Immunohistochemical Characterization of Keratan Sulfate Preferentially Expressed in Papillary Thyroid Carcinoma Using a Novel Monoclonal Antibody 299-1C1.

Keratan sulfate (KS) is a glycosaminoglycan consisting of repeating N-acetyllactosamine disaccharides, in which both galactose and N-acetylglucosamine are often 6-O-sulfated. It has been reported that KS recognized by the monoclonal antibodies 5D4 and 373E1 is preferentially expressed in papillary thyroid carcinoma (PTC) and only minimally in other thyroid tumors/lesions or normal thyroid tissue. However, the precise epitopes recognized by these antibodies remain incompletely characterized, and the expression and extent of low-sulfated KS in PTC have not been systematically evaluated. To better understand the nature of KS expressed in PTC, we generated a novel anti-KS monoclonal antibody, 299-1C1, and performed immunohistochemical analyses using 299-1C1 together with two existing anti-KS monoclonal antibodies, 5D4 and R-10G, in combination with keratanase II and endo-β-galactosidase. The results showed that both highly sulfated and low-sulfated KS are preferentially expressed in PTC, including lymph node metastases, accompanied by upregulation of genes encoding key KS biosynthetic enzymes (B3GNT7, B4GALT4, CHST2, and CHST6) in integrated TCGA/GTEx transcriptomic datasets. Expression of B3GNT7, B4GALT4, and CHST2, but not CHST6, was further increased in BRAF-mutant PTCs. These findings indicate that anti-KS monoclonal antibodies are useful for the pathological diagnosis of PTC, particularly for distinguishing lymph node metastases from intranodal thyroid inclusions.