A Minimally Invasive, Accurate, and Efficient Technique for Intrathymic Injection in Mice

Summary

The present protocol describes an interventional radiology procedure established for intrathymic injection in mice to avoid the risk of open surgery and improve the accuracy of blind percutaneous injections.

Abstract

Intrathymic injection in mouse models is an important technique for studying thymic and immune function, including genetic and acquired T cell disorders. This requires methods for the direct deposition of reagents and/or cells into the thymus of living mice. Traditional methods of intrathymic injection include thoracic surgery or minimally invasive percutaneous blind injections, both of which have significant limitations. Ultra-high frequency ultrasound imaging devices have made image-guided percutaneous injections possible in mice, greatly improving the injection accuracy of the percutaneous injection approach and enabling the injection of smaller targets. However, image-guided injections rely on the utilization of an integrated rail system, making this a rigid and time-consuming procedure. A unique, safe, and efficient method for percutaneous intrathymic injections in mice is presented here, eliminating reliance on the rail system for injections. The technique relies on using a high-resolution micro-ultrasound unit to image the mouse thymus noninvasively. Using a free-hand technique, a radiologist can place a needle tip directly into the mouse thymus under sonographic guidance. Mice are cleaned and anesthetized before imaging. For an experienced radiologist adept at ultrasound-guided procedures, the learning period for the stated technique is quite short, typically within one session. The method has a low morbidity and mortality rate for the mice and is much faster than current mechanically assisted techniques for percutaneous injection. It allows the investigator to efficiently perform precise and reliable percutaneous injections of thymuses of any size (including very small organs such as the thymus of aged or immunodeficient mice) with minimal stress on the animal. This method enables the injection of individual lobes if desired and facilitates large-scale experiments due to the time-saving nature of the procedure.

Introduction

The thymus has an essential role in T cell development and immunity. T cell deficiency, which can be caused by thymic involution, genetic disorders, infections, and cancer treatments, amongst other factors, leads to high mortality and morbidity^1,2. Mouse models are indispensable in both basic and translational immunology research and have been used for decades to study thymic biology and T cell development, as well as to develop treatments for those suffering from thymic dysfunction and T cell deficiency^3,4,5.

Protocol

All procedures were performed in accordance with animal care guidelines at the Center for Discovery and Innovation (IACUC protocol 290). For the present study, C57BL/6 mice (female, 4-6 weeks old), C57BL/6 mice (female, 6 months old), J:NU female mice, NOD scid gamma (NSG) female mice, and B6;CAG-luc, -GFP mice were used as the young mouse model, aged mouse model, athymic nude model, immunodeficient model, and bioluminescence cell source, respectively. The mice were obtained from a commercial source (see Table of Materials). This procedure will typically require two people (one to remain sterile while performing the injections and another to handle th....

Results

The successful implementation of this technique relies on a few key steps to be followed. First, reliable identification of the thymus gland itself has to be ensured. In young mice, this is straightforward due to the gland's large size (Figure 3A). In older mice or immunodeficient mice, it can be more challenging; however, it is still very feasible with modern ultrasound equipment (Figure 3B,C). Second, it is critically important to set the need.......

Discussion

An ultrasound-guided free-hand injection is a highly accurate technique for delivering study materials to the thymus in an efficient and aseptic fashion. Following the initial sterilization of the skin at the injection site, sterility is maintained during the procedure owing to the use of sterile gloves, sterile ultrasound probe covers, and sterile ultrasound gel. In contrast to the blind percutaneous approach^10,17 or relying on surgical incisions for direct visu.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Aquasonic 100 Ultrasound Gel	Parker Laboratories (Fairfield, NJ, USA)	01-01	Sterile Ultrasound Transmission Gel
B6;CAG-luc, -GFP mouse	The Jackson Laboratory (Bar Harbor, ME, USA)	025854	Bioluminescence cell source
BD Insulin Syringes with needle	Becton Dickinson (Franklin Lakes, NJ, USA)	328431	Ultra-fine needle - 12.7 mm, 30 G
C57BL/6 mouse - aged	The Jackson Laboratory (Bar Harbor, ME, USA)	000664	age 6 months old; aged model
C57BL/6 mouse - young	The Jackson Laboratory (Bar Harbor, ME, USA)	000664	age 4-6 weeks; young model
Chloraprep One-step 0.67 mL	CareFusion (El Paso, TX, USA)	260449	chlorhexidine gluconate applicator
Curity Cotton Tipped Applicator	Cardinal Health (Dublin, OH, USA)	A5000-2	Sterile, 6"
D-Luciferin	Gold Biotechnology (St Louis, MO, USA)	LUCK-1G
Isoflurane	Henry Schein (Melville, NY, USA)	1182097
IVIS Lumina X5	PerkinElmer (Melville, NY, USA)	n/a	In vivo bioluminescence imaging system
J:NU mouse	The Jackson Laboratory (Bar Harbor, ME, USA)	007850	Athymic nude model
Kendall Hypoallergenic Paper Tape	Cardinal Health (Dublin, OH, USA)	1914C
Kimtech Surgical Nitrile Gloves	Kimberly-Clark Professional (Irving, TX, USA)	56892	Sterile Gloves
Nair Hair Remover Lotion	Church and Dwight (Trenton, NJ, USA)	n/a	Depilatory agent
NOD scid gamma (NSG) mouse	The Jackson Laboratory (Bar Harbor, ME, USA)	005557	Immunodeficient model
Phosphate-Buffered Saline (PBS), 1x	Corning (Corning, NY, USA)	21-040-CV
Puralube Vet Ointment	Med Vet International	PH-PURALUBE-VET	Eye ointment
Sheathes	Sheathing Technologies (Morgan Hill, CA, USA)	10040	Sterile Ultrasound Probe Covers
Sure-Seal Induction Chamber	Braintree Scientific (Braintree, MA, USA)	EZ-17 85	Anesthesia induction chamber
Transducer MX550D	FUJIFILM VisualSonics (Toronto, ON, Canada)	n/a	Vevo 3100 imaging probe (25-55 MHz, Centre Transmit: 40 MHz)
Trypan Blue, 0.4% solution in PBS	MP Biomedicals (Solon, OH, USA)	91691049
Vevo 3100 Imaging System	FUJIFILM VisualSonics (Toronto, ON, Canada)	n/a	Ultrasound imaging system
Vevo 3100 Lab Software	FUJIFILM VisualSonics (Toronto, ON, Canada)	n/a	Version 3.2.7 for imaging and analysis
Vevo Compact Dual Anesthesia System	FUJIFILM VisualSonics (Toronto, ON, Canada)	n/a	Tabletop isoflurane-based anesthesia unit
Vevo Imaging Station	FUJIFILM VisualSonics (Toronto, ON, Canada)	n/a	Procedural platform