Cross-priming in health and disease

Increasing the efficacy of tumor cell vaccines by enhancing cross priming

Cancer immunotherapy has been attempted for more than a century, and investment has intensified in the last 20 years. The complexity of the immune system is exemplified by the myriad of immunotherapeutic approaches under investigation. While anti-tumor immunity has been achieved experimentally with multiple effector cells and molecules, particular promise is shown for harnessing the CD8 T cell response. Tumor cell-based vaccines have been employed in hundreds of clinical trials to date and offer several advantages over subunit and peptide vaccines. However, tumor cell-based vaccines, often aimed at cross priming tumor-reactive CD8 T cells, have shown modest success in clinical trials. Here we review the mechanisms of cross priming and discuss strategies to increase the efficacy of tumor cell-based vaccines. A synthesis of recent findings on tissue culture conditions, cell death, and dendritic cell activation reveals promising new avenues for clinical investigation.

Human tissues contain CD141hi cross-presenting dendritic cells with functional homology to mouse CD103+ nonlymphoid dendritic cells

Dendritic cell (DC)-mediated cross-presentation of exogenous antigens acquired in the periphery is critical for the initiation of CD8⁺ T cell responses. Several DC subsets are described in human tissues but migratory cross-presenting DCs have not been isolated, despite their potential importance in immunity to pathogens, vaccines, and tumors and tolerance to self. Here, we identified a CD141^hi DC present in human interstitial dermis, liver, and lung that was distinct from the majority of CD1c⁺ and CD14⁺ tissue DCs and superior at cross-presenting soluble antigens. Cutaneous CD141^hi DCs were closely related to blood CD141⁺ DCs, and migratory counterparts were found among skin-draining lymph node DCs. Comparative transcriptomic analysis with mouse showed tissue DC subsets to be conserved between species and permitted close alignment of human and mouse DC subsets. These studies inform the rational design of targeted immunotherapies and facilitate translation of mouse functional DC biology to the human setting.

Hematopoietic-specific targeting of influenza A virus reveals replication requirements for induction of antiviral immune responses

A coordinated innate and adaptive immune response, orchestrated by antigen presenting cells (APCs), is required for effective clearance of influenza A virus (IAV). Although IAV primarily infects epithelial cells of the upper respiratory tract, APCs are also susceptible. To determine if virus transcription in these cells is required to generate protective innate and adaptive immune responses, we engineered IAV to be selectively attenuated in cells of hematopoietic origin. Incorporation of hematopoietic-specific miR-142 target sites into the nucleoprotein of IAV effectively silenced virus transcription in APCs, but had no significant impact in lung epithelial cells. Here we demonstrate that inhibiting IAV replication in APCs in vivo did not alter clearance, or the generation of IAV-specific CD8 T cells, suggesting that cross-presentation is sufficient for cytotoxic T lymphocyte activation. In contrast, loss of in vivo virus infection, selectively in APCs, resulted in a significant reduction of retinoic acid-inducible gene I-dependent type I IFN (IFN-I). These data implicate the formation of virus replication intermediates in APCs as the predominant trigger of IFN-I in vivo. Taking these data together, this research describes a unique platform to study the host response to IAV and provides insights into the mechanism of antigen presentation and the induction of IFN-I.

Layers of dendritic cell-mediated T cell tolerance, their regulation and the prevention of autoimmunity

The last decades of Nobel prize-honored research have unequivocally proven a key role of dendritic cells (DCs) at controlling both T cell immunity and tolerance. A tight balance between these opposing DC functions ensures immune homeostasis and host integrity. Its perturbation could explain pathological conditions such as the attack of self tissues, chronic infections, and tumor immune evasion. While recent insights into the complex DC network help to understand the contribution of individual DC subsets to immunity, the tolerogenic functions of DCs only begin to emerge. As these consist of many different layers, the definition of a “tolerogenic DC” is subjected to variation. Moreover, the implication of DCs and DC subsets in the suppression of autoimmunity are incompletely resolved. In this review, we point out conceptual controversies and dissect the various layers of DC-mediated T cell tolerance. These layers include central tolerance, Foxp3⁺ regulatory T cells (Tregs), anergy/deletion and negative feedback regulation. The mode and kinetics of antigen presentation is highlighted as an additional factor shaping tolerance. Special emphasis is given to the interaction between layers of tolerance as well as their differential regulation during inflammation. Furthermore, potential technical caveats of DC depletion models are considered. Finally, we summarize our current understanding of DC-mediated tolerance and its role for the suppression of autoimmunity. Understanding the mechanisms of DC-mediated tolerance and their complex interplay is fundamental for the development of selective therapeutic strategies, e.g., for the modulation of autoimmune responses or for the immunotherapy of cancer.

RNAi screen for kinases and phosphatases that play a role in antigen presentation by dendritic cells

Effective CD8(+) T-cell responses against tumor or microbial antigens that are not directly expressed in antigen-presenting cells (APCs) depend on the cross-presentation of these antigens on MHC class I in APCs. To identify signaling molecules that regulate cross-presentation, we used lentiviral-based RNA interference to test the roles of hundreds of kinases and phosphatases in this process. Our study uncovered eight previously unknown genes, consisting of one positive and seven negative regulators of antigen cross-presentation. Depletion of Acvr1c, a type I receptor for TGF-β family of signaling molecules, led to an increase in CD80 and CD86 co-stimulator surface expression and secreted IL-12 in mouse bone marrow-derived DCs, as well as antigen-specific T-cell proliferation.

Development of antigen cross-presentation capacity in dendritic cells

Cross-presentation by dendritic cells (DCs) of exogenous antigens on MHC class I is important for the generation of immune responses to intracellular pathogens, as well as for maintenance of self tolerance. In mice, the CD8(+) DC lineage is specialised for this role. However, DCs of this lineage are not born with cross-presentation capacity. Several studies have demonstrated that it must be induced as a later developmental step by cytokines such as granulocyte macrophage colony-stimulating factor (GM-CSF), or by microbial products such as toll-like receptor (TLR) ligands. Increased cross-presentation capacity is thus induced in peripheral CD8 lineage DCs during inflammation or infection. However, this capacity is already fully developed in steady-state thymic CD8(+) DCs, in accordance with their role in the deletion of self-reactive developing T cells.

