Binding and mitogenic properties of a galactosyl-specific lectin from the tunicate Didemnum candidum for murine thymocytes and splenocytes.

The affinity-purified major galactosyl-specific lectin (DCL-I) from the tunicate Didemnum candidum binds and induces blastogenesis in untreated and Vibrio cholerae neuraminidase (VCN)-treated murine splenocytes. However, it only binds and stimulates murine thymocytes when they have been pretreated with VCN. Binding and stimulation of VCN-treated splenocytes and thymocytes is specifically inhibited by D-galactose. Binding of lectin to splenocytes is highly increased by the VCN treatment, but the cells are stimulated to the same extent in both untreated and VCN-treated cells. This suggests that although additional DCL-I carbohydrate acceptors are uncovered by VCN treatment of the splenocytes, the ones responsible for triggering blastogenesis are already exposed on the untreated cells. On the contrary, cleavage of terminal sialic acids from the thymocyte cell surface is required to expose DCL-I carbohydrate acceptors in order to obtain a mitogenic response. Our results suggest that the distribution of cell surface galactosyl carbohydrate moieties to which DCL-I binds is different in thymocytes and splenocytes.