C1q-Dependent Dendritic Cell Cross-Presentation of In Vivo-Formed Antigen-Antibody Complexes

Antibody modulation of B cell responses-Incorporating positive and negative feedback

Antibodies are powerful modulators of ongoing and future B cell responses. While the concept of antibody feedback has been appreciated for over a century, the topic has seen a surge in interest due to the evidence that the broadening of antibody responses to SARS-CoV-2 after a third mRNA vaccination is a consequence of antibody feedback. Moreover, the discovery that slow antigen delivery can lead to more robust humoral immunity has put a spotlight on the capacity for early antibodies to augment B cell responses. Here, we review the mechanisms whereby antibody feedback shapes B cell responses, integrating findings in humans and in mouse models. We consider the major influence of epitope masking and the diverse actions of complement and Fc receptors and provide a framework for conceptualizing the ways antigen-specific antibodies may influence B cell responses to any form of antigen, in conditions as diverse as infectious disease, autoimmunity, and cancer.

How B cells drive T-cell responses: A key role for cross-presentation of antibody-targeted antigens

In this review we discuss an underexposed mechanism in the adaptive immune system where B cell and T cell immunity collaborate. The main function of B cell immunity is the generation of antibodies which are well known for their high affinity and antigen-specificity. Antibodies can bind antigens in soluble form making so-called immune complexes (ICs) or can opsonize antigen-exposing cells or particles for degradation. This leads to well-known effector mechanisms complement activation, antibody-dependent cytotoxicity and phagocytosis. What is less realized is that antibodies can play an important role in the targeting of antigen to dendritic cells (DCs) and thereby can drive T cell immunity. Here we summarize the studies that described this highly efficient process of antibody-mediated antigen uptake in DCs in vitro and in vivo. Only very low doses of antigen can be captured by circulating antibodies and subsequently trapped by DCs in vivo. We studied the handling of these ICs by DCs in subcellular detail. Upon immune complex engulfment DCs can sustain MHC class I and II antigen presentation for many days. Cell biological analysis showed that this function is causally related to intracellular antigen-storage compartments which are functional endolysosomal organelles present in DCs. We speculate that this function is immunologically very important as DCs require time to migrate from the site of infection to the draining lymph nodes to activate T cells. The implications of these findings and the consequences for the immune system, immunotherapy with tumor-specific antibodies and novel vaccination strategies are discussed.

Complement C1q essential for aeroallergen sensitization via CSF1R+ conventional dendritic cells type 2

BACKGROUND

Dendritic cells (DCs) are heterogeneous, comprising multiple subsets with unique functional specifications. Our previous work has demonstrated that the specific conventional type 2 DC subset, CSF1R+cDC2s, plays a critical role in sensing aeroallergens.

OBJECTIVE

It remains to be understood how CSF1R+cDC2s recognize inhaled allergens. We sought to elucidate the transcriptomic programs and receptor-ligand interactions essential for function of this subset in allergen sensitization.

METHODS

We applied single-cell RNA sequencing to mouse lung DCs. Conventional DC-selective knockout mouse models were employed, and mice were subjected to inhaled allergen sensitization with multiple readouts of asthma pathology. Under the clinical arm of this work, human lung transcriptomic data were integrated with mouse data, and bronchoalveolar lavage (BAL) specimens were collected from subjects undergoing allergen provocation, with samples assayed for C1q.

RESULTS

We found that C1q is selectively enriched in lung CSF1R+cDC2s, but not in other lung cDC2 or cDC1 subsets. Depletion of C1q in conventional DCs significantly attenuates allergen sensing and features of asthma. Additionally, we found that C1q binds directly to human dust mite allergen, and the C1q receptor CD91 (LRP1) is required for lung CSF1R+cDC2s to recognize the C1q-allergen complex and induce allergic lung inflammation. Lastly, C1q is enriched in human BAL samples following subsegmental allergen challenge, and human RNA sequencing data demonstrate close homology between lung IGSF21+DCs and mouse CSF1R+cDC2s.