Mycobacterium tuberculosis inhibits neutrophil apoptosis, leading to delayed activation of naive CD4 T cells

Mycobacterium tuberculosis promotes its replication by inhibiting the apoptosis of infected macrophages. A proapoptotic M. tuberculosis mutant lacking nuoG, a subunit of the type I NADH dehydrogenase complex, exhibits attenuated growth in vivo, indicating that this virulence mechanism is essential. We show that M. tuberculosis also suppresses neutrophil apoptosis. Compared to wild-type, the nuoG mutant spread to a larger number of lung phagocytic cells. Consistent with the shorter lifespan of infected neutrophils, infection with the nuoG mutant resulted in fewer bacteria per infected neutrophil, accelerated bacterial acquisition by dendritic cells, earlier trafficking of these dendritic cells to lymph nodes, and faster CD4 T cell priming. Neutrophil depletion abrogated accelerated CD4 T cell priming by the nuoG mutant, suggesting that inhibiting neutrophil apoptosis delays adaptive immunity in tuberculosis. Thus, pathogen modulation of apoptosis is beneficial at multiple levels, and enhancing phagocyte apoptosis promotes CD4 as well as CD8 T cell responses.

Immunological tolerance to muscle autoantigens involves peripheral deletion of autoreactive CD8+ T cells

Muscle potentially represents the most abundant source of autoantigens of the body and can be targeted by a variety of severe autoimmune diseases. Yet, the mechanisms of immunological tolerance toward muscle autoantigens remain mostly unknown. We investigated this issue in transgenic SM-Ova mice that express an ovalbumin (Ova) neo-autoantigen specifically in skeletal muscle. We previously reported that antigen specific CD4⁺ T cell are immunologically ignorant to endogenous Ova in this model but can be stimulated upon immunization. In contrast, Ova-specific CD8⁺ T cells were suspected to be either unresponsive to Ova challenge or functionally defective. We now extend our investigations on the mechanisms governing CD8⁺ tolerance in SM-Ova mice. We show herein that Ova-specific CD8⁺ T cells are not detected upon challenge with strongly immunogenic Ova vaccines even after depletion of regulatory T cells. Ova-specific CD8⁺ T cells from OT-I mice adoptively transferred to SM-Ova mice started to proliferate in vivo, acquired CD69 and PD-1 but subsequently down-regulated Bcl-2 and disappeared from the periphery, suggesting a mechanism of peripheral deletion. Peripheral deletion of endogenous Ova-specific cells was formally demonstrated in chimeric SM-Ova mice engrafted with bone marrow cells containing T cell precursors from OT-I TCR-transgenic mice. Thus, the present findings demonstrate that immunological tolerance to muscle autoantigens involves peripheral deletion of autoreactive CD8⁺ T cells.

NADPH oxidase regulates efficacy of vaccination in aspergillosis

Invasive aspergillosis is often a consequence of immune suppression, and accumulating evidence points to a role for adaptive immunity. Hence, it may be possible to manipulate the adaptive immune system to enhance protective immunity in at-risk individuals. In this issue of the JCI, De Luca and colleagues describe the ontogeny of adaptive immune responses to murine aspergillosis infection in relation to vaccination. Their thought-provoking findings reveal the complexities of vaccine-induced immunity and could be used to improve vaccine efficacy.

RLR-mediated production of interferon-β by a human dendritic cell subset and its role in virus-specific immunity

Cytosolic RIG-I-like helicases (RLR) are PRRs involved in type I IFN production and antiviral immunity. This study focuses to the comparison of the expression, function, and signaling cascades associated to RLR in the previously identified CD14(-)DC-SIGN(+)PPARγ(low)CD1a(+) and CD14(low)DC-SIGN(+)PPARγ(high)CD1a(-) human moDC subsets. Our results revealed that the expression of RLR genes and proteins as well as the activity of the coupled signaling pathways are significantly higher in the CD1a(+) subset than in its phenotypically and functionally distinct counterpart. Specific activation of RLR in moDCs by poly(I:C) or influenza virus was shown to induce the secretion of IFN-β via IRF3, whereas induction of proinflammatory cytokine responses were predominantly controlled by TLR3. The requirement of RLR-mediated signaling in CD1a(+) moDCs for priming naïve CD8(+) T lymphocytes and inducing influenza virus-specific cellular immune responses was confirmed by RIG-I/MDA5 silencing, which abrogated these functions. Our results demonstrate the subset-specific activation of RLR and the underlying mechanisms behind its cytokine secretion profile and identify CD1a(+) moDCs as an inflammatory subset with specialized functional activities. We also provide evidence that this migratory DC subset can be detected in human tonsil and reactive LNs.

Heat shock proteins: conditional mediators of inflammation in tumor immunity

Heat shock protein (HSP)-based anticancer vaccines have undergone successful preclinical testing and are now entering clinical trial. Questions still remain, however regarding the immunological properties of HSPs. It is now accepted that many of the HSPs participate in tumor immunity, at least in part by chaperoning tumor antigenic peptides, introducing them into antigen presenting cells such as dendritic cells (DC) that display the antigens on MHC class I molecules on the cell surface and stimulate cytotoxic lymphocytes (CTL). However, in order for activated CD8+ T cells to function as effective CTL and kill tumor cells, additional signals must be induced to obtain a sturdy CTL response. These include the expression of co-stimulatory molecules on the DC surface and inflammatory events that can induce immunogenic cytokine cascades. That such events occur is indicated by the ability of Hsp70 vaccines to induce antitumor immunity and overcome tolerance to tumor antigens such as mucin1. Secondary activation of CTL can be induced by inflammatory signaling through Toll-like receptors and/or by interaction of antigen-activated T helper cells with the APC. We will discuss the role of the inflammatory properties of HSPs in tumor immunity and the potential role of HSPs in activating T helper cells and DC licensing.

The role of heat shock proteins in antigen cross presentation

Heat shock proteins (HSPs) are molecular chaperones that bind tumor antigens and mediate their uptake into antigen presenting cells. HSP–antigen complexes are then directed toward either the MHC class I pathway through antigen cross presentation or the conventional class II pathway, leading to activation of T cell subsets. Uptake of HSP-chaperoned polypeptides can involve both receptor-mediated and receptor-independent routes, and mechanisms of antigen sorting between the Class I and II pathways after uptake are currently under investigation. The processes involved in internalization of HSP–antigen complexes differ somewhat from the mechanisms previously determined for (unchaperoned) particulate and free soluble antigens. A number of studies show that HSP-facilitated antigen cross presentation requires uptake of the complexes by scavenger receptors (SR) followed by processing in the proteasome, and loading onto MHC class I molecules. In this review we have examined the roles of HSPs and SR in antigen uptake, sorting, processing, cell signaling, and activation of innate and adaptive immunity.

