The Role of Fc Receptors in the Innate Immune System of Flounders Purported to Be Homologs of FcγRII and FcγRIII.

FcγR is a significant opsonin receptor located on the surface of immune cells, playing a crucial role in Ab-dependent cell-mediated immunity. Our previous work revealed opposite expression trends of FcγRII and FcγRIII in flounder mIgM+ B lymphocytes after phagocytosis of antiserum-opsonized Edwardsiella tarda. This observation suggests that FcγRII and FcγRIII might serve distinct functions in Ig-opsonized immune responses. In this study, we prepared rFcγRIII as well as its corresponding Abs to investigate the potential roles of FcγRII and FcγRIII in the Ab-dependent immune response of IgM+ B cells. Our findings indicate that, unlike FcγRII, FcγRIII does not participate in Ab-dependent cellular phagocytosis. Instead, it is involved in cytokine production and bacterial killing in mIgM+ B lymphocytes. Additionally, we identified platelet-derived ADAM17 as a key factor in regulating FcγRIII shedding and cytokine release in mIgM+ B lymphocytes. These results elucidate the functions of FcγRII and FcγRIII in the innate immunology of mIgM+ B lymphocytes and contribute to an improved understanding of the regulatory roles of FcγRs in the phagocytosis of teleost B lymphocytes.

Endodermal apoplastic barriers are linked to osmotic tolerance in meso-xerophytic grass Elymus sibiricus.

Drought is the most serious adversity faced by agriculture and animal husbandry industries. One strategy that plants use to adapt to water deficits is modifying the root growth and architecture. Root endodermis has cell walls reinforced with apoplastic barriers formed by the Casparian strip (CS) and suberin lamellae (SL) deposits, regulates radial nutrient transport and protects the vascular cylinder from abiotic threats. Elymus sibiricus is an economically important meso-xerophytic forage grass, characterized by high nutritional quality and strong environmental adaptability. The purpose of this study was to evaluate the drought tolerance of E. sibiricus genotypes and investigate the root structural adaptation mechanism of drought-tolerant genotypes' responding to drought. Specifically, a drought tolerant (DT) and drought sensitive (DS) genotype were screened out from 52 E. sibiricus genotypes. DT showed less apoplastic bypass flow of water and solutes than DS under control conditions, as determined with a hydraulic conductivity measurement system and an apoplastic fluorescent tracer, specifically PTS trisodium-8-hydroxy-1,3,6-pyrenetrisulphonic acid (PTS). In addition, DT accumulated less Na, Mg, Mn, and Zn and more Ni, Cu, and Al than DS, regardless of osmotic stress. Further study showed more suberin deposition in DT than in DS, which could be induced by osmotic stress in both. Accordingly, the CS and SL were deposited closer to the root tip in DT than in DS. However, osmotic stress induced their deposition closer to the root tips in DS, while likely increasing the thickness of the CS and SL in DT. The stronger and earlier formation of endodermal barriers may determine the radial transport pathways of water and solutes, and contribute to balance growth and drought response in E. sibiricus. These results could help us better understand how altered endodermal apoplastic barriers in roots regulate water and mineral nutrient transport in plants that have adapted to drought environments. Moreover, the current findings will aid in improving future breeding programs to develop drought-tolerant grass or crop cultivars.

The role of Syk phosphorylation in Fc receptor mediated mIgM+ B lymphocyte phagocytosis in flounder (Paralichthys olivaceus).

