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Background Many lines of evidence suggest that accumulation of aggregated alpha-synuclein (αSYN) in the Parkinson’s disease (PD) brain causes infiltration of T cells. However, in which ways the stationary brain cells interact with the T cells remain elusive. Here, we identify astrocytes as potential antigen-presenting cells capable of activating T cells in the PD brain. Astrocytes are a major component of the nervous system, and accumulating data indicate that astrocytes can play a central role during PD progression. Methods To investigate the role of astrocytes in antigen presentation and T-cell activation in the PD brain, we analyzed post mortem brain tissue from PD patients and controls. Moreover, we studied the capacity of cultured human astrocytes and adult human microglia to act as professional antigen-presenting cells following exposure to preformed αSYN fibrils. Results Our analysis of post mortem brain tissue demonstrated that PD patients express high levels of MHC-II, which correlated with the load of pathological, phosphorylated αSYN. Interestingly, a very high proportion of the MHC-II co-localized with astrocytic markers. Importantly, we found both perivascular and infiltrated CD4 + T cells to be surrounded by MHC-II expressing astrocytes, confirming an astrocyte T cell cross-talk in the PD brain. Moreover, we showed that αSYN accumulation in cultured human astrocytes triggered surface expression of co-stimulatory molecules critical for T-cell activation, while cultured human microglia displayed very poor antigen presentation capacity. Notably, intercellular transfer of αSYN/MHC-II deposits occurred between astrocytes via tunneling nanotubes, indicating spreading of inflammation in addition to toxic protein aggregates. Conclusions In conclusion, our data from histology and cell culture studies suggest an important role for astrocytes in antigen presentation and T-cell activation in the PD brain, highlighting astrocytes as a promising therapeutic target in the context of chronic inflammation.

Key Points MHC class II molecules bind antigenic peptides that are generated in endosomal–lysosomal antigen-processing compartments. These peptides are derived from proteins that access these compartments using various endocytic pathways and also as a result of autophagy. Proteolysis in antigen-processing compartments is regulated in antigen-presenting cells (APCs) to favour the formation of antigenic peptides that can bind to MHC class II and to avoid the complete hydrolysis of proteins to very short peptides or to amino acids. Nonspecific endocytosis processes are terminated following dendritic cell (DC) activation, but mature DCs can still internalize antigen by receptor-mediated endocytosis or phagocytosis. Using these pathways, mature DCs can generate peptide–MHC class II complexes and activate naive CD4 + T cells. The formation of antigen-processing compartments is regulated during APC activation. B cell activation results in MHC class II recruitment to endosomes and lysosomes to form these compartments, whereas in DCs, lysosomal proteases relocalize to antigen-processing compartments and enhance antigen proteolysis. APC activation leads to efficient generation of peptide–MHC class II complexes and markedly increases the expression of these complexes on the APC plasma membrane. Increased surface expression of peptide–MHC class II complexes on activated APCs is a result of enhanced MHC class II transcription and/or translation, movement of intracellular peptide–MHC class II complexes to the APC plasma membrane and reduced lysosomal MHC class II degradation. Expression of the E3 ubiquitin ligase MARCH1 by immature APCs promotes rapid turnover of peptide–MHC class II complexes. DC activation terminates MARCH1 expression and ubiquitylation of peptide–MHC class II complexes, thus increasing the half-life of peptide–MHC class II complexes. To play their part in the generation of effective adaptive immune responses, different types of antigen-presenting cell (APC) take up and process antigen in different ways. The length of time that peptide–MHC class II complexes are present on APC surfaces can also vary depending on the cell type. This Review describes the different modes and mechanisms that regulate MHC class II processing and presentation. Antigenic peptide-loaded MHC class II molecules (peptide–MHC class II) are constitutively expressed on the surface of professional antigen-presenting cells (APCs), including dendritic cells, B cells, macrophages and thymic epithelial cells, and are presented to antigen-specific CD4 + T cells. The mechanisms of antigen uptake, the nature of the antigen processing compartments and the lifetime of cell surface peptide–MHC class II complexes can vary depending on the type of APC. It is likely that these differences are important for the function of each distinct APC subset in the generation of effective adaptive immune responses. In this Review, we describe our current knowledge of the mechanisms of uptake and processing of antigens, the intracellular formation of peptide–MHC class II complexes, the intracellular trafficking of peptide–MHC class II complexes to the APC plasma membrane and their ultimate degradation.

