Cross-dressed CD8α+/CD103+ dendritic cells prime CD8+ T cells following vaccination

Neoantigen enriched biomimetic nanovaccine for personalized cancer immunotherapy

Personalized cancer vaccines elicit robust T cell immunity and anti-tumour potency, but identifying tumour-specific antigens remains challenging, severely constraining the therapeutic window. Biomimetic nanovaccines employing cancer cell membranes display inherent biocompatibility and stimulate T-cell responses against diverse tumour antigens, though tumours develop multiple mechanisms to reduce antigen presentation. Here we demonstrate a rapid and general strategy to fabricate personalized nanovaccines based on Antigen-Enriched tumor Cell Membranes (AECM) for early intervention. Interferon-γ potently stimulates antigen presentation across a broad range of cancer cell types. By coupling the generated AECM with PC7A adjuvant, a stimulator of interferon genes (STING)-activating polymer, the AECM@PC7A nanovaccine induces robust poly-neoepitopic T-cell responses even at low dosage, achieving significant tumour regression and metastasis inhibition in multiple murine cancer models. This anti-tumor response relies on MHC-I restricted antigen presentation and CD8+ T-cell activation, with dendritic cells presenting AECM antigens predominantly via cross-dressing to prime T-cells. AECM@PC7A exhibits remarkable anti-tumor efficacy when compared to vaccines with diverse formulations, and demonstrates therapeutic potential in post-surgical and humanized xenograft tumor models. This proof-of-concept study provides a promising universal avenue for the rapid development of personalized cancer vaccines applicable to early intervention for a broad range of patients.

Enhancing Immune Responses Through Modulation of Innate Cell Microenvironments in Lymph Nodes with Virus-Mimetic Vaccines

Nanovaccines hold significant promise for the prevention and treatment of infectious diseases. However, the efficacy of many nanovaccines is often limited by inadequate stimulation of both innate and adaptive immune responses. Herein, we explore a rational vaccine strategy aimed at modulating innate cell microenvironments within lymph nodes (LNs) to enhance the generation of effective immune responses. Inspired by the structure and natural infection process of viruses, we developed a versatile antigen and adjuvant co-delivery platform, termed virus-mimetic vaccines (VMVs). Specifically, polyarginine-tagged antigens were noncovalently assembled onto nucleic acid nanogels containing cytosine-phosphate-guanine oligodeoxynucleotide via a salt-bridge zipper mechanism, which can activate Toll-like receptor 9. Upon intramuscular immunization, VMVs effectively drained into the LNs, recruiting and activating multiple innate cells, including CD8+ dendritic cells (DCs), CD103+ DCs, macrophages, plasmacytoid DCs, and neutrophils. This activation modulates the innate cell microenvironments and relocates antigen-presenting cells within LNs, optimizing adaptive immune responses. VMVs induced a robust antigen-specific immune response, characterized by high levels of neutralizing antibodies, augmented memory T cell activity, and enhanced development of germinal center B cells. Together, our findings demonstrate that dynamic modulation of innate cell microenvironments by VMVs leads to optimized generation of both humoral and cellular immunity against infectious diseases.

MIP-3α-antigen fusion DNA vaccine enhances sex differences in tuberculosis model and alters dendritic cell activity early post vaccination

Background

Tuberculosis (TB) remains a major cause of global morbidity and mortality. Efforts to control TB are hampered by the lengthy and cumbersome treatment required to eradicate the infection. Bacterial persistence during exposure to bactericidal antibiotics is at least partially mediated by the bacterial stringent response enzyme, RelMtb. A therapeutic DNA vaccine targeting RelMtb has been shown to increase the efficacy of antitubercular drugs, and fusing macrophage-inflammatory protein 3α (MIP-3α), which interacts with CCR6 on immature dendritic cells (iDCs), to RelMtb further increases the vaccine's therapeutic efficacy. A secondary analysis of these prior studies elucidated prominent sex-based differences in vaccine therapeutic efficacy, with female mice showing improved microbial outcomes compared to males as a result of the Rel and MIP-3α-Rel vaccine constructs, with a greater sex-associated difference in the MIP-3α-Rel group. In the current study, we addressed the hypothesis that these sex-related differences are due to differential DC activation/function soon after vaccination.

Methods

A EαGFP reporter vaccine model was used to track vaccine antigen presentation by an antibody Y-Ae which binds the Eα peptide tag in complex with I-Ab MHC-II molecules.

Results

MIP-3α-EαGFP groups had more DCs presenting vaccine antigen infiltrating from the periphery, with more abundant Langerhans cells in males and greater CD8 + CD103 + cross-presenting dermal DCs in females. This model also shows there is greater DC activation, as measured by CD80 and MHC II MFI, by MIP-3α compared to EαGFP alone, especially in female mice.

Conclusions

Our findings are consistent with the sex- and MIP-3α-related differences seen in the therapeutic model and supports the hypothesis that in both sexes MIP-3α enhances vaccine uptake and cell activation by peripheral iDCs. Additionally, Female mice showed greater levels of antigen presentation, especially in DCs able to cross-present antigen, explaining why they had the best outcomes. Further studies are required to understand underlying mechanisms and to link APC results directly to T-cell responses.

Targeting Siglec-E facilitates tumor vaccine-induced antitumor immunity in renal carcinoma

BACKGROUND

Siglec-E is an immune checkpoint inhibitory molecule. Expression of Siglec-E on the immune cells has been shown to promote tumor regression. This study aimed to develop an adenovirus (Ad) vaccine targeting Siglec-E and carbonic anhydrase IX (CAIX) (Ad-Siglec-E/CAIX) and to evaluate its potential antitumor effects in several preclinical renal cancer models.

METHODS

Ad vaccines encoding Siglec-E or CAIX were developed and evaluated for their therapeutic potential in mouse subcutaneous, lung metastatic, and orthotopic tumor models. The expression of Ad-Siglec-E/CAIX was confirmed via PCR and flow cytometry. Immune responses induced by Ad-Siglec-E/CAIX were assessed in vitro and in vivo using flow cytometry, immunohistochemistry, ELISA, histological analysis, cell proliferation, enzyme-linked immunosorbent spot, cytotoxic T lymphocytes (CTL) killing, and cell depletion assays.

RESULTS

Ad-Siglec-E/CAIX vaccine induced the increase of tumor-infiltrated immune cells, and significantly suppressed the subcutaneous tumor growth of renal carcinoma. Immunization with Ad-Siglec-E/CAIX promoted the induction and maturation of CD11c+ dendritic cells and their subsets, which in turn enhanced tumor-specific CD8+ T cell immune responses, as evidenced by increased CD8+ T cell proliferation and CTL activity. Importantly, the deletion of CD8+ T cells in vivo abolished the antitumor effect of the Ad-Siglec-E/CAIX vaccine, highlighting the essential role of functional CD8+ T cell responses. The potent therapeutic efficacy of the Ad-Siglec-E/CAIX vaccine was also observed in lung metastasis and orthotopic tumor models through tumor-specific CD8+ T cell immune responses.

CONCLUSIONS

Our results indicate that targeting Siglec-E enhances the therapeutic efficacy of Ad-CAIX against renal carcinoma, providing a promising therapeutic option for solid tumors.

MHC class I-dressing is mediated via phosphatidylserine recognition and is enhanced by polyI:C

In addition to cross-presentation, cross-dressing plays an important role in the induction of CD8+ T cell immunity. In the process of cross-dressing, conventional dendritic cells (DCs) acquire major histocompatibility complex class I (MHCI) from other cells and subsequently prime CD8+ T cells via the pre-formed antigen-MHCI complexes without antigen processing. However, the mechanisms underlying the cross-dressing pathway, as well as the relative contributions of cross-presentation and cross-dressing to CD8+ T cell priming are not fully understood. Here, we demonstrate that DCs rapidly acquire MHCI-containing membrane fragments from dead cells via the phosphatidylserine recognition-dependent mechanism for cross-dressing. The MHCI dressing is enhanced by a TLR3 ligand polyinosinic-polycytidylic acid (polyI:C). Further, polyI:C promotes not only cross-presentation but also cross-dressing in vivo. Taken together, these results suggest that cross-dressing as well as cross-presentation is involved in inflammatory diseases associated with cell death and type I IFN production.

