The role of IgG Fc glycan in cancer

Introduction

IgGs are structurally composed of variable fragment antigen-binding (Fab) domains that confer their binding specificity to the antigen, and an Fc domain that determine their effector function. The effector functions mediated by IgG rely on their interactions with Fcg receptors (FcgRs) that are expressed on various immune cells. While the Fc portion is traditionally considered the invariant region of an IgG molecule, this domain displays considerable heterogeneity. This heterogeneity arises from the differences between the subclasses, allotypes and the composition of the attached glycan complex at the CH2 domain. These variations result in selective engagement of particular classes of FcgRs with distinct effector activities.

While much progress has been made in our understanding the role of specific human IgG Fc glycans in mediating diverse immune responses, there is lack of knowledge regarding the heterogeneity of the mouse IgG Fc glycans, and their impact on the mouse antibody-mediated immune responses in general and during cancer development. The aim of this study is to decipher the mouse Fc fingerprint in naïve mice and in mice inflicted with different kinds of tumors. We will characterize how this affects the IgG Fc interaction with different FcgRs, and evaluate the overall degree of homology between mouse and human IgG glycosylation pathways and their immunological role.

Material and methods

We will initially perform a mass spectrometry-based discovery step characterizing the murine Fc N-linked glycoform repertoire. We will isolate IgG from serum of naïve mice and mice under different physiological conditions (vaccinated mice, tumor-bearing mice, autoimmunity models, etc.) and will pool them together for the discovery step. The discovery will identify the Fc glycans repertoire and will set the database to identify glycans in the following experiments.

We will next compare the Fc fingerprint (total IgG and relevant antigen-specific IgG) in healthy vs. tumor bearing mice and at different time points and assess whether it can serve as a diagnostic, prognostic or any other marker.

o Preliminary Results and discussion

Following the discovery step we have established the glycan repertoire that will serve for the future targeted analysis of our experimental samples. We have several glycan fingerprints of naïve mice that were analyzed following this discovery step. Interestingly, each IgG subclass displayed a unique Fc fingerprint, suggesting this process is a well regulated one. After establishing this step, we plan to analyze samples from tumor bearing mice and assess their significance.

o Conclusion

Fc glycoforms modulate the structure of the Fc to alternate between different FcgR types binding conformations. We saw that each IgG subclass has a unique Fc fingerprint, suggesting that different factors contribute to the final IgG and this is probably a tightly regulated process. We plan to further elucidate these factors and whether they can modulate or predict tumor progression.