Identification of a Novel Major Histocompatibility Complex Class II–restricted Tumor Antigen Resulting from a Chromosomal Rearrangement Recognized by CD4⁺ T Cells

CD4⁺ T cells play an important role in antitumor immune responses and autoimmune and infectious diseases. Although many major histocompatibility complex (MHC) class I–restricted tumor antigens have been identified in the last few years, little is known about MHC class II– restricted human tumor antigens recognized by CD4⁺ T cells. Here, we describe the identification of a novel melanoma antigen recognized by an human histocompatibility leukocyte antigen (HLA)-DR1–restricted CD4⁺ tumor-infiltrating lymphocyte (TIL)1363 using a genetic cloning approach. DNA sequencing analysis indicated that this was a fusion gene generated by a low density lipid receptor (LDLR) gene in the 5′ end fused to a GDP-l-fucose:β-d-galactoside 2-α-l-fucosyltransferase (FUT) in an antisense orientation in the 3′ end. The fusion gene encoded the first five ligand binding repeats of LDLR in the NH₂ terminus followed by a new polypeptide translated in frame with LDLR from the FUT gene in an antisense direction. Southern blot analysis showed that chromosomal DNA rearrangements occurred in the 1363mel cell line. Northern blot analysis detected two fusion RNA transcripts present only in the autologous 1363mel, but not in other cell lines or normal tissues tested. Two minimal peptides were identified from the COOH terminus of the fusion protein. This represents the first demonstration that a fusion protein resulting from a chromosomal rearrangement in tumor cells serves as an immune target recognized by CD4⁺ T cells.