CONCLUSIONS

C1q is secreted from the CSF1R+cDC2 subset among conventional DCs. Our data indicate that the C1q-LRP1 axis represents a candidate for translational therapeutics in the prevention and suppression of allergic lung inflammation.

The show and tell of cross-presentation

Cross-presentation is the culmination of complex subcellular processes that allow the processing of exogenous proteins and the presentation of resultant peptides on major histocompatibility class I (MHC-I) molecules to CD8 T cells. Dendritic cells (DCs) are a cell type that uniquely specializes in cross-presentation, mainly in the context of viral or non-viral infection and cancer. DCs have an extensive network of endovesicular pathways that orchestrate the biogenesis of an ideal cross-presentation compartment where processed antigen, MHC-I molecules, and the MHC-I peptide loading machinery all meet. As a central conveyor of information to CD8 T cells, cross-presentation allows cross-priming of T cells which carry out robust adaptive immune responses for tumor and viral clearance. Cross-presentation can be canonical or noncanonical depending on the functional status of the transporter associated with antigen processing (TAP), which in turn influences the vesicular route of MHC-I delivery to internalized antigen and the cross-presented repertoire of peptides. Because TAP is a central node in MHC-I presentation, it is targeted by immune evasive viruses and cancers. Thus, understanding the differences between canonical and noncanonical cross-presentation may inform new therapeutic avenues against cancer and infectious disease. Defects in cross-presentation on a cellular and genetic level lead to immune-related disease progression, recurrent infection, and cancer progression. In this chapter, we review the process of cross-presentation beginning with the DC subsets that conduct cross-presentation, the signals that regulate cross-presentation, the vesicular trafficking pathways that orchestrate cross-presentation, the modes of cross-presentation, and ending with disease contexts where cross-presentation plays a role.

The evolving biology of cross-presentation

Cross-priming was first recognized in the context of in vivo cytotoxic T lymphocyte (CTL) responses generated against minor histocompatibility antigens induced by immunization with lymphoid cells. Even though the basis for T cell antigen recognition was still largely unclear at that time, these early studies recognized the implication that such minor histocompatibility antigens were derived from the immunizing cells and were obtained exogenously by the host's antigen presenting cells (APCs) that directly prime the CTL response. As antigen recognition by the T cell receptor became understood to involve peptides derived from antigens processed by the APCs and presented by major histocompatibility molecules, the "cross-priming" phenomenon was subsequently recast as "cross-presentation" and the scope considered for examining this process gradually broadened to include many different forms of antigens, including soluble proteins, and different types of APCs that may not be involved in in vivo CTL priming. Many studies of cross-presentation have relied on in vitro cell models that were recently found to differ from in vivo APCs in particular mechanistic details. A recent trend has focused on the APCs and pathways of cross-presentation used in vivo, especially the type 1 dendritic cells. Current efforts are also being directed towards validating the in vivo role of various putative pathways and gene candidates in cross-presentation garnered from various in vitro studies and to determine the relative contributions they make to CTL responses across various forms of antigens and immunologic settings. Thus, cross-presentation appears to be carried by different pathways in various types of cells for different forms under different physiologic settings, which remain to be evaluated in an in vivo physiologic setting.

Regulation of dendritic cell function by Fc-γ-receptors and the neonatal Fc receptor

Dendritic cells (DCs) express receptors to sense pathogens and/or tissue damage and to communicate with other immune cells. Among those receptors, Fc receptors (FcRs) are triggered by the Fc region of antibodies produced during adaptive immunity. In this review, the role of FcγR and neonatal Fc receptor (FcRn) in DC immunity will be discussed. Their expression in DC subsets and impact on antigen uptake and presentation, DC maturation and polarisation of T cell responses will be described. Lastly, we will discuss the importance of FcR-mediated DC function in the context of immunity during viral infection, inflammatory disease, cancer and immunotherapy.

Combating Human Viral Diseases: Will Plant-Based Vaccines Be the Answer?

