We have recently shown that 6-sulfo sialyl N-acetyllactosamine (LacNAc) in O-linked glycan recognized by the CL40 antibody is abundant in the pleural mesothelium under physiological conditions, and that these glycans are complementary synthesized by GlcNAc6ST2 (encoded by Chst4) and GlcNAc6ST3 (encoded by Chst5) in mice. GlcNAc6ST3 is essential for the synthesis of R-10G-positive keratan sulfate (KS) in the brain. The predicted minimum epitope of the R-10G antibody is a dimeric asialo 6-sulfo LacNAc. Whether the R-10G-reactive KS/sulfated LacNAc oligosaccharides are also present in the pleural mesothelium was unknown. If any, the question of which GlcNAc6ST is the responsible enzyme was another issue to be examined as well. Here, we showed that R-10G-reactive glycans are as abundant in the pulmonary pleura as are CL40-reactive glycans, and that GlcNAc6ST3 is only partially involved in the synthesis of these pleural R-10G glycans, unlike in the adult brain. Unexpectedly, GlcNAc6ST2 is essential for the synthesis of R-10G-positive KS/sulfated LacNAc oligosaccharides in the lung pleura. The type of GlcNAc6ST involved in KS glycan biosynthesis and the magnitude of its contribution to the KS synthesis were found to differ among tissues in vivo. We show that GlcNAc6ST2 is required and sufficient for R-10G-reactive KS synthesis in the lung pleura. Interestingly, R-10G immunoreactivity in KSGal6ST (encoded by Chst1) and C6st1 (encoded by Chst3) double-deficient mouse lungs was markedly increased. Muc16, a mucin molecule, was shown to be a candidate carrier protein for the pleural R-10G-reactive glycans. These results suggested that R-10G-positive KS/sulfated LacNAc oligosaccharides may play a role in mesothelial cell proliferation and differentiation. Further elucidation of the functions of sulfated glycans synthesized by GlcNAc6ST2 and GlcNAc6ST3, such as R-10G and CL40 glycans, in pathological conditions may lead to a better understanding of the underlying mechanisms of the physiopathology of lung mesothelium.

Simple cysts (SCs) within the testes are rare in adults. The testicular SC lining comprises mesothelial cells. This report presents a case involving the accidental discovery of a castration-worthy SC in the right testis of a Japanese man in his 90s who was diagnosed with prostatic adenocarcinoma. Microscopic analysis revealed that the cystic wall was covered with flat to cuboidal mesothelial cells. Immunohistochemistry demonstrated mesothelial cell positivity for calretinin, Wilms tumor 1 (WT-1), Hector Battifora mesothelial epitope-1 (HBME-1), and cyclin-dependent kinase inhibitor p16 (p16INK4a). Notably, no human papillomavirus (HPV) in situ hybridization was detected. This report outlines an exceptionally rare instance of a simple cyst within the testis, suggesting the possibility of its origin from ectopic rete testis epithelium. This marks the inaugural documentation in the English literature of an SC within the testis found incidentally during castration for prostatic cancer.