Adjuvants Enhancing Cross-Presentation by Dendritic Cells PDF

REVIEW published: 13 December 2018...

REVIEW published: 13 December 2018 doi: 10.3389/fimmu.2018.02874 Adjuvants Enhancing Cross-Presentation by Dendritic Cells: The Key to More Effective Vaccines? Nataschja I. Ho 1† , Lisa G. M. Huis in ’t Veld 1† , Tonke K. Raaijmakers 1,2 and Gosse J. Adema 1* 1 Radiotherapy and OncoImmunology Laboratory, Department of Radiation Oncology, Radboud Institute for Molecular Life Sciences, Radboud University Medical Center, Nijmegen, Netherlands, 2 Department of Anesthesiology, Pain and Palliative Medicine, Radboud University Medical Center, Nijmegen, Netherlands 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 Edited by: Sandra Tuyaerts, of action for such adjuvants is to prolong antigen exposure to dendritic cells (DCs) KU Leuven, Belgium and to induce their maturation. These mature DCs are extremely effective in the Reviewed by: activation of antigen-specific T cells, which is a pre-requisite for induction of potent and Irina Caminschi, Monash University, Australia long-lasting cellular immunity. For the activation of CD8+ cytotoxic T cell responses, Sandra Stephanie Diebold, however, the exogenous vaccine antigens need to gain access to the endogenous King’s College London, MHCI presentation pathway of DCs, a process referred to as antigen cross-presentation. United Kingdom In this review, we will focus on recent insights in clinically relevant vaccine adjuvants *Correspondence: Gosse J. Adema that impact DC cross-presentation efficiency, including aluminum-based nanoparticles, [email protected] saponin-based adjuvants, and Toll-like receptor ligands. Furthermore, we will discuss † These authors have contributed the importance of adjuvant combinations and highlight new developments in cancer equally to this work 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 Specialty section: This article was submitted to as part of vaccines able to induce strong cellular immunity. Vaccines and Molecular Therapeutics, Keywords: adjuvants, dendritic cell, cross-presentation, aluminum, saponin, TLR, vaccine a section of the journal Frontiers in Immunology Received: 08 October 2018 INTRODUCTION Accepted: 22 November 2018 Published: 13 December 2018 Since the development of the first successful vaccine by Edward Jenner in 1796 against smallpox, Citation: a lot of research has been done on the development of vaccines against other diseases. Current Ho NI, Huis in ’t Veld LGM, vaccines against infectious agents can be divided into live attenuated vaccines (where their virulent Raaijmakers TK and Adema GJ (2018) properties are weakened, e.g., yellow fever, measles), subunit vaccines (containing a fragment of Adjuvants Enhancing Cross-Presentation by Dendritic Cells: the pathogen, e.g., Hepatitis B), toxoid vaccines (with inactivated toxic compounds, e.g., tetanus, The Key to More Effective Vaccines?. diphtheria), and conjugated vaccines (linking polysaccharide coats to protein, e.g., Haemophilus Front. Immunol. 9:2874. influenzae type B) (1). While especially prophylactic vaccines against infectious diseases have been doi: 10.3389/fimmu.2018.02874 developed successfully and are clinically applied, development of therapeutic vaccines against Frontiers in Immunology | www.frontiersin.org 1 December 2018 | Volume 9 | Article 2874 Ho et al. Adjuvants Enhancing Dendritic Cell Cross-Presentation persistent infections or cancer is lagging behind. For the MAPK and NF-κB signaling resulting in TNF-α, IL-12, and development of new vaccines many aspects should be taken into IL-6 production (19, 20). Intracellular TLRs, which are mostly consideration such as the nature of the antigenic material, the found in endosomes, require internalized ligands such as nucleic type of immune memory responses that needs to be induced, acids to activate downstream signaling. Currently, only the TLR4 but also the administration and delivery routes, which might agonist monophosphoryl lipd (MPL), a non-toxic LPS-derived reduce the risk of side effects. Next generation vaccines like TLR4 ligand, is approved for human applications (Table 1). subunit vaccines for infectious diseases mostly aim for higher Other TLR ligands showed effective tumor immunity in animal safety with less side effects, which is often detrimental for their models or clinical trials (21–23). immunogenicity. Therefore, adjuvants are usually required to Alternative pathways for DC maturation include intracellular enhance vaccine potency. Similarly, tumor neoantigen