Macrophages and dendritic cells use the cytosolic pathway to rapidly cross-present antigen from live, vaccinia-infected cells

Merocytophagy is an integrin-stabilized macrophage response to microbes reliant on Syk signaling

Macrophages and dendritic cells acquire bacteria and cytosolic content from other cells without killing the donor cell through a trogocytosis-associated process termed merocytophagy. While characteristics of this behavior have been partially identified, the mechanism and potential contribution to the response to infection are unclear. Here, we reveal that a wide range of distinct species of bacteria stimulate enhanced merocytophagy in macrophages through pattern recognition receptor (PRR). Further, we found that cell-to-cell transfer in response to Francisella tularensis infection occurs in a predominantly MyD88-independent manner, relying on spleen tyrosine kinase (Syk) activity. Syk signaling during this response also results in increased surface expression of cell-to-cell adhesion proteins integrin α4, integrin β1, ICAM-1 and CD44 at the site of merocytophagy transfer, and depleting these surface molecules impairs merocytophagic cell-to-cell transfer. Altogether, our data demonstrate that merocytophagy is a host response to infection facilitated by tight cell-to-cell binding which molecularly resembles an immunological synapse between macrophages.

Dendritic cells overcome Cre/Lox induced gene deficiency by siphoning cytosolic material from surrounding cells

In a previous report, keratinocytes were shown to share their gene expression profile with surrounding Langerhans cells (LCs), influencing LC biology. Here, we investigated whether transferred material could substitute for lost gene products in cells subjected to Cre/Lox conditional gene deletion. We found that in human Langerin-Cre mice, epidermal LCs and CD11b+CD103+ mesenteric DCs overcome gene deletion if the deleted gene was expressed by neighboring cells. The mechanism of material transfer differed from traditional antigen uptake routes, relying on calcium and PI3K, being susceptible to polyguanylic acid inhibition, and remaining unaffected by inflammation. Termed intracellular monitoring, this process was specific to DCs, occurring in all murine DC subsets tested and human monocyte-derived DCs. The transferred material was presented on MHC-I and MHC-II, suggesting a role in regulating immune responses.

Dendritic Cells Overcome Cre/Lox Induced Gene Deficiency by Siphoning Material From Neighboring Cells Using Intracellular Monitoring-a Novel Mechanism of Antigen Acquisition

Macrophages and dendritic cells (DCs) in peripheral tissue interact closely with their local microenvironment by scavenging protein and nucleic acids released by neighboring cells. Material transfer between cell types is necessary for pathogen detection and antigen presentation, but thought to be relatively limited in scale. Recent reports, however, demonstrate that the quantity of transferred material can be quite large when DCs are in direct contact with live cells. This observation may be problematic for conditional gene deletion models that assume gene products will remain in the cell they are produced in. Here, we investigate whether conditional gene deletions induced by the widely used Cre/Lox system can be overcome at the protein level in DCs. Of concern, using the human Langerin Cre mouse model, we find that epidermal Langerhans cells and CD11b+CD103+ mesenteric DCs can overcome gene deletion if the deleted gene is expressed by neighboring cells. Surprisingly, we also find that the mechanism of material transfer does not resemble known mechanisms of antigen uptake, is dependent on extra- and intracellular calcium, PI3K, and scavenger receptors, and mediates a majority of material transfer to DCs. We term this novel process intracellular monitoring, and find that it is specific to DCs, but occurs in all murine DC subsets tested, as well as in human DCs. Transferred material is successfully presented and cross presented on MHC-II and MHC-I, and occurs between allogeneic donor and acceptors cells-implicating this widespread and unique process in immunosurveillance and organ transplantation.

Cross-presentation by the others

The critical role of conventional dendritic cells in physiological cross-priming of immune responses to tumors and pathogens is widely documented and beyond doubt. However, there is ample evidence that a wide range of other cell types can also acquire the capacity to cross-present. These include not only other myeloid cells such as plasmacytoid dendritic cells, macrophages and neutrophils, but also lymphoid populations, endothelial and epithelial cells and stromal cells including fibroblasts. The aim of this review is to provide an overview of the relevant literature that analyzes each report cited for the antigens and readouts used, mechanistic insight and in vivo experimentation addressing physiological relevance. As this analysis shows, many reports rely on the exceptionally sensitive recognition of an ovalbumin peptide by a transgenic T cell receptor, with results that therefore cannot always be extrapolated to physiological settings. Mechanistic studies remain basic in most cases but reveal that the cytosolic pathway is dominant across many cell types, while vacuolar processing is most encountered in macrophages. Studies addressing physiological relevance rigorously remain exceptional but suggest that cross-presentation by non-dendritic cells may have significant impact in anti-tumor immunity and autoimmunity.