Antigen Cross-Priming of Cell-Associated Proteins is Enhanced by Macroautophagy within the Antigen Donor Cell

Phagocytosis of dying cells constitutes an important mechanism of antigen capture for the cross-priming of CD8⁺ T cells. This process has been shown to be critical for achieving tumor and viral immunity. While most studies have focused on the mechanisms inherent in the dendritic cell that account for exogenous antigen accessing MHC I, several recent reports have highlighted the important contribution made by the antigen donor cell. Specifically, the cell stress and cell death pathways that precede antigen transfer are now known to impact cross-presentation and cross-priming. Herein, we review the current literature regarding a role for macroautophagy within the antigen donor cell. Further examination of this point of immune regulation is warranted and may contribute to a better understanding of how to optimize immunotherapy for treatment of cancer and chronic infectious disease.

The C-type lectin receptor CLEC9A mediates antigen uptake and (cross-)presentation by human blood BDCA3+ myeloid dendritic cells

CLEC9A is a recently discovered C-type lectin receptor involved in sensing necrotic cells. In humans, this receptor is selectively expressed by BDCA3+ myeloid dendritic cells (mDCs), which have been proposed to be the main human cross-presenting mDCs and may represent the human homologue of murine CD8+ DCs. In mice, it was demonstrated that antigens delivered with antibodies to CLEC9A are presented by CD8+ DCs to both CD4+ and CD8+ T cells and induce antitumor immunity in a melanoma model. Here we assessed the ability of CLEC9A to mediate antigen presentation by human BDCA3+ mDCs, which represent < 0.05% of peripheral blood leukocytes. We demonstrate that CLEC9A is only expressed on immature BDCA3+ mDCs and that cell surface expression is lost after TLR-mediated maturation. CLEC9A triggering via antibody binding rapidly induces receptor internalization but does not affect TLR-induced cytokine production or expression of costimulatory molecules. More importantly, antigens delivered via CLEC9A antibodies to BDCA3+ mDCs are presented by both MHC class I (cross-presentation) and MHC class II to antigen-specific T cells. We conclude that CLEC9A is a promising target for in vivo antigen delivery in humans to increase the efficiency of vaccines against infectious or malignant diseases.

Living in the liver: hepatic infections

The liver has vital metabolic and clearance functions that involve the uptake of nutrients, waste products and pathogens from the blood. In addition, its unique immunoregulatory functions mediated by local expression of co-inhibitory receptors and immunosuppressive mediators help to prevent inadvertent organ damage. However, these tolerogenic properties render the liver an attractive target site for pathogens. Although most pathogens that reach the liver via the blood are eliminated or controlled by local innate and adaptive immune responses, some pathogens (such as hepatitis viruses) can escape immune control and persist in hepatocytes, causing substantial morbidity and mortality worldwide. Here, we review our current knowledge of the mechanisms of liver targeting by pathogens and describe the interplay between pathogens and host factors that promote pathogen elimination and maintain organ integrity or that allow pathogen persistence.

Heat shock proteins and cancer vaccines: developments in the past decade and chaperoning in the decade to come

Molecular chaperone-peptide complexes extracted from tumors (heat shock protein [HSP] vaccines) have been intensively studied in the preceding two decades, proving to be safe and effective in treating a number of malignant diseases. They offer personalized therapy and target a cross-section of antigens expressed in patients' tumors. Future advances may rely on understanding the molecular underpinnings of this approach to immunotherapy. One property common to HSP vaccines is the ability to stimulate antigen uptake by scavenger receptors on the antigen-presenting cell surface and trigger T-lymphocyte activation. HSPs can also induce signaling through Toll-Like receptors in a range of immune cells and this may mediate the effectiveness of vaccines.

Evaluation of antigen specific recognition and cell mediated cytotoxicity by a modified lysispot assay in a rat colon carcinoma model

Background

Antigen-specific CD8+ cytotoxic T lymphocytes represent potent effector cells of the adaptive immune response against viruses as well as tumours. Therefore assays capable at exploring the generation and function of cytotoxic T lymphocytes represent an important objective for both clinical and experimental settings.

Methods

Here we show a simple and reproducible assay for the evaluation of antigen-specific CD8+ cytotoxic T lymphocytes based on a LysiSpot technique for the simultaneous determination of antigen-specific IFN-γ production and assessment of tumor cytolysis. The assay was developed within an experimental model of colorectal carcinoma, induced by the colorectal tumor cell line DHD-K12 that induces tumors in BDIX rats and, in turn, elicits a tumor- specific immune response.

Results

Using DHD-K12 cells transfected to express Escherichia coli β-galactosidase as target cells, and by the fine setting of spot colours detection, we have developed an in vitro assay that allows the recognition of cytotoxic T lymphocytes induced in BDIX rats as well as the assessment of anti-tumour cytotoxicity. The method highlighted that in the present experimental model the tumour antigen-specific immune response was bound to killing target cells in the proportion of 55%, while 45% of activated cells were not cytotoxic but released IFN-γ. Moreover in this model by an ELISPOT assay we demonstrated the specific recognition of a nonapeptide epitope called CSH-275 constitutionally express in DHD-K12 cells.

Conclusions

The assay proved to be highly sensitive and specific, detecting even low frequencies of cytotoxic/activated cells and providing the evaluation of cytokine-expressing T cells as well as the extent of cytotoxicity against the target cells as independent functions. This assay may represent an important tool to be adopted in experimental settings including the development of vaccines or immune therapeutic strategies

B-Cell Response during Protozoan Parasite Infections

In this review, we discuss how protozoan parasites alter immature and mature B cell compartment. B1 and marginal zone (MZ) B cells, considered innate like B cells, are activated during protozoan parasite infections, and they generate short lived plasma cells providing a prompt antibody source. In addition, protozoan infections induce massive B cell response with polyclonal activation that leads to hypergammaglobulnemia with serum antibodies specific for the parasites and self and/or non related antigens. To protect themselves, the parasites have evolved unique ways to evade B cell immune responses inducing apoptosis of MZ and conventional mature B cells. As a consequence of the parasite induced-apoptosis, the early IgM response and an already establish humoral immunity are affected during the protozoan parasite infection. Moreover, some trypanosomatides trigger bone marrow immature B cell apoptosis, influencing the generation of new mature B cells. Simultaneously with their ability to release antibodies, B cells produce cytokines/quemokines that influence the characteristic of cellular immune response and consequently the progression of parasite infections.

