Microbes and Infection 16 (2014) 269e272www.elsevier.com/locate/micinf

Highlight

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Mycobacteria, meet your messenger*

Biology textbooks contain detailed descriptions about cel-lular communication involving hormones, the interactions ofimmune cells, and ion gradients across neuronal membranes.However, space inmore recent editions will need to be accordedto a new system of cellular communication involving micro-scopic particles called exosomes. These mini-messengers areattracting a great deal of attention from academia and industryalike, notably for their ability to manipulate immune responses.In this issue of Microbes and Infection, Tang and colleaguesexamine how exosomes may shape immune responses duringthe course of infectious disease [1].

Exosomes are small (30e120 nm) membrane vesicles thatare probably secreted by all types of cell. These vesicles areformed inside the cell by the inward budding of cellularcompartments called multivesicular endosomes (MVE).Exosomes internalised in the MVE are released into theextracellular environment when the MVE fuses with theplasma membrane. They are then taken up by target cellseither via the target cell’s endocytic pathway or by fusingdirectly with the plasma membrane and releasing theircontent directly into the cytoplasm. However, their functionin intercellular communication took a long time to be rec-ognised. Their first reported cargo was the transferrinreceptor [2,3] which was shedded by reticulocytes duringtheir differentiation. This led to the initial belief that exo-somes were the garbage bags of a cellular trash disposalsystem, and ten years passed before the discovery of some ofthe more exotic activities of these microvesicles. Raposo andcolleagues found that exosomes secreted by EpsteineBarrvirus-transformed B cells contained MHC class II dimersbound to antigenic peptides that could stimulate T cellresponses [4]. The study of exosomes subsequently explodedand their cargo became meticulously catalogued. Besidesmembrane-associated proteins, mass spectrometry has shownthat over 4400 various protein components can be trans-ported by these microvesicles [5]. One big surprise camewith the discovery of nucleic acid content in the form of

icle highlight of “Proteomic analysis and immune properties of exo-

eleased by macrophages infected with Mycobacterium avium” by J.J.

t al. [1].

x.doi.org/10.1016/j.micinf.2014.03.004

579/� 2014 Institut Pasteur. Published by Elsevier Masson SAS. All rights

mRNA and microRNA raising the possibility of genetictransfer between cells [6].

As a result of this extensive cargo manifest, exosomes havebeen implicated in a wide variety of processes, includingapoptosis, angiogenesis, inflammation, coagulation, and thecreation of polarity during development [7]. Their role in theimmune system has been extensively studied. Besides directantigen presentation, exosomes can also carry antigens fromthe cells from which they originate. These exosomes can betaken up by dendritic cells which present degraded antigenscoupled with MHC molecules to T cells. Exosomes may alsocontain microbial components that stimulate innate immuneresponses. Beatty and colleagues were the first to show that thecell wall components of mycobacteria were trafficked insidethe endocytic compartment of infected macrophages [8].These pathogen-associated molecular patterns (PAMPs) weresomehow passed on to uninfected bystander cells. Not longafter, this relay mechanism was shown to involve exosomes[9]. Exosomes released from mouse macrophages infectedwith Mycobacterium tuberculosis were capable of inducing apro-inflammatory response in uninfected macrophages. Thus,exosomes function in a system of pathogen surveillance,increasing the exposure of intracellular pathogens to theimmune cells and showing these microbial agents that there isnowhere to hide inside the infected cell.

Tang and co-workers now report similar findings in ahuman cell system with Mycobacterium avium, which is aclose relative of M. tuberculosis. M. avium is less virulent thanM. tuberculosis, which makes it much safer to handle. Tangtreated uninfected macrophages either with exosomes derivedfrom macrophages infected with M. avium or exosomesderived from uninfected macrophages and examined theresponse of these resting macrophages. Uninfected macro-phages that were treated with exosomes derived from M.avium infected macrophages secreted higher concentrations ofthe pro-inflammatory cytokines TNF-a and IFN-g than unin-fected macrophages treated with control exosomes. In fact, theresponse effectively mimicked that of infection of macro-phages with M. avium itself, albeit with somewhat delayedkinetics. Exosomes derived from M. avium infected macro-phages also promoted the expression of CD80 and CD86 inuninfected cells. These cell surface molecules work in tandem

reserved.

270 Highlight / Microbes and Infection 16 (2014) 269e272

to activate T cells, suggesting that exosomes may also stim-ulate the adaptive arm of immunity upon infection with M.avium. Exosome isolation is a delicate process, and obtainingenough material to examine their potential microbial contentsis no trivial matter. Although the authors were unable todefinitively say that these exosomes contained M. aviumcomponents, previous reports that exosomes secreted bymacrophages carry microbial antigens and PAMPs [9,10]support the hypothesis that exosomes introduce immuno-genic proteins to uninfected macrophages. Thus, exosomesfunction as relay warning signals that alert resting macro-phages to the presence of infection.

