Haptoglobin buffers lipopolysaccharides to safeguard against aberrant NFκB activation

Haptoglobin, an abundant serum protein, is the major detoxifier of hemoglobin. In addition, haptoglobin modulates the inflammatory response via NFκB. However, in some studies haptoglobin represses and in others it activates NFκB signaling. Here, we show that these conflicting results are explainable by the ability of haptoglobin to bind and buffer lipopolysaccharides. While haptoglobin preparations from human serum induced NFκB-dependent transcription, we found that haptoglobin itself was dispensable. The effect was rather caused by lipopolysaccharides associated with haptoglobin. These were apparently enriched during purification. We show that haptoglobin binds lipopolysaccharides from different bacterial species with low micromolar affinities. This will result in competition with toll-like receptor 4, the lipopolysaccharide receptor, which we demonstrate to be sufficient for induction of NFκB-dependent transcription by haptoglobin. Given its abundance in human serum, haptoglobin constitutes a buffer for serum-borne lipopolysaccharides, shielding them from toll-like receptor 4 to safeguard against aberrant inflammatory reactions caused by small lipopolysaccharide fluctuations. Concordantly, NFκB activation in primary macrophages by purified haptoglobin was delayed relative to stimulation with pure lipopolysaccharide. Our findings warrant evaluation of therapeutic benefits of haptoglobin for non-hemolytic conditions as well as re-evaluation of purification strategies. Finally, they allow to disentangle the mechanistic basis of immunosuppression by oncofetal haptoglobin.