Pathways of MHC I cross-presentation of exogenous antigens

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

Blocking Autophagy in M1 Macrophages Enhances Virus Replication and Eye Disease in Ocularly Infected Transgenic Mice

Macrophages are one of the first innate immune infiltrates in the cornea of mice following ocular infection with herpes simplex virus 1 (HSV-1). Using gamma interferon (IFN-γ) and interleukin-4 (IL-4) injections to polarize macrophages into M1 and M2, respectively, and in M1 and M2 conditional knockout mice, we have shown that M1 macrophages play an important role in suppressing both virus replication in the eye and eye disease in HSV-1-infected mice. Autophagy is also important in controlling HSV infection and integrity of infected cells. To determine if blocking autophagy in M1 and M2 macrophages affects HSV-1 infectivity and eye disease, we generated two transgenic mouse strains expressing the HSV-1 γ34.5 autophagy gene under the M1 promoter (M1-γ34.5) or the M2 promoter (M2-γ34.5). We found that blocking autophagy in M1 macrophages increased both virus replication in the eyes and eye disease in comparison to blocking autophagy in M2 macrophages or wild-type (WT) control mice, but blocked autophagy did not affect latency-reactivation. However, blocking autophagy affected fertility in both M1 and M2 transgenic mice. Analysis of 62 autophagy genes and 32 cytokines/chemokines from infected bone marrow-derived macrophages from M1-γ34.5, M2-γ34.5, and WT mice suggested that upregulation of autophagy-blocking genes (i.e., Hif1a, Mtmr14, mTOR, Mtmr3, Stk11, and ULK2) and the inflammatory tumor necrosis factor alpha (TNF-α) gene in M1-γ34.5 transgenic mice correlated with increased pathogenicity, while upregulation of proautophagy genes (Nrbf2 and Rb1cc1) in M2-γ34.5 macrophages correlated with reduced pathogenicity. The in vivo and in vitro responses of M1-γ34.5 and M2-γ34.5 transgenic mice to HSV-1 infection were independent of the presence of the γ34.5 gene in wild-type HSV-1. Our results suggest that M1 macrophages, but not M2 macrophages, play an important role in autophagy relative to primary virus replication in the eye and eye disease in infected mice. IMPORTANCE Autophagy plays a critical role in clearing, disassembling, and recycling damaged cells, thus limiting inflammation. The HSV-1 γ34.5 gene is involved in neurovirulence and immune evasion by blocking autophagy in infected cells. We found that blocking autophagy in M1 macrophages enhances HSV-1 virus replication in the eye and eye disease in ocularly infected transgenic mice. Our results also show the suppressive effects of γ34.5 on immune responses to infection, suggesting the importance of intact autophagy in M1 but not M2 macrophages in controlling primary infection and eye disease.

Spotlight on TAP and its vital role in antigen presentation and cross-presentation

In the late 1980s and early 1990s, the hunt for a transporter molecule ostensibly responsible for the translocation of peptides across the endoplasmic reticulum (ER) membrane yielded the successful discovery of transporter associated with antigen processing (TAP) protein. TAP is a heterodimer complex comprised of TAP1 and TAP2, which utilizes ATP to transport cytosolic peptides into the ER across its membrane. In the ER, together with other components it forms the peptide loading complex (PLC), which directs loading of high affinity peptides onto nascent major histocompatibility complex class I (MHC-I) molecules that are then transported to the cell surface for presentation to CD8+ T cells. TAP also plays a crucial role in transporting peptides into phagosomes and endosomes during cross-presentation in dendritic cells (DCs). Because of the critical role that TAP plays in both classical MHC-I presentation and cross-presentation, its expression and function are often compromised by numerous types of cancers and viruses to evade recognition by cytotoxic CD8 T cells. Here we review the discovery and function of TAP with a major focus on its role in cross-presentation in DCs. We discuss a recently described emergency route of noncanonical cross-presentation that is mobilized in DCs upon TAP blockade to restore CD8 T cell cross-priming. We also discuss the various strategies employed by cancer cells and viruses to target TAP expression or function to evade immunosurveillance - along with some strategies by which the repertoire of peptides presented by cells which downregulate TAP can be targeted as a therapeutic strategy to mobilize a TAP-independent CD8 T cell response. Lastly, we discuss TAP polymorphisms and the role of TAP in inherited disorders.