Molecular pharming or the technology of application of plants and plant cell culture to manufacture high-value recombinant proteins has progressed a long way over the last three decades. Whether generated in transgenic plants by stable expression or in plant virus-based transient expression systems, biopharmaceuticals have been produced to combat several human viral diseases that have impacted the world in pandemic proportions. Plants have been variously employed in expressing a host of viral antigens as well as monoclonal antibodies. Many of these biopharmaceuticals have shown great promise in animal models and several of them have performed successfully in clinical trials. The current review elaborates the strategies and successes achieved in generating plant-derived vaccines to target several virus-induced health concerns including highly communicable infectious viral diseases. Importantly, plant-made biopharmaceuticals against hepatitis B virus (HBV), hepatitis C virus (HCV), the cancer-causing virus human papillomavirus (HPV), human immunodeficiency virus (HIV), influenza virus, zika virus, and the emerging respiratory virus, severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) have been discussed. The use of plant virus-derived nanoparticles (VNPs) and virus-like particles (VLPs) in generating plant-based vaccines are extensively addressed. The review closes with a critical look at the caveats of plant-based molecular pharming and future prospects towards further advancements in this technology. The use of biopharmed viral vaccines in human medicine and as part of emergency response vaccines and therapeutics in humans looks promising for the near future.

Isotype selection for antibody-based cancer therapy

The clinical application of monoclonal antibodies (mAbs) has revolutionized the field of cancer therapy, as it has enabled the successful treatment of previously untreatable types of cancer. Different mechanisms play a role in the anti-tumour effect of mAbs. These include blocking of tumour-specific growth factor receptors or of immune modulatory molecules as well as complement and cell-mediated tumour cell lysis. Thus, for many mAbs, Fc-mediated effector functions critically contribute to the efficacy of treatment. As immunoglobulin (Ig) isotypes differ in their ability to bind to Fc receptors on immune cells as well as in their ability to activate complement, they differ in the immune responses they activate. Therefore, the choice of antibody isotype for therapeutic mAbs is dictated by its intended mechanism of action. Considering that clinical efficacy of many mAbs is currently achieved only in subsets of patients, optimal isotype selection and Fc optimization during antibody development may represent an important step towards improved patient outcome. Here, we discuss the current knowledge of the therapeutic effector functions of different isotypes and Fc-engineering strategies to improve mAbs application.

A Highly Expressing, Soluble, and Stable Plant-Made IgG Fusion Vaccine Strategy Enhances Antigen Immunogenicity in Mice Without Adjuvant

Therapeutics based on fusing a protein of interest to the IgG Fc domain have been enormously successful, though fewer studies have investigated the vaccine potential of IgG fusions. In this study, we systematically compared the key properties of seven different plant-made human IgG1 fusion vaccine candidates using Zika virus (ZIKV) envelope domain III (ZE3) as a model antigen. Complement protein C1q binding of the IgG fusions was enhanced by: 1) antigen fusion to the IgG N-terminus; 2) removal of the IgG light chain or Fab regions; 3) addition of hexamer-inducing mutations in the IgG Fc; 4) adding a self-binding epitope tag to create recombinant immune complexes (RIC); or 5) producing IgG fusions in plants that lack plant-specific β1,2-linked xylose and α1,3-linked fucose N-linked glycans. We also characterized the expression, solubility, and stability of the IgG fusions. By optimizing immune complex formation, a potently immunogenic vaccine candidate with improved solubility and high stability was produced at 1.5 mg IgG fusion per g leaf fresh weight. In mice, the IgG fusions elicited high titers of Zika-specific antibodies which neutralized ZIKV using only two doses without adjuvant, reaching up to 150-fold higher antibody titers than ZE3 antigen alone. We anticipate these findings will be broadly applicable to the creation of other vaccines and antibody-based therapeutics.

The neonatal Fc receptor in cancer FcRn in cancer

Since the neonatal IgG Fc receptor (FcRn) was discovered, it was found to be involved in immunoglobulin recycling and biodistribution, immune complexes routing, antigen presentation, humoral immune response, and cancer immunosurveillance. The latest data show that FcRn plays a part in cancer pathophysiology. In various types of cancers, such as lung and colorectal cancer, FcRn has been described as an early marker for prognosis. Dysregulation of FcRn expression by cancer cells allows them to increase their metabolism, and this process could be exploited for passive targeting of cytotoxic drugs. However, the roles of this receptor depend on whether the studied cell population is the tumor tissue or the infiltrating cells, bringing forward the need for further studies.