The fetus is thought to be protected from exposure to foreign antigens, yet CD45RO + T cells reside in the fetal intestine. Here we combined functional assays with mass cytometry, single-cell RNA sequencing and high-throughput T cell antigen receptor (TCR) sequencing to characterize the CD4 + T cell compartment in the human fetal intestine. We identified 22 CD4 + T cell clusters, including naive-like, regulatory-like and memory-like subpopulations, which were confirmed and further characterized at the transcriptional level. Memory-like CD4 + T cells had high expression of Ki-67, indicative of cell division, and CD5, a surrogate marker of TCR avidity, and produced the cytokines IFN-γ and IL-2. Pathway analysis revealed a differentiation trajectory associated with cellular activation and proinflammatory effector functions, and TCR repertoire analysis indicated clonal expansions, distinct repertoire characteristics and interconnections between subpopulations of memory-like CD4 + T cells. Imaging mass cytometry indicated that memory-like CD4 + T cells colocalized with antigen-presenting cells. Collectively, these results provide evidence for the generation of memory-like CD4 + T cells in the human fetal intestine that is consistent with exposure to foreign antigens. Koning and colleagues used mass cytometry, single-cell RNA-seq and high-throughput TCR sequencing to characterize the CD4 + T cell compartment in the human fetal intestine.

Purpose Sipuleucel-T, the first FDA-approved autologous cellular immunotherapy for treatment of advanced prostate cancer, is manufactured by activating peripheral blood mononuclear cells, including antigen presenting cells (APCs), with a fusion protein containing prostatic acid phosphatase. Analysis of data from three phase 3 trials was performed to immunologically characterize this therapy during the course of the three doses, and to relate the immunological responses to overall survival (OS). Methods Sipuleucel-T product characteristics [APC numbers, APC activation (CD54 upregulation), and total nucleated cell (TNC) numbers] were assessed in three randomized, controlled phase 3 studies ( N  = 737). Antigen-specific cellular and humoral responses were assessed in a subset of subjects. The relationships between these parameters and OS were assessed. Results APC activation occurred in the first dose preparation [6.2-fold, (4.65, 7.70); median (25th, 75th percentile)] and increased in the second [10.6-fold (7.83, 13.65)] and third [10.5-fold (7.89, 13.65)] dose preparations. Cytokines and chemokines associated with activated APCs were produced during the manufacture of each dose; T-cell activation-associated cytokines were detected in the second and third dose preparations. Antigen-specific T cells were detectable after administration of the first sipuleucel-T dose. Cumulative APC activation, APC number, and TNC number correlated with OS ( P  < 0.05). Antigen-specific immune responses were observed in 78.8 % of monitored subjects and their presence correlated with OS ( P  = 0.003). Conclusion Sipuleucel-T broadly engages the immune system by activating APCs ex vivo and inducing long-lived immune responses in vivo. These data indicate antigen-specific immune activation as a mechanism by which sipuleucel-T prolongs OS.