vaccines receptors, such as Nucleotide binding domain-Like Receptor are devoid of immune activation potential and are fully Protein 3 (NLRP3), which forms a caspase-1 activating dependent on strong adjuvants to induce protective immune complex (inflammasome) together with Cardinal and apoptosis- responses. Adjuvants generally act by activating innate and associated speck-like protein containing a caspase recruitment adaptive immune responses, but can also function to create domain (24). This pathway results in cleavage and release of an antigen depot, slowly releasing the antigen for prolonged the pro-inflammatory cytokines IL-1β, IL-18, and IL-33 (25). A presentation and stimulation of the immune system (2). One very important characteristic of adjuvants that has received much of the first licensed carrier-adjuvants was alum, an inorganic less attention is their ability to induce presentation of exogenous adjuvant widely used in vaccines against e.g., hepatitis B virus, antigens not only in MHCII to CD4+ T cells but also in MHCI human papillomavirus, and diphtheria. Like most of the early to CD8+ T cells. This latter process is essential for efficient CD8+ adjuvants, they were mainly aimed at inducing protective T cell priming and is called antigen cross-presentation. In this antibody responses and hence strongly Th2 biased immunity. review, we will focus on recent insights in clinically relevant The discovery of microbe sensing pattern recognition receptors adjuvants that impact DC cross-presentation. Understanding DC (PRRs), such as Toll-like receptors (TLRs) and nucleotide- cross-presentation will be important to design novel adjuvants binding oligomerization domain (NOD)-like receptors, has able to induce strong cellular immunity for future vaccine boosted research into vaccine adjuvants aiming to induce development. cellular immune responses that are essential to fight intracellular pathogens and cancer cells. Interaction of PRR with their corresponding ligands potentiate and shape the adaptive immune MOLECULAR MECHANISMS OF responses (3). Since then, several types of immune potentiating DENDRITIC CELL CROSS-PRESENTATION adjuvants (e.g., TLR agonists and saponin QS-21) have been licensed and used in the clinic against various diseases (Table 1). Dendritic cells are the professional APCs of our immune Each adjuvant has a unique immunological signature that system that are key in linking innate and adaptive immunity. can be used in highly different types of diseases. Choosing the DCs are especially known for their ability to cross-present, right adjuvant to combine with the best target antigen for a as they process and present exogenous antigens on MHCI given disease is a challenging task (12). Next generation vaccine molecules much more efficiently than other phagocytes. The adjuvants are now mostly designed to contain both the function efficiency of CD8+ T cell priming called cross-priming by of a carrier and a potent immune response inducer to boost the DCs is dependent on both antigen cross-presentation efficiency efficacy of the vaccine. Although many prophylactic vaccines rely (number of a given MHCI/peptide complex on the cell surface) on neutralizing antibody responses, especially diseases such as and the level of DC maturation (expression levels of co- cancer, HIV, tuberculosis, and malaria are in need of a vaccine stimulatory molecules and cytokines). It has been reported that eliciting strong T cell responses (13–17). As a consequence, many cross-presentation is important for inducing T cell responses studies investigated the potency of next generation adjuvants specific for tumor antigens and infectious diseases (26–28). for their capacity to induce antigen specific CD8+ and CD4+ How exogenous antigens are processed in DCs and presented T cell responses. An important characteristic of adjuvants able on MHCI to CD8+ T cells is still not fully understood. Two to induce cellular immunity is the efficient delivery of the main pathways of antigen cross-presentation in DCs have been target antigen into professional antigen presenting dendritic cells proposed: the cytosolic pathway and the vacuolar pathway. In (DCs) and its potency in activating these DCs. In general, DC the cytosolic pathway, exogenous antigens or protein fragments maturation enhances their antigen presentation capacity and derived from it are transported from endosomal vesicles into the ability to activate T cells and is a prerequisite for induction cytosol where they are degraded by the proteasome. The trimmed of