Concise Review: Developing Best-Practice Models for the Therapeutic Use of Extracellular Vesicles

Growing interest in extracellular vesicles (EVs, including exosomes and microvesicles) as therapeutic entities, particularly in stem cell-related approaches, has underlined the need for standardization and coordination of development efforts. Members of the International Society for Extracellular Vesicles and the Society for Clinical Research and Translation of Extracellular Vesicles Singapore convened a Workshop on this topic to discuss the opportunities and challenges associated with development of EV-based therapeutics at the preclinical and clinical levels. This review outlines topic-specific action items that, if addressed, will enhance the development of best-practice models for EV therapies. Stem Cells Translational Medicine 2017;6:1730-1739.

Applying extracellular vesicles based therapeutics in clinical trials - an ISEV position paper

Extracellular vesicles (EVs), such as exosomes and microvesicles, are released by different cell types and participate in physiological and pathophysiological processes. EVs mediate intercellular communication as cell-derived extracellular signalling organelles that transmit specific information from their cell of origin to their target cells. As a result of these properties, EVs of defined cell types may serve as novel tools for various therapeutic approaches, including (a) anti-tumour therapy, (b) pathogen vaccination, (c) immune-modulatory and regenerative therapies and (d) drug delivery. The translation of EVs into clinical therapies requires the categorization of EV-based therapeutics in compliance with existing regulatory frameworks. As the classification defines subsequent requirements for manufacturing, quality control and clinical investigation, it is of major importance to define whether EVs are considered the active drug components or primarily serve as drug delivery vehicles. For an effective and particularly safe translation of EV-based therapies into clinical practice, a high level of cooperation between researchers, clinicians and competent authorities is essential. In this position statement, basic and clinical scientists, as members of the International Society for Extracellular Vesicles (ISEV) and of the European Cooperation in Science and Technology (COST) program of the European Union, namely European Network on Microvesicles and Exosomes in Health and Disease (ME-HaD), summarize recent developments and the current knowledge of EV-based therapies. Aspects of safety and regulatory requirements that must be considered for pharmaceutical manufacturing and clinical application are highlighted. Production and quality control processes are discussed. Strategies to promote the therapeutic application of EVs in future clinical studies are addressed.

Challenges with advanced therapy medicinal products and how to meet them

Advanced therapy medicinal products (ATMPs), which include gene therapy medicinal products, somatic cell therapy medicinal products and tissue-engineered products, are at the cutting edge of innovation and offer a major hope for various diseases for which there are limited or no therapeutic options. They have therefore been subject to considerable interest and debate. Following the European regulation on ATMPs, a consolidated regulatory framework for these innovative medicines has recently been established. Central to this framework is the Committee for Advanced Therapies (CAT) at the European Medicines Agency (EMA), comprising a multidisciplinary scientific expert committee, representing all EU member states and European Free Trade Association countries, as well as patient and medical associations. In this article, the CAT discusses some of the typical issues raised by developers of ATMPs, and highlights the opportunities for such companies and research groups to approach the EMA and the CAT as a regulatory advisor during development.

Dual role of SLP-76 in mediating T cell receptor-induced activation of phospholipase C-gamma1

Phospholipase C-gamma1 (PLC-gamma1) activation depends on a heterotrimeric complex of adaptor proteins composed of LAT, Gads, and SLP-76. Upon T cell receptor stimulation, a portion of PLC-gamma1 is recruited to a detergent-resistant membrane fraction known as the glycosphingolipid-enriched membrane microdomains (GEMs), or lipid rafts, to which LAT is constitutively localized. In addition to LAT, PLC-gamma1 GEM recruitment depended on SLP-76, and, in particular, required the Gads-binding domain of SLP-76. The N-terminal tyrosine phosphorylation sites and P-I region of SLP-76 were not required for PLC-gamma1 GEM recruitment, but were required for PLC-gamma1 phosphorylation at Tyr(783). Thus, GEM recruitment can be insufficient for full activation of PLC-gamma1 in the absence of a second SLP-76-mediated event. Indeed, a GEM-targeted derivative of PLC-gamma1 depended on SLP-76 for T cell receptor-induced phosphorylation at Tyr783 and subsequent NFAT activation. On a biochemical level, SLP-76 inducibly associated with both Vav and catalytically active ITK, which efficiently phosphorylated a PLC-gamma1 fragment at Tyr783 in vitro. Both associations were disrupted upon mutation of the N-terminal tyrosine phosphorylation sites of SLP-76. The P-I region deletion disrupted Vav association and reduced SLP-76-associated kinase activity. A smaller deletion within the P-I region, which does not impair PLC-gamma1 activation, did not impair the association with Vav, but reduced SLP-76-associated kinase activity. These results provide new insight into the multiple roles of SLP-76 and the functional importance of its interactions with other signaling proteins.

