[Preparation and function of human soluble FcγRIIb]

To detect the concentration of soluble FcγRIIb in blood sera of SLE patients and healthy controls, then to obtain recombinant human soluble FcγRIIb protein (husFcγRIIb) in Escherichia coli (E.coli) and examine its binding capability with immune complexes (IC) and its effect on IgM secretion by B cells.

The concentration of husFcγRIIb in blood sera of SLE patients and healthy controls was detected by ELISA. E.coli BL21, containing pET-sFcγRIIb, was stimulated by IPTG to induce husFcγRIIb expression, and then husFcγRIIb protein was purified by Ni-NTA agarose bead system. The IC-binding ability of husFcγRIIb was detected by ELISA. Furthermore, B cells were sorted by immune magnetic bead from human peripheral blood and challenged by different stimulators under the condition of husFcγRIIb or not for 10 d, then the concentration of IgM in supernatant was detected by ELISA.

The concentration of husFcγRIIb in the serum of SLE patients was lower than that in the controls (P<0.05). The recombinant husFcγRIIb protein was successfully expressed and purified with M(r); being 41 500. It could combine with IC and the absorption became higher with the increasing concentration of IC. After 10-day stimulation on the B cells, the titer of IgM between SPA and SPA+husFcγRIIb groups was not significantly different (P>0.05), and the titer was higher in SPA+anti-IgM group than SPA group (P<0.01). Interestingly, the titer of IgM in SPA+husFcγRIIb+anti-IgM group was lower than SPA+anti-IgM group (P<0.01), SPA group (P<0.01) and SPA+husFcγRIIb group (P<0.01).

The concentration of husFcγRIIb in the serum of SLE patients is lower than that in the healthy controls. The recombinant husFcγRIIb protein can combine with IC and inhibit IgM antibody secretion by B cells.