potent and long-lasting immunity. One of the best studied peptides are then transported by the transporter associated with DC maturation stimuli are TLR ligands, including poly(I:C), antigen processing (TAP) to the endoplasmic reticulum (ER) LPS, CpG, R848, and Pam3 CSK4 , which can activate DCs to where they are loaded on MHCI molecules (29–31). However, upregulate co-stimulatory molecules such as CD40, CD80, and there have been suggestions that the protein fragments can be CD86 (18). TLRs can be expressed extracellularly (TLRs 1, 2, transported back into endocytic compartments and trimmed by 4, 5, and 6) and intracellularly (TLRs 3, 7, 8, and 9) (3). All insulin-regulated aminopeptidase (IRAP) and loaded on MHCI TLRs, except TLR3, utilize the adaptor molecule MyD88 to (32). In the vacuolar pathway antigens are degraded by proteases trigger activation of TGF-β Activated Kinase 1 which activates in endo/lysosomal compartments and directly loaded on MHCI Frontiers in Immunology | www.frontiersin.org 2 December 2018 | Volume 9 | Article 2874 Ho et al. Adjuvants Enhancing Dendritic Cell Cross-Presentation TABLE 1 | Clinically approved adjuvants. Adjuvant Description Proposed immune mechanism Clinical application Aluminum salts Hydroxide, phosphate, alum Activation of NLRP3 inflammasome and caspase-1 in HBV, HPV, diphtheria, and tetanus DCs, induces Th2 response (4, 5). AS01 Liposome (containing MPL Activates APCs expressing TLR4, stimulates cytokine Malaria, Herpes Zoster and QS-21) and co-stimulatory molecules production, promotes antigen-specific antibody responses and stimulates CD8+ T cells (6). AS02 Oil-in-water emulsion Antigen specific CD8+ and CD4+ T cell responses and Malaria (containing MPL and QS-21) antibody responses (7) AS03 Oil-in-water emulsion NF-κB activation, production of cytokines and Pandemic influenza (containing squalene, chemokines in muscle and draining LN, provoke polysorbate 80 and migration of monocytes, DCs and granulocytes into α-tocopherol) draining LN, enhancing CD4+ T cell immune responses (8). AS04 MPL formulated in Activates TLR4 on DCs, induction of cytokines and HBV, HPV aluminum salt antigen specific T cell activation (9). MF59 Oil-in-water emulsion Rapid influx of CD11b+ cells, upregulation of Seasonal and pandemic influenza inflammatory cytokines and chemokines, recruitment of APCs (10). Virosomes Lipid vesicle containing Virosomal-adjuvanted influenza vaccine (Inflexal® V) Influenza, Hepatitis A inactivated viral proteins increases antibody titer (11). NLRP3, nucleotide binding domain-like receptor protein 3; DCs, dendritic cells; HBV, Hepatitis B virus; HPV, human papillomavirus; MPL, monophosphoryl lipid; LN, lymph node. molecules (33, 34). A comprehensive overview of these and suggested (41). Although Sec22b seems to regulate antigen cross- alternative cross-presentation pathways in DCs has recently been presentation in the vacuolar pathway, it is not ruled out that it reviewed (35). can play a role in the cytosolic pathway. How antigens are transported from the endosomes to the Two recent studies report on regulation of antigen cross- cytosol is still under debate. Extensive studies in murine models presentation in DCs by stromal interaction molecule 1 (STIM1), identified the ER-associated degradation (ERAD) member, Sec61, a calcium sensor that conveys the calcium content of the as a possible translocator for antigen from the endosomes into ER to store-operated channels of a cell (42, 43). Nunes- the cytosol. Applying a Sec61-specific intracellular antibody, Hasler and colleagues showed that STIM1 can promote the Zehner et al. showed that they could trap Sec61 in the ER contact sites between the ER and phagosomes (42). This and prevent its transport toward endosomes, thereby blocking induces Ca2+ signaling and thereby the migration and fusion antigen translocation and cross-presentation (36). However, a of phagosomes with endosomes or lysosomes to enable efficient more recent study using mycolactone, which binds specifically cross-presentation in DCs. In a companion study it was shown to Sec61α, showed severe inhibition of protein import into the that the ER membrane protein uncoordinated 93 homolog ER but no inhibition of ERAD or protein export from endocytic B1 (UCN93B1) interacts with STIM1 and can control cross- compartments (37). Although, both studies showed inhibition presentation in DCs (43). Ablation of UCN93B1 impairs phago- of DC cross-presentation upon blocking of Sec61, it seems lysosomal fusion, proteolytic activity, and antigen export to