Glycoproteomic Analysis of MGL-Binding Proteins on Acute T‑Cell Leukemia Cells

C-type lectins are
a diverse group of proteins involved in many
human physiological and pathological processes. Most C-type lectins
are glycan-binding proteins, some of which are pivotal for innate
immune responses against pathogens. Other C-type lectins, such as
the macrophage galactose-type lectin (MGL), have been shown to induce
immunosuppressive responses upon the recognition of aberrant glycosylation
on cancer cells. MGL is known to recognize terminal N-acetylgalactosamine (GalNAc), such as the Tn antigen, which is commonly
found on malignant cells. Even though this glycan specificity of MGL
is well described, there is a lack of understanding of the actual
glycoproteins that bind MGL. We present a glycoproteomic workflow
for the identification of MGL-binding proteins, which we applied to
study MGL ligands on the human Jurkat leukemia cell line. In addition
to the known MGL ligands and Tn antigen-carrying proteins CD43 and
CD45 on these cells, we have identified a set of novel cell-surface
ligands for MGL. Importantly, for several of these, O-glycosylation
has hitherto not been described. Altogether, our data provide new
insight into the identification and structure of novel MGL ligands
that presumably act as modulatory molecules in cancer immune responses.