Major Histocompatibility Complex II Expression on Oral Langerhans Cells Differentially Regulates Mucosal CD4 and CD8 T Cells

In murine periodontitis, the T helper (Th)17 response against Porphyromonas gingivalis in cervical lymph node is abrogated by diphtheria toxin-driven depletion of Langerhans cells (LCs). We determined the impact of major histocompatibility complex class II (MHC-II) presentation in LCs on Th17 cells in the oral mucosa of mice. Using an established human-Langerin promoter-Cre mouse model, we generated LC-specific deletion of the H2-Ab1 (MHC-II) gene. MHC-II expression was ablated in 81.2% of oral-resident LCs compared with >99% of skin-resident LCs. MHC-II (LCΔMHC-II) depletion did not reduce the number of CD4 T cells nor the frequency of Th17 cells compared with that in wild-type mice. However, the frequencies of Th1 cells decreased, and Helios+ T-regulatory cells increased. In ligature-induced periodontitis, the numbers of CD4 T cells and Th17 cells were similar in LCΔMHC-II and wild-type mice. Normal numbers of Th17 cells can therefore be sustained by as little as 18.8% of MHC-II-expressing LCs in oral mucosa. Unexpectedly, oral mucosa CD8 T cells increased >25-fold in LCΔMHC-II mice. Hence, these residual MHC-II-expressing LCs appear unable to suppress the local expansion of CD8 T cells while sufficient to sustain a homeostatic CD4 T-cell response. Reducing the expression of MHC-II on specific LC subpopulations may ultimately boost CD8-mediated intraepithelial surveillance at mucosal surfaces.

Intercellular Transfer of Immune Regulatory Molecules Via Trogocytosis

Trogocytosis, an active cellular process involving the transfer of plasma membrane and attached cytosol during cell-to-cell contact, has been observed prominently in CD4 T cells interacting with antigen-presenting cells carrying antigen-loaded major histocompatibility complex (MHC) class II molecules. Despite the inherent absence of MHC class II molecules in CD4 T cells, they actively acquire these molecules from encountered antigen-presenting cells, leading to the formation of antigen-loaded MHC class II molecules-dressed CD4 T cells. Subsequently, these dressed CD4 T cells engage in antigen presentation to other CD4 T cells, revealing a dynamic mechanism of immune communication. The transferred membrane proteins through trogocytosis retain their surface localization, thereby altering cellular functions. Concurrently, the donor cells experience a loss of membrane proteins, resulting in functional changes due to the altered membrane properties. This chapter provides a focused exploration into trogocytosis-mediated transfer of immune regulatory molecules and its consequential impact on diverse immune responses.

The mosquito effect: regulatory and effector T cells acquire cytoplasmic material from tumor cells through intercellular transfer

The phenomenon of intercellular transfer of cellular material, including membranes, cytoplasm, and even organelles, has been observed for decades. The functional impact and molecular mechanisms of such transfer in the immune system remain largely elusive due to the absence of a robust in vivo model. Here, we introduce a new tumor mouse model, where tumor cells express the soluble ultra-bright fluorescent protein ZsGreen, which allows detection and measurement of intercellular transfer of cytoplasm from tumor cells to infiltrating immune cells. We found that in addition to various types of myeloid lineage cells, a large fraction of T regulatory cells and effector CD8 T cells acquire tumor material. Based on the distribution of tumor-derived ZsGreen, the majority of T cells integrate captured cytoplasm into their own, while most myeloid cells store tumor material in granules. Furthermore, scRNA-seq analysis revealed significant alterations in transcriptomes of T cells that acquired tumor cell cytoplasm, suggesting potential impact on T cell function. We identified that the participation of T cells in intercellular transfer requires cell-cell contact and is strictly dependent on the activation status of T lymphocytes. Finally, we propose to name the described phenomenon of intercellular transfer for tumor infiltrating T cells the "mosquito effect".

Do RNA modifications contribute to modulation of immune responses in allergic diseases?

RNA modifications have emerged as a fundamental mechanism of post-transcriptional gene regulation, playing vital roles in cellular physiology and the development of various diseases. While the investigation of RNA modifications has seen significant advancements, the exploration of their implication in allergic diseases has been comparatively overlooked. Allergic reactions, including hay fever, asthma, eczema and food allergies, result from hypersensitive immune responses, affecting a considerable population worldwide. Despite the high prevalence, the molecular mechanisms underlying these responses remain partially understood. The potential role of RNA modifications in modulating the hypersensitive immune responses has yet to be fully elucidated. This mini-review seeks to shed light on potential connections between RNA modifications and allergy, highlighting recent findings and potential future research directions. By expanding our understanding of the complex interplay between RNA modifications and allergic responses, we hope to unlock new avenues for allergy diagnosis, prognosis, and therapeutic intervention.

AXL receptor tyrosine kinase inhibition improves the anti-tumor effects of CD8+ T cells by inducing CD103+ dendritic cell-mediated T cell priming

AXL is a member of TAM receptor family and has been highlighted as a potential target for cancer treatment. Accumulating evidence has uncovered the critical role of the AXL signaling pathway in tumor growth, metastasis, and resistance against anti-cancer drugs, as well as its association with cancer immune escape. However, the function of AXL as a manipulator of the immune system in the tumor microenvironment (TME) remains unclear. Therefore, in this study, we investigated the impact of AXL on immune cells in the TME of a syngeneic tumor model using AXL knockout (AXL-/-) mice. Compared to AXL wild-type (AXL+/+) mice, tumor growth was significantly suppressed in AXL-/- mice, and an induced population of tumor-infiltrated CD8+ T cells and CD103+ dendritic cells (DCs) was observed. The change of CD8+ T cells and CD103+ DCs was also confirmed in tumor-draining lymph nodes (TdLN). In addition, the clonal expansion of OVA-specific CD8+ T cells was dominant in AXL-/- mice. Finally, anti-PD-1 treatment evidenced synergistic anti-cancer effects in AXL-/- mice. Overall, our data indicate that AXL signaling may inhibit the clonal expansion of tumor-specific CD8+ T cells through the regulation of the migration of CD8+ T cells and DCs in TME. Thus, AXL may be a powerful molecular target to improve anti-cancer effects through single or combined therapy with immune checkpoint inhibitors (ICI).

MHC cross-dressing in antigen presentation

Dendritic cells (DCs) orchestrate T cell responses by presenting antigenic peptides on major histocompatibility complex (MHC) and providing costimulation and other instructive signals. Professional antigen presenting cells (APCs), including DCs, are uniquely capable of generating and presenting peptide antigens derived from exogenous proteins. In addition to these canonical cross-presentation and MHC-II presentation pathways, APCs can also display exogenous peptide/MHC (p/MHC) acquired from neighboring cells and extracellular vesicles (EVs). This process, known as MHC cross-dressing, has been implicated in the regulation of T cell responses in a variety of in vivo contexts, including allogeneic solid organ transplantation, tumors, and viral infection. Although the occurrence of MHC cross-dressing has been clearly demonstrated, the importance of this antigen presentation mechanism continues to be elucidated. The contribution of MHC cross-dressing to overall antigen presentation has been obfuscated by the fact that DCs express the same MHC alleles as all other cells in the host, making it difficult to distinguish p/MHC generated within the DC from p/MHC acquired from another cell. As a result, much of what is known about MHC cross-dressing comes from studies using allogeneic organ transplantation and bone marrow chimeric mice, though recent development of mice bearing conditional knockout MHC and β2-microglobulin alleles should facilitate substantial progress in the coming years. In this review, we highlight recent advances in our understanding of MHC cross-dressing and its role in activating T cell responses in various contexts, as well as the experimental insights into the mechanism by which it occurs.