that Sec61 plays a more dominant role in inhibiting protein the cytosol, resulting in a decrease of antigen degradation and translocation into the ER and altering antigen cross-presentation cross-presentation. Others showed that antigen transportation at a different level than antigen export to the cytosol. into the cytosol is enhanced by NADPH-oxidase complex Another ongoing debate is how ER proteins are translocated (NOX2) and reactive oxygen species (ROS) production in to endosomes in DCs for efficient cross-presentation. The the endosomes (44). Reactive oxygen species causes lipid group of Amigorena proposed that recruitment of ER and peroxidation, which disrupts the endosomal membrane, resulting ER-Golgi intermediate compartment (ERGIC) components to in antigen leakage from endosomes. Furthermore, it has been phagosomes is mediated by the ER-resident SNARE Sec22b shown that NOX2 can be recruited to the endosomes to induce (38). Silencing of Sec22b uncovered that phagosome-lysosome alkalization upon ROS release (45). This will cause an increase interactions were delayed, thereby limiting proteolysis and of endosomal pH thereby preventing rapid antigen degradation, preserving antigenic fragments for cross-presentation, which resulting in enhanced antigen cross-presentation. The group of was recently also confirmed in conditional Sec22b-knockout Guermonprez suggested that lipid bodies (LBs) are involved in DCs (39). Conflicting results were found using similar Sec22b- DC cross-presentation (46). They showed that the Immunity- knockout DCs (40) and based on a review of both studies related GTPase family member 3 (Irgm3) controls accumulation with respect to technical differences, a role for Sec22b as well of LBs induced by cell activation stimuli including INF-γ and as for unidentified new regulators of cross-presentation was Poly(I:C). LBs are organelles composed of a central core of Frontiers in Immunology | www.frontiersin.org 3 December 2018 | Volume 9 | Article 2874 Ho et al. Adjuvants Enhancing Dendritic Cell Cross-Presentation cholesteryl esters and triglycerides that are surrounded by a single cross-present antibody-bound antigen efficiently after activation layer of phospholipids also containing LB proteins (47). The of Fcγ-receptors (66), but a more recent study showed that Irgm3 protein is localized in the ER and in LBs where it interacts complement factor C1q plays a dominant role in antibody-bound with the LB coat protein adipose differentiation-related protein antigen uptake and cross-presentation in DCs (67). Although, (ADRP). Mice deficient in either Irgm3 or ADRP showed defects some studies have shown the ability of pDCs to cross-present in in LB formation and impaired cross-presentation in DCs. Further vitro or ex vivo (34, 68, 69), their role in cross-presentation in research is needed to understand how LBs control antigen cross- vivo seems lacking during viral infections despite the fact that presentation by DCs and to determine the molecular pathways they are known for their ability in producing large amounts of that control the involvement of LBs. type I interferons (70, 71). However, a recent study showed that upon TLR ligand activation, mitochrondial ROS production is increased independently of NOX2 in pDCs (72). Increased ROS ANTIGEN CROSS-PRESENTATION AND DC production resulted in high endosomal pH, antigen protection SUBSETS from endosomal degradation, and induced export to the cytosol, ultimately leading to enhanced antigen cross-presentation and An important aspect to take into account when choosing an CD8+ T cell priming. adjuvant to induce DC cross-presentation is the type of DC that In human, the cDC subset in blood can roughly be divided will be affected. Intensive research has shown that there are many into BDCA1+ (CD1c+ ) and BDCA3+ (CD141+ ) DCs (73). The DC subsets present in mice as well in human, with still room for BDCA1+ and BDCA3+ subsets are proposed as the human newly unidentified subsets. Murine DCs in secondary lymphoid counterparts of murine CD8α− and CD8α+ DCs, respectively. organs can be divided roughly into conventional DCs (cDCs) It has been shown that BDCA1+ DCs are capable of cross- and plasmacytoid DCs (pDCs). cDCs can be further divided presentation of extracellular antigen (74). Upon activation with into cDC1 (CD8α+ and CD103+ ) and cDC2 (CD8α− , CD11b+ , TLR ligands, BDCA1+ DCs showed similar efficiency in cross- and CD172a+ ) DCs (48). The development of CD8α+ DCs is presentation compared to BDCA3+ DCs (75). A recent study regulated