Glycosylation‐dependent interaction of Jacalin with CD45 induces T lymphocyte activation and Th1/Th2 cytokine secretion

Jacalin, an α‐ O ‐glycoside of the disaccharide Thomsen‐Friedenreich antigen (galactose β1‐3 N ‐acetylgalactosamine, T‐antigen)‐specific lectin from jackfruit seeds, has been shown to induce mitogenic responses and to block infection by HIV‐1 in CD4 + T lymphocytes. The molecular mechanism underlying Jacalin‐induced T cell activation has not been elucidated completely yet. In the present study, protein tyrosine phosphatase (PTPase) CD45 was isolated from a Jurkat T cell membrane fraction as a major receptor for Jacalin through affinity chromatography and mass spectrometry. CD45, which is highly glycosylated and expressed exclusively on the surface of lymphocytes, is a key regulator of lymphocyte signaling, playing a pivotal role in activation and development. We found that the lectin induced significant IL‐2 production by a CD45‐positive Jurkat T cell line (JE6.1) and primary T cells. However, this effect did not occur in a CD45‐negative Jurkat T cell line (J45.01) and was blocked completely by a specific CD45 PTPase inhibitor in Jurkat T (JE6.1) and primary T cells. Furthermore, we also observed that Jacalin caused a marked increase in IL‐2 secretion in response to TCR ligation and CD28 costimulation and contributed to Th1/Th2 cytokine production by activating CD45. Jacalin increased CD45 tyrosine phosphatase activity, which resulted in activation of the ERK1/2 and p38 MAPK cascades. Based on these findings, we propose a new, immunoregulatory model for Jacalin, wherein glycosylation‐dependent interactions of Jacalin with CD45 on T cells elevate TCR‐mediated signaling, which thereby up‐regulate T cell activation thresholds and Th1/Th2 cytokine secretion.