A New Tyrosine Phosphorylation Site in PLCγ1: The Role of Tyrosine 775 in Immune Receptor Signaling

Phospholipase Cgamma (PLCgamma) is a ubiquitous gatekeeper of calcium mobilization and diacylglycerol-mediated events induced by the activation of Ag and growth factor receptors. The activity of PLCgamma is regulated through its controlled membrane translocation and tyrosine (Y) phosphorylation. Four activation-induced tyrosine phosphorylation sites have been previously described (Y472, Y771, Y783, and Y1254), but their specific roles in Ag receptor-induced PLCgamma1 activation are not fully elucidated. Unexpectedly, we found that the phosphorylation of a PLCgamma1 construct with all four sites mutated to phenylalanine was comparable with that observed with wild-type PLCgamma1, suggesting the existence of an unidentified site(s). Sequence alignment with known phosphorylation sites in PLCgamma2 indicated homology of PLCgamma1 tyrosine residue 775 (Y775) with PLCgamma2 Y753, a characterized phosphorylation site. Tyrosine 775 was characterized as a phosphorylation site using phospho-specific anti-Y775 antiserum, and by mutational analysis. Phosphorylation of Y775 did not depend on the other tyrosines, and point mutation of PLCgamma1 Y775, or the previously described Y783, substantially reduced AgR-induced calcium, NF-AT, and AP-1 activation. Mutation of Y472, Y771, and Y1254 had no effect on overall PLCgamma1 phosphorylation or activation. Although the concomitant mutation of Y775 and Y783 abolished downstream PLCgamma1 signaling, these two tyrosines were sufficient to reconstitute the wild-type response in the absence of functional Y472, Y771, and Y1254. These data establish Y775 as a critical phosphorylation site for PLCgamma1 activation and confirm the functional importance of Y783.

Surface expression of Fc epsilon RI on Langerhans' cells of clinically uninvolved skin is associated with disease activity in atopic dermatitis, allergic asthma, and rhinitis

BACKGROUND

Fc epsilon RI expressed on the surface of human epidermal Langerhans' cells facilitates uptake of IgE-associated allergens and plays a pivotal role in the pathogenesis of atopic dermatitis. Seminal results from studies investigating Langerhans' cell Fc epsilon RI in skin biopsy sections or epidermal cell suspensions demonstrate the highest receptor expression in lesional skin of patients with active atopic dermatitis.

OBJECTIVE

We sought to investigate and localize Fc epsilon RI expression on Langerhans' cells within a minimally disturbed tissue environment in clinically uninvolved skin and to compare receptor expression between healthy donors and patients with atopic dermatitis or other allergic diseases.

METHODS

Intact epidermal sheets from skin suction blisters, immunofluorescently stained with Langerhans' cell markers and anti-Fc epsilon RI alpha (mAbs 15E5 and 22E7) or anti-IgE, were examined by means of confocal microscopy. Samples incubated with anti-Fc epsilon RI alpha before or after cell fixation-permeabilization were compared to discriminate between cytoplasmic and membrane localization.

RESULTS

Cytoplasmic Fc epsilon RI alpha chain was found in Langerhans' cells from all donors, irrespective of atopic status. Surface Fc epsilon RI-bound IgE was detected in the skin of individuals with active atopic dermatitis and in the skin of those with active asthma or rhinitis. No surface Fc epsilon RI was expressed in the skin of patients with a clinical history of atopic dermatitis, asthma, or rhinitis whose disease was in remission or in the skin of nonatopic individuals.

CONCLUSION

In clinically uninvolved skin, Langerhans' cell-surface Fc epsilon RI expression is not only linked to atopic dermatitis but is also generally associated with allergic disease. This supports the concept of a systemic regulatory mechanism associated with active allergic disease, which is further aggravated by local inflammation in atopic skin lesions.

Regulation of Fc epsilonRI expression on human monocytic cells by ligand and IL-4

BACKGROUND

The Fc epsilonRI subunit composition and kinetic of expression differ between antigen-presenting cells and mast cells. Up to now, there has been no human in vitro model available that mimics the characteristics on monocytes.

