Differential Binding of Two Chicken β-Galactoside-Specific Lectins to Homologous Lymphocyte Subpopulations and Evidence for Inhibitor Activity of the Dimeric Lectin on Stimulated T Cells

Plant lectins can be potent modulators of vertebrate immune functions. Biochemical characterization of lectins from animal tissues enables the determination of whether these endogenous activities display a comparable immunological potency. Focusing on chicken β-galactoside-binding lectins, the monomeric intestinal (CL-14) and the dimeric liver lectin (CL-16) were purified and the lack of cross-contamination was ascertained. In very close agreement with the molecular masses of 14,974 and 14,976 calculated on the basis of the available sequence data (Y. Sakakuraet al., J. Biol. Chem.265, 21573–21579, 1990), electrospray mass spectrometric analysis yielded values of 14,969 (CL-14) and 14,972 (CL-16), the reasons for the deviation in gel electrophoretic behavior being unclear. Solid-phase assays with immobilized lactosylated poly-L-lysine demonstrated a comparatively lower affinity and higher extent of binding at saturation for the monomeric lectin than for the dimeric protein, whose properties were similar to those of an immunomodulatory plant lectin. Flow cytometry revealed homogeneous and strong binding of the dimeric lectin within the chicken peripheral blood lymphocyte population, whereas the monomeric lectin stained two subpopulations at different intensities. Two-color flow cytometry disclosed preferential binding of this lectin to B cells. When a B cell line was employed for determination of affinity constants and extents of binding at saturation, qualitatively comparable parameters to those for the solid-phase assays were obtained. The similar profile of lectin-binding glycoproteins in blots of cellular extracts underscored that accessibility to ligands, not qualitatively different ligand display, may explain the differences for the cell line. At up to a concentration of 10 μg/ml of the lectins no stimulation of [3H]thymidine incorporation was seen for blood and spleen cell populations. However, the dimeric lectin reduced stimulation of cells that were responsive to an anti-TcR2 antibody. Thus, this lectin can apparently exhibit inhibitory activity to this kind of T cell activationin vitro.