Codelivery of improved immune complex and virus-like particle vaccines containing Zika virus envelope domain III synergistically enhances immunogenicity

Zika virus (ZIKV) reemergence poses a significant health threat especially due to its risks to fetal development, necessitating safe and effective vaccines that can protect pregnant women. Zika envelope domain III (ZE3) has been identified as a safe and effective vaccine candidate, however it is poorly immunogenic. We previously showed that plant-made recombinant immune complex (RIC) vaccines are a robust platform to improve the immunogenicity of weak antigens. In this study, we altered the antigen fusion site on the RIC platform to accommodate N-terminal fusion to the IgG heavy chain (N-RIC), and thus a wider range of antigens, with a resulting 40% improvement in RIC expression over the normal C-terminal fusion (C-RIC). Both types of RICs containing ZE3 were efficiently assembled in plants and purified to >95% homogeneity with a simple one-step purification. Both ZE3 RICs strongly bound complement receptor C1q and elicited strong ZE3-specific antibody titers that correlated with ZIKV neutralization. When either N-RIC or C-RIC was codelivered with plant-produced hepatitis B core (HBc) virus-like particles (VLP) displaying ZE3, the combination elicited 5-fold greater antibody titers (>1,000,000) and more strongly neutralized ZIKV than either RICs or VLPs alone, after only two doses without adjuvant. These findings demonstrate that antigens that require a free N-terminus for optimal antigen display can now be used with the RIC system, and that plant-made RICs and VLPs are highly effective vaccines targeting ZE3. Thus, the RIC platform can be more generally applied to a wider variety of antigens.

The Complex Association of FcγRIIb With Autoimmune Susceptibility

FcγRIIb is the only inhibitory Fc receptor and controls many aspects of immune and inflammatory responses. The observation 19 years ago that Fc γ RIIb ^-/- mice generated by gene targeting in 129 derived ES cells developed severe lupus like disease when backcrossed more than 7 generations into C57BL/6 background initiated extensive research on the functional understanding of this strong autoimmune phenotype. The genomic region in the distal part of Chr1 both in human and mice in which the Fc γ R gene cluster is located shows a high level of complexity in relation to the susceptibility to SLE. Specific haplotypes of closely linked genes including the Fc γ RIIb and Slamf genes are associated with increased susceptibility to SLE both in mice and human. Using forward and reverse genetic approaches including in human GWAS and in mice congenic strains, KO mice (germline and cell type specific, on different genetic background), knockin mice, overexpressing transgenic mice combined with immunological models such as adoptive transfer of B cells from Ig transgenic mice the involved genes and the causal mutations and their associated functional alterations were analyzed. In this review the results of this 19 years extensive research are discussed with a focus on (genetically modified) mouse models.

Adjuvants Enhancing Cross-Presentation by Dendritic Cells: The Key to More Effective Vaccines?

Over the last decades, vaccine development has advanced significantly in pursuing higher safety with less side effects. However, this is often accompanied by a reduction in vaccine immunogenicity and an increased dependency on adjuvants to enhance vaccine potency. Especially for diseases like cancer, it is important that therapeutic vaccines contain adjuvants that promote strong T cell responses. An important mode of action for such adjuvants is to prolong antigen exposure to dendritic cells (DCs) and to induce their maturation. These mature DCs are extremely effective in the activation of antigen-specific T cells, which is a pre-requisite for induction of potent and long-lasting cellular immunity. For the activation of CD8⁺ cytotoxic T cell responses, however, the exogenous vaccine antigens need to gain access to the endogenous MHCI presentation pathway of DCs, a process referred to as antigen cross-presentation. In this review, we will focus on recent insights in clinically relevant vaccine adjuvants that impact DC cross-presentation efficiency, including aluminum-based nanoparticles, saponin-based adjuvants, and Toll-like receptor ligands. Furthermore, we will discuss the importance of adjuvant combinations and highlight new developments in cancer vaccines. Understanding the mode of action of adjuvants in general and on antigen cross-presentation in DCs in particular will be important for the design of novel adjuvants as part of vaccines able to induce strong cellular immunity.