OBJECTIVE

The characterization of a natural human monocytic cell line (THP1), which expresses Fc epsilonRI, and the comparison to primary human monocytes and other monocytic cell lines, which only express Fc epsilonRI after transfection with the human Fc epsilonRI alpha-chain gene.

METHODS

Surface receptor expression was characterized by flow cytometry, the human Fc epsilonRI alpha-chain gene was introduced by electroporation, and induction of Fc epsilonRI alpha-chain message was detected by semiquantitative RT PCR.

RESULTS

Here we show that the parental human cell line THP1, but none of the other cell lines tested, displays surface Fc epsilonRI in response to IL-4 or incubation with receptor ligand (IgE, antibody). Transfection of Fc epsilonRI alpha-chain resulted in receptor expression on all cell lines, all of which increased surface Fc epsilonRI in the presence of IgE. Only the THP1-alpha transfectant, however, further increased receptor levels in response to IL-4, resulting from mRNA induction for the Fc epsilonRI-alpha, but not the beta- or gamma-subunit.

CONCLUSION

Based on THP1, U937 and HL60 and their alpha-chain transfectants we present a model system for the study of Fc epsilonRI regulation and signalling on human cells. THP1 in particular, due to its responsiveness to both ligand and IL-4, even without prior manipulation, is ideally suited to address questions on Fc epsilonRI modulation in an 'allergic environment'.

Molecular consequences of human mast cell activation following immunoglobulin E-high-affinity immunoglobulin E receptor (IgE-FcepsilonRI) interaction

The cross-linking by immunoglobulin E of its high-affinity receptor, FcepsilonRI, on mast cells initiates a complex series of biochemical events leading to degranulation and the synthesis and secretion of eicosanoids and cytokines through the action of transcription factors, such as nuclear factor-kappaB. The initial activation involves the phosphorylation of FcepsilonRI beta- and gamma-subunits through the actions of the tyrosine kinases lyn and syk. For the purposes of description, the subsequent events may be grouped in three cascades characterized by the key proteins involved. First, the phospholipase C-inositol phosphate cascade activates protein kinase C and is largely responsible for calcium mobilization and influx. Second, activation of Ras and Raf via mitogen-activated protein kinase causes the production of arachidonic acid metabolites. Third, the generation of sphingosine and sphingosine-1-phosphate occurs through activation of sphingomyelinase. While the early signaling events tend to be specific for the cited cascades, there is an increasing overlap of activated proteins with the downstream propagation of the signal. It is the balanced interaction between these proteins that culminates in degranulation, synthesis, and release of eicosanoids and cytokines.

Characterization of Fc epsilonRI expressing human monocytic cell lines. 1. The role of CD45 on signal transduction in primary monocytes and cell lines

BACKGROUND

Recently, the high affinity receptor for IgE (Fc epsilonRI), which plays a major role in allergies, has been identified on a number of different antigen-presenting cell types, including human monocytes from atopic and nonatopic donors. In this report human monocytic cell lines were used to test for the expression of Fc epsilonRI, reasoning that a monocytic cell line expressing Fc epsilonRI constitutively would be a useful tool for large scale studies on the regulation of IgE binding and signal transduction.

METHODS

Reverse transduction polymerase chain reaction was applied to identify Fc epsilonRI alpha-chain message, flow cytometry to detect Fc epsilonRI surface expression and signal transduction on the cell lines generated by transfection.

RESULTS

We report the establishment of monocytic cell lines constitutively expressing Fc epsilonRI (THP1-alpha01 to THP1-alpha40) generated by transfection of the cell line THP1 with a plasmid encoding the Fc epsilonRI alpha-chain only. Fc epsilonRI on the THP1-alpha lines specifically binds IgE and is functional with regard to ligand binding and signal transduction. Comparative studies between the transfectants and primary human monocytes from nonatopic donors demonstrated the regulatory role of the tyrosine phosphatase CD45 on Fc epsilonRI-mediated cell activation.

CONCLUSIONS

Monocytic cell lines carry Fc epsilonRI alpha-chain RNA and enhancement by transfection results in surface Fc epsilonRI expression on THP1. Triggering the receptor on the THP1-alpha lines or on human monocytes, which express native Fc epsilonRI, elicits a rapid and transient calcium mobilization, prevented by co-cross-linking of Fc epsilonRI and CD45.

Function and regulation of Fc epsilon RI expression on monocytes from non-atopic donors

BACKGROUND

The high affinity receptor for IgE (Fc epsilon RI) has recently been identified on antigen presenting cells, i.e. Langerhans cells and monocytes from atopic donors and it was hypothesized that Fc epsilon RI expression levels correlated with allergy.

