Characterization of lectin isolated from Momordica charantia seed as a B cell activator

Lectin isolated from the seeds of Momordica charantia (MCL) is a galactose-specific glycoprotein. To investigate the effects of MCL on cell activation, we analyzed the responses of BALB/c splenocytes, thymocytes, T cells and B cells on MCL stimulation. Proliferation assays showed that MCL selectively stimulates the B cell subset of splenocytes (p < 0.05) in a dose and time dependent manner and that this activation proceeds without the involvement of T cells. Flow cytometric analysis revealed that the fluorescein isothiocyanate (FITC)-labeled MCL binds to B cells, which was inhibited by specific sugars, including galactose. Mouse immunoglobulin (Ig) was able to inhibit MCL-induced proliferation of mouse B cells, suggesting MCL stimulates B cell activation via membrane Ig in the B cell surface. Moreover, after 96-h co-culture, MCL triggered splenocytes to produce a large amount of non-specific IgM in culture supernatants (p < 0.01). Additionally, MCL was shown to up-regulate the cell activation marker CD86, in a B cell subpopulation distinct from that affected by LPS. These data suggest that MCL is a T cell-independent B cell activator and a polyclonal Ig inducer, and provide further information on the immunomodulatory effect of MCL.