Gene expression analysis of porcine whole blood cells infected with foot-and-mouth disease virus using high-throughput sequencing technology

Foot-and-mouth disease virus (FMDV) is a single-stranded positive RNA virus that belongs to the family Picornaviridae. FMDV infects cloven-hoofed animals, such as pigs, sheep, goats, cattle and diverse wildlife species, and remains a major threat to the livestock industry worldwide. In this study, a transcriptome analysis of whole blood from pigs infected with FMDV was performed using the paired-end Illumina sequencing technique to understand the interactions between the pathogen and its host cells. During infection with FMDV, a total of 120 differentially expressed genes (DEGs) were identified, including 110 up-regulated genes and 10 down-regulated genes. To further investigate the DEGs involved in interactions between the virus and its host, gene ontology (GO) annotation and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment were conducted. GO annotation indicated that a number of DEGs were enriched in categories involved in host-virus interactions, such as response to stimulus, immune system process and regulation of biological process. KEGG enrichment analysis indicated that the DEGs were primarily involved in the ribosome signaling pathway and immune-related signaling pathways. Ten DEGs, including the immune-related genes BTK1, C1QB, TIMD4 and CXCL10, were selected and validated using quantitative PCR, which showed that the expression patterns of these genes are consistent with the results of the in silico expression analysis. In conclusion, this study presents the first transcriptome analysis of pig whole blood cells infected with FMDV, and the results obtained in this study improve our understanding of the interactions between FMDV and host cells as well as the diagnosis and control of FMD.

Sustained cross-presentation capacity of murine splenic dendritic cell subsets in vivo

An exclusive feature of dendritic cells (DCs) is their ability to cross‐present exogenous antigens in MHC class I molecules. We analyzed the fate of protein antigen in antigen presenting cell (APC) subsets after uptake of naturally formed antigen‐antibody complexes in vivo. We observed that murine splenic DC subsets were able to present antigen in vivo for at least a week. After ex vivo isolation of four APC subsets, the presence of antigen in the storage compartments was visualized by confocal microscopy. Although all APC subsets stored antigen for many days, their ability and kinetics in antigen presentation was remarkably different. CD8α⁺ DCs showed sustained MHC class I‐peptide specific CD8⁺ T‐cell activation for more than 4 days. CD8α⁻ DCs also presented antigenic peptides in MHC class I but presentation decreased after 48 h. In contrast, only the CD8α⁻ DCs were able to present antigen in MHC class II to specific CD4⁺ T cells. Plasmacytoid DCs and macrophages were unable to activate any of the two T‐cell types despite detectable antigen uptake. These results indicate that naturally occurring DC subsets have functional antigen storage capacity for prolonged T‐cell activation and have distinct roles in antigen presentation to specific T cells in vivo.

Formation of Immune Complexes with a Tetanus-Derived B Cell Epitope Boosts Human T Cell Responses to Covalently Linked Peptides in an Ex Vivo Blood Loop System

Enhancing T cell responses against both viral and tumor Ags requires efficient costimulation and directed delivery of peptide Ags into APCs. Long peptide vaccines are considered favorable vaccine moieties from a clinical perspective, as they can harbor more than one immunogenic epitope enabling treatment of a broader target population. In addition, longer peptides are not extracellularly loaded on MHC class I; rather, they require intracellular processing and will thereby be presented to T cells mainly by professional APCs, thereby avoiding the risk of tolerance induction. The drawback of peptide vaccines regardless of peptide length is that naked peptides are not actively targeted to and taken up by APCs, and the standard nonconjugated adjuvant-peptide mixtures do not ensure cotargeting of the two to the same APC. We have identified a tetanus toxin-derived B cell epitope that can mediate the formation of immune complexes in the presence of circulating Abs. In this study, we show that these immune complexes improve both Ag uptake by APCs (blood monocytes and CD1c⁺ dendritic cells) and consequently improve CD8⁺ T cell recall responses in a human ex vivo blood loop system. The uptake of the peptide conjugate by blood monocytes is dependent on Abs and the complement component C1q. We envision that this strategy can be used to facilitate active uptake of Ags into APCs to improve T cell responses against pathogens or cancer.