Gender Differences in Urothelial Bladder Cancer: Effects of Natural Killer Lymphocyte Immunity

Men are more likely to develop cancer than women. In fact, male predominance is one of the most consistent cancer epidemiology findings. Additionally, men have a poorer prognosis and an increased risk of secondary malignancies compared to women. These differences have been investigated in order to better understand cancer and to better treat both men and women. In this review, we discuss factors that may cause this gender difference, focusing on urothelial bladder cancer (UBC) pathogenesis. We consider physiological factors that may cause higher male cancer rates, including differences in X chromosome gene expression. We discuss how androgens may promote bladder cancer development directly by stimulating bladder urothelium and indirectly by suppressing immunity. We are particularly interested in the role of natural killer (NK) cells in anti-cancer immunity.

Viral infection modulates Qa-1b in infected and bystander cells to properly direct NK cell killing

Natural killer (NK) cell activation depends on the signaling balance of activating and inhibitory receptors. CD94 forms inhibitory receptors with NKG2A and activating receptors with NKG2E or NKG2C. We previously demonstrated that CD94-NKG2 on NK cells and its ligand Qa-1b are important for the resistance of C57BL/6 mice to lethal ectromelia virus (ECTV) infection. We now show that NKG2C or NKG2E deficiency does not increase susceptibility to lethal ECTV infection, but overexpression of Qa-1b in infected cells does. We also demonstrate that Qa-1b is down-regulated in infected and up-regulated in bystander inflammatory monocytes and B cells. Moreover, NK cells activated by ECTV infection kill Qa-1b-deficient cells in vitro and in vivo. Thus, during viral infection, recognition of Qa-1b by activating CD94/NKG2 receptors is not critical. Instead, the levels of Qa-1b expression are down-regulated in infected cells but increased in some bystander immune cells to respectively promote or inhibit their killing by activated NK cells.

I'm Infected, Eat Me! Innate Immunity Mediated by Live, Infected Cells Signaling To Be Phagocytosed

Innate immunity against pathogens is known to be mediated by barriers to pathogen invasion, activation of complement, recruitment of immune cells, immune cell phagocytosis of pathogens, death of infected cells, and activation of the adaptive immunity via antigen presentation. Here, we propose and review evidence for a novel mode of innate immunity whereby live, infected host cells induce phagocytes to phagocytose the infected cell, thereby potentially reducing infection. We discuss evidence that host cells, infected by virus, bacteria, or other intracellular pathogens (i) release nucleotides and chemokines as find-me signals, (ii) expose on their surface phosphatidylserine and calreticulin as eat-me signals, (iii) release and bind opsonins to induce phagocytosis, and (iv) downregulate don't-eat-me signals CD47, major histocompatibility complex class I (MHC1), and sialic acid. As long as the pathogens of the host cell are destroyed within the phagocyte, then infection can be curtailed; if antigens from the pathogens are cross-presented by the phagocyte, then an adaptive response would also be induced. Phagocytosis of live infected cells may thereby mediate innate immunity.

Vpr Is a VIP: HIV Vpr and Infected Macrophages Promote Viral Pathogenesis

HIV infects several cell types in the body, including CD4+ T cells and macrophages. Here we review the role of macrophages in HIV infection and describe complex interactions between viral proteins and host defenses in these cells. Macrophages exist in many forms throughout the body, where they play numerous roles in healthy and diseased states. They express pattern-recognition receptors (PRRs) that bind viral, bacterial, fungal, and parasitic pathogens, making them both a key player in innate immunity and a potential target of infection by pathogens, including HIV. Among these PRRs is mannose receptor, a macrophage-specific protein that binds oligosaccharides, restricts HIV replication, and is downregulated by the HIV accessory protein Vpr. Vpr significantly enhances infection in vivo, but the mechanism by which this occurs is controversial. It is well established that Vpr alters the expression of numerous host proteins by using its co-factor DCAF1, a component of the DCAF1-DDB1-CUL4 ubiquitin ligase complex. The host proteins targeted by Vpr and their role in viral replication are described in detail. We also discuss the structure and function of the viral protein Env, which is stabilized by Vpr in macrophages. Overall, this literature review provides an updated understanding of the contributions of macrophages and Vpr to HIV pathogenesis.

