What are extracellular vesicles, outer membrane vesicles and exosomes?

Extracellular vesicles (EVs) refers to particles released from cells (eukaryotic and prokaryotic), encased in a lipid bilayer, and unable to replicate independently as they lack a functional nucleus. They are small particles typically between 10 and 300nm depending in origin that play essential roles in cell-to-cell communication. They carry a variety of molecules, including proteins, lipids, and nucleic acids. EVs are categorized into three main types:

• Exosomes: Small vesicles formed inside cells and released when multivesicular bodies fuse with the plasma membrane.
• Microvesicles: Larger vesicles that bud directly from the plasma membrane.
• Apoptotic bodies: Large vesicles released during programmed cell death (apoptosis).

EVs are involved in numerous biological processes and have potential therapeutic applications.

All exosomes are EVs, but not all EVs are exosomes. Extracellular vesicles are the umbrella term for all lipid bilayer membrane-bound particles released from cells of microbial or mammalian origin.

Exosomes are specifically referring to EVs formed in mammalian cell lines through the endosomal pathway and then released to the extra cellular space through fusion to the plasma membrane. For a scientifically correct description of this we recommend that you consult the MISEV (Minimal information for Studies of Extracellular Vesicles) published by the International Society of Extracellular vesicles.

Outer membrane vesicles (OMVs) are a specific type of extracellular vesicle produced by Gram-negative bacteria. They are derived from the outer membrane and contain various components from the bacterial cell envelope, including lipopolysaccharides (LPS), outer membrane proteins, and periplasmic contents.

Extracellular vesicles (EVs) are being explored for various therapeutic applications due to their ability to deliver molecules and facilitate communication between cells. Key applications include:

• Vaccine: Extracellular vesicles can be used as vaccines or adjuvants in vaccine applications. Especially bacterial derived vesicles are known to carry immune active antigens. Vesicles can also be engineered by decorating them with specific antigens to address specific infections.
• Gene therapy: EVs can be engineered to carry genetic material, such as RNA and DNA, to target cells, offering a non-viral delivery method that can cross biological barriers and is less likely to provoke immune responses.
• Cancer treatment: EVs derived from tumor cells can be modified to deliver immune-stimulating neoantigens, enhancing immunotherapy by activating T cells to target and destroy cancer cells.
• Drug delivery: EVs can be loaded with therapeutic drugs and targeted to specific cells or tissues, improving treatment precision and effectiveness.
• Regenerative medicine: EVs from stem cells can promote tissue repair and regeneration by delivering growth factors and signaling molecules to damaged tissues.

Using extracellular vesicles (EVs) for therapy presents several challenges:

• Scalability and standardization: Producing EVs in large quantities while maintaining consistent quality is difficult. Sometimes the EVs are not secreted in feasible yields for clinical applications and selection or engineering of the strains are needed. Traditionally the methods used for vesicle isolation have not been designed for large scale applications and therefore need to be exchanged with maintained functionality of the vesicles.
• Mechanisms of action: Incomplete understanding makes it hard to predict their behavior and efficacy in different therapeutic contexts.
• Safety and regulatory concerns: Ensuring that EVs are free from contaminants and safe for patients requires rigorous testing and regulatory approval.
• Targeted delivery: Achieving precise delivery of EVs to specific tissues or cells without off-target effects is complex.

The regulatory process for extracellular vesicles (EVs) involves several key steps to ensure their safety, efficacy, and quality for therapeutic use:

• Preclinical research: Extensive laboratory and animal studies to understand the mechanisms of action, safety, and potential therapeutic benefits of EVs. Researchers must demonstrate that EVs can be produced consistently and are free from contaminants.
• Clinical trials: Rigorous testing in humans, typically in three phases:
  • Phase I: Tests the safety and dosage in a small group of healthy volunteers or patients.
  • Phase II: Evaluates the efficacy and side effects in a larger group of patients.
  • Phase III: Confirms the efficacy, monitors side effects, and compares EVs to standard treatments in a large patient population.
• Regulatory approval: Agencies like the FDA in the United States or the EMA in Europe review the data from preclinical and clinical studies, assessing the safety, efficacy, and manufacturing quality of EVs before granting approval for therapeutic use.
• Quality control and manufacturing: Ensures consistent production quality, involving standardized protocols for EV isolation, characterization, and storage. Regulatory guidelines require that EVs are produced under Good Manufacturing Practices (GMP) to ensure their safety and efficacy.
• Post-market surveillance: Ongoing monitoring of EVs to detect any long-term or rare side effects, ensuring continued safety and effectiveness in the broader patient population.