News FocusCell Biology

The Ins and Outs of Exosomes

See allHide authors and affiliations

Science  24 Jun 2005:
Vol. 308, Issue 5730, pp. 1862-1863
DOI: 10.1126/science.308.5730.1862

This article has a correction. Please see:

Cells spit out these mysterious vesicles, but what they do and whether they boast medical uses, such as in cancer vaccines, is up in the air

MONTREAL, CANADA—”Alice in Blunderland” is how biochemist Rose Johnstone describes her 1970s investigations into curious, lipid-encased particles burped out by the sheep red blood cells she was studying. Baffled by the material, which resembled “little florets” under a microscope, Johnstone, of McGill University in Montreal, nevertheless pursued them, gradually drawing in a small cohort of scientists. She anointed the vesicles exosomes; unlike similar-looking structures called endosomes, which carry material into cells, these particles seemed to cart stuff away.

Three decades later, exosomes are beginning to surrender their secrets. Scientists now know that they have been conserved across species, suggesting useful, even life-preserving functions. Exosomes secreted by white blood cells, for example, appear to mediate immune responses, activating and perhaps also suppressing the immune system. Indeed, cancer physicians are already trying to exploit exosomes to trigger immune response against a variety of tumors. And other researchers are exploring whether these vesicles assist the spread of HIV and prions.

In one indication of the exosome's coming of age, the first-ever meeting* devoted to these particles was held here last month, organized by Johnstone and sponsored by the Leukemia and Lymphoma Society. About two dozen biologists from eight countries converged in a classroom at McGill University for 2 days of spirited discussion. Although exosomes are no longer dismissed as simple cell fragments, their biological significance remains tantalizing but uncertain. “Many people think exosomes are pieces of cell running around with no specific function,” said Sebastian Amigorena of the Curie Institute in Paris. “I want to believe they're doing something.”

Unexpected finds

Most cell biologists enter the exosome field after they stumble across the particles in their experiments. That was the case for Johnstone. She and her colleagues first saw what she would later call exosomes while hunting for an elusive amino acid transporter. Examining blood drawn from sheep, they saw that the immature red blood cells were ejecting “tons of stuff” after binding to an antibody, says Johnstone. The red blood cells of chicks, piglets, frogs, rats, and humans all produced these vesicles. Her group ultimately reported in 1983 that exosomes help nascent red blood cells develop by carting away proteins that are no longer needed.

But exosomes lingered in obscurity until 1996. That year, Graça Raposo and Hans Geuze of Utrecht University in the Netherlands and their colleagues reported that other blood cells, called B cells, secrete exosomes that offer up antigens—tiny bits of pathogens—to the T cells of the immune system. Such antigen presentation is a key initial step in launching an immune response. And exosomes were found to sport the crucial major histocompatibility complex molecules that bind to and display antigens, just as dendritic cells of the immune system do. Indeed, the presence of these immune molecules is one of the identifying signatures of an exosome, along with a characteristic lipid membrane.

Taking shape.

Microscopic vesicles called exosomes are secreted from many cell types, everything from intestinal cells to blood to cancer cells; here, exosomes appear as cup-shaped vesicles (left) or are labeled as black dots inside a larger cell structure (right). Scientists are only beginning to piece together their functions and importance.

CREDITS: B. FÉVRIER AND G. RAPOSO, CURRENT OPINION IN CELL BIOLOGY 16, 1 (2004), ELSEVIER LTD.

Raposo later moved to the Curie Institute, where she and her exosomes attracted other scientists interested in the role these vesicles play in T cell activation. One question is whether exosomes need a partner to arouse T cells, and if so, how that dance is choreographed. Clotilde Théry of the Curie Institute believes that exosomes and dendritic cells work in concert. She and her colleague Laurence Zitvogel, of Paris's Gustave Roussy Institute, have found that, like a relay racer handing off a baton, exosomes in a test tube pass certain antigens onto dendritic cells. The dendritic cells then use those antigens to activate T cells. Whether this also happens inside an animal is unknown.

Dendritic cells themselves appear to produce exosomes, as do an ever-lengthening list of other cell types. For instance, cells at the outer edges of the intestine and cancer cells have also been shown to secrete the vesicles.

Lassoing a skittish target

Finding that additional cell types make exosomes has only added to confusion over what they do. Further complicating matters is that the particles are maddeningly awkward to work with. “It's difficult to manipulate [and] purify them,” says Amigorena.

Aled Clayton of Cardiff University in Wales, U.K., who has followed up on reports that cancer cells secrete exosomes, can attest to that. He theorizes that these exosomes might suppress the immune system rather than activate it, letting the cancer flourish. But despite examining exosomes ejected by various cancer cells, including breast cancer and mesothelioma, he can't produce sufficient data to back up the conjecture. After months of painstaking work with the particles, “we've failed miserably at generating any immune responses from exosomes,” he confessed at the meeting.