The co-delivery of adenovirus-based immune checkpoint vaccine elicits a potent anti-tumor effect in renal carcinoma

Immune-based checkpoint therapy has made significant progress in cancer treatment, but its therapeutic effect is limited. A replication-defective adenovirus (Ad) vaccine encoding tumor antigen carbonic anhydrase IX (CAIX) combined with Ad-encoding immune checkpoint PD-L1 was developed to treat renal carcinoma. Three tumor models, subcutaneous, lung metastasis and orthotopic tumor were established, and Ad vaccines were used to immunize them and evaluate the vaccine's therapeutic effect. Compared to the single Ad vaccine group, the subcutaneous tumor growth was significantly reduced in Ad-CAIX/Ad-PD-L1 combination group. Co-immunization of Ad-CAIX/Ad-PD-L1 enhanced the induction and maturation of CD11c+ or CD8+CD11c+ DCs in the spleen and tumor and promoted the strong tumor-specific CD8+ T cell immune responses. In vivo CD8 T cell deletion assay showed that the anti-tumor effect of the Ad-CAIX/Ad-PD-L1 vaccine was mainly dependent on functional CD8+ T cell immune responses. Furthermore, the Ad-CAIX/Ad-PD-L1 vaccine effectively inhibited tumor growth and lung metastasis in metastatic or orthotopic models. These results indicate that the combination strategy of the immune checkpoint vaccine shows promising potential as an approach for malignant tumor therapy.

TIME Is Ticking for Cervical Cancer

Cervical cancer (CC) is a major health problem among reproductive-age females and comprises a leading cause of cancer-related deaths. Human papillomavirus (HPV) is the major risk factor associated with CC incidence. However, lifestyle is also a critical factor in CC pathogenesis. Despite HPV vaccination introduction, the incidence of CC is increasing worldwide. Therefore, it becomes critical to understand the CC tumor immune microenvironment (TIME) to develop immune cell-based vaccination and immunotherapeutic approaches. The current article discusses the immune environment in the normal cervix of adult females and its role in HPV infection. The subsequent sections discuss the alteration of different immune cells comprising CC TIME and their targeting as future therapeutic approaches.

Trogocytosis and cross-dressing in antigen presentation

Antigen (Ag)-presenting cells capture or synthesize Ags that are processed into peptides bound and displayed on the plasma membrane by major histocompatibility complex (MHC) molecules. Here, we review a mechanism that enables cells to present Ag-loaded MHC molecules that they have not produced themselves, namely trogocytosis. During trogocytosis, a cell acquires fragments from another living cell without, in most cases, affecting the viability of the donor cell. The trogocytic cell can incorporate into its own plasma membrane (becoming cross-dressed) proteins acquired from the donor cell, including intact Ag and MHC molecules. Trogocytosis and cross-dressing expand the immunological functions that immune and nonimmune cells are able to carry out, with both beneficial and deleterious consequences.

Pathways of MHC I cross-presentation of exogenous antigens

Phagocytes, particularly dendritic cells (DCs), generate peptide-major histocompatibility complex (MHC) I complexes from antigens they have collected from cells in tissues and report this information to CD8 T cells in a process called cross-presentation. This process allows CD8 T cells to detect, respond and eliminate abnormal cells, such as cancers or cells infected with viruses or intracellular microbes. In some settings, cross-presentation can help tolerize CD8 T cells to self-antigens. One of the principal ways that DCs acquire tissue antigens is by ingesting this material through phagocytosis. The resulting phagosomes are key hubs in the cross-presentation (XPT) process and in fact experimentally conferring the ability to phagocytize antigens can be sufficient to allow non-professional antigen presenting cells (APCs) to cross-present. Once in phagosomes, exogenous antigens can be cross-presented (XPTed) through three distinct pathways. There is a vacuolar pathway in which peptides are generated and then bind to MHC I molecules within the confines of the vacuole. Ingested exogenous antigens can also be exported from phagosomes to the cytosol upon vesicular rupture and/or possibly transport. Once in the cytosol, the antigen is degraded by the proteasome and the resulting oligopeptides can be transported to MHC I molecule in the endoplasmic reticulum (ER) (a phagosome-to-cytosol (P2C) pathway) or in phagosomes (a phagosome-to-cytosol-to-phagosome (P2C2P) pathway). Here we review how phagosomes acquire the necessary molecular components that support these three mechanisms and the contribution of these pathways. We describe what is known as well as the gaps in our understanding of these processes.

MHC-dressing on dendritic cells: Boosting anti-tumor immunity via unconventional tumor antigen presentation

Dendritic cells are crucial for anti-tumor immune responses due to their ability to activate cytotoxic effector CD8⁺ T cells. Canonically, in anti-tumor immunity, dendritic cells activate CD8⁺ T cells in a process termed cross-presentation. Recent studies have demonstrated that another type of antigen presentation, MHC-dressing, also serves to activate CD8⁺ T cells against tumor cell-derived antigens. Understanding MHC-dressing's specific contributions to anti-tumor immunity can open up novel therapeutic avenues. In this review, we summarize the early studies that identified MHC-dressing as a relevant antigen presentation pathway before diving into a deeper discussion of the biology of MHC-dressing, focusing in particular on which dendritic cell subsets are most capable of performing MHC-dressing and how MHC-dressing compares to other forms of antigen presentation. We conclude by discussing the implications MHC-dressing has for anti-tumor immunity.

Therapeutic Adenovirus Vaccine Combined Immunization with IL-12 Induces Potent CD8+ T Cell Anti-Tumor Immunity in Hepatocellular Carcinoma

Simple Summary

Hepatocellular carcinoma is a kind of tumor with a high malignant degree and mortality rate, and there is no effective treatment method. Currently, immunotherapy has shown good prospects in treating hepatocellular carcinoma. As an important approach of immunotherapy, the vaccine has become an attractive method for tumor treatment. This study developed an adenovirus vaccine containing tumor antigen glypican-3 and adjuvant interleukin 12. The subcutaneous tumor model was intramuscularly immunized three times with vaccines at a ten-day interval. Compared with the control group, the proliferation of CD 8⁺ T cell, the induction of multifunctional CD 8⁺ T cell and dendritic cells, and cytotoxic T lymphocyte activity were significantly increased in the combined immunization group, and the growth of tumor was inhibited obviously. The therapeutic effect of the vaccine of glypican-3 and interleukin 12 mainly depends on the anti-tumor effect of CD 8⁺ T cells mediated by dendritic cells. Likewise, this vaccine also showed a good therapeutic effect in the lung metastasis model of hepatocellular carcinoma. Therefore, the adenovirus vaccine of glypican-3 and interleukin 12 might become a potential way to treat hepatocellular carcinoma.

Abstract

Hepatocellular carcinoma (HCC) is one of the cancers with the highest morbidity and mortality in the world. However, clinical progress in the treatment of HCC has not shown a satisfactory therapeutic effect. Here, we have developed a novel strategy to treat HCC with an adenovirus (Ad)-based vaccine, which contains a specific antigen glypican-3 (GPC3) and an immunostimulatory cytokine IL-12. In the subcutaneous tumor model, Ad-IL-12/GPC3 vaccine was injected into muscles three times to evaluate its therapeutic effect. Compared with the control immunization group, the Ad-IL-12/GPC3 immunization group showed a significant tumor growth inhibition effect, which was confirmed by the reduced tumor volume and the increased tumor inhibition. Ad-IL-12/GPC3 co-immunization promoted the induction and maturation of CD11c⁺ or CD8⁺CD11c⁺ DCs and increased the number of tumor-infiltrating CD8⁺ T cells. Furthermore, in the Ad-IL-12/GPC3 group, the proliferation of CD8⁺ T cells, the induction of multifunctional CD8⁺ T cells, and CTL activity were significantly increased. Interestingly, the deletion of CD8⁺ T cells abolished tumor growth inhibition by Ad-IL-12/GPC3 treatment, suggesting that CD8⁺ T cell immune responses were required to eliminate the tumor. Likewise, Ad-IL-12/GPC3 vaccine also effectively inhibited lung tumor growth or metastasis by enhancing CD8⁺ DCs-mediated multifunctional CD8⁺ T cell immune responses in the lung metastasis model. Therefore, these results indicate that IL-12 combined with Ad-GPC3 vaccine co-immunization might provide a promising therapeutic strategy for HCC patients.