Recognition of nucleic acids by pattern-recognition receptors and its relevance in autoimmunity

Summary:  Host cells trigger signals for innate immune responses upon recognition of conserved structures in microbial pathogens. Nucleic acids, which are critical components for inheriting genetic information in all species including pathogens, are key structures sensed by the innate immune system. The corresponding receptors for foreign nucleic acids include members of Toll‐like receptors, RIG‐I‐like receptors, and intracellular DNA sensors. While nucleic acid recognition by these receptors is required for host defense against the pathogen, there is a potential risk to the host of self‐nucleic acids recognition, thus precipitating autoimmune and autoinflammatory diseases. In this review, we discuss the roles of nucleic acid‐sensing receptors in guarding against pathogen invasion, discriminating between self and non‐self, and contributing to autoimmunity and autoinflammatory diseases.

Tumor evasion from T cell surveillance

An intact immune system is essential to prevent the development and progression of neoplastic cells in a process termed immune surveillance. During this process the innate and the adaptive immune systems closely cooperate and especially T cells play an important role to detect and eliminate tumor cells. Due to the mechanism of central tolerance the frequency of T cells displaying appropriate arranged tumor-peptide-specific-T-cell receptors is very low and their activation by professional antigen-presenting cells, such as dendritic cells, is frequently hampered by insufficient costimulation resulting in peripheral tolerance. In addition, inhibitory immune circuits can impair an efficient antitumoral response of reactive T cells. It also has been demonstrated that large tumor burden can promote a state of immunosuppression that in turn can facilitate neoplastic progression. Moreover, tumor cells, which mostly are genetically instable, can gain rescue mechanisms which further impair immune surveillance by T cells. Herein, we summarize the data on how tumor cells evade T-cell immune surveillance with the focus on solid tumors and describe approaches to improve anticancer capacity of T cells.

Towards a systems understanding of MHC class I and MHC class II antigen presentation

The molecular details of antigen processing and presentation by MHC class I and class II molecules have been studied extensively for almost three decades. Although the basic principles of these processes were laid out approximately 10 years ago, the recent years have revealed many details and provided new insights into their control and specificity. MHC molecules use various biochemical reactions to achieve successful presentation of antigenic fragments to the immune system. Here we present a timely evaluation of the biology of antigen presentation and a survey of issues that are considered unresolved. The continuing flow of new details into our understanding of the biology of MHC class I and class II antigen presentation builds a system involving several cell biological processes, which is discussed in this Review.

Towards a systems understanding of MHC class I and MHC class II antigen presentation

The molecular details of antigen processing and presentation by MHC class I and class II molecules have been studied extensively for almost three decades. Although the basic principles of these processes were laid out approximately 10 years ago, the recent years have revealed many details and provided new insights into their control and specificity. MHC molecules use various biochemical reactions to achieve successful presentation of antigenic fragments to the immune system. Here we present a timely evaluation of the biology of antigen presentation and a survey of issues that are considered unresolved. The continuing flow of new details into our understanding of the biology of MHC class I and class II antigen presentation builds a system involving several cell biological processes, which is discussed in this Review.

The existence of two types of proteasome, the constitutive proteasome and the immunoproteasome, may serve as another layer of protection against autoimmunity

Negative selection of CD8 single positive thymocytes is based on the presentation through the major histocompatibility complex (MHC) class I pathway of peptides derived from degradation of self-proteins by the constitutive proteasome and the immunoproteasome in the thymus. Then naïve CD8+ T-cells can be primed by mature dendritic cells. In mature dendritic cells peptides presented by MHC class I molecules are derived from degradation of endogenous self-proteins or through the process of cross-presentation from degradation of exogenous proteins by the immunoproteasome. In the absence of infection, peripheral cells display peptides on MHC class I molecules derived from degradation of endogenous self-proteins by the constitutive proteasome. The pool of peptides derived from protein degradation by the constitutive proteasome differs from the pool peptides derived from protein degradation by the immunoproteasome. Thus the probability of an autoreactive naïve CD8+ T-cell that escaped negative selection, and converted by a mature dendritic cell to autoreactive cytolytic T-cell, to kill a normal cell in the periphery, is reduced.

How to connect an IgE-driven response with CTL activity?

One of the goals of cell-based immune therapy in cancer is the induction of tumor-specific cytotoxic T-lymphocyte (CTL) responses. To achieve this objective, the ability of dendritic cells (DC) to cross-present tumor antigens can be exploited. One of the most efficient pathways for the induction of CTLs by cross-presentation is mediated by immunoglobulins of the IgG class, which are used by DCs to sample antigen in the form of immune complexes via Fc-gamma receptors. Could DCs use an IgE-mediated cross-presentation mechanism in a comparable manner to induce CTLs? We here discuss the potential of two human IgE Fc receptors, FcεRI and FcεRII, to serve as antigen uptake receptors for IgE-mediated cross-presentation. We conclude that the existence of an IgE-mediated cross-presentation pathway would provide a direct link between IgE-driven immune responses and CTL activity.

Expression of type I interferon by splenic macrophages suppresses adaptive immunity during sepsis

Early during Gram-negative sepsis, excessive release of pro-inflammatory cytokines can cause septic shock that is often followed by a state of immune paralysis characterized by the failure to mount adaptive immunity towards secondary microbial infections. Especially, the early mechanisms responsible for such immune hypo-responsiveness are unclear. Here, we show that TLR4 is the key immune sensing receptor to initiate paralysis of T-cell immunity after bacterial sepsis. Downstream of TLR4, signalling through TRIF but not MyD88 impaired the development of specific T-cell immunity against secondary infections. We identified type I interferon (IFN) released from splenic macrophages as the critical factor causing T-cell immune paralysis. Early during sepsis, type I IFN acted selectively on dendritic cells (DCs) by impairing antigen presentation and secretion of pro-inflammatory cytokines. Our results reveal a novel immune regulatory role for type I IFN in the initiation of septic immune paralysis, which is distinct from its well-known immune stimulatory effects. Moreover, we identify potential molecular targets for therapeutic intervention to overcome impairment of T-cell immunity after sepsis.