OBJECTIVE

The aims of the study was to investigate the function and expression of Fc epsilon RI on monocytes from non-atopic donors.

METHODS

Purified monocytes or peripheral blood mononuclear cells were used to study Fc epsilon RI expression and signal transduction on CD14 positive cells by flow cytometry and/or confocal laser microscopy.

RESULTS

Freshly isolated monocytes from healthy individuals (n = 58) were shown to express Fc epsilon RI (median 18%, range 2-66%). No IgE was bound to these receptors in vivo, and in vitro no significant binding of complete IgE molecules could be obtained. IgE positive monocytes from atopic donors were also found to have free Fc epsilon RI incapable of binding IgE in vitro. On all CD14 positive cells free Fc epsilon RI expression was rapidly and completely lost during culture in conventional culture media (IMDM, RPMI) but not in phosphate buffered saline (PBS). Moreover, signal transduction through free Fc epsilon RI appeared to be inhibited. However, both IgE binding and calcium mobilization were restored by treatment of fresh non-atopic monocytes with neuraminidase. Importantly, culturing these monocytes overnight in conventional medium containing 2 micrograms/mL IgE induced a cycloheximide insensitive accumulation of IgE bound to Fc epsilon RI and, in addition, led to cell activation.

CONCLUSION

Monocytes from both atopic donors and healthy individuals express Fc epsilon RI, but the previously reported different expression levels between the two groups seem to be directly related to the absence or presence of IgE in the serum. This may be due to the fact that Fc epsilon RI is subjected to a constant turnover process which is slowed down but not prevented by ligand binding. In addition, free Fc epsilon RI on non-atopic monocytes are under control of a neuramindase sensitive structure(s), which influences signal transduction and IgE binding.

The 'monocytic' cell line I937 displays typical B cell characteristics

The monocytic cell line I937 was derived from U937 by sorting for cells with high expression of MHC class II molecules. Further analysis of these class II molecules revealed the presence of the HLA-DR3 subtype suggesting that the cell line was a potential candidate for testing antigen presentation to T cells restricted by HLA-DR3. We found that the T cell clones CFTS4:2.80 and CFTS4:2.6 with the required restriction element responded to the house dust mite antigen DPT presented by I937 but not U937, whereas CFTS4:3.1, which is not HLA-DR restricted, did not respond to either cell line. Subsequent analysis of surface markers on I937, however, indicated that the cell line is of B cell origin. In contrast to the parental cell line U937, I937 was tested negative for CD4, CD31 and CD64 but expressed CD19, CD21 and CD40. Although neither surface nor cytoplasmic Ig molecules were detected in either I937 or U937, Southern blot analysis revealed IgH gene rearrangement in I937. In addition, a fragment specific for Epstein-Barr virus nuclear antigen (EBNA2) was amplified in I937 by PCR technique. Therefore, we conclude that I937 is an EBV-transformed B cell line, presumably derived from the same donor and not as reported originally as a subline of U937, which expresses high MHC class II levels.

Modulation of Fc gamma receptor-mediated early events by the tyrosine phosphatase CD45 in primary human monocytes. Consequences for interleukin-6 production

Stimulation of primary human monocytes from several donors by cross-linking of Fc gamma receptor type I (Fc gamma RI) and Fc gamma RII gave rise to calcium mobilization and protein tyrosine phosphorylation. These early events were not observed without cross-linking. CD45, a transmembrane tyrosine phosphatase, when co-cross-linked with either Fc gamma RI or Fc gamma RII, could prevent Fc gamma RI and Fc gamma RII-mediated calcium mobilization and protein tyrosine phosphorylation. When interleukin (IL)-6 production was measured, we noted a strong IL-6 production after activation of primary human monocytes by cross-linking of Fc gamma RI or Fc gamma RII. In contrast to calcium mobilization and tyrosine phosphorylation of proteins, IL-6 production was not affected by co-cross-linking of CD45 with either Fc gamma RI or Fc gamma RII. Interestingly, cross-linking of the CD45 itself was sufficient to induce IL-6 production. Our results show that the CD45 molecule is important in modulating early events following stimulation of primary human monocytes by cross-linking of Fc gamma RI or Fc gamma RII. However, triggering of CD45 alone can also induce IL-6 production, indicating that CD45 ligation itself can give signals and may have an important role in cytokine induction pathways.