Exploiting Manipulated Small Extracellular Vesicles to Subvert Immunosuppression at the Tumor Microenvironment Through Mannose Receptor/CD206 Targeting
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International Journal of Molecular Sciences Review Exploiting Manipulated Small Extracellular Vesicles to Subvert Immunosuppression at the Tumor Microenvironment through Mannose Receptor/CD206 Targeting Maria Luisa Fiani *, Valeria Barreca, Massimo Sargiacomo, Flavia Ferrantelli, Francesco Manfredi and Maurizio Federico * National Center for Global Health, Istituto Superiore di Sanità, 00161 Rome, Italy; [email protected] (V.B.); [email protected] (M.S.); fl[email protected] (F.F.); [email protected] (F.M.) * Correspondence: maria.fi[email protected] (M.L.F.); [email protected] (M.F.); Tel.: +39-06-4990-2518 (M.L.F.); +39-06-4990-6016 (M.F.) Received: 6 July 2020; Accepted: 27 August 2020; Published: 31 August 2020 Abstract: Immunosuppression at tumor microenvironment (TME) is one of the major obstacles to be overcome for an effective therapeutic intervention against solid tumors. Tumor-associated macrophages (TAMs) comprise a sub-population that plays multiple pro-tumoral roles in tumor development including general immunosuppression, which can be identified in terms of high expression of mannose receptor (MR or CD206). Immunosuppressive TAMs, like other macrophage sub-populations, display functional plasticity that allows them to be re-programmed to inflammatory macrophages. In order to mitigate immunosuppression at the TME, several efforts are ongoing to effectively re-educate pro-tumoral TAMs. Extracellular vesicles (EVs), released by both normal and tumor cells types, are emerging as key mediators of the cell to cell communication and have been shown to have a role in the modulation of immune responses in the TME. Recent studies demonstrated the enrichment of high mannose glycans on the surface of small EVs (sEVs), a subtype of EVs of endosomal origin of 30–150 nm in diameter. This characteristic renders sEVs an ideal tool for the delivery of therapeutic molecules into MR/CD206-expressing TAMs. In this review, we report the most recent literature data highlighting the critical role of TAMs in tumor development, as well as the experimental evidences that has emerged from the biochemical characterization of sEV membranes. In addition, we propose an original way to target immunosuppressive TAMs at the TME by endogenously engineered sEVs for a new therapeutic approach against solid tumors. Keywords: tumor-associated macrophages; tumor microenvironment; macrophage polarization; mannose receptor; exosomes; extracellular vesicles; HIV-1 Nef 1. Introduction Both immunosuppression and genetic escape are formidable weapons through which tumors can elude host immune surveillance. Solid tumors develop in a quite complex context, referred to as tumor microenvironment (TME) [1,2], which is composed of both cellular and non-cellular elements, usually resulting in an immunosuppressive behavior. Counteracting such a general effect would favor both spontaneous and therapeutic anti-tumor immunity, hence critically contributing to control tumor cell growth. Therefore, subverting TME immunosuppression represents a major goal for anticancer immunotherapies. Int. J. Mol. Sci. 2020, 21, 6318; doi:10.3390/ijms21176318 www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2020, 21, 6318 2 of 20 Int.Both J. Mol. normal Sci. 2020, and21, x tumor cells constitutively release membrane-bilayered vesicles, commonly2 of 20 referred to as extracellular vesicles (EVs) [3,4]. They differ in the mechanisms of biogenesis and secretion,secretion, giving giving rise rise to to the the generation generation of of a a heterogeneous heterogeneous populationpopulation of of vesicles vesicles with with different different sizes sizes andand contents contents [5 ,6[5,6],], which which include include small small EVs EVs (sEVs)(sEVs) oror exosomesexosomes and and microvesicles microvesicles or or ectosomes. ectosomes. ExosomesExosomes are vesiclesare vesicles of 30–150 of 30–150 nm diameter nm diameter generated generated by inward by buddinginward ofbudding endosomal of endosomal membranes to formmembranes intraluminal to form vesicles intraluminal that accumulate vesicles in that intracellular accumulate organelles in intracellular called multivesicular organelles called bodies (MVBs).multivesicular MVBs ultimately bodies (MVBs). fuse with MVBs the plasmaultimately membrane, fuse with thereby the plasma releasing membrane, intraluminal thereby vesicles releasing into theintraluminal extracellular vesicles environment into the (Figure extracellular1). On envi theronment contrary, (Figure ectosomes 1). On are the 100–500 contrary, nm ectosomes vesicles shed are by direct100–500 budding nm vesicles from shed the by plasma direct membrane budding from [7,8 the]. Di plasmafferent membrane types of EVs [7,8]. often Different show types overlapping of EVs featuresoften thatshow make overlapping difficult tofeatures obtain that relatively make puredifficul preparationst to obtain whenrelatively purified pure from preparations cell-conditioned when mediapurified or biological from cell-conditioned fluids. In this media review, or webiological will use fluids. the term In this sEV review, to refer we to will EV use types the co-isolated term sEV to by refer to EV types co-isolated by typical purification methods and exosomes to distinguish EV whose typical purification methods and exosomes to distinguish EV whose subcellular biogenesis strictly subcellular biogenesis strictly derives from multivesicular bodies/endosomes [8]. derives from multivesicular bodies/endosomes [8]. Figure 1. Electron microscopy of multivesicular bodies (MVB) and small extracellular vesicles (sEVs) Figure 1. Electron microscopy of multivesicular bodies (MVB) and small extracellular vesicles (sEVs) (a) TEM micrograph of multivesicular bodies with intraluminal vesicles in Mel501, a melanoma cell (a) TEM micrograph of multivesicular bodies with intraluminal vesicles in Mel501, a melanoma cell line (b) SEM (Scanning Electron Microscope). Micrograph of sEVs purified from conditioned medium line (b) SEM (Scanning Electron Microscope). Micrograph of sEVs purified from conditioned medium of Mel501of Mel501 cells cells by by di ffdifferentialerential centrifugations. centrifugations. Courtesy Courtesy ofof FrancescaFrancesca Iosi and the the Microscopy Microscopy Area Area of of thethe ISS ISS Core Core Facilities. Facilities. SEVs carry a complex cargo of nucleic acids, proteins, and lipids that largely reflects the SEVs carry a complex cargo of nucleic acids, proteins, and lipids that largely reflects the characteristics and the functional state of the cells they originate from, and that will be delivered to characteristics and the functional state of the cells they originate from, and that will be delivered to neighboringneighboring or distantor distant cells cells [9 ,[9,10].10]. As As a a result, result, the the functionsfunctions ofof those recipient cells cells will will be be modulated modulated by sEVsby sEVs in ain manner a manner that that is is strictly strictly dependent dependent on on the the naturenature ofof producerproducer cells, making making sEVs sEVs central central playersplayers in intercellularin intercellular communication communication and and reprogramming reprogramming ofof targettarget cells [11]. [11]. Ectosomes Ectosomes generation generation is ais much a much less less known known process process that that requires requires the the accumulation accumulation ofof theirtheir cargo at at the the cytosolic cytosolic surface surface of of specificspecific plasma plasma membrane membrane microdomains microdomains [7 [7,12].,12]. SEVs-mediatedSEVs-mediated transfer transfer of of molecular molecular and and genetic genetic materialmaterial from one cell cell to to another, another, either either locally locally or ator long at long distance, distance, is ais keya key contributor contributor to to the the mechanisms mechanisms ofof intercellularintercellular communication involved involved in variousin various physiological physiological and and pathological pathological conditions conditions [[13–15].13–15]. Moreover, for for these these reasons, reasons, sEVs sEVs are are nownow considered considered powerful powerful tools fortools clinical for applications,clinical applications, including advancedincluding diagnostics,advanced diagnostics, therapeutics, andtherapeutics, regenerative and medicine regenerative [16–19 medicine]. [16–19]. TheThe molecular molecular composition composition ofof sEVssEVs is determined determined by by the the cell cell type type of origin of origin as well as wellas by asthe by theintracellular intracellular pathway pathway followed followed en enroute route to their to their release release into intothe extracellu the extracellularlar space space[8,20,21]. [8, 20This,21 ]. Thisheterogeneity heterogeneity confers confers to to sEVs sEVs dist distinctinct properties, properties, such such as as tropism tropism to to certain certain organs, organs, and and uptake uptake by by specificspecific cell cell types. types. In In the the case case of of tumor-derived tumor-derived sEVs,sEVs, thesethese eventsevents often lead lead to to the the impairment impairment of of immuneimmune responses responses at TMEat TME [22 [22],], also also favoring favoring pre-metastatic pre-metastatic nicheniche formationformation and metastasis metastasis [23,24]. [23,24]. Int. J. Mol. Sci. 2020, 21, 6318 3 of 20 In tumor cells, sEV biogenesis and ultimately sEV composition is a complex and regulated process, which involves many different molecules associated with the