Synthetic biology is a powerful tool to create therapeutics which can be rationally designed to enable unique and combinatorial functionalities. Here we utilize non-pathogenic E coli Nissle as a versatile platform for the development of a living biotherapeutic for the treatment of cancer. The engineered bacterial strain, referred to as SYNB1891, targets STING-activation to phagocytic antigen-presenting cells (APCs) in the tumor and activates complementary innate immune pathways. SYNB1891 treatment results in efficacious antitumor immunity with the formation of immunological memory in murine tumor models and robust activation of human APCs. SYNB1891 is designed to meet manufacturability and regulatory requirements with built in biocontainment features which do not compromise its efficacy. This work provides a roadmap for the development of future therapeutics and demonstrates the transformative potential of synthetic biology for the treatment of human disease when drug development criteria are incorporated into the design process for a living medicine. Synthetic biology can be used to create rationally designed living therapeutics. Here the authors engineer E. coli Nissle to target STING activation in antigen presenting cells for the treatment of solid tumors and demonstrate preclinical activity in murine models.

mRNA vaccines are rapidly emerging as a powerful platform for infectious diseases because they are well tolerated, immunogenic, and scalable and are built on precise but adaptable antigen design. We show that two immunizations of modified non-replicating mRNA encoding influenza H10 hemagglutinin (HA) and encapsulated in lipid nanoparticles (LNP) induce protective HA inhibition titers and H10-specific CD4+ T cell responses after intramuscular or intradermal delivery in rhesus macaques. Administration of LNP/mRNA induced rapid and local infiltration of neutrophils, monocytes, and dendritic cells (DCs) to the site of administration and the draining lymph nodes (LNs). While these cells efficiently internalized LNP, mainly monocytes and DCs translated the mRNA and upregulated key co-stimulatory receptors (CD80 and CD86). This coincided with upregulation of type I IFN-inducible genes, including MX1 and CXCL10. The innate immune activation was transient and resulted in priming of H10-specific CD4+ T cells exclusively in the vaccine-draining LNs. Collectively, this demonstrates that mRNA-based vaccines induce type-I IFN-polarized innate immunity and, when combined with antigen production by antigen-presenting cells, lead to generation of potent vaccine-specific responses. mRNA vaccines have been proven to be suitable and efficient against pandemic pathogens. However, the immune processes after mRNA vaccination that lead to robust responses remain elusive. Now in Molecular Therapy, Liang et al. (2017) define the target cells and immune responses at the vaccination sites and in the lymph nodes that result in vaccine immunity.

Checkpoint inhibitors and T-cell therapies have highlighted the critical role of T cells in anti-cancer immunity. However, limitations associated with these treatments drive the need for alternative approaches. Here, we engineer red blood cells into artificial antigen-presenting cells (aAPCs) presenting a peptide bound to the major histocompatibility complex I, the costimulatory ligand 4-1BBL, and interleukin (IL)-12. This leads to robust, antigen-specific T-cell expansion, memory formation, additional immune activation, tumor control, and antigen spreading in tumor models in vivo. The presence of 4-1BBL and IL-12 induces minimal toxicities due to restriction to the vasculature and spleen. The allogeneic aAPC, RTX-321, comprised of human leukocyte antigen-A*02:01 presenting the human papilloma virus (HPV) peptide HPV16 E7 11-19 , 4-1BBL, and IL-12 on the surface, activates HPV-specific T cells and promotes effector function in vitro. Thus, RTX-321 is a potential ‘off-the-shelf’ in vivo cellular immunotherapy for treating HPV + cancers, including cervical and head/neck cancers. Red blood cells (RBCs) have unique properties that have been exploited for therapeutic uses. Here the authors engineer RBCs to co-express tumor associated antigens on MHC I, 4-1BBL and IL-12, generating artificial antigen presenting cells that can induce antigen-specific T cell responses and antitumor immune responses in preclinical models.

