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Immune cells sense and react to a multitude of factors including both host and microbe-derived signals. Understanding how cells translate these cues into particular cellular behaviors is a complex yet critical area of study. We have previously shown that both neutrophils and macrophages are important for controlling the fish pathogen Streptococcus iniae. Here, we report both host and bacterial determinants leading to the formation of organized macrophage aggregates as part of the host inflammatory response in a subset of infected larvae. Streptococcal capsule was a required signal for aggregate formation. Macrophage aggregation coincided with NFκB activity, and the formation of these aggregates is mediated by leukotriene B4 (LTB4) produced by neutrophils. Depletion, inhibition, or genetic deletion of leukotriene A4 hydrolase (Lta4h), which catalyzes the last step in LTB4 synthesis, resulted in the absence of macrophage aggregation. Larvae with impaired neutrophil function also had impaired macrophage aggregation; however, aggregate formation was partially rescued with the addition of exogenous LTB4. Neutrophil-specific expression of lta4h was sufficient to rescue macrophage aggregation in Lta4h-deficient larvae and increased host survival following infection. In summary, our findings highlight a novel innate immune response to infection in which specific bacterial products drive neutrophils that modulate macrophage behavior through eicosanoid signaling.

Streptococcus iniae causes systemic infection characterized by meningitis and sepsis. Here, we report a larval zebrafish model of S. iniae infection. Injection of wild-type S. iniae into the otic vesicle induced a lethal infection by 24 h postinfection. In contrast, an S. iniae mutant deficient in polysaccharide capsule ( cpsA mutant) was not lethal, with greater than 90% survival at 24 h postinfection. Live imaging demonstrated that both neutrophils and macrophages were recruited to localized otic infection with mutant and wild-type S. iniae and were able to phagocytose bacteria. Depletion of neutrophils and macrophages impaired host survival following infection with wild-type S. iniae and the cpsA mutant, suggesting that leukocytes are critical for host survival in the presence of both the wild-type and mutant bacteria. However, zebrafish larvae with impaired neutrophil function but normal macrophage function had increased susceptibility to wild-type bacteria but not the cpsA mutant. Taking these findings together, we have developed a larval zebrafish model of S. iniae infection and have found that although neutrophils are important for controlling infection with wild-type S. iniae , neutrophils are not necessary for host defense against the cpsA mutant.

The aquatic pathogen, Streptococcus iniae, is responsible for over 100 million dollars in annual losses for the aquaculture industry and is capable of causing systemic disease in both fish and humans. A better understanding of S. iniae disease pathogenesis requires an appropriate model system. The genetic tractability and the optical transparency of the early developmental stages of zebrafish allow for the generation and non-invasive imaging of transgenic lines with fluorescently tagged immune cells. The adaptive immune system is not fully functional until several weeks post fertilization, but zebrafish larvae have a conserved vertebrate innate immune system with both neutrophils and macrophages. Thus, the generation of a larval infection model allows the study of the specific contribution of innate immunity in controlling S. iniae infection. The site of microinjection will determine whether an infection is systemic or initially localized. Here, we present our protocols for otic vesicle injection of zebrafish aged 2-3 days post fertilization as well as our techniques for fluorescent confocal imaging of infection. A localized infection site allows observation of initial microbe invasion, recruitment of host cells and dissemination of infection. Our findings using the zebrafish larval model of S. iniae infection indicate that zebrafish can be used to examine the differing contributions of host neutrophils and macrophages in localized bacterial infections. In addition, we describe how photolabeling of immune cells can be used to track individual host cell fate during the course of infection.

Neutrophils are highly motile phagocytic cells that play a critical role in the immune response to infection. Zebrafish (Danio rerio) are increasingly used to study neutrophil function and host-pathogen interactions. The generation of transgenic zebrafish lines with fluorescently labeled leukocytes has made it possible to visualize the neutrophil response to infection in real time by use of optically transparent zebrafish larvae. In addition, the genetic tractability of zebrafish has allowed for the generation of models of inherited neutrophil disorders. In this review, we discuss several zebrafish models of infectious disease, both in the context of immunocompetent, as well as neutrophil-deficient hosts and how these models have shed light on neutrophil behavior during infection.

Heat shock is a routine method used for inducible gene expression in animal models including zebrafish. Environmental temperature plays an important role in the immune system and infection progression of ectotherms. In this study, we analyzed the impact of short-term heat shock on neutrophil function using zebrafish (Danio rerio) as an animal model. Short-term heat shock decreased neutrophil recruitment to localized Streptococcus iniae infection and tail fin wounding. Heat shock also increased random neutrophil motility transiently and increased the number of circulating neutrophils. With the use of the translating ribosome affinity purification (TRAP) method for RNA isolation from specific cell types such as neutrophils, macrophages and epithelial cells, we found that heat shock induced the immediate expression of heat shock protein 70 (hsp70) and a prolonged expression of heat shock protein 27 (hsp27). Heat shock also induced cell stress as detected by the splicing of X-box binding protein 1 (xbp1) mRNA, a marker for endoplasmic reticulum (ER) stress. Exogenous expression of Hsp70, Hsp27 and spliced Xbp1 in neutrophils or epithelial cells did not reproduce the heat shock induced effects on neutrophil recruitment. The effect of heat shock on neutrophils is likely due to a combination of complex changes, including, but not limited to changes in gene expression. Our results indicate that routine heat shock can alter neutrophil function in zebrafish. The findings suggest that caution should be taken when employing a heat shock-dependent inducible system to study the innate immune response.

Recent research on a wide variety of systems has demonstrated that animals generally coevolve with their microbial symbionts. Although such relationships are most often established anew each generation, the partners associate with fidelity, i.e., they form exclusive alliances within the context of rich communities of non-symbiotic environmental microbes. The mechanisms by which this exclusivity is achieved and maintained remain largely unknown. Studies of the model symbiosis between the Hawaiian squid Euprymna scolopes and the marine luminous bacterium Vibrio fischeri provide evidence that the interplay between evolutionarily conserved features of the innate immune system, most notably MAMP/PRR interactions, and a specific feature of this association, i.e., luminescence, are critical for development and maintenance of this association. As such, in this partnership and perhaps others, symbiotic exclusivity is mediated by the synergism between a general animal-microbe 'language' and a 'secret language' that is decipherable only by the specific partners involved.