RESUMO
Recent studies have shown that dendritic cells (DCs) regulate B cell functions. In this study, we report that bone marrow (BM)-derived immature DCs, but not mature DCs, can inhibit BCR-induced proliferation of B cells in a contact-dependent manner. This inhibition is overcome by treatment with BAFF and is dependent on the BCR coreceptor CD22; however, it is not dependent on expression of the CD22 glycan ligand(s) produced by ST6Gal-I sialyltransferase. We found that a second CD22 ligand (CD22L) is expressed on CD11c(+) splenic and BM-derived DCs, which does not contain ST6Gal-I-generated sialic acids and which, unlike the B cell-associated CD22L, is resistant to neuraminidase treatment and sodium metaperiodate oxidation. Examination of splenic and BM B cell subsets in CD22 and ST6Gal-I knockout mice revealed that ST6Gal-I-generated B cell CD22L plays a role in splenic B cell development, whereas the maintenance of long-lived mature BM B cells depends only on CD22 and not on alpha2,6-sialic acids produced by ST6Gal-I. We propose that the two distinct CD22L have different functions. The alpha2,6-sialic acid-containing glycoprotein is important for splenic B cell subset development, whereas the DC-associated ST6Gal-I-independent CD22L may be required for the maintenance of long-lived mature B cells in the BM.
Assuntos
Linfócitos B/citologia , Linfócitos B/metabolismo , Células Dendríticas/imunologia , Lectina 2 Semelhante a Ig de Ligação ao Ácido Siálico/imunologia , Lectina 2 Semelhante a Ig de Ligação ao Ácido Siálico/metabolismo , Animais , Linfócitos B/imunologia , Diferenciação Celular/imunologia , Proliferação de Células , Células Cultivadas , Células Dendríticas/citologia , Camundongos , Camundongos Endogâmicos C57BL , Camundongos Knockout , Ácido N-Acetilneuramínico/metabolismo , Neuraminidase/metabolismo , Oxirredução , Ácido Periódico/metabolismo , Ligação Proteica , Receptores de Antígenos de Linfócitos B/imunologia , Lectina 2 Semelhante a Ig de Ligação ao Ácido Siálico/genética , Sialiltransferases/deficiência , Sialiltransferases/genética , Sialiltransferases/metabolismo , beta-D-Galactosídeo alfa 2-6-SialiltransferaseRESUMO
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 Igalpha/beta, Syk, and phospholipase C-gamma2 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.