Mature dendritic cell-derived dendrosomes swallow oxaliplatin-loaded nanoparticles to boost immunogenic chemotherapy and tumor antigen-specific immunotherapy

The cytomembrane-derived delivery platform represents a promising biomimetic strategy in oncotherapy. To achieve durable and reliable tumor inhibition, mature dendrosomes (mDs), which were isolated from bone marrow-derived dendritic cells undergoing CT26 tumor antigen (TA) stimulation, were fused with redox-responsive nanoparticles (NPs) that were composed of poly(disulfide ester amide) polymers with an intensified disulfide density and hydrophobic oxaliplatin (OXA) prodrugs with the ability to potentiate immunogenicity. In vitro and in vivo results revealed that NP/mDs could induce tumor cell death through mitochondrial pathway and thus created immunogenic microenvironments, but also elicited immunocyte differentiation by TA cross-dressing and infiltration by direct presentation. By further neutralizing immune-regulatory interaction, the administration of PD-L1 antibody (αPD-L1) greatly improved antitumor efficiency of NP/mDs. Furthermore, the effectors of host immune systems effectively inhibited the growth and metastasis of distal tumors, likely because the autologous TA evoked by OXA and allogeneic TA delivered by mDs acted as additional stimuli to reinforce the immune response of tumor-specific T cells and immunosurveillance toward oncogenesis. These results demonstrated that NP/mDs could simultaneously realize immunogenic chemotherapeutics and specific TA delivery. In combination with αPD-L1, the antitumor effect was further enhanced. Therefore, NP/mDs provide a promising strategy for the comprehensive treatment of malignancy.

Antigen transfer and its effect on vaccine-induced immune amplification and tolerance

Antigen transfer refers to the process of intercellular information exchange, where antigenic components including nucleic acids, antigen proteins/peptides and peptide-major histocompatibility complexes (p-MHCs) are transmitted from donor cells to recipient cells at the thymus, secondary lymphoid organs (SLOs), intestine, allergic sites, allografts, pathological lesions and vaccine injection sites via trogocytosis, gap junctions, tunnel nanotubes (TNTs), or extracellular vesicles (EVs). In the context of vaccine inoculation, antigen transfer is manipulated by the vaccine type and administration route, which consequently influences, even alters the immunological outcome, i.e., immune amplification and tolerance. Mainly focused on dendritic cells (DCs)-based antigen receptors, this review systematically introduces the biological process, molecular basis and clinical manifestation of antigen transfer.

Dendritic cells can prime anti-tumor CD8+ T cell responses through major histocompatibility complex cross-dressing

Antigen cross-presentation, wherein dendritic cells (DCs) present exogenous antigen on major histocompatibility class I (MHC-I) molecules, is considered the primary mechanism by which DCs initiate tumor-specific CD8+ T cell responses. Here, we demonstrate that MHC-I cross-dressing, an antigen presentation pathway in which DCs acquire and display intact tumor-derived peptide:MHC-I molecules, is also important in orchestrating anti-tumor immunity. Cancer cell MHC-I expression was required for optimal CD8+ T cell activation in two subcutaneous tumor models. In vivo acquisition of tumor-derived peptide:MHC-I molecules by DCs was sufficient to induce antigen-specific CD8+ T cell priming. Transfer of tumor-derived human leukocyte antigen (HLA) molecules to myeloid cells was detected in vitro and in human tumor xenografts. In conclusion, MHC-I cross-dressing is crucial for anti-tumor CD8+ T cell priming by DCs. In addition to quantitatively enhancing tumor antigen presentation, MHC cross-dressing might also enable DCs to more faithfully and efficiently mirror the cancer cell peptidome.

Gnawing Between Cells and Cells in the Immune System: Friend or Foe? A Review of Trogocytosis

Trogocytosis occurs when one cell contacts and quickly nibbles another cell and is characterized by contact between living cells and rapid transfer of membrane fragments with functional integrity. Many immune cells are involved in this process, such as T cells, B cells, NK cells, APCs. The transferred membrane molecules including MHC molecules, costimulatory molecules, receptors, antigens, etc. An increasing number of studies have shown that trogocytosis plays an important role in the immune system and the occurrence of relevant diseases. Thus, whether trogocytosis is a friend or foe of the immune system is puzzling, and the precise mechanism underlying it has not yet been fully elucidated. Here, we provide an integrated view of the acquired findings on the connections between trogocytosis and the immune system.

Distinct roles but cooperative effect of TLR3/9 agonists and PD-1 blockade in converting the immunotolerant microenvironment of irreversible electroporation-ablated tumors

Irreversible electroporation (IRE) is a new cancer ablation technology, but methods to improve IRE-induced therapeutic immunity are only beginning to be investigated. We developed a mouse model bearing large primary (300 mm³) and medium distant (100 mm³) EG7 lymphomas engineered to express ovalbumin (OVA) as a nominal tumor antigen. We established experimental protocols including IRE alone and IRE combined with Toll-like receptor (TLR)3/9 agonists (poly I:C/CpG) (IRE + pIC/CpG), PD-1 blockade (IRE + PD-1 blockade), or both (IRE + Combo) to investigate therapeutic effects on primary and distant EG7 tumors and conversion-promoting effects on the immunotolerant tumor microenvironment (TME). We demonstrated that IRE alone simulated very weak OVA-specific CD8⁺ T cell responses and did not inhibit primary tumor growth. IRE + pIC/CpG synergistically stimulated more efficient OVA-specific CD8⁺ T cell responses and primary tumor growth inhibition than IRE + PD-1 blockade. IRE + pIC/CpG played a major role in the modulation of immune cell profiles but a minor role in the downregulation of PD-L1 expression in the TME and vice versa for IRE + PD-1 blockade. IRE + Combo cooperatively induced potent OVA-specific CD8⁺ T cell immunity and rescued exhausted intratumoral CD8⁺ T cells, leading to eradication of not only primary tumors but also untreated concomitant distant tumors and lung metastases. IRE + Combo efficiently modulated immune cell profiles, as evidenced by reductions in immunotolerant type-2 (M2) macrophages, myeloid-derived suppressor-cells, plasmacytoid dendritic cells, and regulatory T cells and by increases in immunogenic M1 macrophages, CD169⁺ macrophages, type-1 conventional dendritic cells, and CD8⁺ T cells, leading to conversion of immunotolerance in not only primary TMEs but also untreated distant TMEs. IRE + Combo also showed effective therapeutic effects in two breast cancer models. Therefore, our results suggest that IRE + Combo is a promising strategy to improve IRE ablation therapy in cancer.

New insights on the monitoring of solid-organ allografts based on immune cell signatures

Attaining a fair long-term allograft survival remains a challenge for allogeneic transplantation worldwide. Although the emergence of immunosuppressants has caused noticeable progress in the management of immunologic rejection, proper application of these therapeutics and dose adjustments require delicate and real-time monitoring of recipients. Nevertheless, the majority of conventional allograft monitoring approaches are based on organ damage or functional tests that render them unable to predict the rejection events in early time points before the establishment of a functional alloimmune response. On the other hand, biopsy-based methods include invasive practices and are accompanied by serious complications. In recent years, there have been a myriad of attempts on the discovery of reliable and non-invasive approaches for the monitoring of allografts that regarding a close relationship between allografts and hosts' immune system, most of the attempts have been devoted to the studies on the immune response-associated biomarkers. The discovery of gene and protein expression patterns in immune cells along with their phenotypic characterization and secretome analysis as well as tracking the immune responses in allograft tissues and clinical specimens are among the notable attempts taken to discover the non-invasive predictive markers with a proper coincidence to the pathologic condition. Collectively, these studies suggest a list of candidate biomarkers with ideal potentials for early and non-invasive prediction of allograft rejection and shed light on the way towards developing more standardized and reproducible approaches for monitoring the allograft rejection.

