Non-invasive Imaging of the Innate Immune Response in a Zebrafish Larval Model of Streptococcus iniae Infection

Summary

Here, we present a protocol for the generation and imaging of a localized bacterial infection in the zebrafish otic vesicle.

Abstract

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.

Introduction

Streptococcus iniae is a major aquatic pathogen that is capable of causing systemic disease in both fish and humans¹. While S. iniae is responsible for large losses in the aquaculture industry, it is also a potential zoonotic pathogen, capable of causing disease in immunocompromised human hosts with clinical pathologies similar to those caused by other streptococcal human pathogens. Given its similarities with human pathogens, it is important to study S. iniae disease pathogenesis in the context of a natural host. An adult zebrafish model of S. iniae infection revealed robust infiltration of host leukocytes to the localiz....

Protocol

Adult and embryonic zebrafish were maintained in accordance with the University of Wisconsin-Madison Research Animal Resources Center.

1. Preparing Microinjection Needles

1. Prepare thin wall glass capillary injection needles (1.0 OD/0.75 ID) using a micropipette puller device with the following settings: air pressure 200, heat 502, pull 90, velocity 80, time 70, air time at start of pull 5, air time at end of pull 5.
2. Using fine tweezers, break off the tip of the pulled needle so that the tip opening has a diameter of approximately 10 µm.

2. Preparing Larval Injection Dishes

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Results

Microinjection of S. iniae into the otic vesicle (Figure 1 and Figure 2) results in an initially localized host response. When injected correctly, the bacteria should only be seen in the otic vesicle and not in the surrounding tissue or blood. This can be visualized during microinjection using phenol red dye (Figure 1A). Alternatively, if labeled bacteria are injected, a quick scan of infected larvae immediately post injection can confirm the bacteria are only i.......

Discussion

The infection method used here is useful for the study of the host immune response to an initially localized infection in 2-3 dpf embryos and larvae. The focus of an inflammatory stimulus, such as infection, in a closed cavity such as the otic vesicle allows for the study of neutrophil and macrophage chemotaxis and phagocytosis. One caveat of injecting bacteria into the otic vesicle is that the ability of neutrophils to efficiently phagocytose bacteria in fluid-filled cavities may be dependent on the particular microbe. .......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
1.7 ml Eppendorfs	MidSci	AVSS1700
14 ml Falcon tube	BD Falcon	352059
27 G x ½ in. needle	BD Biosciences	305109
96-well Plate	Corning Incorporated	3596
Agar	BD Biosciences	214030
CellTracker Red	Molecular Probes, Invitrogen	C34552
CNA agar	Dot Scientific, Inc	7126A
Disposable transfer pipets	Fisher Scientific	13-711-7m
Dissecting scope	Nikon	SMZ745
DMSO	Sigma Aldrich	D2650
Ethanol 200 proof	MDS	2292
Fine tweezers	Fine Science Tools	11251-20
Gel comb	VWR	27372-482	4.2 mm width, 1.5 mm thick
Glass bottom dishes			Custom made by drilling a 16 - 18 mm hole in the center of a 35 mm tissue culture dish bottom and placing a 22 mm round #1 coverslip in the hole and sealing with a thin layer of Norland Optical Adhesive 68 cured by UV light.
Glycerol	Fisher Scientific	G33-4
High melt agarose	Denville Scientific, Inc.	CA3510-6
Hydrogen peroxide	Fisher Scientific	H325
Laser scanning confocal microscope	Olympus	with FV-1000 system
Low melt agarose	Fisher	BP165-25
Magnetic stand	Tritech (Narishige)	GJ-1
Microinjection system	Parker	Picospritzer III
Microloader pipet tips	Eppendorf	930001007
Micromanipulator	Tritech (Narishige)	M-152
Micropipette puller	Sutter Instrument Company	Flaming/Brown P-97
Nanodrop spectrophotmeter	Thermo Scientific	ND-1000
N-Phenylthiourea (PTU)	Sigma Aldrich	P7629
Paraformaldheyde	Electron Microscopy Sciences	15710
Petri dishes	Fisher Scientific	FB0875712	100 x 15 mm
Phenol	Sigma Aldrich	P-4557
Phenol Red	Ricca Chemoical Company	572516
Phosphate buffered saline	Fisher Scientific	BP665-1
Potassium hydroxide	Sigma Aldrich	P-6310
Pronase	Roche	165921
Protease peptone	Fluka Biochemika	29185
Small cell culture dish	Corning Incorporated	430165	35 x 10 mm
Sudan Black	Sigma Aldrich	S2380
Thin wall glass capillary injection needles	World Precision Instruments, Inc.	TW100-3
Todd Hewitt	Sigma Aldrich/Fluka Analytical	T1438
Tricaine (ethyl 3-aminobenzoate)	Argent Chemical Laboratory/Finquel	C-FINQ-UE-100G
Triton X-100	Fisher Scientific	BP151-500
Tween 20	Fisher Scientific	BP337-500
Yeast extract	Fluka Biochemika	92144