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Haemophilus influenzae

I. General Information

1. NCBI Taxonomy ID:

2. Disease:

Meningitis

3. Introduction

Haemophilus influenzae are Gram negative, pleomorphic rods first described by Pfeiffer in 1892. Haemophilus influenzae includes six encapsulated types, a through f, as well as nonencapsulated or untypeable strains, and these are found as both respiratory tract commensals and respiratory and invasive pathogens (Kilian et al, 1991; Tristram et al., 2007). Invasive H. influenzae infections, such as bacteremia, meningitis, pneumonia in children, epiglottis, and septic arthritis, are usually caused by H. influenzae strains possessing the type b capsule. Furthermore, these infections occur much more frequently in non-immune children between ages 2 months and 5 years, and this age-related susceptibility is inversely correlated with the bactericidal capacity of the blood , which is mediated by antibodies directed against the type b capsule.Although non-typable H. influenzae rarely cause invasive infections in normal hosts, they are important causes of respiratory infections, such as bronchitis and pneumonia in adults with underlying pulmonary disease and otitis media and sinusitis in healthy individuals. Recent studies show that H. influenzae cause 23-27% of acute otitis media in children, the same incidence as S. pneumoniae (Marrs et al., 2001). Prior to introduction of Hib vaccines, Hib was estimated to be responsible worldwide for some three million serious illnesses and an estimated 386,000 deaths per year, chiefly through meningitis and pneumonia, with 95% of cases and 98% of deaths occurring in patients in developing countries. Almost all victims are children under the age of 5 years, with those between 4 and 18 months of age especially vulnerable . Large, population-based disease burden studies showed annual rates of Hib meningitis at 10 to 60 cases per 100,000 children under 5 years of age in countries where Hib vaccine is not used . In developing countries, where the vast majority of Hib deaths occur, pneumonia accounts for a larger number of deaths than meningitis. However, Hib meningitis is also a serious problem in such countries with mortality rates several times higher than seen in developed countries, and 15 to 35% of survivors have permanent disabilities such as mental retardation or deafness. Systematic vaccination has virtually eliminated Hib disease in most industrialized nations, with 89 countries offering infant immunization against Hib by the end of 2004 compared with 38 in 1999. However, Hib vaccine is not available in Japan. Ninety-two percent of the populations of developed countries were vaccinated against Hib as of 2003, while vaccination coverage was 42% for developing countries and 8% for least-developed countries (Tristram et al., 2007).

4. Microbial Pathogenesis

The most important virulence factor defining pathogenic H. influenzae strains is capsule, of which six serotypes, a-f, have been described. Among encapsulated strains, those possessing the type b capsule (Hib strains), which is composed of polyribosylribitol phosphate (PRP), are the most virulent, capable of causing sustained bacteremia and subsequent focal infection in non-immune, normal hosts. Like the capsules of other gram negative organisms, H. influenzae capsules enhance the pathogenicity of the bacteria by protecting them from phagocytosis. The strong negative charge of the polysaccharide capsules may provide electrostatic repulsion to phagocytic cells and the capsular material itself may sterically interfere with the binding of opsonizing antibodies and complement to the bacterial surface (Marrs et al., 2001).

H. influenzae pili (sometimes called fimbriae) was the first well studied adherence factors of H. influenzae. These filamentous organelles are seen by electron microscopy distributed in a peritrichous manner on the H. influenzae bacterial cell surface of isolates that hemagglutinate and adhere to buccal epithelial cells, but are absent from non-hemagglutinating and non-adherent bacteria. Generally strains isolated from invasive infections do not express pili, while those colonizing the nasopharynx more commonly do. piliated H. influenzae binds to erythrocytes possessing the Anton (AnWj) antigen. Additional studies have shown that sialic acid-containing lactosylceramides, the gangliosides GM1, GM2, GM3 and GD1a, inhibit pilus- mediated H. influenzae binding to both buccal epithelial cells and erythrocytes (Marrs et al., 2001).

Highly similar high-molecular-weight proteins (HMW1, HMW2) were first identified as H. influenzae-associated antigens by their ability to induce robust antibody responses in individuals with acute otitis media . Subsequent studies have revealed that these HMW proteins function as adherence factors and can facilitate the colonization of cultured human epithelial cells . HMW1 is a 160 kDa product of the hmw1A gene and HMW2 is a 155 kDa protein encoded by the hmw2A gene; hmw1A and hmw2A are 71% identical and 80% similar . To date the hmw genes have only been detected in nontypable strains, and a majority of HMW-positive strains retain both chromosomal loci (Marrs et al., 2001).

