Fig. 8: Evolution and resolution of the C. violaceum-induced granuloma.

During the first hours of C. violaceum infection in the liver of WT mice, a single bacterium infects a hepatocyte. Infected hepatocytes are visible 12 hpi, in which bacteria rapidly replicate with a 1.5 hour doubling time. Shortly thereafter at 24 hpi, a neutrophil swarm appears and forms a microabscess. The ROS produced by neutrophils helps to slow the replication rate to a 14.5 hour doubling time but fails to clear the infection. At 3 dpi, macrophages arrive and begin to organize at the periphery of the microabscess, and bacterial replication halts. By 5 dpi this macrophage zone is pronounced, defining the pathology as a granuloma. Macrophages continue to form a barrier around the necrotic core that is composed mostly of C. violaceum and debris from dead neutrophils. Pyroptosis is critical to maintain the integrity of the granuloma, likely by killing any macrophage that becomes infected by C. violaceum, thus closing the macrophage replicative niche. Using nitric oxide, macrophages kill C. violaceum at the border of the necrotic core of the granuloma as early as 7 dpi. Once the bacteria are killed, macrophages can then infiltrate the inner layers of the granuloma to efferocytose the necrotic debris and dead bacteria. At later times when all the C. violaceum are dead, bacterial antigens can now be seen within the outer macrophage zone. Finally, the granuloma pathology is fully resolved and the liver returns to homeostasis. All this is accomplished by the innate immune system without the need for adaptive immune help during the primary infection.