Ross G D, Cain J A, Lachmann P J
J Immunol. 1985 May;134(5):3307-15.
Human leukocyte complement receptor type three (CR3) was shown to be lectin-like and to resemble bovine serum conglutinin (K) in that it bound to both iC3b and unopsonized yeast (Saccharomyces cerevisiae), and was inhibited by EDTA or N-acetyl-D-glucosamine (NADG). CR3 and K also bound to zymosan (Z), a yeast cell wall extract that contains primarily polysaccharide and no detectable protein. However, structural differences and the absence of K on bovine phagocytes indicated that CR3 was not the human homologue of bovine K. Phagocytic and respiratory responses to unopsonized Z were CR3 dependent because they were inhibited by monoclonal antibodies specific for the alpha-chain of CR3 and did not occur with phagocytes from patients with a genetic deficiency of CR3. The binding of CR3 to Z did not require opsonization of the Z with neutrophil-secreted C3, as Z binding and responses were not inhibited by Fab anti-C3. In addition, CR3-dependent binding of yeast occurred with neutrophils from which protein secretion was blocked by fixation with paraformaldehyde. Rabbit erythrocytes (RaE) also bound weakly to neutrophil CR3 and triggered ingestion. Anti-CR3 not only blocked the binding and ingestion of RaE but also blocked selectively the ingestion of RaEC3b without affecting the strong binding mediated by CR1. Even though sheep E and sheep EC3b were not ingested by neutrophils, a weak binding of CR3 to sheep E was suggested by the finding of 20 to 40% inhibition of sheep EAIgG ingestion by anti-CR3. Such inhibition was only observed in buffers that allowed activity of the CR3 binding site and not in buffers containing either EDTA or NADG. An apparently contradictory finding was that the weak CR3-dependent binding of Z triggered neutrophil ingestion and a superoxide burst, whereas the avid CR3-dependent binding of sheep EC3bi did not induce significant ingestion or a respiratory burst. Blocking studies with monoclonal antibodies specific for different epitopes of the alpha-chain of CR3 suggested that this might result from the presence of two distinct binding sites in CR3: one site for fixed iC3b that did not trigger functions, and a second function-triggering site for Z that did not bind to fixed iC3b.
人白细胞三型补体受体（CR3）显示出类似凝集素的特性，并且在与iC3b和未调理的酵母（酿酒酵母）结合方面类似于牛血清胶固素（K），并且被乙二胺四乙酸（EDTA）或N-乙酰-D-葡萄糖胺（NADG）抑制。CR3和K也与酵母聚糖（Z）结合，酵母聚糖是一种主要包含多糖且未检测到蛋白质的酵母细胞壁提取物。然而，结构差异以及牛吞噬细胞上不存在K表明CR3不是牛K的人类同源物。对未调理的Z的吞噬和呼吸反应是CR3依赖性的，因为它们被针对CR3α链的单克隆抗体抑制，并且在患有CR3基因缺陷的患者的吞噬细胞中不发生。CR3与Z的结合不需要用中性粒细胞分泌的C3对Z进行调理，因为Z的结合和反应不受Fab抗C3的抑制。此外，酵母的CR3依赖性结合发生在蛋白质分泌被多聚甲醛固定阻断的中性粒细胞中。兔红细胞（RaE）也与中性粒细胞CR3弱结合并引发吞噬。抗CR3不仅阻断了RaE的结合和吞噬，还选择性地阻断了RaEC3b的吞噬，而不影响由CR1介导的强结合。尽管绵羊红细胞（E）和绵羊红细胞C3b（EC3b）不被中性粒细胞吞噬，但抗CR3对绵羊红细胞IgG吞噬的20%至40%抑制表明CR3与绵羊红细胞有弱结合。这种抑制仅在允许CR3结合位点活性的缓冲液中观察到，而在含有EDTA或NADG的缓冲液中未观察到。一个明显矛盾的发现是，Z的弱CR3依赖性结合引发中性粒细胞吞噬和超氧化物爆发，而绵羊红细胞C3bi（EC3bi）的强CR3依赖性结合并未诱导明显的吞噬或呼吸爆发。用针对CR3α链不同表位的单克隆抗体进行的阻断研究表明，这可能是由于CR3中存在两个不同的结合位点：一个是与固定的iC3b结合的位点，不触发功能，另一个是与Z结合的功能触发位点，不与固定的iC3b结合。