Unraveling the regulatory role of endoplasmic-reticulum-associated degradation in tumor immunity

During malignant transformation and cancer progression, tumor cells face both intrinsic and extrinsic stress, endoplasmic reticulum (ER) stress in particular. To survive and proliferate, tumor cells use multiple stress response pathways to mitigate ER stress, promoting disease aggression and treatment resistance. Among the stress response pathways is ER-associated degradation (ERAD), which consists of multiple components and steps working together to ensure protein quality and quantity. In addition to its established role in stress responses and tumor cell survival, ERAD has recently been shown to regulate tumor immunity. Here we summarize current knowledge on how ERAD promotes protein degradation, regulates immune cell development and function, participates in antigen presentation, exerts paradoxical roles on tumorigenesis and immunity, and thus impacts current cancer therapy. Collectively, ERAD is a critical protein homeostasis pathway intertwined with cancer development and tumor immunity. Of particular importance is the need to further unveil ERAD's enigmatic roles in tumor immunity to develop effective targeted and combination therapy for successful treatment of cancer.

Francisella tularensis enters a double membraned compartment following cell-cell transfer

Previously, we found that phagocytic cells ingest bacteria directly from the cytosol of infected cells without killing the initially infected cell (Steele et al., 2016). Here, we explored the events immediately following bacterial transfer. Francisella tularensis bacteria acquired from infected cells were found within double-membrane vesicles partially composed from the donor cell plasma membrane. As with phagosomal escape, the F. tularensis Type VI Secretion System (T6SS) was required for vacuole escape. We constructed a T6SS inducible strain and established conditions where this strain is trapped in vacuoles of cells infected through bacterial transfer. Using this strain we identified bacterial transfer events in the lungs of infected mice, demonstrating that this process occurs in infected animals. These data and electron microscopy analysis of the transfer event revealed that macrophages acquire cytoplasm and membrane components of other cells through a process that is distinct from, but related to phagocytosis.

Essential PTX3 Biology (not only) for Cardiologists and Cardiac Surgeons

Inflammation has been recognized to form an integral part of the atherosclerotic process. Much consideration has been given lately to the role played in atherogenesis by C-reactive protein (CRP). Although not accepted unequivocally, CRP appears to be not only a marker, but also an active mediator of the atherosclerotic process. Pentraxin 3 (PTX3) is a newly identified acute phase reactant which shares some structural and some functional properties with CRP. On the other hand, pentraxin 3 displays unique biological properties of its own, including a possible role in the pathogenesis of cardiovascular diseases and in processes accompanying the natural evolution of surgical wounds. This review article discusses recent information concerning basic pentraxin 3 biology in inflammation and in innate immunity reactions as viewed by a cardiologist in the context of acute coronary events and by a surgeon in patients struck with multiple wounds who are at the same time menaced by bacterial infections.

Roles of M1 and M2 Macrophages in Herpes Simplex Virus 1 Infectivity

Macrophages are the predominant infiltrate in the corneas of mice that have been ocularly infected with herpes simplex virus 1 (HSV-1). However, very little is known about the relative roles of M1 (classically activated or polarized) and M2 (alternatively activated or polarized) macrophages in ocular HSV-1 infection. To better understand these relationships, we assessed the impact of directed M1 or M2 activation of RAW264.7 macrophages and peritoneal macrophages (PM) on subsequent HSV-1 infection. In both the RAW264.7 macrophage and PM in vitro models, HSV-1 replication in M1 macrophages was markedly lower than in M2 macrophages and unstimulated controls. The M1 macrophages expressed significantly higher levels of 28 of the 32 tested cytokines and chemokines than M2 macrophages, with HSV-1 infection significantly increasing the levels of proinflammatory cytokines and chemokines in the M1 versus the M2 macrophages. To examine the effects of shifting the immune response toward either M1 or M2 macrophages in vivo, wild-type mice were injected with gamma interferon (IFN-γ) DNA or colony-stimulating factor 1 (CSF-1) DNA prior to ocular infection with HSV-1. Virus replication in the eye, latency in trigeminal ganglia (TG), and markers of T cell exhaustion in the TG were determined. We found that injection of mice with IFN-γ DNA, which enhances the development of M1 macrophages, increased virus replication in the eye; increased latency; and also increased CD4, CD8, IFN-γ, and PD-1 transcripts in the TG of latently infected mice. Conversely, injection of mice with CSF-1 DNA, which enhances the development of M2 macrophages, was associated with reduced virus replication in the eye and reduced latency and reduced the levels of CD4, CD8, IFN-γ,and PD-1 transcripts in the TG. Collectively, these results suggest that M2 macrophages directly reduce the levels of HSV-1 latency and, thus, T-cell exhaustion in the TG of ocularly infected mice.IMPORTANCE Our findings demonstrate a novel approach to further reducing HSV-1 replication in the eye and latency in the TG by modulating immune components, specifically, by altering the phenotype of macrophages. We suggest that inclusion of CSF-1 as part of any vaccination regimen against HSV infection to coax responses of macrophages toward an M2, rather than an M1, response may further improve vaccine efficacy against ocular HSV-1 replication and latency.