Shaping of T Cell Functions by Trogocytosis

Trogocytosis is an active process whereby plasma membrane proteins are transferred from one cell to the other cell in a cell-cell contact-dependent manner. Since the discovery of the intercellular transfer of major histocompatibility complex (MHC) molecules in the 1970s, trogocytosis of MHC molecules between various immune cells has been frequently observed. For instance, antigen-presenting cells (APCs) acquire MHC class I (MHCI) from allografts, tumors, and virally infected cells, and these APCs are subsequently able to prime CD8⁺ T cells without antigen processing via the preformed antigen-MHCI complexes, in a process called cross-dressing. T cells also acquire MHC molecules from APCs or other target cells via the immunological synapse formed at the cell-cell contact area, and this phenomenon impacts T cell activation. Compared with naïve and effector T cells, T regulatory cells have increased trogocytosis activity in order to remove MHC class II and costimulatory molecules from APCs, resulting in the induction of tolerance. Accumulating evidence suggests that trogocytosis shapes T cell functions in cancer, transplantation, and during microbial infections. In this review, we focus on T cell trogocytosis and the related inflammatory diseases.

DNGR-1 limits Flt3L-mediated antitumor immunity by restraining tumor-infiltrating type I conventional dendritic cells

BACKGROUND

Conventional type 1 dendritic cells (cDC1s) are central to antitumor immunity and their presence in the tumor microenvironment associates with improved outcomes in patients with cancer. DNGR-1 (CLEC9A) is a dead cell-sensing receptor highly restricted to cDC1s. DNGR-1 has been involved in both cross-presentation of dead cell-associated antigens and processes of disease tolerance, but its role in antitumor immunity has not been clarified yet.

METHODS

B16 and MC38 tumor cell lines were inoculated subcutaneously into wild-type (WT) and DNGR-1-deficient mice. To overexpress Flt3L systemically, we performed gene therapy through the hydrodynamic injection of an Flt3L-encoding plasmid. To characterize the immune response, we performed flow cytometry and RNA-Seq of tumor-infiltrating cDC1s.

RESULTS

Here, we found that cross-presentation of tumor antigens in the steady state was DNGR-1-independent. However, on Flt3L systemic overexpression, tumor growth was delayed in DNGR-1-deficient mice compared with WT mice. Of note, this protection was recapitulated by anti-DNGR-1-blocking antibodies in mice following Flt3L gene therapy. This improved antitumor immunity was associated with Batf3-dependent enhanced accumulation of CD8+ T cells and cDC1s within tumors. Mechanistically, the deficiency in DNGR-1 boosted an Flt3L-induced specific inflammatory gene signature in cDC1s, including Ccl5 expression. Indeed, the increased infiltration of cDC1s within tumors and their protective effect rely on CCL5/CCR5 chemoattraction. Moreover, FLT3LG and CCL5 or CCR5 gene expression signatures correlate with an enhanced cDC1 signature and a favorable overall survival in patients with cancer. Notably, cyclophosphamide elevated serum Flt3L levels and, in combination with the absence of DNGR-1, synergized against tumor growth.

CONCLUSION

DNGR-1 limits the accumulation of tumor-infiltrating cDC1s promoted by Flt3L. Thus, DNGR-1 blockade may improve antitumor immunity in tumor therapy settings associated to high Flt3L expression.

Antigen presentation in cancer: insights into tumour immunogenicity and immune evasion

Immune checkpoint blockade, which blocks inhibitory signals of T cell activation, has shown tremendous success in treating cancer, although success still remains limited to a fraction of patients. To date, clinically effective CD8⁺ T cell responses appear to target predominantly antigens derived from tumour-specific mutations that accumulate in cancer, also called neoantigens. Tumour antigens are displayed on the surface of cells by class I human leukocyte antigens (HLA-I). To elicit an effective antitumour response, antigen presentation has to be successful at two distinct events: first, cancer antigens have to be taken up by dendritic cells (DCs) and cross-presented for CD8⁺ T cell priming. Second, the antigens have to be directly presented by the tumour for recognition by primed CD8⁺ T cells and killing. Tumours exploit multiple escape mechanisms to evade immune recognition at both of these steps. Here, we review the tumour-derived factors modulating DC function, and we summarize evidence of immune evasion by means of quantitative modulation or qualitative alteration of the antigen repertoire presented on tumours. These mechanisms include modulation of antigen expression, HLA-I surface levels, alterations in the antigen processing and presentation machinery in tumour cells. Lastly, as complete abrogation of antigen presentation can lead to natural killer (NK) cell-mediated tumour killing, we also discuss how tumours can harbour antigen presentation defects and still evade NK cell recognition.

Bioengineered tissue models for the development of dynamic immuno-associated tumor models and high-throughput immunotherapy cytotoxicity assays

Cancer immunotherapy is rapidly developing, with numerous therapies approved over the past decade and more therapies expected to gain approval in the future. However, immunotherapy of solid tumors has been less successful because immunosuppressive barriers limit immune cell trafficking and function against cancer cells. Interactions between suppressive immune cells, cytokines, and inhibitory factors are central to cancer immunotherapy approaches. In this review, we discuss recent advances in utilizing microfluidic platforms for understanding cancer-suppressive immune system interactions. Dendritic cell (DC)-mediated tumor models, infiltrated lymphocyte-mediated tumor models [e.g., natural killer (NK) cells, T cells, chimeric antigen receptor (CAR) T cells, and macrophages], monocyte-mediated tumor models, and immune checkpoint blockade (ICB) tumor models are among the various bioengineered immune cell-cancer cell interactions that we reviewed herein.

Plasmacytoid dendritic cells cross-prime naive CD8 T cells by transferring antigen to conventional dendritic cells through exosomes

Although plasmacytoid dendritic cells (pDCs) have been shown to play a critical role in generating viral immunity and promoting tolerance to suppress antitumor immunity, whether and how pDCs cross-prime CD8 T cells in vivo remain controversial. Using a pDC-targeted vaccine model to deliver antigens specifically to pDCs, we have demonstrated that pDC-targeted vaccination led to strong cross-priming and durable CD8 T cell immunity. Surprisingly, cross-presenting pDCs required conventional DCs (cDCs) to achieve cross-priming in vivo by transferring antigens to cDCs. Taking advantage of an in vitro system where only pDCs had access to antigens, we further demonstrated that cross-presenting pDCs were unable to efficiently prime CD8 T cells by themselves, but conferred antigen-naive cDCs the capability of cross-priming CD8 T cells by transferring antigens to cDCs. Although both cDC1s and cDC2s exhibited similar efficiency in acquiring antigens from pDCs, cDC1s but not cDC2s were required for cross-priming upon pDC-targeted vaccination, suggesting that cDC1s played a critical role in pDC-mediated cross-priming independent of their function in antigen presentation. Antigen transfer from pDCs to cDCs was mediated by previously unreported pDC-derived exosomes (pDCexos), that were also produced by pDCs under various conditions. Importantly, all these pDCexos primed naive antigen-specific CD8 T cells only in the presence of bystander cDCs, similarly to cross-presenting pDCs, thus identifying pDCexo-mediated antigen transfer to cDCs as a mechanism for pDCs to achieve cross-priming. In summary, our data suggest that pDCs employ a unique mechanism of pDCexo-mediated antigen transfer to cDCs for cross-priming.