Nanoparticle conjugation of antigen enhances cytotoxic T-cell responses in pulmonary vaccination

The ability of vaccines to induce memory cytotoxic T-cell responses in the lung is crucial in stemming and treating pulmonary diseases caused by viruses and bacteria. However, most approaches to subunit vaccines produce primarily humoral and only to a lesser extent cellular immune responses. We developed a nanoparticle (NP)-based carrier that, upon delivery to the lung, specifically targets pulmonary dendritic cells, thus enhancing antigen uptake and transport to the draining lymph node; antigen coupling via a disulfide link promotes highly efficient cross-presentation after uptake, inducing potent protective mucosal and systemic CD8(+) T-cell immunity. Pulmonary immunization with NP-conjugated ovalbumin (NP-ova) with CpG induced a threefold enhancement of splenic antigen-specific CD8(+) T cells displaying increased CD107a expression and IFN-γ production compared with immunization with soluble (i.e., unconjugated) ova with CpG. This enhanced response was accompanied by a potent Th17 cytokine profile in CD4(+) T cells. After 50 d, NP-ova and CpG also led to substantial enhancements in memory CD8(+) T-cell effector functions. Importantly, pulmonary vaccination with NP-ova and CpG induced as much as 10-fold increased frequencies of antigen-specific effector CD8(+) T cells to the lung and completely protected mice from morbidity following influenza-ova infection. Here, we highlight recruitment to the lung of a long-lasting pool of protective effector memory cytotoxic T-cells by our disulfide-linked antigen-conjugated NP formulation. These results suggest the reduction-reversible NP system is a highly promising platform for vaccines specifically targeting intracellular pathogens infecting the lung.

Cross-presentation of antigen by diverse subsets of murine liver cells

Antigen cross-presentation is a principal function of specialized antigen-presenting cells of bone marrow origin such as dendritic cells. Although these cells are sometimes known as "professional" antigen-presenting cells, nonbone marrow-derived cells may also act as antigen-presenting cells. Here, using four-way liver cell isolation and parallel comparison of candidate antigen-presenting cells, we show that, depending on the abundance of antigen-donor cells, different subsets of liver cells could cross-present a hepatocyte-associated antigen. This function was observed in both liver sinusoidal endothelial cells and Kupffer cells even at very low antigen concentration, as well as when using soluble protein. Antigen cross-presentation by liver cells induced efficient CD8+ T-cell proliferation in a similar manner to classical dendritic cells from spleen. However, proliferated cells expressed a lower level of T-cell activation markers and intracellular interferon-gamma levels. In contrast to classical spleen dendritic cells, cross-presentation by liver antigen-presenting cells was predominantly dependent on intercellular adhesion molecule-1.

CONCLUSION

Hepatic sinusoids are an environment rich in antigen cross-presenting activity. However, the liver's resident antigen-presenting cells cause partial T-cell activation. These results clarify how the liver can act as a primary site of CD8+ T-cell activation, and why immunity against hepatocyte pathogens is sometimes ineffective.

Immunization with apoptotic phagocytes containing Histoplasma capsulatum activates functional CD8(+) T cells to protect against histoplasmosis

We have previously revealed the protective role of CD8(+) T cells in host defense against Histoplasma capsulatum in animals with CD4(+) T cell deficiency and demonstrated that sensitized CD8(+) T cells are restimulated in vitro by dendritic cells that have ingested apoptotic macrophage-associated Histoplasma antigen. Here we show that immunization with apoptotic phagocytes containing heat-killed Histoplasma efficiently activated functional CD8(+) T cells whose contribution was equal to that of CD4(+) T cells in protection against Histoplasma challenge. Inhibition of macrophage apoptosis due to inducible nitric oxide synthase (iNOS) deficiency or by caspase inhibitor treatment dampened the CD8(+) T cell but not the CD4(+) T cell response to pulmonary Histoplasma infection. In mice subcutaneously immunized with viable Histoplasma yeasts whose CD8(+) T cells are protective against Histoplasma challenge, there was heavy granulocyte and macrophage infiltration and the infiltrating cells became apoptotic. In mice subcutaneously immunized with carboxyfluorescein diacetate succinimidyl ester (CFSE)-labeled apoptotic macrophages containing heat-killed Histoplasma, the CFSE-labeled macrophage material was found to localize within dendritic cells in the draining lymph node. Moreover, depleting dendritic cells in immunized CD11c-DTR mice significantly reduced CD8(+) T cell activation. Taken together, our results revealed that phagocyte apoptosis in the Histoplasma-infected host is associated with CD8(+) T cell activation and that immunization with apoptotic phagocytes containing heat-killed Histoplasma efficiently evokes a protective CD8(+) T cell response. These results suggest that employing apoptotic phagocytes as antigen donor cells is a viable approach for the development of efficacious vaccines to elicit strong CD8(+) T cell as well as CD4(+) T cell responses to Histoplasma infection.

Antitumor effect of malaria parasite infection in a murine Lewis lung cancer model through induction of innate and adaptive immunity

Background

Lung cancer is the most common malignancy in humans and its high fatality means that no effective treatment is available. Developing new therapeutic strategies for lung cancer is urgently needed. Malaria has been reported to stimulate host immune responses, which are believed to be efficacious for combating some clinical cancers. This study is aimed to provide evidence that malaria parasite infection is therapeutic for lung cancer.

