The Amaranthus leucocarpus lectin enhances the anti-CD3 antibody-mediated activation of human peripheral blood CD4⁺ T cells

Activation of CD4⁺ T cells plays a main role in adaptive immune response by regulating cellular and humoral immunity via processes associated with changes in cell surface oligosaccharide receptors. Lectins are glycoproteins that specifically recognize oligosaccharides and have been used to characterize changes in oligosaccharides present on T cell surface and their effects on activation. A lectin from Amaranthus leucocarpus seeds (ALL) is specific for glycoprotein structures containing galactose-W-acetylgalactosamine and is able to bind to human and murine CD4⁺ T cells, however, its effect on activation remains unclear. We examined the effect of ALL on the activation of peripheral blood human CD4⁺ T cells and analyzed cell proliferation, expression of the activation-associated molecule CD25, secretion of the activation-dependent cytokine interleukin (IL)-2 and intracellular calcium influx changes using flow cytometry. CD4⁺ T cells were stimulated with anti-CD3 antibodies that provided the first activation signal in the presence or absence of ALL. ALL alone did not induce CD4⁺ T cell activation but when also stimulated with anti-CD3 antibodies, ALL up-regulated CD25 expression, cell proliferation, IL-2 secretion and an intracellular calcium influx in a dose-dependent manner. In addition, ALL recognized CD4⁺ T cells expressing the CD69 and Ki67 molecules expressed only by activated T cells and induced production of the TH1-type cytokine interferon-^. Our findings indicate that ALL binds to human activated CD4⁺ T cells and enhances the degree of activation of CD4⁺ T cells that are stimulated with anti-CD3 antibodies. ALL provides a new tool for analyzing T cell activation mechanisms.