Neisseria meningitidis, are the cause of fulminant sepsis or meningitis leading to death within hours after appearance of initial unspecific symptoms (fever, headache, nausea). Invasive meningococcal disease (IMD) is afflicted with an overall mortality rate of about 10% and, furthermore, 10-20% of survivors suffer from lifelong sequelae such as deafness, learning disabilities or limb amputations. Infants and toddlers are at highest risk of developing IMD, complicating a timely intervention due to the non-specific early symptoms of disease, usually mistaken for flu or flu-like illness until more specific symptoms (neck stiffness, purpura, loss of consciousness) appear at later stages of IMD.
Their capacity to kill within within hours after appearance of initial symptoms has drawn the attention of physicians and researchers towards N. meningitidis. However, this is certainly not a trait of their ‘normal’ lifestyle since killing their host inevitably leads to their own demise since it stops the chain of host-to-host transmission. Indeed, the incidence of IMD is very low (~1-5 per 100.000 in most developed countries) compared to the rate at which meningococci can be found in the throats of healthy individuals (~10% of general population). The natural niche of N. meningitidis is the human nasopharynx, where the nasal passages and throat meet, and they can persist here for up to several months without noticeable symptoms. The triggers that precipitate IMD from asymptomatic carriage are unclear, but they must involve crossing of the epithelial layer and entering the blood stream. Once inside the blood stream, N. meningitidis are protected against innate immune defence mechanisms of the host by expression of a polysaccharide capsule and by their unique ability to inactivate the bacterial-killing activity of blood, which allows for their unbridled multiplication.
A key aspect of the meningococcal lifestyle is their strict human-specific host tropism, which has hampered experimental in vivo approaches to understand the balance between commensalism and virulence of N. meningitidis as a common inhabitant of the human throat. Many virulence determinants of meningococci target their interaction partners in a human-specific fashion. This includes their adhesins (e.g. type IV pili, opacity-associated proteins (Opa)), their iron acquisition systems (transferrin binding protein, lactoferrin binding protein) and their immune evasion strategies (IgA-protease, factor H binding protein (fHbp), C4BP binding by PorA).
In our lab, we use a ‘humanized’ transgenic mouse line that expresses a human protein to which N. meningitidis adhere within the nasopharyngeal mucosa. These mice express human carcinoembryonic antigen-like cellular adhesion protein 1 (CEACAM1) [Figure 1] which is a cell surface protein recognized by Opa adhesins in the meningococcal outer membrane, thereby enabling tight adherence of meningococci to the epithelial cells. Intriguingly, the CEACAM1-humanized mice can harbour N. meningitidis in their upper respiratory tract, while their wild-type littermates do not facilitate colonization.
This in vivo model enables us to analyse the host response towards meningococcal intranasal infection and has revealed a contribution of innate immune effectors such as neutrophils, potent bacterial-killing white blood cells [Figure 2].
While we cannot experimentally infect humans with N. meningitidis, we can study how human cells respond to the bacteria by growing human cell lines in the laboratory. In order to approach mechanistic insights into mechanisms of host cell attachment and transcytosis of N. meningitidis, we use tissue culture based approaches. Calu-3 cells [Figure 3] are a standard cell line derived from the respiratory tract epithelium. In addition to this, primary human and mouse epithelial cells are used. The cells are grown in tissue culture inserts at the liquid-air interface to promote their differentiation into a polarized columnar epithelium, such as is found in situ in the living organism.