by DM331 CTL and restricted by HLA-AI or by HLA-A *0201. Recognition of these CTL epitopes cannot be explained by defined, sh ared melanoma antigens, instead, unique or undefined antigens must be respo nsible for the autologous-cell-specific anti-melanoma response, These findi ngs suggest that immunotherapy directed against shared melanoma antigens sh ould be supplemented with immunotherapy directed against unique antigens or other undefined antigens, especially in patients whose tumors do not expre ss MDP.