Abstract
We previously showed that cultured human umbilical vein endothelial cells (HEC) exposed to the inflammatory cytokines tumor necrosis factor-alpha or interleukin-1 display increased activity of beta-galactoside alpha 2,6-sialyltransferase. This is associated with enhanced expression of ligands for the B cell receptor CD22 beta, which recognizes alpha 2-6-linked sialic acids (Hanasaki, K., Varki, A., Stamenkovic, I., and Bevilacqua, M. P. (1994) J. Biol. Chem. 269, 10637-10643). Here we report that increased expression of CD22 ligands is a feature of dermal microvascular endothelial cells as well, and is also observed in response to the cytokine interleukin-4. Tumor necrosis factor-alpha stimulation of HEC causes no change in the profile of endothelial glycoproteins recognized by CD22, but doubles the proportion of total cellular N-linked oligosaccharides capable of binding tightly to CD22. This modest change is sufficient to cause a marked increase in alpha 2-6-linked sialic acid-dependent binding of Chinese hamster ovary (CHO) cells expressing recombinant human CD22. In contrast, B lymphoma cell lines expressing higher levels of cell surface CD22 do not show such sialic acid-dependent binding to activated HEC. Since B lymphoma cells themselves also express high levels of alpha 2-6-linked sialic acids, their CD22 molecules might be rendered nonfunctional by endogenous ligands. In support of this, the lectin function of CD22 can be directly detected on transfected CHO cells, but not on B lymphoma cells. Furthermore, coexpression of beta-galactoside alpha 2,6-sialyltransferase with CD22 in the CHO cells abrogates sialic acid-dependent binding to cytokine-activated HEC. However, such co-transfected cells can bind to B lymphoma cells in a manner apparently less dependent upon alpha 2-6-linked sialic acid, suggesting CD22-mediated interactions that may not be directly dependent on its lectin function. Thus, CD22-mediated interactions between B cells and activated vascular endothelium may be positively regulated by induction of alpha 2-6-linked sialic acid-bearing endothelial cell ligands, but negatively regulated by such ligands on the B cells expressing CD22. Since expression of both CD22 and beta-galactoside alpha 2,6-sialyltransferase are regulated during B cell ontogeny, these findings could be of importance in B cell function and/or trafficking.
Highlights
From the Glycobiology Program, UCSD Cancer Center, and Division of Cellular and Molecular Medicine, University of California at San Diego, La Jolla, California 92093
We previously showed that cultured human umbilical vein endothelial cells (HEC) exposed to the inflammatory cytokines tumor necrosis factor-a or interleukin-l display increased activity of p-galactoside a2,6-sialyltransferase
A soluble chimeric form of CD22{3(CD22Rg), containing the three amino-terminal Ig-like domains of human CD22{3 fused to the COOH-terminal Fc domains of human IgG can bind and precipitate potential glycoprotein ligands on activated T and B cells, one of which is the tyrosine phosphatase CD45 [9, 13,20, 21], Sialic acid (Sia) is an essential component of recognition by CD22 [3], Several studies have established that the structural motif recognized is Siaa2-6Gal{31-4GlcNAc- [9, 13, 19,21] (see the accompanying paper (22», which is found in varying numbers on the antennae of N-linked oligosaccharides of some cell surface glycoproteins 0, 23)
Summary
Sialic acids; AGP, aracid glycoprotein; CD22Rg, CD22-immunoglobulin chimera containing Ig domains 1-3 of human CD22f3; FCS, fetal calf serum; CD22Rg-PAS, CD22Rgcoupled to protein A-Sepharose; HEC, human umbilical vein endothelial cells; ICAM-I, intercellular adhesion molecule-I; Ig, immunoglobulin; IL-l, interleukin-I; LPS, lipopolysaccharide; mAb, monoclonal antibody; PBS, phosphate-buffered saline; PE, phycoerythrin; sLac, sialyllactose; SNA, Sambucus nigra agglutinin; ST6N, f3-galactoside a2,6-sialyltransferase; TNF-a, tumor necrosis factor-a; VCAM-l, vascular cell adhesion molecule-I; BSA, bovine serum albumin; FITC, fluorescein isothiocyanate; MEM, minimal essential medium; DMEM, Dulbecco's modified Eagle's medium; CHO, Chinese hamster ovary; WT, wild-type. The latter fits well with the recent work of BraeschAndersen and Stamenkovic [39], who showed that CD22 molecules bearing a2-6-linked Sia are functionally inactive
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