Pneumococcal meningitis : from the pathogenesis of disease to the development of novel therapeutics

<p dir="ltr">Bacterial meningitis is a life-threatening inflammation of the meninges caused by a bacterial infection in the brain. The global burden of disease is mainly found in low-income countries, where a case-fatality rate of 50% is reported. In contrast, in high-income countries, the incidence is lower, but stable, with a case fatality rate of 7-30%. Accessibility to good health care practice is crucial for outcomes, and the delayed onset of treatment correlates with an increased risk of mortality and neurological sequelae. The most common cause of bacterial meningitis is the Gram-positive bacterium Streptococcus pneumoniae, the pneumococcus, which is associated with an increased risk of death, complications, and long-term neurological sequelae, the latter observed in approximately 50% of survivors. Bacterial presence in the brain causes a vast neuroinflammatory response, which aims to clear the infection, but is inefficient and becomes detrimental. The prolonged presence of the bacterium, its toxin, and the neuroinflammatory process leads to neuronal damage, ultimately causing neurological sequelae in patients. Antibiotic treatment is given to clear the infection; the success of this treatment relies on the sensitivity of the pneumococcal strain to antibiotics. This poses a significant threat in clinics, as antimicrobial resistance is on the rise. In addition, in some countries, administration of dexamethasone is recommended to reduce neuroinflammation, although dampening of the inflammatory response has given inconsistent results. Importantly, despite successful treatment, the incidence of mortality and long-term neurological sequelae remains high, highlighting the need for new treatment strategies to improve outcomes of pneumococcal meningitis.</p><p dir="ltr">In order to develop new therapies, knowledge of the pathogenesis of the disease is crucial. In paper I, we demonstrated that S. pneumoniae invades the brain in a temporal, but not in a spatial pattern, with all brain regions equally affected by using, for the first time in the field, 3D whole brain imaging. The host responded dynamically, with neuronal damage observed early in the disease. We further indicated the importance of neutralising the pneumococcal toxin pneumolysin for future therapies. In paper II, we investigated the innate immune response to pneumococcal meningitis and the importance of microglia, the main innate immune cell in the brain parenchyma, in the generation of this response. Our results demonstrated that the microglial population inhibited the infiltration of peripheral monocytes and neutrophils into the brain, which was detrimental as the infiltration of peripheral immune cells in the brain increased protection in mice depleted of microglia, by reducing the bacterial load and lowering cytokine release. Furthermore, the expansion of neutrophilic and monocytic populations in the periphery also contributed to the clearance of the pneumococcus and reduced the levels of cytokines in the absence of resident microglia.</p><p dir="ltr">The antibiotic resistance observed among pneumococcal strains must be addressed by developing new antimicrobial peptides. In paper III, we demonstrated how the bacteriophage-derived lytic enzyme cpl-1, a pneumococcal endolysin, could, due to its efficient and fast antimicrobial activity, make B-lactam and macrolide-resistant pneumococcal strains susceptible to antibiotics again. This had neuroprotective effects in vitro and dramatically increased survival in vivo, thereby highlighting the potential of cpl-1 as an adjuvant treatment in antibiotic-resistant pneumococcal meningitis cases.</p><p dir="ltr">In paper IV, we isolated bioengineered human extracellular vesicles to address the major challenges in current treatment strategies of pneumococcal meningitis: the release of pneumolysin and the neuroinflammation. We demonstrated that treatment with rabies viral glycoprotein-expressing extracellular vesicles, which target acetylcholine receptors on neurons, increased survival in the bacteremia- derived meningitis model, due to its capacity to enter the brain parenchyma, where it sequestered and neutralised pneumolysin, protected neurons, and reduced inflammation.</p><p dir="ltr">In conclusion, this thesis focused on the host response to infection and the potential of new treatment strategies in improving the outcome of pneumococcal meningitis.</p><h3>List of scientific papers</h3><p dir="ltr">I. Spatio-temporal brain invasion pattern of Streptococcus pneumoniae and dynamic changes in the cellular environment in bacteremia-derived meningitis.</p><p dir="ltr"><b>Farmen, K.</b>, Tofino-Vian, M., Wellfelt, K., Olson, L., & lovino, F. Neurobiology of Disease, 195, 106484, (2024). <a href="https://doi.org/10.1016/j.nbd.2024.106484" rel="noreferrer" target="_blank">https://doi.org/10.1016/j.nbd.2024.106484</a></p><p dir="ltr">II. Microglial Depletion is Protective in Experimental Bacterial Meningitis Due to an Expansion of Neutrophils and Monocytes.</p><p dir="ltr"><b>Farmen, K.</b>, Tofino-Vian, M., Benito Cuesta, I., Sarlus, H., Harris, R. A., & lovino, F. [Submitted]</p><p dir="ltr">III. Bacteriophage-derived endolysins restore antibiotic susceptibility in ß- lactam-and macrolide-resistant Streptococcus pneumoniae infections.</p><p dir="ltr">Vander Elst, N., <b>Farmen, K.</b>, Knorr, L., Merlijn, L., & lovino, F. Molecular Medicine, 31(1), 170, (2025). <a href="https://doi.org/10.1186/s10020-025-01226-1" rel="noreferrer" target="_blank">https://doi.org/10.1186/s10020-025-01226-1</a></p><p dir="ltr">IV. Bioengineered extracellular vesicles dampen inflammation, increase survival, and neutralise pneumolysin in experimental pneumococcal meningitis</p><p dir="ltr"><b>Farmen K.</b>, Tofino-Vian, M., Mamand D.R., Liang X., Zhou H., Hou W.Q.V., Andaloussi E.S., Wiklander P.B.O., & Iovino F. [Submitted]</p>