Davis and colleagues observed the early stages of zebrafish embryo infection by the spirochete Leptospira interrogans, as described in a recent issue of PLoS Neglected Tropical Diseases. They injected 10-100 spirochetes into the hindbrain of zebrafish embryos at 30 hours post-fertilization, when the innate immune response is fully functional. Macrophages rushed to the hindbrain and engulfed the invaders within the first 4 hours following inoculation of the spirochetes. L. interrogans was also rapidly phagocytosed when injected into the caudal vein.
Figure 1a from Lesley and Ramakrishnan, 2008. A zebrafish embryo 30 hours following fertilization. The hindbrain and caudal vein are indicated with the bracket and arrow, respectively.
Macrophages typically kill and destroy their prey following phagocytosis. However, spirochetes were still observed in the macrophages 24 hours following inoculation, suggesting that L. interrogans can survive inside macrophages.
Their most striking observation was the accumulation of infected macrophages near the dorsal aorta in a region known as the aorta-gonad-mesonephros (AGM), where hematopoietic stem cells are born. This was not a general phenomenon of bacterial infections as macrophages harboring Pseudomonas aeruginosa failed to accumlate at the AGM following its injection into zebrafish embryos.
Figures 2C and 2D from Davis et al., 2009. The embryo was infected with fluorescently stained L. interrogans. 24 hours following infection, most of the spirochetes were found near the dorsal aorta (brackets), with a few scattered around the ventral tail. Scale bar, 300 µm in panel C, 100 µm in panel D.
Here's what the authors concluded in the final sentence of the paper:
The strikingly specific delivery of leptospires to [the AGM] by phagocytes provides insights into pathogenesis by suggesting a novel mechanism for targeting of organs during leptospiral dissemination.In other words, L. interrogans may be capable of steering macrophages towards specific organs. Once the macrophages arrive at their destination, the spirochetes may escape from the macrophage and colonize the organ.
The challenge for the authors in future studies will be to demonstrate the relevance of the zebrafish model to leptospirosis. Hamsters and guinea pigs are appropriate models for leptospirosis because the pathology and lethality of Leptospira infection in these rodents is similar to what's observed in human leptospirosis patients. The fate of Leptospira that macrophages capture in these rodents differs from what is seen in zebrafish embryos. Leptospira that are found inside macrophages in tissue sections from infected rodents often appear to be disintegrating. Nevertheless, it's possible that a few Leptospira survive phagocytosis and subsequently guide the macrophage towards the target organs.
Davis, J.M., Haake, D.A., & Ramakrishnan, L. (2009). Leptospira interrogans stably infects zebrafish embryos, altering phagocyte behavior and homing to specific tissues PLoS Neglected Tropical Diseases, 3 (6) DOI: 10.1371/journal.pntd.0000463
Lesley, R. and Ramakrishnan, L. (2008). Insights into early mycobacterial pathogenesis from the zebrafish. Current Opinions in Microbiology 11(3):277-283. DOI: 10.1016/j.mib.2008.05.013