ST6Gal-I Restrains CD22-Dependent Antigen Receptor Endocytosis and Shp-1 Recruitment in Normal and Pathogenic Immune Signaling

Abstract

The ST6Gal-I sialyltransferase produces Siglec ligands for the B-cell-specific CD22 lectin and sustains humoral immune responses. Using multiple experimental approaches to elucidate the mechanisms involved, we report that ST6Gal-I deficiency induces immunoglobulin M (IgM) antigen receptor endocytosis in the absence of immune stimulation. This coincides with increased antigen receptor colocalization with CD22 in both clathrin-deficient and clathrin-enriched membrane microdomains concurrent with diminished tyrosine phosphorylation of Igα/β, Syk, and phospholipase C-γ2 upon immune activation. Codeficiency with CD22 restores IgM antigen receptor half-life at the cell surface in addition to reversing alterations in membrane trafficking and immune signaling. Diminished immune responses due to ST6Gal-I deficiency further correlate with constitutive recruitment of Shp-1 to CD22 in unstimulated B cells independent of Lyn tyrosine kinase activity and prevent autoimmune disease pathogenesis in the Lyn-deficient model of systemic lupus erythematosus, resulting in a significant extension of life span. Protein glycosylation by ST6Gal-I restricts access of antigen receptors and Shp-1 to CD22 and operates by a CD22-dependent mechanism that decreases the basal rate of IgM antigen receptor endocytosis in altering the threshold of B-cell immune activation.

Supplemental material for this article may be found at http://mcb.asm.org/.

We thank Linda Walker and Maria Conejo for excellent technical support. Fluorescent microscopy was performed with the UCSD Cancer Center Digital Imaging Shared Resource Facility directed by James Feramisco. CD22-deficient and ST6Gal-I/CD22 doubly deficient mice were provided by James C. Paulson of The Scripps Research Institute (La Jolla, Calif.).

J.D.M. is a founder of Abaron Biosciences, Inc., a company that is developing drugs related to the research described in the manuscript. The University of California—San Diego is also an equity holder. The terms of this arrangement have been reviewed and approved by the University of California—San Diego, in accordance with its conflict of interest policies.

This research was funded by NIH grants HL57345 (J.D.M.), AI050143 (J.C.P.), and GM25042 (B.E.C.). J.D.M. is supported as an Investigator of the Howard Hughes Medical Institute.