Methodology/Principal Findings

Antitumor effect of malaria infection was examined in both subcutaneously and intravenously implanted murine Lewis lung cancer (LLC) model. The results showed that malaria infection inhibited LLC growth and metastasis and prolonged the survival of tumor-bearing mice. Histological analysis of tumors from mice infected with malaria revealed that angiogenesis was inhibited, which correlated with increased terminal deoxynucleotidyl transferase-mediated (TUNEL) staining and decreased Ki-67 expression in tumors. Through natural killer (NK) cell cytotoxicity activity, cytokine assays, enzyme-linked immunospot assay, lymphocyte proliferation, and flow cytometry, we demonstrated that malaria infection provided anti-tumor effects by inducing both a potent anti-tumor innate immune response, including the secretion of IFN-γ and TNF-α and the activation of NK cells as well as adaptive anti-tumor immunity with increasing tumor-specific T-cell proliferation and cytolytic activity of CD8⁺ T cells. Notably, tumor-bearing mice infected with the parasite developed long-lasting and effective tumor-specific immunity. Consequently, we found that malaria parasite infection could enhance the immune response of lung cancer DNA vaccine pcDNA3.1-hMUC1 and the combination produced a synergistic antitumor effect.

Conclusions/Significance

Malaria infection significantly suppresses LLC growth via induction of innate and adaptive antitumor responses in a mouse model. These data suggest that the malaria parasite may provide a novel strategy or therapeutic vaccine vector for anti-lung cancer immune-based therapy.

Lymphoid organ-resident dendritic cells exhibit unique transcriptional fingerprints based on subset and site

Lymphoid organ-resident DC subsets are thought to play unique roles in determining the fate of T cell responses. Recent studies focusing on a single lymphoid organ identified molecular pathways that are differentially operative in each DC subset and led to the assumption that a given DC subset would more or less exhibit the same genomic and functional profiles throughout the body. Whether the local milieu in different anatomical sites can also influence the transcriptome of DC subsets has remained largely unexplored. Here, we interrogated the transcriptional relationships between lymphoid organ-resident DC subsets from spleen, gut- and skin-draining lymph nodes, and thymus of C57BL/6 mice. For this purpose, major resident DC subsets including CD4 and CD8 DCs were sorted at high purity and gene expression profiles were compared using microarray analysis. This investigation revealed that lymphoid organ-resident DC subsets exhibit divergent genomic programs across lymphoid organs. Interestingly, we also found that transcriptional and biochemical properties of a given DC subset can differ between lymphoid organs for lymphoid organ-resident DC subsets, but not plasmacytoid DCs, suggesting that determinants of the tissue milieu program resident DCs for essential site-specific functions.

TRAIL/DR5 plays a critical role in NK cell-mediated negative regulation of dendritic cell cross-priming of T cells

Dendritic cells (DCs) are critical in initiating immune responses by cross-priming of tumor Ags to T cells. Previous results showed that NK cells inhibited DC-mediated cross-presentation of tumor Ags both in vivo and in vitro. In this study, enhanced Ag presentation was observed in draining lymph nodes in TRAIL(-/-) and DR5(-/-) mice compared with that of wild-type mice. NK cells inhibit DC cross-priming of tumor Ags in vitro, but not direct presentation of endogenous Ags. NK cells lacking TRAIL, but not perforin, were not able to inhibit DC cross-priming of tumor Ags. DCs that lack expression of TRAIL receptor DR5 were less susceptible to NK cell-mediated inhibition of cross-priming, and cross-linking of DR5 receptor led to reduced generation of MHC class I-Ag peptide complexes, followed by attenuated cross-priming of CD8(+) T cells. In addition, key molecules involved in the TRAIL/DR5 pathway during DC/NK cell interactions were determined. In summary, these data indicate a novel alternative pathway for DC/NK cell interactions in antitumor immunity and may reflect homeostasis of both DCs and NK cells for regulation of CD8(+) T cell function in physiological conditions.

Batf3 transcription factor-dependent DC subsets in murine CMV infection: differential impact on T-cell priming and memory inflation

Priming of CD8(+) T cells specific for viruses that interfere with the MHC class I presentation pathway is a challenge for the immune system and is believed to rely on cross-presentation. Cytomegalovirus (CMV) infection induces vigorous CD8(+) T-cell responses despite its potent immune evasion strategies. Furthermore, CD8(+) T cells specific for a subset of viral epitopes accumulate and are maintained at high levels exhibiting an activated phenotype - referred to as "inflationary T cells". Taking advantage Batf3(-/-) mice in which the development of cross-presenting CD8α(+) and CD103(+) DCs is severely compromised, we analyzed their role in the induction and inflation of murine (M)CMV-specific CD8(+) T-cell responses. We found that priming of MCMV-specific CD8(+) T cells was severely impaired in the absence of cross-presenting DCs. However, inflation of two immuno-dominant MCMV-specific CD8(+) T-cell populations was largely normal in the absence of cross-presenting DCs, indicating that inflation during latency was mainly dependent on direct antigen presentation. These results highlight differential antigen presentation requirements during acute and latent MCMV infection.

Properties and applications of single-chain major histocompatibility complex class I molecules

Stable major histocompatibility complex (MHC) class I molecules at the cell surface consist of three separate, noncovalently associated components: the class I heavy chain, the β(2)-microglobulin light chain, and a presented peptide. These three components are assembled inside cells via complex pathways involving many other proteins that have been studied extensively. Correct formation of disulfide bonds in the endoplasmic reticulum is central to this process of MHC class I assembly. For a single specific peptide to be presented at the cell surface for possible immune recognition, between hundreds and thousands of peptide-containing precursor polypeptides are required, so the overall process is relatively inefficient. To increase the efficiency of antigen presentation by MHC class I molecules, and for possible therapeutic purposes, single-chain molecules have been developed in which the three, normally separate components have been joined together via flexible linker sequences in a single polypeptide chain. Remarkably, these single-chain MHC class I molecules fold up correctly, as judged by functional recognition by cells of the immune system, and more recently by X-ray crystallographic structural data. This review focuses on the interesting properties and potential of this new type of engineered MHC class I molecule.

Chemokines: a new dendritic cell signal for T cell activation

Dendritic cells (DCs) are the main inducers and regulators of cytotoxic T lymphocyte (CTL) responses against viruses and tumors. One checkpoint to avoid misguided CTL activation, which might damage healthy cells of the body, is the necessity for multiple activation signals, involving both antigenic as well as additional signals that reflect the presence of pathogens. DCs provide both signals when activated by ligands of pattern recognition receptors and “licensed” by helper lymphocytes. Recently, it has been established that such T cell licensing can be facilitated by CD4⁺ T helper cells (“classical licensing”) or by natural killer T cells (“alternative licensing”). Licensing regulates the DC/CTL cross-talk at multiple layers. Direct recruitment of CTLs through chemokines released by licensed DCs has recently emerged as a common theme and has a crucial impact on the efficiency of CTL responses. Here, we discuss recent advances in our understanding of DC licensing for cross-priming and implications for the temporal and spatial regulation underlying this process. Future vaccination strategies will benefit from a deeper insight into the mechanisms that govern CTL activation.

