Parotin Subunit as a Potent Polyclonal B Cell Activator Binds to Newly Found Glycosylphosphatidylinositol (GPI)-Anchored Proteins on Human B Cell Surfaces

Parotin subunit (PS) is a unique glycoprotein, isolated from bovine parotid glands, which possesses the ability to induce polyclonal antibody production and IL-1-like activity. The present studies investigated the existence of receptors for PS on B cell surfaces using PS-affinity chromatography. No PS-binding proteins (PSR) solubilized from human B cell surfaces with Triton X-100 were detected, whereas the PSR released from human B cell membranes with phosphatidylinositol-specific phospholiphase C (PI-PLC) treatment were composed of 75- and 40-kDa proteins. PI-PLC treatment markedly reduced polyclonal antibody responses to PS but weakly inhibited the responses to PWM, xanthan gum, and LPS in human and mouse lymphocytes. Addition of PSR caused a dose-related reduction in polyclonal IgM and IgG antibody responses to PS. These results suggest that PSR can act as glycosylphosphatidylinositol-anchored receptors for PS.