The COVID-19 pandemic has fostered major advances in vaccination technologies 1 – 4 ; however, there are urgent needs for vaccines that induce mucosal immune responses and for single-dose, non-invasive administration 4 – 6 . Here we develop an inhalable, single-dose, dry powder aerosol SARS-CoV-2 vaccine that induces potent systemic and mucosal immune responses. The vaccine encapsulates assembled nanoparticles comprising proteinaceous cholera toxin B subunits displaying the SARS-CoV-2 RBD antigen within microcapsules of optimal aerodynamic size, and this unique nano–micro coupled structure supports efficient alveoli delivery, sustained antigen release and antigen-presenting cell uptake, which are favourable features for the induction of immune responses. Moreover, this vaccine induces strong production of IgG and IgA, as well as a local T cell response, collectively conferring effective protection against SARS-CoV-2 in mice, hamsters and nonhuman primates. Finally, we also demonstrate a mosaic iteration of the vaccine that co-displays ancestral and Omicron antigens, extending the breadth of antibody response against co-circulating strains and transmission of the Omicron variant. These findings support the use of this inhaled vaccine as a promising multivalent platform for fighting COVID-19 and other respiratory infectious diseases. An inhalable, single-dose dry powder aerosol SARS-CoV-2 vaccine shows good storage stability, results in sustained antigen delivery to antigen-presenting cells in the lungs and induces a potent antiviral immune response.

Targeted approaches to treat autoimmune diseases would improve upon current therapies that broadly suppress the immune system and lead to detrimental side effects. Antigen-specific tolerance was induced using poly(lactide-co-glycolide) nanoparticles conjugated with disease-relevant antigen to treat a model of multiple sclerosis. Increasing the nanoparticle dose and amount of conjugated antigen both resulted in more durable immune tolerance. To identify active tolerance mechanisms, we investigated downstream cellular and molecular events following nanoparticle internalization by antigen-presenting cells. The initial cell response to nanoparticles indicated suppression of inflammatory signaling pathways. Direct and functional measurement of surface MHC-restricted antigen showed positive correlation with both increasing particle dose from 1 to 100 μg/mL and increasing peptide conjugation by 2-fold. Co-stimulatory analysis of cells expressing MHC-restricted antigen revealed most significant decreases in positive co-stimulatory molecules (CD86, CD80, and CD40) following high doses of nanoparticles with higher peptide conjugation, whereas expression of a negative co-stimulatory molecule (PD-L1) remained high. T cells isolated from mice immunized against myelin proteolipid protein (PLP139–151) were co-cultured with antigen-presenting cells administered PLP139–151-conjugated nanoparticles, which resulted in reduced T cell proliferation, increased T cell apoptosis, and a stronger anti-inflammatory response. These findings indicate several potential mechanisms used by peptide-conjugated nanoparticles to induce antigen-specific tolerance. Peptide-conjugated nanoparticles induce antigen-specific tolerance in models of autoimmunity. Kuo et al. investigated cellular and molecular mechanisms of antigen-presenting cells following nanoparticle internalization. Increasing peptide conjugation and delivering higher nanoparticle doses both contributed to enhanced antigen presentation, as well as reductions in co-stimulatory expression and effector T cell responses.

Mounting evidence suggests that the tumor microenvironment is profoundly immunosuppressive. Thus, mitigating tumor immunosuppression is crucial for inducing sustained antitumor immunity. Whereas previous studies involved intratumoral injection, we report here an inhalable nanoparticle-immunotherapy system targeting pulmonary antigen presenting cells (APCs) to enhance anticancer immunity against lung metastases. Inhalation of phosphatidylserine coated liposome loaded with STING agonist cyclic guanosine monophosphate–adenosine monophosphate (NP-cGAMP) in mouse models of lung metastases enables rapid distribution of NP-cGAMP to both lungs and subsequent uptake by APCs without causing immunopathology. NP-cGAMP designed for enhanced cytosolic release of cGAMP stimulates STING signaling and type I interferons production in APCs, resulting in the pro-inflammatory tumor microenvironment in multifocal lung metastases. Furthermore, fractionated radiation delivered to one tumor-bearing lung synergizes with inhaled NP-cGAMP, eliciting systemic anticancer immunity, controlling metastases in both lungs, and conferring long-term survival in mice with lung metastases and with repeated tumor challenge. Successful anticancer immunotherapy should induce robust systemic immunity against metastases. Here, the authors engineer an inhalable nano-STING agonist, which synergizes with fractionated radiation to control lung metastases and confers long-term systemic antitumor immunity in mice.