Ganglioside (GM₁)-treated T cells shed CD4

In previous studies (Morrison et al., 1990), we showed that ganglioside (GM₁) modulation of CD4 was associated with activation of phospholipase C and increased production of inositol triphosphate, but not with activation of protein kinase C. These results demonstrated a unique signal transduction pathway related to GM₁ modulation of CD4 on T cells and raised the question as to whether intracellular Ca²⁺ levels and related protein kinases would be affected by GM₁-induced signalling. We now show that GM₁ modulation of CD4 from human T cells corresponds to decreased cellular Ca²⁺ without significant changes in cellular photein phosphorylation. In the course of this study we discovered that T cells challenged with GM₁ exhibited new proteins in their surrounding media. Fractionation of cellular and supernatant proteins show that cells treated with GM₁ released proteins with an approximate molecular weight (M_r) of 49 000. This was exclusive of GM₁ protein association and GM₁-induced protein phosphorylation. Immunoblots demonstrated the presence of CD4 in GM₁-treated cell supernatants. Western immunoblots using anti-CD4 antibodies detected a lower M_r form (49 000) of CD4 in the supernatants of GM₁-treated cells. These studies further define the unique nature of GM₁ signalling relating to modulation of CD4 and demonstrate that the fate of GM₁ modulated CD4, in part, involves protein shedding.