| Budget Amount *help |
¥2,000,000 (Direct Cost: ¥2,000,000)
Fiscal Year 1989: ¥1,200,000 (Direct Cost: ¥1,200,000)
Fiscal Year 1988: ¥800,000 (Direct Cost: ¥800,000)
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| Research Abstract |
Whether membrane IgM (mIgM) of B cells can activate complement (C) upon antigen reception has been investigated, the working hypothesis being that mIgM molecules, though inactive in their monomeric form, assume a polymeric structure mimicking secretory IgM when they are assembled on the cell membrane by antigen-mediated crosslinking.
The attempt to test this possibility by assaying C3 deposition on the B cell membrane was initially obstructed by non-specific deposition of C3 on B cell in vitro. This non-specific reaction took place when lymphocytes alone were incubated with mouse serum even in the presence of serum regulatory components, Mg-EDTA or protease inhibitors. However, the non-specific C activation could be diminished in the presence of erythrocytes, presumably owing to the trans-acting regulatory function of CRI on the surrounding cells (Iida & Nussenzweig,1983).
In order to test the specific C activation, a Percoll fraction of murine blood or spleen containing lymphocytes and erythrocytes were incubated with an unsaturating dosis of F(ab')_2 of rabbit antibodies to mIg then with mouse serum for various periods and the number of C3-bearing cells was analyzed by FACS. The bivalent, but not monovalent, F(ab')_2 of anti-mu or anti-kappa indeed induced significant deposition of C3 on the B cell membrane, whereas crosslinking the non-C activating mIgD by anti-delta F(ab')_2 was without effect. The activation of the classical pathway of C by crosslinked mIgM was implicated by the inhibition by Mg-EDTA. The slow kinetics of C3 deposition induced by anti-IgM suggested involvement of redistribution and/or rearrangement processes of antibody-ligated mIgM molecules on the membrane. The results demonstrate that crosslinking of mIgM, but not mIgD, results in activation of C via the classical pathway on the B cell surface in the face of various regulators of complement activation. The biological significance of this phenomenon remains to be investigated.
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