Activation of innate immune responses by Haemophilus influenzae lipooligosaccharide

A Gram-negative pathogenHaemophilus influenzaehas a truncated endotoxin known as lipooligosaccharide (LOS). Recent studies onH. influenzaeLOS highlighted its structural and compositional implications for bacterial virulence; however, the role of LOS in the activation of innate and adaptive immunity is poorly understood. THP-1 monocytes were stimulated with either lipopolysaccharide (LPS) fromEscherichia colior LOS compounds derived fromH. influenzaeEagan, Rd, and Rdlic1 lpsAstrains. Cell surface expression of key antigen-presenting, costimulatory, and adhesion molecules, as well as gene expression of some cytokines and pattern recognition receptors, were studied. Eagan and Rd LOS had a lower capacity to induce the expression of ICAM-1, CD40, CD58, tumor necrosis factor alpha (TNF-α), and interleukin-1β (IL-1β) compared to LPS. In contrast, antigen-presenting (HLA-ABC or HLA-DR) and costimulatory (CD86) molecules and NOD2 were similarly upregulated in response to LOS and LPS. LOS from a mutant Rd strain (Rdlic1 lpsA) consistently induced higher expression of innate immune molecules than the wild-type LOS, suggesting the importance of phosphorylcholine and/or oligosaccharide extension in cellular responses to LOS. An LOS compound with a strong ability to upregulate antigen-presenting and costimulatory molecules combined with a low proinflammatory activity may be considered a vaccine candidate to immunize againstH. influenzae.