Copresentation of Antigen and Ligands of Siglec-G Induces B Cell Tolerance Independent of CD22

Abstract

Differentiation of self from non-self is indispensable for maintaining B cell tolerance in peripheral tissues. CD22 and Siglec-G are two inhibitory co-receptors of the BCR that are implicated in maintenance of tolerance to self-antigens. Enforced ligation of CD22 and the BCR by a nanoparticle displaying both antigen and CD22 ligands induces a tolerogenic circuit resulting in apoptosis of the antigen reactive B cell. Whether Siglec-G also has this property has not be investigated in large part due to the lack of a selective Siglec-G ligand. Here, we report the development of a selective high-affinity ligand for Siglec-G and its application as a chemical tool to investigate the tolerogenic potential of Siglec-G. We find that liposomal nanoparticles decorated with antigen and Siglec-G ligand inhibit BCR signaling in both B1 and B2 B cells compared to liposomes displaying antigen alone. Not only is inhibition of B cell activation observed by ligating the BCR with Siglec-G, but robust tolerance towards T-independent and T-dependent antigens is also induced in mice. The ability of Siglec-G to inhibit B cell activation equally in both B1 and B2 subsets is consistent with our observation that Siglec-G is expressed at a relatively constant level throughout numerous B cell subsets. These results suggest that Siglec-G may contribute to maintenance of B cell tolerance towards self-antigens in various B cell compartments.

The Ligand-Binding Domain of Siglec-G Is Crucial for Its Selective Inhibitory Function on B1 Cells

“…Whether Siglec-G can be induced on the surface of macrophages was not addressed in this study but can easily be done with the new mAb in the future. Our Ab shows a Siglec-G cellular staining pattern very comparable to that of a rat mAb recently discussed in a published paper (20).…”

“…Our findings are therefore highly relevant for the better understanding of B cell-driven autoimmune diseases and offer new treatment concepts for patients with these diseases. In this context, a Siglec-G ligand-dependent B cell tolerization strategy has been published recently (20).…”

Siglec-G Deficiency Leads to Autoimmunity in Aging C57BL/6 Mice

“…2A). Because Siglec-G is also expressed on dendritic cells (DCs) (19,21), we analyzed DC numbers in aging mice and also determined activation markers such as MHC class II and CD86 on DCs. We did not detect any significant changes in the three knockout mouse strains (not shown).…”

“…For Siglec-G, it was shown that it binds and activates the phosphatase SHP-1, whereas Siglec-10 binds both SHP-1 and SHP-2 phosphatases. These tyrosine phosphatases can dephosphorylate positive regulators of the BCR signaling cascade and cause a dampening of the BCR signal (19,20). Although Siglec-G is expressed on all B cell subsets (19,21), deficiency of Siglec-G in mice causes a strong enlargement of the B1 cell population, but not of B2 cells, indicating that Siglec-G has mainly a B1 cell-restricted inhibitory function.…”

“…These tyrosine phosphatases can dephosphorylate positive regulators of the BCR signaling cascade and cause a dampening of the BCR signal (19,20). Although Siglec-G is expressed on all B cell subsets (19,21), deficiency of Siglec-G in mice causes a strong enlargement of the B1 cell population, but not of B2 cells, indicating that Siglec-G has mainly a B1 cell-restricted inhibitory function. Additionally, Siglec-G-deficient B cells are hyperreactive and exhibit a highly elevated Ca 2+ flux as well as a prolonged live span only in B1 but not in B2 cells (9).…”