Analysis of Intracellular Trafficking of Dendritic Cell Receptors for Antigen Targeting

Quantitative Measurement of Plasma Membrane Protein Internalisation and Recycling in Heterogenous Cellular Samples by Flow Cytometry

Plasma membrane proteins mediate important aspects of physiology, including nutrient acquisition, cell-cell interactions, and monitoring homeostasis. The trafficking of these proteins, involving internalisation from and/or recycling back to the cell surface, is often critical to their functions. These processes can vary among different proteins and cell types and states and are still being elucidated. Current strategies to measure surface protein internalisation and recycling are typically microscopy or biochemical assays; these are accurate but generally limited to analysing a homogenous cell population and are often low throughput. Here, we present flow cytometry-based methods involving probe-conjugated antibodies that enable quantification of internalisation or recycling rates at the single-cell level in complex samples. To measure internalisation, we detail an assay where the protein of interest is labelled with a specific antibody conjugated to a fluorescent oligonucleotide-labelled probe. To measure recycling, a specific antibody conjugated to a cleavable biotin group is employed. These probes permit the differentiation of molecules that have been internalised or recycled from those that have not. When combined with cell-specific marker panels, these methods allow the quantitative study of plasma membrane protein trafficking dynamics in a heterogenous cell mixture at the single-cell level. Key features • These assays allow sensitive quantification of internalised or recycled surface molecules using oligonucleotide or cleavable biotin-conjugated probes, respectively, and detected by flow cytometry. • They can be adapted to any membrane protein that transits via the cell surface and for which a specific purified antibody is available. • The dynamics of a cell surface protein can be measured in heterogenous cell populations simultaneously, including various cellular activation states. • The internalisation assay builds upon the method developed by Liu et al. [1,2] and extends its application to heterogenous human peripheral blood mononuclear cells. • These assays have been extensively used on suspension cells but have not been tested on adherent cells.

Trial watch: Dendritic cell (DC)-based immunotherapy for cancer

Dendritic cell (DC)-based vaccination for cancer treatment has seen considerable development over recent decades. However, this field is currently in a state of flux toward niche-applications, owing to recent paradigm-shifts in immuno-oncology mobilized by T cell-targeting immunotherapies. DC vaccines are typically generated using autologous (patient-derived) DCs exposed to tumor-associated or -specific antigens (TAAs or TSAs), in the presence of immunostimulatory molecules to induce DC maturation, followed by reinfusion into patients. Accordingly, DC vaccines can induce TAA/TSA-specific CD8+/CD4+ T cell responses. Yet, DC vaccination still shows suboptimal anti-tumor efficacy in the clinic. Extensive efforts are ongoing to improve the immunogenicity and efficacy of DC vaccines, often by employing combinatorial chemo-immunotherapy regimens. In this Trial Watch, we summarize the recent preclinical and clinical developments in this field and discuss the ongoing trends and future perspectives of DC-based immunotherapy for oncological indications.

Ubiquitin-like protein 3 (UBL3) is required for MARCH ubiquitination of major histocompatibility complex class II and CD86

The MARCH E3 ubiquitin (Ub) ligase MARCH1 regulates trafficking of major histocompatibility complex class II (MHC II) and CD86, molecules of critical importance to immunity. Here we show, using a genome-wide CRISPR knockout screen, that ubiquitin-like protein 3 (UBL3) is a necessary component of ubiquitination-mediated trafficking of these molecules in mice and in humans. Ubl3-deficient mice have elevated MHC II and CD86 expression on the surface of professional and atypical antigen presenting cells. UBL3 also regulates MHC II and CD86 in human dendritic cells (DCs) and macrophages. UBL3 impacts ubiquitination of MARCH1 substrates, a mechanism that requires UBL3 plasma membrane anchoring via prenylation. Loss of UBL3 alters adaptive immunity with impaired development of thymic regulatory T cells, loss of conventional type 1 DCs, increased number of trogocytic marginal zone B cells, and defective in vivo MHC II and MHC I antigen presentation. In summary, we identify UBL3 as a conserved, critical factor in MARCH1-mediated ubiquitination with important roles in immune responses.

A specific hybridisation internalisation probe (SHIP) enables precise live-cell and super-resolution imaging of internalized cargo

Facilitated by the advancements in microscopy, our understanding of the complexity of intracellular vesicle traffic has dramatically increased in recent years. However, distinguishing between plasma membrane-bound or internalised ligands remains a major challenge for the studies of cargo sorting to endosomal compartments, especially in small and round cells such as lymphocytes. The specific hybridization internalisation probe (SHIP) assay, developed for flow cytometry studies, employs a ssDNA fluorescence internalisation probe and a complementary ssDNA quenching probe to unambiguously detect the internalized receptors/cargo. Here, we adopted the SHIP assay to study the trafficking of receptor/ligand complexes using B lymphocytes and B cell receptor-mediated antigen internalization as a model system. Our study demonstrates the potential of the SHIP assay for improving the imaging of internalized receptor/ligand complexes and establishes the compatibility of this assay with multiple imaging modalities, including live-cell imaging and super-resolution microscopy.