Spleen tyrosine kinase (Syk) is a non-receptor protein tyrosine kinase, and it mediates downstream signaling of FcR-mediated immune responses. Our previous work revealed that the expression of Syk was significantly up-regulated in flounder mIgM+ B lymphocytes after phagocytosis of antiserum-opsonized Edwardsiella tarda, which suggested Syk might be involved in Ig-opsonized phagocytosis. In this paper, phospho-Syk (pSyk) inhibitor was used to investigate the potential role of phosphorylated Syk in FcR-mediated phagocytosis of IgM+ B cells. Indirect immunofluorescence assay (IFA) and Western blotting showed that the level of phosphorylated Syk in the mIgM+ B lymphocytes treated with pSyk inhibitor was significantly lower compared to the control group after stimulation with flounder antiserum. Flow cytometry analysis showed that after 3 h incubation with antiserum-opsonized E. tarda, the phagocytosis rates of mIgM+ B lymphocytes from peripheral blood, spleen and head kidney pre-treated with pSyk inhibitor were 48.1%, 40.1% and 43.6% respectively, which were significantly lower than that of the control groups with 58.7%, 53.2% and 57.4%, respectively. And likewise, after pSyk inhibitor treatment, the proportions of mIgM+ B lymphocytes with higher intracellular reactive oxygen species (ROS) levels in peripheral blood, spleen and head kidney decreased to 15.2%, 12.0% and 12.1% from the control level of 26.5%, 25.9% and 26.3%, respectively. Moreover, the expression of three genes affected by pSyk, including phospholipase Cγ1 (PLCγ1), phospholipase Cγ2 (PLCγ2) and phosphatidylinositol 3 kinase (PI3K) were found to be significantly down-regulated in pSyk inhibitor-treated mIgM+ B lymphocytes post phagocytosis. These results suggest that pSyk plays a key role in FcR-mediated phagocytosis and bactericidal activity of mIgM+ B lymphocytes, which promotes further understanding of the regulatory role of pSyk in teleost B cells phagocytosis.

Regulatory Role of Fc Receptor in mIgM+ B Lymphocyte Phagocytosis in Flounder (Paralichthys olivaceus).

Fc receptor (FcR) is an important opsonin receptor on the surface of immune cells, playing an important role in antibody-dependent cell-mediated immunity. Our previous work found that the FcR of flounder showed a marked expression response in phagocytizing IgM+ B cell, which suggested that FcR might participate in regulating Ig-opsonized phagocytosis. In this paper, in order to elucidate the potential role of FcR in mediating phagocytosis of IgM+ B cell, flounder anti-E. tarda serum was prepared and complement-inactivated for the use of E. tarda opsonization, and the sera of healthy flounder were used as control. Flow cytometric analysis showed that the phagocytosis rates of antiserum-opsonized E. tarda in peripheral blood mIgM+ B lymphocytes were significantly higher than the control group, and higher phagocytosis rates of mIgM+ B lymphocyte could be detected with an increasing incubation time ranging from 1 to 5 h. The phagocytosis rates of antiserum-opsonized E. tarda by mIgM+ B lymphocyte for an incubation time of 1, 3 or 5 h were 51.1, 63.0, and 77.5% respectively, which were significantly higher than the phagocytosis rates in the control groups with 40.2, 50.9, and 63.8%, respectively. While the Fc fragment of IgM on the surface of opsonized E. tarda was blocked by rabbit anti-flounder IgM polyclonal antibodies, phagocytosis rates of mIgM+ B lymphocyte decreased significantly compared with the unblocked group. Moreover, the proportion of mIgM+ B lymphocytes with higher intracellular reactive oxygen species (ROS) levels rose to 32.1% from the control level of 23.0% after phagocytosis of antiserum-opsonized E. tarda. FcγRII and Syk were found to be significantly upregulated, while FcγRIII was significantly downregulated in the mIgM+ B lymphocytes post phagocytosis. Furthermore, when FcγRII of mIgM+ B lymphocytes was blocked by the prepared antibodies, their phagocytosis rate of antiserum-opsonized E. tarda was 39.0%, which was significantly lower than the unblocked group of 54.0%. These results demonstrate that FcR plays a critical role in mediating phagocytosis and bactericidal activity of mIgM+ B lymphocytes, which would facilitate an improved understanding of the regulatory roles of FcR in phagocytosis of teleost B lymphocytes.