Laboratory of Aquacultural Biology, Graduate School of Agricultural Sciences, Tohoku University, Aoba-ku, Sendai 980-0845, Japan; Oyster Research Institute, Aoba-ku, Sendai 989-3204, Japan.
National Institute of Radiological Sciences, QST, Inage-ku, Chiba-shi 263-8555, Japan.
Fish Shellfish Immunol. 2017 Nov;70:575-582. doi: 10.1016/j.fsi.2017.09.027. Epub 2017 Sep 9.
For a marine bivalve mollusk such as Pacific oyster Crassostrea gigas, the elimination of foreign particles via hemocyte phagocytosis plays an important role in host defense mechanisms. The hemocytes of C. gigas have a high phagocytic ability for baker's yeast (Saccharomyces cerevisiae) and its cell-wall product zymosan. C. gigas hemocytes might phagocytose yeast cells after binding to polysaccharides on the cell-wall surface, but it is unknown how and what kinds of polysaccharide molecules are recognized. We conducted experiments to determine differences in the phagocytic ability of C. gigas hemocytes against heat-killed yeast (HK yeast), zymosan and zymocel, which are similarly sized and shaped but differ in the polysaccharide composition of their particle surface. We found that both the agranulocytes and granulocytes exerted strong phagocytic ability on all tested particles. The phagocytic index (PI) of granulocytes for zymosan was 9.4 ± 1.7, which significantly differed with that for HK yeast and zymocel (P < 0.05). To evaluate the PI for the three types of particles, and especially to understand the outcome of the much higher PI for zymosan, PI was gauged in increments of 5 (1-5, 6-10, 11-15, and ≥16), and the phagocytic frequencies were compared according to these increments. The results show that a markedly high PI of ≥16 was exhibited by 18.1% of granulocytes for zymosan, significantly higher than 1.7% and 3.9% shown for HK yeast and zymocel, respectively (P < 0.05). These findings indicate that the relatively high PI for zymosan could not be attributed to a situation wherein all phagocytic hemocytes shared a high mean PI, but rather to the ability of some hemocytes to phagocytose a larger portion of zymosan. To determine whether the phagocytosis of these respective particles depended on the recognition of specific polysaccharide receptors on the hemocyte surface, C. gigas hemocytes were pretreated with soluble α-mannan or β-laminarin and then allowed to phagocytose the three types of the particles. The percentage of phagocytic cells of β-laminarin-treated granulocytes decreased significantly for zymosan and zymocel, but not for yeast. These results suggest that C. gigas might possess at least two types of hemocytes, and that one type of the hemocytes (granulocytes) is more active for phagocytosis. The granulocytes were found to have multiple subtypes with different phagocytic abilities and multiple phagocytic receptors. Some of the granulocyte subtypes revealed a much stronger phagocytic ability, depending on the presence of β-glucan receptors for phagocytosis.
对于像太平洋牡蛎（Crassostrea gigas）这样的海洋双壳贝类软体动物，通过血细胞吞噬作用消除外来颗粒在宿主防御机制中起着重要作用。太平洋牡蛎的血细胞具有吞噬面包酵母（酿酒酵母）及其细胞壁产物酵母聚糖的高吞噬能力。太平洋牡蛎的血细胞可能通过结合到细胞壁表面的多糖上而吞噬酵母细胞，但尚不清楚如何以及识别哪种多糖分子。我们进行了实验以确定太平洋牡蛎血细胞对热灭活酵母（HK 酵母）、酵母聚糖和酵母纤维素的吞噬能力的差异，这三种物质大小和形状相似，但颗粒表面的多糖组成不同。我们发现，颗粒细胞和粒细胞都对所有测试颗粒具有很强的吞噬能力。粒细胞对酵母聚糖的吞噬指数（PI）为 9.4 ± 1.7，与 HK 酵母和酵母纤维素的 PI 显著不同（P < 0.05）。为了评估三种类型颗粒的 PI，特别是为了了解酵母聚糖更高的 PI 的结果，我们以 5 为增量（1-5、6-10、11-15 和≥16）来衡量 PI，并根据这些增量比较吞噬频率。结果表明，粒细胞对酵母聚糖的 PI 明显较高，有 18.1%达到≥16，显著高于 HK 酵母和酵母纤维素的 1.7%和 3.9%（P < 0.05）。这些发现表明，酵母聚糖相对较高的 PI 不能归因于所有吞噬血细胞具有较高的平均 PI，而是归因于一些血细胞能够吞噬更大比例的酵母聚糖。为了确定这些颗粒的吞噬作用是否取决于血细胞表面特定多糖受体的识别，用可溶性α-甘露聚糖或β- 藻聚糖预处理太平洋牡蛎血细胞，然后让它们吞噬三种类型的颗粒。β- 藻聚糖处理的粒细胞对酵母聚糖和酵母纤维素的吞噬细胞百分比显著下降，但对酵母没有下降。这些结果表明，太平洋牡蛎可能至少有两种类型的血细胞，其中一种血细胞（粒细胞）更活跃于吞噬作用。发现粒细胞具有不同吞噬能力的多种亚型和多种吞噬受体。一些粒细胞亚型表现出更强的吞噬能力，这取决于吞噬β-葡聚糖受体的存在。
