A functional spleen contributes to afucosylated IgG in humans

Abstract

As a lymphoid organ, the spleen hosts a wide range of immune cell populations, which not only remove blood-borne antigens, but also generate and regulate antigen-specific immune responses. In particular, the splenic microenvironment has been demonstrated to play a prominent role in adaptive immune responses to enveloped viral infections and alloantigens. During both types of immunizations, antigen-specific immunoglobulins G (IgGs) have been characterized by the reduced amount of fucose present on N-linked glycans of the fragment crystallizable (Fc) region. These glycans are essential for mediating the induction of immune effector functions. Therefore, we hypothesized that a spleen may modulate humoral responses and serve as a preferential site for afucosylated IgG responses, which potentially play a role in immune thrombocytopenia (ITP) pathogenesis. To determine the role of the spleen in IgG-Fc glycosylation, we performed IgG subclass-specific liquid chromatography–mass spectrometry (LC–MS) analysis of Fc glycosylation in a large cohort of individuals splenectomized due to trauma, due to ITP, or spherocytosis. IgG-Fc fucosylation was consistently increased after splenectomy, while no effects for IgG-Fc galactosylation and sialylation were observed. An increase in IgG1- and IgG2/3-Fc fucosylation level upon splenectomy has been reported here for the first time, suggesting that immune responses occurring in the spleen may be particularly prone to generate afucosylated IgG responses. Surprisingly, the level of total IgG-Fc fucosylation was decreased in ITP patients compared to healthy controls. Overall, our results suggest a yet unrecognized role of the spleen in either the induction or maintenance of afucosylated IgG responses by B cells.

Role of Fc Core Fucosylation in the Effector Function of IgG1 Antibodies

“…The B cell response to infections that induce afucosylated IgGs probably explains the increased afucosylation during childhood (see below), but the mechanisms are unclear. Recently the spleen has been suggested to be an important site of fucosylation regulation: IgG1-Fc core fucosylation was observed to be increased in trauma splenectomized healthy individuals as well as in spherocytosis or immune thrombocytopenia (ITP) patients that were splenectomized, compared to non-splenectomized controls ( 133 ). There are several possible explanations for this observation: 1) The spleen may be a site of afucosylated IgG generation.…”

“…Indeed in ITP, the spleen is also a source of anti-platelets IgGs which are highly afucosylated compared to total IgGs. 2) The spleen removes preferentially immune complexes containing afucosylated IgGs via FcɣRIIIA positive myeloid cells, so that splenectomy induces a change in the ratio of fucosylated/afucosylated IgGs ( 133 ). Further investigation of these possible mechanisms is warranted.…”

Detection of Antibody-Dependent Cell-Mediated Cytotoxicity—Supporting Antibodies by NK-92-CD16A Cell Externalization of CD107a: Recognition of Antibody Afucosylation and Assay Optimization

“…The second component is an internal antibody standard for calibration to control for intra-experimental variations. The standard for reference is the monoclonal antibody (mAb) GA101-WT anti-CD20 that is ~10% afucosylated, comparable to human afucosylation of total circulating IgG1 [ 31 , 32 ]. The third component is ‘target’ cells with antigen expression.…”