Mechanisms of host species-specific tissue macrophage immunity to Streptococcus pneumoniae and Klebsiella pneumoniae infections.

Encapsulated bacterial pathogens such as Streptococcus pneumoniae and Klebsiella pneumoniae are major causes of invasive human disease, yet the early events that determine infection outcome remain poorly defined. This thesis investigates the initial stages of host–pathogen interaction, focusing on the spleen and liver as key sites of bacterial clearance, and exploring mechanisms of host specificity in hypervirulent K. pneumoniae. To achieve this, we established novel translational platforms, including ex vivo perfusion of human spleen and human liver segments as well as primary human macrophage cultures, which together provide powerful tools to study infection biology in a physiologically relevant human context. In the human spleen, we demonstrate that clearance of encapsulated bacteria relies on mannose receptor–mediated capture by sinusoidal lining cells, which is indispensable for subsequent macrophage-mediated killing. These findings revise existing paradigms derived from murine models and have direct implications for vaccine evaluation and host-directed therapies. In the human liver, we show that Kupffer cells clear pneumococcal isolates in a non-selective manner, irrespective of capsule type or predicted invasive potential, thereby underscoring the robustness of hepatic innate defences. While mice models showed the critical role of the liver in discriminating bacteria based on their capsules composition. Finally, we dissect the host adaptation of hypervirulent K. pneumoniae ST25 clones, revealing host-species-specific cytotoxic mechanisms that restrict cross-species infection between human and pigs and emphasize the importance of the host context in pathogen evolution. Together, these studies not only advance understanding of the earliest stages of S. pneumoniae and K. pneumoniae infection in humans, but also establish innovative ex vivo and cellular systems as tools for dissecting human immune responses to bacterial pathogens. By challenging assumptions derived from murine studies, this work refines the framework for developing effective vaccines, host-directed therapies, and strategies for infection control at the human–animal interface.