Cross-dressing of CD8α+ Dendritic Cells with Antigens from Live Mouse Tumor Cells Is a Major Mechanism of Cross-priming

Live cells are the most abundant sources of antigen in a tumor-bearing host. Here, we used live tumor cells as source of antigens to investigate the mechanism underlying their immunogenicity in murine tumor models. The live tumor cells were significantly more immunogenic than irradiated or apoptotic tumor cells. We examined the interaction of live and apoptotic tumor cells with major subsets of antigen-presenting cells, i.e., CD8α⁺ dendritic cells (DC), CD8α^- DCs, plasmacytoid DCs, and CD169⁺ macrophages at skin draining lymph nodes. The CD8α⁺ DCs captured cell-associated antigens from both live and apoptotic tumor cells, whereas CD169⁺ macrophages picked up cell-associated antigens mostly from apoptotic tumor cells. Trogocytosis and cross-dressing of membrane-associated antigenic material from live tumor cells to CD8α⁺ DCs was the primary mechanism for cross-priming of tumor antigens upon immunization with live cells. Phagocytosis of apoptotic tumor cells was the primary mechanism for cross-priming of tumor antigens upon immunization with apoptotic or irradiated cells. These findings clarify the mechanism of cross-priming of cancer antigens by DCs, allowing for a greater understanding of antitumor immune responses.

Host CD8α+ and CD103+ dendritic cells prime transplant antigen-specific CD8+ T cells via cross-dressing

The participation of dendritic cells (DCs) in CD8⁺ T-cell-mediated allograft rejection is a long-standing question of great clinical relevance for tissue transplantation. Here, we show that Batf3^-/- mice demonstrate delayed allo-skin graft rejection and are deficient in priming allo-specific CD8⁺ T cells. Batf3^-/- mouse priming is restored by transferring either purified CD8α⁺ or CD103⁺ DCs, demonstrating the critical role of these cells in alloreactivity. Using D^b -restricted antiviral F5 transgenic T-cell receptor T cells, which we demonstrate are alloreactive with H-2K^d , we show that cross-dressing of CD8α⁺ and CD103⁺ primes CD8⁺ T-cell or allo-specific responses in vitro and in vivo. These findings suggest novel strategies for moderating tissue rejection based on interfering with DC cross-dressing or subsequent interaction with T cells.

Co-immunizing with PD-L1 induces CD8+ DCs-mediated anti-tumor immunity in multiple myeloma

Tumor therapeutic vaccines have faced a challenge for effective protection against malignant tumors by inducing tumor-specific CD8⁺ T cell responses. Here, we designed a DNA vaccine containing a tumor-specific antigen of Dickkopf-1 (DKK-1) and an immune checkpoint of programmed death ligand 1 (PD-L1) delivered by PLGA/PEI nanoparticle-mediated delivery system for multiple myeloma therapy. Murine subcutaneous tumor model established with human DKK1 (hDKK-1)-SP2/0 cells were intramuscularly immunized with PLGA/PEI-pPD-L1/pDDK-1 vaccine and equal amount of control 3 times at 10 day-intervals. Compared with PLGA/PEI-pDKK1 immunization group, PLGA/PEI-pPD-L1/pDKK-1 co-immunization enhanced the induction and mature of CD11c⁺ DCs and CD8⁺CD11c⁺ DCs, and promoted antigen-specific Th1 responses and cytotoxic T lymphocyte (CTL) responses. The reduced tumor volume and weight as well as increased tumor inhibition rate were observed in PLGA/PEI-pPD-L1/pDKK-1 vaccine co-immunization group, indicated that the vaccine could effectively inhibit the tumor growth of multiple myeloma. The anti-tumor activity of PLGA/PEI-pPD-L1/pDKK-1 vaccine was abrogated by CD8 cell depletion accompanied with the reduced percentages of CD8⁺CD11c⁺ DCs and CD8⁺ T cells in the spleen and TILs. These results indicated that the anti-tumor efficacy of PLGA/PEI-pPD-L1/pDKK-1 vaccine was required for CD8⁺CD11c⁺ DCs-mediated CD8⁺ T cell immunity responses. This vaccine strategy may represent a potential and promising approach for hematological malignancy treatment.

Antigen processing and presentation

Dendritic cells are at the center of immune responses. They are defined by their ability to sense the environment, take up and process antigen, migrate to secondary lymphoid organs, where they present antigens to the adaptive immune system. In particular, they present lipids and proteins from pathogens, which they encountered in peripheral tissues, to T cells in order to induce a specific effector immune response. These complex antigens need to be broken down into peptides of a certain length in association with Major Histocompatibility Complex (MHC) molecules. Presentation of MHC/antigen complexes alongside costimulatory molecules and secretion of proinflammatory cytokines will induce an appropriate immune response. This interaction between dendritic cells and T cells takes place at defined locations within secondary lymphoid organs. In this review, we discuss the current knowledge and recent advances on the cellular and molecular mechanisms that underlie antigen processing and the subsequent presentation to T lymphocytes.

Batf3-Dependent Genes Control Tumor Rejection Induced by Dendritic Cells Independently of Cross-Presentation

The BATF3-dependent cDC1 lineage of conventional dendritic cells (cDC) is required for rejection of immunogenic sarcomas and for rejection of progressive sarcomas during checkpoint blockade therapy. One unique function of the cDC1 lineage is the efficient cross-presentation of tumor-derived neoantigens to CD8+ T cells, but it is not clear that this is the only unique function of cDC1 required for tumor rejection. We previously showed that BATF3 functions during cDC1 lineage commitment to maintain IRF8 expression in the specified cDC1 progenitor. However, since cDC1 progenitors do not develop into mature cDC1s in Batf3 -/- mice, it is still unclear whether BATF3 has additional functions in mature cDC1 cells. A transgenic Irf8-Venus reporter allele increases IRF8 protein concentration sufficiently to allow autonomous cDC1 development in spleens of Batf3 -/- mice. These restored Batf3 -/- cDC1s are transcriptionally similar to control wild-type cDC1s but have reduced expression of a restricted set of cDC1-specific genes. Restored Batf3 -/- cDC1s are able to cross-present cell-associated antigens both in vitro and in vivo However, Batf3 -/- cDC1 exhibit altered characteristics in vivo and are unable to mediate tumor rejection. These results show that BATF3, in addition to regulating Irf8 expression to stabilize cDC1 lineage commitment, also controls expression of a small set of genes required for cDC1-mediated tumor rejection. These BATF3-regulated genes may be useful targets in immunotherapies aimed at promoting tumor rejection.

Dendritic Cell Targeting mRNA Lipopolyplexes Combine Strong Antitumor T-Cell Immunity with Improved Inflammatory Safety

In vitro transcribed mRNA constitutes a versatile platform to encode antigens and to evoke CD8 T-cell responses. Systemic delivery of mRNA packaged into cationic liposomes (lipoplexes) has proven particularly powerful in achieving effective antitumor immunity in animal models. Yet, T-cell responses to mRNA lipoplexes critically depend on the induction of type I interferons (IFN), potent pro-inflammatory cytokines, which inflict dose-limiting toxicities. Here, we explored an advanced hybrid lipid polymer shell mRNA nanoparticle (lipopolyplex) endowed with a trimannose sugar tree as an alternative delivery vehicle for systemic mRNA vaccination. Like mRNA lipoplexes, mRNA lipopolyplexes were extremely effective in conferring antitumor T-cell immunity upon systemic administration. Conversely to mRNA lipoplexes, mRNA lipopolyplexes did not rely on type I IFN for effective T-cell immunity. This differential mode of action of mRNA lipopolyplexes enabled the incorporation of N1 methyl pseudouridine nucleoside modified mRNA to reduce inflammatory responses without hampering T-cell immunity. This feature was attributed to mRNA lipopolyplexes, as the incorporation of thus modified mRNA into lipoplexes resulted in strongly weakened T-cell immunity. Taken together, we have identified lipopolyplexes containing N1 methyl pseudouridine nucleoside modified mRNA as potent yet low-inflammatory alternatives to the mRNA lipoplexes currently explored in early phase clinical trials.

