Arabinogalactan proteins contribute to the immunostimulatory properties of New Zealand honeys

Context: Factors in honey that improve wound healing are poorly understood, but are thought to include lipopolysaccharide (LPS), apalbumin-1 and -2, and a 5.8 kDa component that stimulate cytokine release from macrophages. Objective: To characterize the ability of New Zealand honeys to elicit the release of tumor necrosis factor-alpha (TNF-alpha) from monocytic cell lines as a model for early events within a wound site. Materials and methods: The ability of kanuka (Kunzea ericoides), manuka (Leptospermum scoparium), and clover (Trifolium spp.) honeys to stimulate the release of TNF-alpha from monocytic cell lines THP-1 and U937 was assayed by ELISA. Results: All three honeys stimulated TNF-alpha release from THP-1 cells, with kanuka honey being the most active. The activity of kanuka honey was associated with a high molecular weight (> 30 kDa) component that was partially heat labile and inhibitable with polymyxin B. LPS concentrations in the honeys were too low to adequately explain the level of immunostimulation. The contribution of type II arabinogalactan proteins (AGPs) we recently identified in kanuka honey was tested, as AGPs are known immunostimulators. AGPs purified from kanuka honey stimulated the release of TNF-alpha from THP-1 and U937 cells. Discussion: Here we demonstrated that AGPs we recently identified in kanuka honey have immunostimulatory activity. We propose that the immunostimulatory properties of individual honeys relate to their particular content of LPS, apalbumins, the 5.8 kDa component and AGPs. Conclusion: The immunostimulatory activity of kanuka honey may be particularly dependent on AGPs derived from the nectar of kanuka flowers.