Characterization of onion lectin (Allium cepa agglutinin) as an immunomodulatory protein inducing Th1-type immune response in vitro

Onion (Allium cepa), a bulb crop of economic importance, is known to have many health benefits. The major objective of the present study is to address the immunomodulatory properties of onion lectin (A. cepa agglutinin; ACA). ACA was purified from onion extract by c-mannose-agarose chromatography (yield: similar to 1 mg/kg). ACA is non-glycosylated and showed a molecular mass of similar to 12 kDa under reducing/non-reducing SDS-PAGE; glutaraldehyde cross-linking indicated that ACA is a non-covalent tetramer of similar to 12 kDa subunits. Its N-terminal sequence (RNVLLNNEGL; UniProt KB Accn. COHJM8) showed 70-90% homology to mannose-specific Allium agglutinins. ACA showed specific hemagglutination activity of 8200 units/mg and is stable in the pH range 6-10 and up to 45 degrees C. The immunomodulatory activity of ACA was assessed using the macrophage cell line, RAW264.7 and rat peritoneal macrophages; at 0.1 mu g/well, it showed a significant increase (6-8-fold vs. control) in the production of nitric oxide at 24 h, and significantly stimulated (2-4-fold vs. control) the production of pro-inflammatory cytokines (TNE-alpha and IL-12) at 24 h. ACA (0.1 mu g/well) enhanced the proliferation of murine thymocytes by similar to 4 fold (vs. control) at 24 h; however, ACA does not proliferate B cell-enriched rat splenocytes. Further, it significantly elevated the expression levels of cytokines (IFN-gamma and IL-2) over the control in murine thymocytes. Taken together, purified ACA induces a Th1-type immune response in vitro. Though present in low amounts,ACA may contribute to the immune-boosting potential of the popular spice onion since considerable amounts are consumed on a daily basis universally. (C) 2015 Elsevier B.V. All rights reserved.