Secretory IgA fulfills many protective functions on the mucosal surface; these include the neutralization of toxins, inhibition of pathogen attachment to the epithelium, and agglutination of organisms within mucus . To surmount this immunological obstacle, H. influenzae constitutively secrete IgA1 proteases which are enzymes that inactivate the predominant IgA species, IgA1, by cleaving the immunoglobulin molecule at the heavy chain hinge region, resulting in the production of two fragments (Fab and Fc). (Reinholdt et al., 1997)

H. influenzae produces a lipopolysaccharide structure that lacks an O-specific antigen and, instead, incorporates short oligosaccharide side chains onto three conserved heptoses and is thus termed lipooligosaccharide (LOS). A wide variety of glyco-modifications to the core LOS molecule have been detected and include the addition of glucose, galactose, lactose, phosphatidylcholine (ChoP), and sialic acid. This repertoire of sugars allows the organism to mimic common eukaryotic glycolipid structures, and is presumably an adaptive strategy for subverting immune defenses (Marrs et al., 2001).

5. Host Ranges and Animal Models

H. influenzae live exclusively in humans, and although they may cause disease in a variety of body sites (such as the central nervous system, joints, skin, lungs, bronchi and genito-urinary tract), they are most commonly isolated from the nasopharynx, where they are carried asymptomatically . Throat or nasopharyngeal culture surveys of both healthy and ill individuals reveal the carriage rate to be between 20 and 85%, and the majority of colonizing strains lack a capsule. Person to person transmission of H. influenzae is assumed to occur by contact with infected respiratory droplets and has resulted in clusters of H. influenzae type b (Hib) invasive infections among children in households and day care centers (Marrs et al., 2001).

The infant rat model of invasive Hib disease played a substantial role in the identification of Hib virulence factors. The model allows for respiratory tract colonization in rats less than three weeks of age by inhalation of only a few organisms. Colonization is followed closely by bacteremia and meningitis. Therefore, this model mimics the invasive disease seen in children . Older rats (up to three months of age) can also develop meningitis through intraperitoneal injection of Hib and subsequent bacteremia. Rabbits have also been used for experimental Hib infections and have contributed to understanding the pathophysiology of meningitis and in the design of treatment regiments. Rabbits require direct inoculation into the CNS or the perturbation of the CNS prior to i.v. injection (Marrs et al., 2001).

Experimental infection studies for otitis media most often employ the chinchilla but the gerbil and rat have also been used . In the chinchilla, NTHi are often inoculated directly into the bulla and disease is monitored over a two- to eight-week period for fluid pressure changes using otoscopy and tympanometry. Tympanocentesis and nasopharyngeal lavage are performed to assess colonization and immune responses present in the middle ear . Direct inoculation of the middle ear results in 100% infectivity but bacterial and host factors involved in initial nasopharyngeal colonization cannot be studied through this route. While NTHi can colonize the nasopharynx of chinchillas after experimental inoculation, colonization does not consistently lead to otitis media. An otitis media model has, however, been developed that allows for initial colonization with NTHi. Chinchillas are first infected with adenovirus, and then NTHi, administered nasally, will colonize the nasopharynx and sometimes result in otitis media. This scenario mimics development of otitis media in children. This method has been used to assess the role of bacterial components in colonization and their potential as vaccine candidates .

Pulmonary infection models used for vaccine efficacy tests have utilized both mice and rats. In these models, animals are immunized intravenously or at the mucosal surface and then challenged by direct administration of NTHi to the lungs. Bacterial clearance is then assessed a few hours after challenge by culturing homogenized lung tissue or lavage fluid. These pulmonary clearance models, however, do not allow for the study of persistent colonization as is seen with chronic infections in the lungs of adults with COPD. Attempts have been made to reflect this scenario in rats by administering NTHi encapsulated within agarose beads. This method can lead to infections that persist for weeks resulting in pathology and immune responses that parallel those seen in humans .

6. Host Protective Immunity

Newborns are protected by transplacental maternal type b antibodies and older children and adults are protected by "natural antibodies", which are H. influenzae specific antibodies in a host who has not had a known H. influenzae infection. Most likely these antibodies result from subclinical infection or asymptomatic colonization (Talan et al., 1999).

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