Regulation of the Cell Biology of Antigen Cross-Presentation

Antigen cross-presentation is an adaptation of the cellular process of loading MHC-I molecules with endogenous peptides during their biosynthesis within the endoplasmic reticulum. Cross-presented peptides derive from internalized proteins, microbial pathogens, and transformed or dying cells. The physical separation of internalized cargo from the endoplasmic reticulum, where the machinery for assembling peptide-MHC-I complexes resides, poses a challenge. To solve this problem, deliberate rewiring of organelle communication within cells is necessary to prepare for cross-presentation, and different endocytic receptors and vesicular traffic patterns customize the emergent cross-presentation compartment to the nature of the peptide source. Three distinct pathways of vesicular traffic converge to form the ideal cross-presentation compartment, each regulated differently to supply a unique component that enables cross-presentation of a diverse repertoire of peptides. Delivery of centerpiece MHC-I molecules is the critical step regulated by microbe-sensitive Toll-like receptors. Defining the subcellular sources of MHC-I and identifying sites of peptide loading during cross-presentation remain key challenges.

CD4+ T Cell and NK Cell Interplay Key to Regression of MHC Class Ilow Tumors upon TLR7/8 Agonist Therapy

One of the next challenges in cancer immunotherapy is the resistance of tumors to T-cell-based treatments through loss of MHC class I. Here, we show that under these circumstances, the Toll-like receptor (TLR)-7/8 ligand imiquimod, but not the TLR3 ligand poly I:C or TLR9 ligand CpG, mediated an effective antitumor response. The rejection of these immune-escaped cancers was mediated by NK cells and CD4⁺ T cells, whereas activated CD8⁺ T cells were dispensable. Application of the innate immune stimulator at a distant site activated NK cells and thereby elicited tumor-specific T-cell responses in tumor-bearing mice. Mechanistically, imiquimod activated NK cells to kill tumor cells, resulting in release of tumor antigens and induction of tumor-specific CD4⁺ T cells. These T helper cells provoked a strong induction of CXCL9 and CXCL10 in the tumor environment. Simultaneously, imiquimod induced the expression of the cognate chemokine receptor CXCR3 on peripheral lymphocytes. This ignited intratumoral CD4⁺ T-cell infiltration and accumulation, which was critical for tumor rejection; CXCR3 blocking antibodies mitigated the clinical response. In the effector phase, NK cell recruitment to tumors and their activation depended on CD4⁺ T cells. Together, we have uncovered a potent immune axis of tumor-specific CD4⁺ T cells and NK cells that eliminates escaped MHC-I^low tumors. Cancer Immunol Res; 5(8); 642-53. ©2017 AACR.

C1q: A fresh look upon an old molecule

Originally discovered as part of C1, the initiation component of the classical complement pathway, it is now appreciated that C1q regulates a variety of cellular processes independent of complement activation. C1q is a complex glycoprotein assembled from 18 polypeptide chains, with a C-terminal globular head region that mediates recognition of diverse molecular structures, and an N-terminal collagen-like tail that mediates immune effector mechanisms. C1q mediates a variety of immunoregulatory functions considered important in the prevention of autoimmunity such as the enhancement of phagocytosis, regulation of cytokine production by antigen presenting cells, and subsequent alteration in T-lymphocyte maturation. Furthermore, recent advances indicate additional roles for C1q in diverse physiologic and pathologic processes including pregnancy, tissue repair, and cancer. Finally, C1q is emerging as a critical component of neuronal network refinement and homeostatic regulation within the central nervous system. This review summarizes the classical functions of C1q and reviews novel discoveries within the field.