A Dendritic Cell-Targeted Adenoviral Vector Facilitates Adaptive Immune Response Against Human Glioma Antigen (CMV-IE) and Prolongs Survival in a Human Glioma Tumor Model

Antitumor immunotherapeutic strategies represent an especially promising set of approaches with rapid translational potential considering the dismal clinical context of high-grade gliomas. Dendritic cells (DCs) are the body's most professional antigen-presenting cells, able to recruit and activate T cells to stimulate an adaptive immune response. In this regard, specific loading of tumor-specific antigen onto dendritic cells potentially represents one of the most advanced strategies to achieve effective antitumor immunization. In this study, we developed a DC-specific adenoviral (Ad) vector, named Ad5scFvDEC205FF, targeting the DC surface receptor, DEC205. In vitro analysis shows that 60% of DCs was infected by this vector while the infectivity of other control adenoviral vectors was less than 10%, demonstrating superior infectivity on DCs. Moreover, an average of 14% of DCs were infected by Ad5scFvDEC205FF-GFP, while less than 3% of non-DCs were infected following in vivo administration, demonstrating highly selective in vivo DC infection. Importantly, vaccination with this vehicle expressing human glioma-specific antigen, Ad5scFvDEC205FF-CMV-IE, shows a prolonged survival benefit in GL261CMV-IE-implanted murine glioma models (p < 0.0007). Furthermore, when rechallenged, cancerous cells were completely rejected. In conclusion, our novel, viral-mediated, DC-based immunization approach has the significant therapeutic potential for patients with high-grade gliomas.

Transfer of Cell-Surface Antigens by Scavenger Receptor CD36 Promotes Thymic Regulatory T Cell Receptor Repertoire Development and Allo-tolerance

The development of T cell tolerance in the thymus requires the presentation of host proteins by multiple antigen-presenting cell (APC) types. However, the importance of transferring host antigens from transcription factor AIRE-dependent medullary thymic epithelial cells (mTECs) to bone marrow (BM) APCs is unknown. We report that antigen was primarily transferred from mTECs to CD8α⁺ dendritic cells (DCs) and showed that CD36, a scavenger receptor selectively expressed on CD8α⁺ DCs, mediated the transfer of cell-surface, but not cytoplasmic, antigens. The absence of CD8α⁺ DCs or CD36 altered thymic T cell selection, as evidenced by TCR repertoire analysis and the loss of allo-tolerance in murine allogeneic BM transplantation (allo-BMT) studies. Decreases in these DCs and CD36 expression in peripheral blood of human allo-BMT patients correlated with graft-versus-host disease. Our findings suggest that CD36 facilitates transfer of mTEC-derived cell-surface antigen on CD8α⁺ DCs to promote tolerance to host antigens during homeostasis and allo-BMT.

Extracellular vesicle-mediated MHC cross-dressing in immune homeostasis, transplantation, infectious diseases, and cancer

Eukaryotic cells employ different types of extracellular vesicles (EVs) to exchange proteins, mRNAs, non-coding regulatory RNAs, carbohydrates, and lipids. Cells of the immune system, in particular antigen (Ag)-presenting cells (APCs), acquire major histocompatibility complex (MHC) class I and II molecules loaded with antigenic peptides from leukocytes and tissue parenchymal and stromal cells, through a mechanism known as MHC cross-dressing. Increasing evidence indicates that cross-dressing of APCs with pre-formed Ag-peptide/MHC complexes (pMHCs) is mediated via passage of clusters of EVs with characteristics of exosomes. A percentage of the transferred EVs remain attached to the acceptor APCs, with the appropriate orientation, at sufficient concentration within localized areas of the plasma membrane, and for sufficient time, so the preformed pMHCs carried by the EVs are presented without further processing, to cognate T cells. Although its biological relevance is not fully understood, numerous studies have demonstrated that MHC cross-dressing of APCs represents a pathway of Ag presentation of acquired pre-formed pMHCs to T cells-alternative to direct and cross-presentation-participate in immune homeostasis and T cell tolerance, cross-regulate alloreactive T cells with different MHC restricted specificities, and is a mechanism of Ag spreading for autologous, allogeneic, microbial, tumor, or vaccine-delivered Ags. Here, we compare MHC cross-dressing with other mechanisms and terminologies used for pMHC transfer, including trogocytosis. We discuss the experimental evidence, mostly from in vitro and ex vivo studies, of the role of MHC cross-dressing of APCs via EVs in positive or negative regulation of T cell immunity in the steady state, transplantation, microbial diseases, and cancer.

Cross-priming induces immunodomination in the presence of viral MHC class I inhibition

Viruses have evolved mechanisms of MHCI inhibition in order to evade recognition by cytotoxic CD8⁺ T cells (CTLs), which is well-illustrated by our prior studies on cowpox virus (CPXV) that encodes potent MHCI inhibitors. Deletion of CPXV viral MHCI inhibitors markedly attenuated in vivo infection due to effects on CTL effector function, not priming. However, the CTL response to CPXV in C57BL/6 mice is dominated by a single peptide antigen presented by H-2K^b. Here we evaluated the effect of viral MHCI inhibition on immunodominant (IDE) and subdominant epitopes (SDE) as this has not been thoroughly examined. We found that cross-priming, but not cross-dressing, is the main mechanism driving IDE and SDE CTL responses following CPXV infection. Secretion of the immunodominant antigen was not required for immunodominance. Instead, immunodominance was caused by CTL interference, known as immunodomination. Both immunodomination and cross-priming of SDEs were not affected by MHCI inhibition. SDE-specific CTLs were also capable of exerting immunodomination during primary and secondary responses, which was in part dependent on antigen abundance. Furthermore, CTL responses directed solely against SDEs protected against lethal CPXV infection, but only in the absence of the CPXV MHCI inhibitors. Thus, both SDE and IDE responses can contribute to protective immunity against poxviruses, implying that these principles apply to poxvirus-based vaccines.

The use of vaccinia virus (VACV) to eradicate smallpox is the arguably the most successful demonstration of vaccination. The VACV vaccine also provides cross-protection against related zoonotic orthopoxviruses, including monkey poxvirus (MXPV) and CPXV, which circulate between various animal hosts and humans. Interestingly, Edward Jenner first demonstrated the concept of vaccination against smallpox in the late 1700s using CPXV. He also made the curious observation that CPXV vaccination did not always protect against recurrent exposure to CPXV. Jenner’s observations may be explained by the ability for CPXV to evade antiviral CD8⁺ T cell immune responses. To evade CD8⁺ T cells, CPXV inhibits MHCI antigen presentation, which is required to prime CD8⁺ T cells. Importantly, CPXV is the only orthopoxvirus that inhibits MHCI and thus provides a unique opportunity to investigate the effects of viral MHCI inhibition on CD8⁺ T cell priming. Here, we examine the factors that contribute to priming of CPXV-specific CD8⁺ T cells and show that viral MHCI inhibition does not affect CD8⁺ T cell priming, but prior CPXV immunization does inhibit priming during subsequent exposure to CPXV. The effects of pre-existing poxvirus immunity are therefore important to consider if poxvirus-based vaccines against various diseases are to be widely used.

Development of an adenovirus vector vaccine platform for targeting dendritic cells

Adenoviral (Ad) vector vaccines represent one of the most promising modern vaccine platforms, and Ad vector vaccines are currently being investigated in human clinical trials for infectious disease and cancer. Our studies have shown that specific targeting of adenovirus to dendritic cells dramatically enhanced vaccine efficacy. However, this was achieved using a molecular adapter, thereby necessitating a two component vector approach. To address the mandates of clinical translation of our strategy, we here sought to accomplish the goal of DC targeting with a single-component adenovirus vector approach. To redirect the specificity of Ad vector vaccines, we replaced the Ad fiber knob with fiber-fibritin chimeras fused to DC1.8, a single-domain antibody (sdAb) specific for murine immature DC. We engineered a fiber-fibritin-sdAb chimeric molecule using the coding sequence for DC1.8, and then replaced the native Ad5 fiber knob sequence by homologous recombination. The resulting Ad5 virus, Ad5FF1.8, expresses the chimeric fiber-fibritin sdAb chimera. Infection with Ad5FF1.8 dramatically enhances transgene expression in DC2.4 dendritic cells compared with infection with native Ad5. Ad5FF1.8 infection of bone marrow-derived DC demonstrates that Ad5FF1.8 selectively infects immature DC consistent with the known specificity of DC1.8. Thus, sdAb can be used to selectively redirect the tropism of Ad5 vector vaccines, providing the opportunity to engineer Ad vector vaccines that are specifically targeted to DC, or specific DC subsets.

