IgG Fab glycosylation: a novel mediator of humoral tolerance (LSBR 1626)
Project leader: Dr. Theo Rispens (Sanquin Research, Dept. of Immunopatholgy)
PhD student: Jana Koers (Apr. 2017 – Apr. 2021)

Our immune system protects against infection by invading pathogens. B cells are an important part of the immune system. These cells can recognize pathogens and make specific antibodies. To protect our bodies against a range of pathogens, a wide repertoire of antibody specificities must be generated. This is achieved because the variable region is constructed from a combination of V,D and J gene segments. Furthermore, point mutations can be introduced into the variable region, creating even more variation in B cell specificity. The introduction of such a mutation can lead to the creation of a so-called N-glycosylation site and thus the generation of antibodies with an extra glycan (‘Fab glycan’). In this project we investigated this Fab glycosylation.

Despite previously found associations between Fab glycans and various aspects of immunity, little is known about the origin and functional consequences of Fab glycosylation. We investigated how and how often Fab glycans are introduced into the variable region of the different types of B cells found in human blood and bone marrow. We show that Fab glycans are present to various degrees depending on the antibody class, and that they were most present in the antibodies of class IgG4 and IgE. IgE antibodies are involved in allergies, and IgG4 antibodies are also often made in response to allergens, which possibly dampens allergic responses, but are also sometimes autoantibodies.

We have set up a culture system to be able to culture B cells in the lab in order to investigate more specifically how the N-glycosylation sites are formed and what role Fab glycans can play in the development of B cells and what possible role the antigen plays therein. We have optimized our B cell culture system and further research should show whether Fab glycosylation actually occurs in the cultured B cells.

For several autoimmune diseases, disease-associated antibodies have been characterized with increased Fab glycosylation. An example of this are the so-called ACPAs, autoantibodies in rheumatoid arthritis (RA). We investigated the Fab glycans for another type of autoantibodies, the anti-hinge antibodies, which are found to an increased extent in RA patients. We have shown that anti-hinge antibodies directed against different hinge epitopes all have increased Fab glycosylation, but that the levels are found equally in healthy individuals. These data suggest that selection for elevated levels of Fab glycans may be independent of inflammation, or that it is triggered by bacteria or endogenous stimulation.

The results of this study show that Fab glycans may be elevated in autoimmune diseases as well as in healthy individuals. More research is needed to gain insight into the possible immune-modulating role of Fab glycans in immunity and to learn more about the pathogenesis of B-cell mediated autoimmune diseases.