Tracking Exosomes by Photoacoustic Imaging

Exosomes act as nanocarriers for intercellular communication allowing the exchange of complex information between cells, thereby regulating processes such as homeostasis, immune responses, and carcinogenesis under physiological and pathological conditions. In addition, due to the unique ability of exosomes to migrate, target, and selectively internalize, they are promising drug delivery vectors, offering a potential new approach to diagnosis and therapy. In vivo tracking of exosomes can provide essential knowledge about biodistribution, migratory capacity, biological roles, and action mechanisms, contributing to a better understanding of their biological functions and potential as therapeutic and drug delivery vectors. Therefore, there is a need to develop efficient and sensitive exosome imaging techniques. Photoacoustics is an emerging biomedical imaging modality that will greatly advance exosome-based nanomedicine applications.

Figure 1. Schematic of the biogenesis and uptake of extracellular vesicles. (Liu Q, et al., 2022)

What is Photoacoustic Imaging?

Photoacoustic imaging (PAI) is a non-destructive medical imaging method developed in recent years that combines the high contrast of optical imaging with the high depth of penetration of ultrasound imaging to provide high-resolution imaging of tissues. It permits imaging of optical absorbers in tissues using an acoustic detector (light input - sound output). The technology detects endogenous or exogenous chromophores excited by pulsed laser irradiation. The non-radiative release of the absorbed energy generates micro-heating and thermoelastic expansion in the vicinity of the chromophore, resulting in ultrasound waves. The sound waves are detected by a transducer and converted into an image. With high resolution, sufficient imaging depth, multiple endogenous and exogenous contrasts, and freedom from ionizing radiation, this technique has great potential for clinical translation. In recent years, PAI instruments and visualizers have undergone tremendous development and have broad applications in the early diagnosis of cancer, detection of oxygen supply to various organs, and research.

Figure 2. Various configurations of PAI. (Steinberg I, et al., 2019)

What is the Principle of PAI?

When the laser irradiates the tissue, due to the scattering effect of the biological tissue on the light wave, although the light wave cannot be effectively focused, the electromagnetic wave energy can effectively enter into the tissue at a depth of about 50mm. The light absorber inside the biological tissue absorbs the electromagnetic energy and converts it to heat, and the thermal expansion and contraction of the absorber make it a source of sound. Since soft tissue is a good medium for acoustic wave propagation, the photoacoustic signal at the absorber can be efficiently radiated to the surrounding medium and propagated through the tissue with low scattering and low loss. The ultrasonic transducer located around the tissue acquires the generated photoacoustic wave, and through signal processing and photoacoustic image reconstruction, a photoacoustic image reflecting the internal structure and function of the tissue can be formed. The biggest advantage of the PAI technology is that the phase and amplitude of the photoacoustic signal can be measured without pre-processing of the specimen, which is not only simple to operate, but also maintains the natural morphology of the biological sample, and can be examined in vivo.

What are the Chromophores that can be used to Generate PAI Contrast?

• Endogenous molecules (e.g., oxygenated and deoxyhemoglobin, myoglobin, melanin).
• Exogenous probes (e.g., organic dyes such as fluorescein and ICG).
• Nanosystems (e.g., nanoparticles containing noble metals).
• Quantum dots, carbon nanotubes.
• Fluorescent proteins and reporter genes.

How to Track Exosomes with PAI?

1. Labeling: Labeling exosomes with a photoacoustic signal emitter, which is usually a specially designed nanoparticle or dye that absorbs light and emits sound waves.

2. Injection: Introduction of the labeled exosomes into the animal model.

3. Illumination: Light is absorbed by the labeled exosomes and converted into ultrasound due to the irradiation of non-ionizing laser pulses of a specific wavelength.

4. Detection: Using an ultrasonic transducer, these sound waves are detected and converted into a photoacoustic signal.

5. Imaging: The signals are collected and processed to produce a final image representing the distribution of exosomes in the subject.

Research Advances in Tracking Exosomes Using PAI

Major limitations of applying gold-based nanomaterials to biomedical research include a lack of active tumor targeting capability, penetration efficiency, and stability. In recent years, researchers have proposed a novel tumor cell-derived stellate plasmonic exosome (TDSP-Exos) for penetrating tumor-targeted NIR-II thermo-radiotherapy and PAI. TDSP-Exos is produced in large quantities by incubation of tumor cells with gold nanopillars, which promote the exocytosis of tumor cell exosomes. TDSP-Exos has significant accumulation in deep tumor tissues and has good performance in PAI. This work suggests that tumor cell-derived exosomes have the potential to serve as a versatile carrier for photothermal agents.

Figure 3. Schematic of stellate plasmonic exosomes for penetrative targeting tumor NIR-II thermo-radiotherapy. (Zhu D, et al., 2020)

Immunotherapy is an emerging technology that utilizes a patient's innate immune system. In immunotherapy, immune cells are stimulated by antibodies, other immune cells, and genetic modifications for purposes such as cancer treatment. Researchers have developed PAI-assisted photodynamic and immunotherapy. The photosensitizer chloramphenicol e6 is loaded into isolated tumor cell-derived exosomes by sonication. The generated nanoparticles can be visualized in vivo by PAI and produce cytotoxic reactive oxygen species (ROS) within the tumor cells under tissue-specific laser irradiation. Tumor cells were damaged by tumor cell-derived EV-stimulated ROS and cytokines from immune cells.

Figure 4. Photoacoustic signal (PA) enhancement of Ce6-R-Exo. (Jang Y, et al., 2021)

Tools to trigger dendritic cell (DC) activation and validate DC migration in vivo are essential to guide successful DC immunotherapy. Researchers used gold nanoparticle (GN) labeling and ultrasound (US)-guided photoacoustic imaging (PAI) to track tumor cell-derived exosomes (TEX). The research found that TEX is effective in activating DCs and that GN-labeled DC migration to the LN is successfully monitored using ultrasound-guided PAI, suggesting that TEX is a good source of DC activation and that ultrasound-guided PAI is a cost-effective and easy-to-use imaging modality.

Figure 5. Analysis of in vitro US-guided PAI of GN-labeled DC2.4 cells. (Piao YJ, et al., 2020)

Our Services and Products

Creative Biostructure is a leader in exosome research, providing clients with one-stop exosome services, including exosome isolation, characterization, labeling, tracking, targeting, and functional analysis. We also offer a comprehensive range of exosome products to help clients explore the potential applications of exosomes as biomarkers and drug carriers.

If you have any questions, please feel free to contact us for more information.

References

1. Liu Q, et al. Tracking tools of extracellular vesicles for biomedical research. Front Bioeng Biotechnol. 2022. 10: 943712.
2. Steinberg I, et al. Photoacoustic clinical imaging. Photoacoustics. 2019. 14: 77-98.
3. Zhu D, et al. Stellate Plasmonic Exosomes for Penetrative Targeting Tumor NIR-II Thermo-Radiotherapy. ACS Appl Mater Interfaces. 2020. 12(33): 36928-36937.
4. Jang Y, et al. Exosome-based photoacoustic imaging guided photodynamic and immunotherapy for the treatment of pancreatic cancer. J Control Release. 2021. 330: 293-304.
5. Piao YJ, et al. Noninvasive Photoacoustic Imaging of Dendritic Cell Stimulated with Tumor Cell-Derived Exosome. Mol Imaging Biol. 2020. 22(3): 612-622.