Dissecting antigen processing and presentation routes in dermal vaccination strategies

The skin is an attractive site for vaccination due to its accessibility and presence of immune cells surveilling this barrier. However, knowledge of antigen processing and presentation upon dermal vaccination is sparse. In this study we determined antigen processing routes that lead to CD8⁺ T cell activation following dermal DNA tattoo immunization, exploiting a model antigen that contains an immunoproteasome-dependent epitope. In agreement with earlier reports, we found that DNA tattoo immunization of wild type (WT) mice triggered vigorous responses to the immunoproteasome-dependent model epitope, whereas gene-deficient mice lacking the immunoproteasome subunits β5i/LMP7 and β2i/MECL1 failed to respond. Unexpectedly, dermal immunization both of irradiated bone marrow (BM) reconstituted mice in which the BM transplant was of WT origin, and of WT mice transplanted with immunoproteasome subunit-deficient BM induced a CD8⁺ T cell response to the immunoproteasome-dependent epitope, implying that both BM and host-derived cells contributed to processing of delivered model antigen. Depletion of radiation-resistant Langerhans cells (LC) from chimeric mice did not diminish tattoo-immunization induced CD8⁺ T cell responses in most mice, illustrating that LC were not responsible for antigen processing and CD8⁺ T cell priming in tattoo-immunized hosts. We conclude that both BM and non-BM-derived cells contribute to processing and cross-presentation of antigens delivered by dermal DNA tattoo immunization.

Antigen-presenting cell-targeted lentiviral vectors do not support the development of productive T-cell effector responses: implications for in vivo targeted vaccine delivery

Targeting transgene expression specifically to antigen-presenting cells (APCs) has been put forward as a promising strategy to direct the immune system towards immunity. We developed the nanobody-display technology to restrict the tropism of lentiviral vectors (LVs) to APCs. However, we observed that immunization with APC-targeted LVs (DC2.1-LVs) did not evoke strong antigen-specific T-cell immunity when compared to immunization with broad tropism LVs (VSV.G-LVs). In this study, we report that VSV.G-LVs are more immunogenic than DC2.1-LVs because they transduce stromal cells, which has a role in activating antigen-specific T cells. Moreover, VSV.G-LVs trigger a pro-inflammatory innate immune response through transduction of APCs and stromal cells, while DC2.1-LVs trigger a type I interferon response with anti-viral capacity. These findings question the rationale of targeting LVs to APCs and argue for the development of VSV.G-LVs with an improved safety profile.

Innate Control of Adaptive Immunity: Beyond the Three-Signal Paradigm

Activation of cells in the adaptive immune system is a highly orchestrated process dictated by multiples cues from the innate immune system. Although the fundamental principles of innate control of adaptive immunity are well established, it is not fully understood how innate cells integrate qualitative pathogenic information to generate tailored protective adaptive immune responses. In this review, we discuss complexities involved in the innate control of adaptive immunity that extend beyond TCR engagement, costimulation, and priming cytokine production but are critical for the generation of protective T cell immunity.

The Biology and Underlying Mechanisms of Cross-Presentation of Exogenous Antigens on MHC-I Molecules

To monitor the health of cells, the immune system tasks antigen-presenting cells with gathering antigens from other cells and bringing them to CD8 T cells in the form of peptides bound to MHC-I molecules. Most cells would be unable to perform this function because they use their MHC-I molecules to exclusively present peptides derived from the cell's own proteins. However, the immune system evolved mechanisms for dendritic cells and some other phagocytes to sample and present antigens from the extracellular milieu on MHC-I through a process called cross-presentation. How this important task is accomplished, its role in health and disease, and its potential for exploitation are the subject of this review.

MyD88/CD40 Genetic Adjuvant Function in Cutaneous Atypical Antigen-Presenting Cells Contributes to DNA Vaccine Immunogenicity

Therapeutic DNA-based vaccines aim to prime an adaptive host immune response against tumor-associated antigens, eliminating cancer cells primarily through CD8+ cytotoxic T cell-mediated destruction. To be optimally effective, immunological adjuvants are required for the activation of tumor-specific CD8+ T cells responses by DNA vaccination. Here, we describe enhanced anti-tumor efficacy of an in vivo electroporation-delivered DNA vaccine by inclusion of a genetically encoded chimeric MyD88/CD40 (MC) adjuvant, which integrates both innate and adaptive immune signaling pathways. When incorporated into a DNA vaccine, signaling by the MC adjuvant increased antigen-specific CD8+ T cells and promoted elimination of pre-established tumors. Interestingly, MC-enhanced vaccine efficacy did not require direct-expression of either antigen or adjuvant by local antigen-presenting cells, but rather our data supports a key role for MC function in "atypical" antigen-presenting cells of skin. In particular, MC adjuvant-modified keratinocytes increased inflammatory cytokine secretion, upregulated surface MHC class I, and were able to increase in vitro and in vivo priming of antigen-specific CD8+ T cells. Furthermore, in the absence of critical CD8α+/CD103+ cross-priming dendritic cells, MC was still able to promote immune priming in vivo, albeit at a reduced level. Altogether, our data support a mechanism by which MC signaling activates an inflammatory phenotype in atypical antigen-presenting cells within the cutaneous vaccination site, leading to an enhanced CD8+ T cell response against DNA vaccine-encoded antigens, through both CD8α+/CD103+ dendritic cell-dependent and independent pathways.

Molecular and Cellular Mechanisms of Antitumor Immune Response Activation by Dendritic Cells

Dendritic cells (DCs) play a crucial role in the initiation and regulation of the antitumor immune response. Already , DC-based antitumor vaccines have been thoroughly explored both in animal tumor models and in clinical trials. DC-based vaccines are commonly produced from DC progenitors isolated from peripheral blood or bone marrow by culturing in the presence of cytokines, followed by loading the DCs with tumor-specific antigens, such as DNA, RNA, viral vectors, or a tumor cell lysate. However, the efficacy of DC-based vaccines remains low. Undoubtedly, a deeper understanding of the molecular mechanisms by which DCs function would allow us to enhance the antitumor efficacy of DC-based vaccines in clinical applications. This review describes the origin and major subsets of mouse and human DCs, as well as the differences between them. The cellular mechanisms of presentation and cross-presentation of exogenous antigens by DCs to T cells are described. We discuss intracellular antigen processing in DCs, cross-dressing, and the acquisition of the antigen cross-presentation function. A particular section in the review describes the mechanisms of tumor escape from immune surveillance through the suppression of DCs functions.

Trogocytosis-associated cell to cell spread of intracellular bacterial pathogens

Macrophages are myeloid-derived phagocytic cells and one of the first immune cell types to respond to microbial infections. However, a number of bacterial pathogens are resistant to the antimicrobial activities of macrophages and can grow within these cells. Macrophages have other immune surveillance roles including the acquisition of cytosolic components from multiple types of cells. We hypothesized that intracellular pathogens that can replicate within macrophages could also exploit cytosolic transfer to facilitate bacterial spread. We found that viable Francisella tularensis, as well as Salmonella enterica bacteria transferred from infected cells to uninfected macrophages along with other cytosolic material through a transient, contact dependent mechanism. Bacterial transfer occurred when the host cells exchanged plasma membrane proteins and cytosol via a trogocytosis related process leaving both donor and recipient cells intact and viable. Trogocytosis was strongly associated with infection in mice, suggesting that direct bacterial transfer occurs by this process in vivo.