In Vivo T Cell-Targeting Nanoparticle Drug Delivery Systems: Considerations for Rational Design

T cells play an important role in immunity and repair and are implicated in diseases, including blood cancers, viral infections, and inflammation, making them attractive targets for the treatment and prevention of diseases. Over recent years, the advent of nanomedicine has shown an increase in studies that use nanoparticles as carriers to deliver therapeutic cargo to T cells for ex vivo and in vivo applications. Nanoparticle-based delivery has several advantages, including the ability to load and protect a variety of drugs, control drug release, improve drug pharmacokinetics and biodistribution, and site- or cell-specific targeting. However, the delivery of nanoparticles to T cells remains a major technological challenge, which is primarily due to the nonphagocytic nature of T cells. In this review, we discuss the physiological barriers to effective T cell targeting and describe the different approaches used to deliver cargo-loaded nanoparticles to T cells for the treatment of disease such as T cell lymphoma and human immunodeficiency virus (HIV). In particular, engineering strategies that aim to improve nanoparticle internalization by T cells, including ligand-based targeting, will be highlighted. These nanoparticle engineering approaches are expected to inspire the development of effective nanomaterials that can target or manipulate the function of T cells for the treatment of T cell-related diseases.

Ubiquitination of MHC Class II Is Required for Development of Regulatory but Not Conventional CD4+ T Cells

MHC class II (MHC II) displays peptides at the cell surface, a process critical for CD4⁺ T cell development and priming. Ubiquitination is a mechanism that dictates surface MHC II with the attachment of a polyubiquitin chain to peptide-loaded MHC II, promoting its traffic away from the plasma membrane. In this study, we have examined how MHC II ubiquitination impacts the composition and function of both conventional CD4⁺ T cell and regulatory T cell (T_reg) compartments. Responses were examined in two models of altered MHC II ubiquitination: MHCIIKR^{KI /KI} mice that express a mutant MHC II unable to be ubiquitinated or mice that lack membrane-associated RING-CH 8 (MARCH8), the E3 ubiquitin ligase responsible for MHC II ubiquitination specifically in thymic epithelial cells. Conventional CD4⁺ T cell populations in thymus, blood, and spleen of MHCIIKR^KI/KI and March8 ^-/- mice were largely unaltered. In MLRs, March8 ^-/-, but not MHCIIKR^KI/KI, CD4⁺ T cells had reduced reactivity to both self- and allogeneic MHC II. Thymic T_reg were significantly reduced in MHCIIKR^KI/KI mice, but not March8 ^-/- mice, whereas splenic T_reg were unaffected. Neither scenario provoked autoimmunity, with no evidence of immunohistopathology and normal levels of autoantibody. In summary, MHC II ubiquitination in specific APC types does not have a major impact on the conventional CD4⁺ T cell compartment but is important for T_reg development.

MARCH1-mediated ubiquitination of MHC II impacts the MHC I antigen presentation pathway

Major histocompatibility complex class II (MHC II) expression and turn-over are regulated via its ubiquitination by the membrane associated RING-CH 1 (MARCH1) E3 ligase. Unexpectedly, we show that MHC II ubiquitination also impacts MHC I. Lack of MARCH1 in B cells and dendritic cells (DCs) resulted in a significant reduction in surface MHC I expression. This decrease was not directly caused by changes in MARCH1 ubiquitination of MHC I but indirectly by altered MHC II trafficking in the absence of its ubiquitination. Deletion of MHC II in March1^-/- cells restored normal MHC I surface expression and replacement of wild type MHC II by a variant that could not be ubiquitinated caused a reduction in MHC I expression. Furthermore, these cells displayed inefficient presentation of peptide and protein antigen via MHC I to CD8⁺ T cells. In summary, we describe an unexpected intersection between MHC I and MHC II such that the surface expression of both molecules are indirectly and directly regulated by MARCH1 ubiquitination, respectively.

Antibody-mediated targeting of antigen to C-type lectin-like receptors Clec9A and Clec12A elicits different vaccination outcomes

Targeting antigen (Ag) to dendritic cell (DC) surface receptors is a potential new mode of vaccination. C-type lectin-like receptors Clec9A and Clec12A are attractive receptor targets however their targeting in vivo elicits significantly different outcomes for unknown reasons. To gain insight into the mechanisms responsible, we have examined the intrinsic capacity of Clec9A and Clec12A to elicit MHC I and MHC II Ag presentation following ex vivo targeting with primary murine DC. Both receptors exhibited high rates of internalization by CD8⁺ DCs, while Clec12A delivered a significantly higher Ag owing to its higher expression level. Targeting Ag to immature CD8⁺ DCs via both Clec9A and Clec12A failed to elicit MHC I cross-presentation above that of controls, while Clec12A was the superior receptor to target following CD8⁺ DC maturation. CD8^- DCs were unable to elicit MHC I cross-presentation regardless of the receptor targeted. For MHC II presentation, targeting Ag to Clec12A enabled significant responses by both immature CD8⁺ and CD8^- DCs, whereas Clec9A did not elicit significant MHC II Ag presentation by either DC subset, resting or mature. Therefore, Clec9A and Clec12A exhibit different intrinsic capacities to elicit MHC I and MHC II presentation following direct Ag targeting, though they can only elicit MHC I responses if the DC expressing the receptor is equipped with the capacity to cross-present. Our conclusions have consequences for the exploitation of these receptors for vaccination purposes, in addition to providing insight into their roles as Ag targets in vivo.