by the transcription factors including inhibitor of DCN showed that in vivo generated monocyte-derived DCs (moDCs) binding 2 (Id2), interferon regulatory factor (IRF) 8, basic leucine and monocyte-derived macrophages can both cross-present zipper ATF-like 3 transcription factor (BATF3), and the nuclear efficiently in a vacuolar-dependent pathway (76). In contrast factor interleukin 3 regulated (NFIL3) (49). The development of to murine pDCs, the human counterpart has been reported CD8α− DCs is orchestrated by the transcription factors including to cross-present soluble, cell-associated antigen efficiently (77). RelB, NOTCH2, RBP-J, IRF2, and IRF4. Deletion of either of However, recent work by the group of Ginhoux has identified a these genes can lead to developmental defects of the DC subsets. pre-DC subset that bears the classical pDC markers, including Mice in which a given DC subset has been selectively depleted, CD123, CD303, and CD304 (78). This pre-DC subset can be e.g., Batf3−/− mice or zinc finger transcription factor knockout distinguished from the classical pDCs by additional markers, studies, have provided important insight in the functional such as CD33, CX3CR1, CD2, CD5, and CD327. Importantly, role of DC subsets in antigen presentation (50, 51). However, they showed that only pre-DCs could induce CD4+ T cell the interpretation of the data in these mice regarding cross- proliferation and IL-12 production compared to classical pDCs. presentation is not always straightforward due to incomplete These data imply that the antigen presenting ability of pDCs depletion, depletion associated side effects, and DC cross- might be a result of “contaminating” pre-DCs. Whether these talk. In general, CD8α+ DCs are considered to be the most pre-DCs can also cross-present to CD8+ T cells is currently potent cross-presenting subset of antigens including proteins, unknown. It will be important to use additional markers to isolate antibody-bound-, cell-associated, and other types of antigens pure pDC subset for future analysis of their antigen presenting in vivo and ex vivo (50, 52–55). The explanations for the capacity. superior cross-presentation ability of CD8α+ DCs include lower So far, most of the aforementioned studies investigating the degradation of antigen in endosomes by ROS production (56), molecular mechanisms of antigen cross-presentation make use more efficient transfer of exogenous antigens into the cytosol of murine DC model systems and require confirmation in the (57), and higher expression of components that are associated human DC setting. Nevertheless, it seems that choosing specific with MHCI processing pathway (55). Emerging data, however, antigen targeting routes can determine the outcome of DC suggest that the cross-presenting ability of each DC subset is cross-presentation efficiency of different subsets. Deciphering the tuned by and dependent on factors such as DC location and molecular mechanisms of cross-presentation in the different DC activation status, the type of antigen, and local inflammatory subtypes in mice and human is needed for the optimal design of signals (58). Indeed, the main DC subset responsible for cross- therapeutic vaccines. presentation in lung, intestine and skin is the migratory CD103+ DCs (59, 60). Although CD8α− DCs are generally considered to be the most potent MHCII antigen presenting subset to CD4+ CLINICALLY RELEVANT ADJUVANTS AND T cells, it has been shown that CD8α− DCs can efficiently ANTIGEN CROSS-PRESENTATION cross-present antibody-bound antigen, antigens from Salmonella typhimurium and S. cerevisiae, or antigen in the presence of During the last years, many groups have been developing saponin adjuvants (61–65). CD8α− DCs have been shown to adjuvants that facilitate uptake by APCs, protect antigens against Frontiers in Immunology | www.frontiersin.org 4 December 2018 | Volume 9 | Article 2874 Ho et al. Adjuvants Enhancing Dendritic Cell Cross-Presentation degradation and stimulate strong immune memory responses consistent with the cytosolic route being the dominant cross- (79). Here, we will focus on new insights in the mode of action presentation pathway activated by the nanoparticle. Interestingly, of clinically relevant adjuvants on antigen cross-presentation while the size and positive charge at neutral pH of AlO(OH) by DCs and subsequent induction of cellular immunity. Many in the traditional vaccine prevented its targeting to lymph studies analysing adjuvants show an enhancement of CD8+ T nodes, AlO(OH) packed into nanoparticles of