Three CpG oligodeoxynucleotide classes differentially enhance antigen-specific humoral and cellular immune responses in mice

Synthetic oligodeoxynucleotides containing unmethylated CpG-dinucleotides (CpG-ODNs) are immunostimulatory in a broad spectrum of species. Extensive studies provide evidence that CpG-ODNs are effective as immunotherapeutics and vaccine adjuvants in various clinical settings. Three major classes of immunostimulatory CpG-ODNs are well characterized according to their in vitro activities and chemical compositions. However, it remains largely unclear whether and how these differences translate in vivo and in particular when used as vaccine adjuvants. In the present study, a panel of CpG-ODNs, including four representative sequences respectively from each class, was used to characterize their adjuvant activities in mice. The results demonstrated that three CpG-ODN classes can differentially affect antigen-specific humoral and cellular immune responses. Specifically, the B- and C-class CpG-ODNs induce a potent Th1-biased immunity with comparable antibody levels as well as CD4⁺ and CD8⁺ T cell responses. In contrast, although the A-class CpG-ODNs can weakly enhance antibody titers and CD8⁺ T cell response regarding cytotoxic activity, they are not able to change the IgG1/IgG2a ratio or elicit antigen-specific, IFN-γ-secreting CD4⁺ and CD8⁺ T cells. Consistent with this, three CpG-ODN classes provide differential antigen-specific protection against Listeria monocytogenes, an intracellular bacterial infection. In conclusion, our study provides not only better knowledge about the adjuvant activities of three CpG-ODN classes but also implications for the rational design of CpG-ODN adjuvants.

Vagal activities are involved in antigen-specific immune inflammation in the intestine

BACKGROUND AND AIMS

The mechanism of intestinal immune inflammation, such as food allergy, remains to be further understood. The present study aims to investigate the role of the vagal nerve in the pathogenesis of skewed T-helper 2 (Th2) responses in the intestine.

METHODS

The expression of the immunoglobulin E (IgE) receptor on the vagus nerve in the mouse intestine was observed by immunohistochemistry. Vagus ganglion neurons (VGN) were isolated from mice and cultured in vitro. The IgE receptor/IgE complex on vagus neurons was examined by immune precipitation assay. A food allergy mouse model was developed; the effect of the partial removal of the vagal nerve (PRVn) via surgery or administration with anticholinergic agents on the suppression of Th2 inflammation was evaluated.

RESULTS

The high-affinity IgE receptor was detected on the intestinal vagus nerve. An increase in the expression of the IgE receptor on the vagus nerve was observed in the intestines of mice with intestinal immune inflammation. Isolated mouse VGN express IgE receptor I, which could form complexes with IgE. Re-exposure to specific antigens activated the sensitized VGN, manifesting the release of transmitter glutamate that could activate dendritic cells by increasing the expression of CD80 and major compatibility complex class II and suppressing interleukin-12. The PRVn suppressed Th2 inflammation in the intestine.

CONCLUSIONS

The intestinal vagus nerve in mice expresses a high-affinity IgE receptor. An antigen-specific immune response can activate the vagus nerve in the intestine and induces the release of transmitters to modulate dendritic cell phenotypes that facilitate the development of skewed Th2 polarization in the intestine.

Antigen-presenting cells of haematopoietic origin prime cytomegalovirus-specific CD8 T-cells but are not sufficient for driving memory inflation during viral latency

Expansion of the CD8 T-cell memory pool, also known as 'memory inflation', for certain but not all viral epitopes in latently infected host tissues is a special feature of the immune response to cytomegalovirus. The L(d)-presented murine cytomegalovirus (mCMV) immediate-early (IE) 1 peptide is the prototype of an epitope that is associated with memory inflation. Based on the detection of IE1 transcripts in latently infected lungs it was previously proposed that episodes of viral gene expression and antigenic activity due to desilencing of a limited number of viral genes may drive epitope-specific memory inflation. This would imply direct antigen presentation through latently infected host tissue cells rather than cell death-associated cross-presentation of viral antigens derived from productively infected cells through uninfected, professional antigen-presenting cells (profAPCs). To address the role of bone marrow-derived profAPCs in CD8 T-cell priming and memory to mCMV, we have used here a combined sex-mismatched and MHC class-I mismatched dual-marker bone marrow chimera model in which presentation of the IE1 epitope is restricted to donor-derived sry(+)L(d+) cells of haematopoietic differentiation lineages. Successful CD8 T-cell priming specific for the L(d)- and D(d)-presented inflationary epitopes IE1 and m164, respectively, but selective failure in IE1 epitope-specific memory inflation in these chimeras indicates different modes of antigen presentation involved in CD8 T-cell priming and memory inflation. These data suggest that memory inflation during mCMV latency requires expression of the epitope-presenting MHC class-I molecule by latently infected non-haematopoietic host tissue cells and thus predicts a role for direct antigen presentation in memory inflation.

Neonatal Fc receptor for IgG (FcRn) regulates cross-presentation of IgG immune complexes by CD8-CD11b+ dendritic cells

Cross-presentation of IgG-containing immune complexes (ICs) is an important means by which dendritic cells (DCs) activate CD8(+) T cells, yet it proceeds by an incompletely understood mechanism. We show that monocyte-derived CD8(-)CD11b(+) DCs require the neonatal Fc receptor for IgG (FcRn) to conduct cross-presentation of IgG ICs. Consequently, in the absence of FcRn, Fcγ receptor (FcγR)-mediated antigen uptake fails to initiate cross-presentation. FcRn is shown to regulate the intracellular sorting of IgG ICs to the proper destination for such cross-presentation to occur. We demonstrate that FcRn traps antigen and protects it from degradation within an acidic loading compartment in association with the rapid recruitment of key components of the phagosome-to-cytosol cross-presentation machinery. This unique mechanism thus enables cross-presentation to evolve from an atypically acidic loading compartment. FcRn-driven cross-presentation is further shown to control cross-priming of CD8(+) T-cell responses in vivo such that during chronic inflammation, FcRn deficiency results in inadequate induction of CD8(+) T cells. These studies thus demonstrate that cross-presentation in CD8(-)CD11b(+) DCs requires a two-step mechanism that involves FcγR-mediated internalization and FcRn-directed intracellular sorting of IgG ICs. Given the centrality of FcRn in controlling cross-presentation, these studies lay the foundation for a unique means to therapeutically manipulate CD8(+) T-cell responses.