While most exosome researchers have focused on the vesicles' apparent ability to switch on the immune system, Clayton and others continue to probe whether the vesicles play an opposing role in immune suppression. Last month, in the Journal of Immunology, molecular biologist Paul Robbins of the University of Pittsburgh, Pennsylvania, reported that exosomes he stumbled across while studying gene therapy for arthritis could heal the autoimmune disease. Trying to understand why delivering gene therapy to one arthritic joint in a mouse helps nearby joints too, Robbins and his colleagues noticed small vesicles traveling from treated joints to untreated joints. “I thought it was cell debris,” says Robbins. But when he and his team culled these vesicles from dendritic cells and reinjected them, they found that a single injection into an arthritic animal eliminated the disease. “Our exosomes are very suppressive,” says Robbins.

Switching on.

Experiments suggest that exosomes can help activate T cells like this one, but debate continues over how they do so.

CREDIT: STEM JEMS/PHOTO RESEARCHERS INC.

Supporting that premise, Helen O'Neill of Australian National University in Canberra told the Montreal gathering of studies showing that exosomes appear to reflect the behavior of the cells that release them; immature dendritic cells and their exosomes, which Robbins used, tend to suppress immunity, whereas mature ones and their exosomes stimulate it. Work by Amigorena, Théry, and others published in March in Blood did not find this distinction, however.

These finer questions were sometimes overshadowed by more fundamental ones. Do all the vesicles being classed as exosomes really fall into that category? In other words, is everyone studying the same thing? Meeting participants reached no consensus on whether the definition of an exosome should be based on the vesicle's chemical makeup, on how it was formed, or on its purpose. Cell biologist Stephen Gould of Johns Hopkins University in Baltimore, Maryland, argued for a broad characterization. “The more narrow you make the definition, the less interesting this field will be to everyone else,” he said.

Tackling cancer

Such fuzziness hasn't stopped oncologists from incorporating exosomes into so-called cancer vaccines, which seek to trigger immune response against tumors. One strategy involves filtering exosomes produced by dendritic cells from a patient's blood. The theory is that these exosomes, produced by dendritic cells that can activate the immune system, sport tumor antigens and can induce a strong immune attack on an individual's cancer if redelivered in sufficient volume.

A Menlo Park, California, company called Anosys, co-founded by Jean-Bernard Le Pecq, funded a small trial in lung cancer and another in melanoma before folding this spring. The exosomes failed to help most of the melanoma patients, Zitvogel, who ran the trial, reported in Montreal. But those with lung cancer fared relatively well, Le Pecq noted, with a handful surviving several years—unusually long for people with the aggressive cancer. One unexpected complication was that not enough exosomes for an individualized vaccine could be extracted from the blood of every volunteer.

Meanwhile, oncologist Malcolm Adams of Cardiff University is working with Zitvogel to launch an ovarian cancer trial of an exosome-based vaccine. Their strategy is slightly different from that used in the lung cancer and melanoma trials: Here, exosomes will be harvested from tumor cells in abdominal fluid. Such cells and their exosomes should bear antigens specific to the individual's cancer. And if the exosomes are administered along with a substance that stimulates the immune system, the vesicles may train the immune system to recognize cancer cells as foreign and attack the cells, suggest Adams and Zitvogel.

Some exosome biologists are investigating whether the particles play a nefarious role in infectious diseases, such as spreading viruses from cell to cell. In 2003, Gould and his colleagues published a provocative theory about HIV dubbed the “Trojan exosome hypothesis.” They proposed that retroviruses, such as HIV, hide as exosomes secreted from an infected cell.

In Gould's theory, the HIV exosomes are released from an infected cell and drift toward one that's not infected. That cell takes them up, internalizing the deadly virus. The theory doesn't discount that HIV can directly infect cells, but it suggests that exosomes offer an alternative way for the virus to spread. In Montreal, Gould reported that exosomes bud from domains in the plasma membrane of T cells, which HIV infects. Bolstering his theory, he's also found that HIV Gag, a protein the virus needs to exit a cell, congregates in exosomes.

But HIV researcher Michael Marsh of University College London questions parts of the Trojan exosome hypothesis. Although Marsh believes that HIV and other retroviruses might assemble in the same endosomal cell compartments that spit out exosomes, he says he's never seen the exosome budding from the cell surface that Gould reports.

In addition to HIV, exosomes may act as infectious pawns for prions, misshapen proteins suspected in several neurodegenerative conditions such as “mad cow disease.” Raposo has recently published data indicating that exosomes act as vehicles for prion transport between cells. She noted that prions appear to accumulate in endosomes as they near the point of spilling their exosome cargo. Exosomes “probably have something to do with transmission of the infectious agent,” she says. In the case of ingested beef causing mad cow disease in people, this could mean ferrying prions from the gut to the central nervous system.

Raposo's prion work—which, she says, elucidates “the dark side” of exosomes—hints at the breadth of potential roles for these vesicles in health and disease. But like so much exosome work, it remains dogged by uncertainties and questions.

  • *Exosomes: Biological Significance, Montreal, Canada, 20–21 May.

View Abstract

Stay Connected to Science

Navigate This Article