Collins Brian E, Blixt Ola, DeSieno Alexis R, Bovin Nicolai, Marth Jamey D, Paulson James C
Department of Molecular Biology, The Scripps Research Institute, La Jolla, CA 92037, USA.
Proc Natl Acad Sci U S A. 2004 Apr 20;101(16):6104-9. doi: 10.1073/pnas.0400851101. Epub 2004 Apr 12.
CD22, a negative regulator of B cell signaling, is a member of the siglec family that binds to alpha2-6-linked sialic acids on glycoproteins. Previous reports demonstrated that binding of multivalent sialoside probes to CD22 is blocked, or "masked," by endogenous (cis) ligands, unless they are first destroyed by sialidase treatment. These results suggest that cis ligands on B cells make CD22 functionally unavailable for binding to ligands in trans. Through immunofluorescence microscopy, however, we observed that CD22 on resting B cells redistributes to the site of contact with other B or T lymphocytes. Redistribution is mediated by interaction with trans ligands on the opposing cell because it does not occur with ligand-deficient lymphocytes from ST6GalI-null mice. Surprisingly, CD45, proposed as both a cis and trans ligand of CD22, was not required for redistribution to sites of cell contact, given that redistribution of CD22 was independent of CD45 and was observed with lymphocytes from CD45-deficient mice. Furthermore, CD45 is not required for CD22 masking as similar levels of masking were observed in the WT and null mice. Comparison of the widely used sialoside-polyacrylamide probe with a sialoside-streptavidin probe revealed that the latter bound a subset of B cells without sialidase treatment, suggesting that cis ligands differentially impacted the binding of these two probes in trans. The combined results suggest that equilibrium binding to cis ligands does not preclude binding of CD22 to ligands in trans, and allows for its redistribution to sites of contact between lymphocytes.
CD22是B细胞信号传导的负调节因子，是唾液酸结合免疫球蛋白样凝集素（siglec）家族的成员，可与糖蛋白上的α2-6连接的唾液酸结合。先前的报道表明，多价唾液酸苷探针与CD22的结合会被内源性（顺式）配体阻断或“掩盖”，除非它们首先被唾液酸酶处理破坏。这些结果表明，B细胞上的顺式配体使CD22在功能上无法与反式配体结合。然而，通过免疫荧光显微镜观察，我们发现静息B细胞上的CD22会重新分布到与其他B或T淋巴细胞接触的部位。重新分布是由与相对细胞上的反式配体相互作用介导的，因为来自ST6GalI基因敲除小鼠的缺乏配体的淋巴细胞不会发生这种情况。令人惊讶的是，鉴于CD22的重新分布独立于CD45，并且在来自CD45缺陷小鼠的淋巴细胞中也观察到了这种情况，因此CD22重新分布到细胞接触部位并不需要被认为是CD22顺式和反式配体的CD45。此外，CD45对于CD22的掩盖不是必需的，因为在野生型和基因敲除小鼠中观察到了相似水平的掩盖。将广泛使用的唾液酸苷-聚丙烯酰胺探针与唾液酸苷-链霉亲和素探针进行比较，发现后者在未经唾液酸酶处理的情况下结合了一部分B细胞，这表明顺式配体对这两种探针的反式结合有不同的影响。综合结果表明，与顺式配体的平衡结合并不排除CD22与反式配体的结合，并允许其重新分布到淋巴细胞之间的接触部位。