Antigen cross-presentation: extending recent laboratory findings to therapeutic intervention

The initiation of adaptive immune responses requires antigen presentation to lymphocytes. In particular, dendritic cells (DCs) are equipped with specialized machinery that promote effective display of peptide/major histocompatibility complexes (MHC), rendering them the most potent stimulators of naive T lymphocytes. Antigen cross-presentation to CD8(+) T cells is an important mechanism for the development of specific cytotoxic T lymphocyte (CTL) responses against tumours and viruses that do not infect antigen-presenting cells. Here, we review recent findings concerning antigen cross-presentation to CD8(+) T lymphocytes. Specific subtypes of DCs in the mouse have been defined as being especially endowed for antigen cross-presentation, and a human homologue of these DCs has recently been described. DC vaccination strategies for the prevention and treatment of human diseases have been under investigation in recent years, but have not generally reached satisfying results. We here provide an overview of new findings in antigen cross-presentation research and how they can be used for development of the next generation of human DC vaccines.

Dendritic cells crosspresent antigens from live B16 cells more efficiently than from apoptotic cells and protect from melanoma in a therapeutic model

Dendritic cells (DC) are able to elicit anti-tumoral CD8⁺ T cell responses by cross-presenting exogenous antigens in association with major histocompatibility complex (MHC) class I molecules. Therefore they are crucial actors in cell-based cancer immunotherapy. Although apoptotic cells are usually considered to be the best source of antigens, live cells are also able to provide antigens for cross-presentation by DC. We have recently shown that prophylactic immunotherapy by DC after capture of antigens from live B16 melanoma cells induced strong CD8⁺ T-cell responses and protection against a lethal tumor challenge in vivo in C57Bl/6 mice. Here, we showed that DC cross-presenting antigens from live B16 cells can also inhibit melanoma lung dissemination in a therapeutic protocol in mice. DC were first incubated with live tumor cells for antigen uptake and processing, then purified and irradiated for safety prior to injection. This treatment induced stronger tumor-specific CD8⁺ T-cell responses than treatment by DC cross-presenting antigens from apoptotic cells. Apoptotic B16 cells induced more IL-10 secretion by DC than live B16 cells. They underwent strong native antigen degradation and led to the expression of fewer MHC class I/epitope complexes on the surface of DC than live cells. Therefore, the possibility to use live cells as sources of tumor antigens must be taken into account to improve the efficiency of cancer immunotherapy.

Immunologic considerations for generating memory CD8 T cells through vaccination

Following infection or vaccination, naïve CD8 T cells that receive the appropriate integration of antigenic, co-stimulatory and inflammatory signals undergo a programmed series of biological changes that ultimately results in the generation of memory cells. Memory CD8 T cells, in contrast to naïve cells, more effectively limit or prevent pathogen re-infection because of both qualitative and quantitative changes that occur following their induction. Unlike vaccination strategies aimed at generating antibody production, the ability to generate protective memory CD8 T cells has proven more complicated and problematic. However, recent experimental results have revealed important principles regarding the molecular and genetic basis for memory CD8 T cell formation, as well as identified ways to manipulate their development through vaccination, resulting in potential new avenues to enhance protective immunity.

Lipids, apoptosis, and cross-presentation: links in the chain of host defense against Mycobacterium tuberculosis

Eicosanoids regulate whether human and murine macrophages infected with Mycobacterium tuberculosis die by apoptosis or necrosis. The death modality is important since apoptosis is associated with diminished pathogen viability and should be viewed as a form of innate immunity. Apoptotic vesicles derived from infected macrophages are also an important source of bacterial antigens that can be acquired by dendritic cells to prime antigen-specific T cells. This review integrates in vitro and in vivo data on how apoptosis of infected macrophages is linked to development of T cell immunity against M. tuberculosis.

Type I IFNs control antigen retention and survival of CD8α(+) dendritic cells after uptake of tumor apoptotic cells leading to cross-priming

Cross-presentation is a crucial mechanism for generating CD8 T cell responses against exogenous Ags, such as dead cell-derived Ag, and is mainly fulfilled by CD8α(+) dendritic cells (DC). Apoptotic cell death occurring in steady-state conditions is largely tolerogenic, thus hampering the onset of effector CD8 T cell responses. Type I IFNs (IFN-I) have been shown to promote cross-priming of CD8 T cells against soluble or viral Ags, partly through stimulation of DC. By using UV-irradiated OVA-expressing mouse EG7 thymoma cells, we show that IFN-I promote intracellular Ag persistence in CD8α(+) DC that have engulfed apoptotic EG7 cells, regulating intracellular pH, thus enhancing cross-presentation of apoptotic EG7-derived OVA Ag by CD8α(+) DC. Notably, IFN-I also sustain the survival of Ag-bearing CD8α(+) DC by selective upmodulation of antiapoptotic genes and stimulate the activation of cross-presenting DC. The ensemble of these effects results in the induction of CD8 T cell effector response in vitro and in vivo. Overall, our data indicate that IFN-I cross-prime CD8 T cells against apoptotic cell-derived Ag both by licensing DC and by enhancing cross-presentation.

Exploiting human herpesvirus immune evasion for therapeutic gain: potential and pitfalls

Herpesviruses stand out for their capacity to establish lifelong infections of immunocompetent hosts, generally without causing overt symptoms. Herpesviruses are equipped with sophisticated immune evasion strategies, allowing these viruses to persist for life despite the presence of a strong antiviral immune response. Although viral evasion tactics appear to target virtually any stage of the innate and adaptive host immune response, detailed knowledge is now available on the molecular mechanisms underlying herpesvirus obstruction of MHC class I-restricted antigen presentation to T cells. This opens the way for clinical application. Here, we review and discuss recent efforts to exploit human herpesvirus MHC class I evasion strategies for the rational design of novel strategies for vaccine development, cancer treatment, transplant protection and gene therapy.