The Study of Pro-Inflammatory Molecules Induced by Genetically and Phenotypically Diverse Strains of Haemophilus influenzae Type a in an in vitro Infection Model

Introduction: Haemophilus influenzae type a (Hia) has recently emerged as a cause of invasive disease in North American Indigenous children. Factors determining the outcomes of exposure to the pathogen, from asymptomatic carriage to fatal disease, are poorly understood. The role of innate immune activation in the pathogenesis of invasive Hia disease remains unexplored. We used clinical Hia isolates to determine whether innate immune responses depended on the presence of the capsule, strain genetic background, and abilities to cause invasive disease. Methods: Differentiated THP-1 cells and HL-60 neutrophil-like cells were stimulated with four Hia strains (invasive or noninvasive; encapsulated or nonencapsulated), in comparison to 1 invasive and 1 noninvasive non-typeable H. influenzae. Surface expression of ICAM-1 and CD64 and release of pro-inflammatory cytokines TNF-α and IL-1β were quantified. Results: In vitro Hia infection resulted in robust activation of inflammatory responses in terms of expression of ICAM-1 and release of TNF-α and IL-1β, irrespective of the presence or absence of the capsule, or abilities to cause invasive disease. Inhibition of TLR4 decreased TNF-α release by THP-1 cells stimulated by Hia. Conclusion: Powerful activation of pro-inflammatory responses induced by Hia may contribute to the pathogenesis of invasive Hia disease. As the activation of macrophages and neutrophils did not depend on encapsulation or source of Hia isolation, the functional abilities of phagocytic cells unlikely represent a limiting factor in host defenses. The development of invasive versus noninvasive disease may depend on the functional abilities of the adaptive immune system.