Although in this instance exosomes appeared to boostimmune responses, they may be a double edged sword, nota-bly during immune responses to cancer. Exosomes secreted bytumour cells can transport tumour antigens to dendritic cells[11]. However, exosomes secreted by some tumours may alsocontain immunosuppressive molecules which can (at leastin vitro) impair the proliferation of immune cells [12,13].Cancerous cells may even impart the malignant gift of onco-genic gene products to their unwitting neighbours [14].Nonetheless, the applications of exosomes are bountiful.

Exosomes may serve as diagnostic markers for various dis-eases. These microvesicles represent a neat little packingsystem because all the protein and RNA cargo is encased in amembrane so they are impervious to circulating nucleases andproteases in the blood. They have a very long half-life in thebloodstream, permitting their transport between anatomicallydistant locations. As a testament to their stability, they havebeen found in many bodily fluids including blood, saliva,urine, epididymal fluid, amniotic liquid, synovial fluid, andbreast milk. Custom made exosomes may be used to delivervaccines or therapeutic agents. In the last decade, severaltherapeutic approaches involving exosomes have been devisedand tested and some have even made it to Phase II clinical

trials [15]. It appears that exosomes may just be the next smallthing.

1. Biosketch e Li-Jung Tang

Dr. Li-Jung Tang completed his Ph.D at XiangYa School ofMedicine, Central South University, China during which hecloned several novel genes related to human multiple mye-loma and carried out preliminary functional analyses of thesegenes. He then carried out postdoctoral training with Dr.Garcia Rodolfo in the International Center for Genetic Engi-neering and Biotechnology (ICGEB) in Trieste, Italy between2005 and 2007. As a post-doctorate he used 2DE MALDITOF/TOF MS to examine protein patterns in human bodyfluids upon infection. He was also involved in research of themacrophage responses to mycobacterium via the Toll-likereceptor pathway. He returned to China with his family in2007 and became an associate professor at the School ofBiological Science and Technology, Central South Universitywhere he studied macrophage defence mechanisms againstmycobacterium that involve exosomes secreted from cells. In2013 he became a professor at Central South University.

2. Interview with Li-Jung Tang

1. What triggered your interest in the immune functions ofexosomes?

I studied at the International Centre for Genetic andEngineering Biotechnology (ICGEB) in Trieste, Italy whereI first started working with exosomes with my PostDocsupervisor. I realised that exosomes are mysterious andwonderful biological products that deserved to be resear-ched.

2. What is the take-home message of the article?

Background

Exosomes are small (30e120 nm) membrane

vesicles that are probably secreted by all types of

cell. Although they were initially believed to be a

by-product of the in vitro manipulation of cells,

and subsequently as a vehicle for cells to shed

unwanted proteins, their important role as

molecular messengers enabling cell communi-

cation between distant sites is becoming

increasingly studied. Exosomes are a stable

packaging device for the transport of proteins

and nucleic acid between cells and are found in a

large number of bodily fluids. They have been

implicated in a wide variety of biological pro-

cesses. Notably, their contents can modulate

both adaptive and innate immune responses

during the course of infection. There is hope that

one day custom made exosomes may be used to

deliver therapeutic agents in the fight against

disease.

In a Nutshell

� Uninfected macrophages that were exposed to exo-somes derived from macrophages infected with M.avium expressed higher levels of molecules involvedin T cell activation, CD80 and CD86, than macro-phages treated with exosomes derived from non-infected macrophages.

� Exosomes derived from macrophages infected withM. avium induced a pro-inflammatory response in

271Highlight / Microbes and Infection 16 (2014) 269e272

Exosomes are involved in the immune system’s defencemechanisms against mycobacteria.

3. Do you have a personal motto, quote or leading sentence?

Persist and you will succeed.

4. What advice would you give to the young next-generationscientists?

The most important thing is to be passionate about whatyou are doing, and then of course you need to work hard.

5. What is your favourite hang-out method after a tough dayat the lab?

Jogging and swimming.

6. In your opinion, what is the most important (scientific)discovery of the last fifty years?

The finding that cells can communicate throughexosomes.

7. If you could travel back in time what scientific discoverywould like to assist to?

The discovery of the structure of DNA by Watson, Crick,and Franklin.

8. If you could travel forward in time e what eventualinvention would you like to check out?

The use of exosomes to treat diseases. Exosomes are veryimportant molecular carriers. But we still have a lot of workto do!

uninfected macrophages, involving the higher pro-duction of TNFa and IFNg than uninfected macro-phages treated with control exosomes. Thus,exosomes released by infected cells mimic the effectof infection.

� The abundance of twelve proteins was differentbetween exosomes derived from macrophages infec-ted with M. avium and those derived from non-infected macrophages. Five of these proteins wereidentified by mass spectrometry. These were cytos-keletal proteins, a protein involved in protein pro-cessing, and a protein involved in signal transduction.The presence of bacterial proteins in exosomes wasnot investigated due to technical limitations.

Drawing by Sophia Hafner.

References

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immune properties of exosomes released by macrophages infected with

Mycobacterium avium. Microbes Infect 2014;16:283e91.

272 Highlight / Microbes and Infection 16 (2014) 269e272

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Emma Louise Walton24 rue ampere, Malakoff 92240, FranceE-mail address: ewalton86@gmail.com

11 March 2014

Available online 21 March 2014