Razi N, Varki A
Glycobiology Program, University of California, San Diego Cancer Center, Divisions of Hematology-Oncology and Cellular and Molecular Medicine, University of California, San Diego, La Jolla, CA 92093-0687, USA.
Proc Natl Acad Sci U S A. 1998 Jun 23;95(13):7469-74. doi: 10.1073/pnas.95.13.7469.
CD22 is a B cell-restricted glycoprotein involved in signal transduction and modulation of cellular activation. It is also an I-type lectin (now designated Siglec-2), whose extracellular domain can specifically recognize alpha2-6-linked sialic acid (Sia) residues. This activity is postulated to mediate intercellular adhesion and/or to act as a coreceptor in antigen-induced B cell activation. However, studies with recombinant CD22 indicate that the lectin function can be inactivated by expression of alpha2-6-linked Sia residues on the same cell surface. To explore whether this masking phenomenon affects native CD22 on B cells, we first developed a probe to detect the lectin activity of recombinant CD22 expressed on Chinese hamster ovary cells (which have no endogenous alpha2-6-linked Sia residues). This probe is inactive against CD22-positive B lymphoma cells and Epstein-Barr virus-transformed lymphoblasts which express high levels of alpha2-6-linked Sia residues. Enzymatic desialylation unmasks the CD22 lectin activity, indicating that endogenous Sia residues block the CD22 lectin-binding site. Truncation of the side chains of cell surface Sia residues by mild periodate oxidation (known to abrogate Sia recognition by CD22) also had this unmasking effect, indicating that the effects of desialylation are not due to a loss of negative charge. Normal resting B cells from human peripheral blood gave similar findings. However, the lectin is partially unmasked during in vitro activation of these cells. Thus, the lectin activity of CD22 is restricted by endogenous sialylation in resting B cells and may be transiently unmasked during in vivo activation, perhaps to modulate intercellular or intracellular interactions at this critical stage in the humoral response.
CD22是一种B细胞限制性糖蛋白，参与信号转导和细胞活化调节。它也是一种I型凝集素（现称为Siglec-2），其细胞外结构域可特异性识别α2-6连接的唾液酸（Sia）残基。据推测，这种活性可介导细胞间黏附，和/或作为抗原诱导的B细胞活化中的共受体。然而，对重组CD22的研究表明，凝集素功能可因同一细胞表面α2-6连接的Sia残基的表达而失活。为了探究这种掩盖现象是否影响B细胞上的天然CD22，我们首先开发了一种探针，用于检测在中国仓鼠卵巢细胞（无内源性α2-6连接的Sia残基）上表达的重组CD22的凝集素活性。该探针对表达高水平α2-6连接的Sia残基的CD22阳性B淋巴瘤细胞和爱泼斯坦-巴尔病毒转化的淋巴母细胞无活性。酶促去唾液酸化可使CD22凝集素活性暴露，表明内源性Sia残基阻断了CD22凝集素结合位点。通过温和的高碘酸盐氧化截断细胞表面Sia残基的侧链（已知可消除CD22对Sia的识别）也有这种暴露效应，表明去唾液酸化的作用不是由于负电荷的丧失。来自人外周血的正常静息B细胞也有类似的发现。然而，在这些细胞的体外活化过程中，凝集素部分暴露。因此，CD22的凝集素活性在静息B细胞中受到内源性唾液酸化的限制，并且可能在体内活化过程中短暂暴露，也许是为了在体液反应的这个关键阶段调节细胞间或细胞内相互作用。
