The Role Of IgG Fc Fingerprint In Cancer

Introduction

IgGs are structurally composed of variable fragment antigen-binding (Fab) domains that confer their binding specificity to the antigen, and a fragment crystallizable (Fc) domain that determine their effector function. The effector functions mediated by IgG rely on their interactions with Fcγ receptors (FcγRs) that are expressed mainly on 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 of the Fc region. 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, allowing for a more in depth mechanistic understanding of this fingerprint. 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 initially performed a mass spectrometry-based discovery step characterizing the murine Fc N-linked glycoform repertoire. We isolated IgG from serum of naïve mice and mice under different immunological conditions (vaccinated mice, tumor-bearing mice etc.) and pooled them together for the discovery step. We have identified the Fc glycan repertoire which will be used as a reference library to identify glycans in the following experiments.

We plan to compare the Fc fingerprint (total IgG and relevant antigen-specific IgG) in healthy vs. tumor bearing mice, at different time points during disease progression and upon immunotherapy in order to assess whether it can serve as a diagnostic or prognostic marker.

Preliminary Results and discussion

Following the discovery step we have established the glycan repertoire and identified several Fc fingerprints that will serve for the future targeted analysis of our experimental samples. Interestingly, each IgG subclass displayed a unique Fc fingerprint, suggesting this process is well-regulated. Following, we plan to analyze samples from tumor bearing mice and assess the significance of Fc fingerprint in this context.

Conclusion

Fc glycoforms modulate the structure of the Fc moiety to accommodate different binding affinities to the FcγRs. We have demonstrated that in mice each IgG subclass has a unique Fc fingerprint, suggesting that different factors contribute to the final IgG functions and that this is probably a tightly regulated process. We plan to further elucidate these factors and whether they can modulate or predict tumor progression.