Beta-catenin signaling drives differentiation and proinflammatory function of IRF8-dependent dendritic cells

Beta-catenin signaling has recently been tied to the emergence of tolerogenic dendritic cells (DCs). In this article, we demonstrate a novel role for beta-catenin in directing DC subset development through IFN regulatory factor 8 (IRF8) activation. We found that splenic DC precursors express beta-catenin, and DCs from mice with CD11c-specific constitutive beta-catenin activation upregulated IRF8 through targeting of the Irf8 promoter, leading to in vivo expansion of IRF8-dependent CD8a+, plasmacytoid, and CD103+ CD11b2 DCs. beta-catenin–stabilized CD8a+ DCs secreted elevated IL-12 upon in vitro microbial stimulation, and pharmacological beta-catenin inhibition blocked this response in wild-type cells. Upon infections with Toxoplasma gondii and vaccinia virus, mice with stabilized DC beta-catenin displayed abnormally high Th1 and CD8+ T lymphocyte responses, respectively. Collectively, these results reveal a novel and unexpected function for beta-catenin in programming DC differentiation toward subsets that orchestrate proinflammatory immunity to infection.

Trogocytosis-associated cell to cell spread of intracellular bacterial pathogens

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

Activation of CD8 T Lymphocytes during Viral Infections

CD8 T lymphocytes are a major cell population of the adaptive immune system. A fundamental characteristic of the CD8 T lymphocyte pool is that it is composed of millions of clones; each with a unique T cell receptor capable of recognizing a limited number of peptides displayed at the cell surface bound to the grooves of major histocompatibility complex class I (MHC I) molecules. Naïve CD8 T lymphocytes are normally resting and circulate between the blood and secondary lymphoid organs in search of their cognate peptide–MHC complexes. During viral infections, bone marrow–derived professional antigen-presenting cells (pAPCs) in secondary lymphoid organs display viral peptides on their MHC I molecules. Specific CD8 T lymphocytes that recognize these peptide–MHC adducts become activated (primed), proliferate extensively, and develop into effectors capable of killing infected cells, identified by the presence at their surface of the pertinent viral peptide–MHC complexes. This article describes how the process of priming naïve CD8 T lymphocytes occurs.

Cross-Presentation of Cell-Associated Antigens by MHC Class I in Dendritic Cell Subsets

Dendritic cells (DCs) have the unique ability to pick up dead cells carrying antigens in tissue and migrate to the lymph nodes where they can cross-present cell-associated antigens by MHC class I to CD8⁺ T cells. There is strong in vivo evidence that the mouse XCR1⁺ DCs subset acts as a key player in this process. The intracellular processes underlying cross-presentation remain controversial and several pathways have been proposed. Indeed, a wide number of studies have addressed the cellular process of cross-presentation in vitro using a variety of sources of antigen and antigen-presenting cells. Here, we review the in vivo and in vitro evidence supporting the current mechanistic models and disscuss their physiological relevance to the cross-presentation of cell-associated antigens by DCs subsets.

Mannosylation of virus-like particles enhances internalization by antigen presenting cells

Internalization of peptides by antigen presenting cells is crucial for the initiation of the adaptive immune response. Mannosylation has been demonstrated to enhance antigen uptake through mannose receptors, leading to improved immune responses. In this study we test the effect of surface mannosylation of protein-based virus-like particles (VLP) derived from Rabbit hemorrhagic disease virus (RHDV) on uptake by murine and human antigen presenting cells. A monomannoside and a novel dimannoside were synthesized and successfully conjugated to RHDV VLP capsid protein, providing approximately 270 mannose groups on the surface of each virus particle. VLP conjugated to the mannoside or dimannoside exhibited significantly enhanced binding and internalization by murine dendritic cells, macrophages and B cells as well as human dendritic cells and macrophages. This uptake was inhibited by the inclusion of mannan as a specific inhibitor of mannose specific uptake, demonstrating that mannosylation of VLP targets mannose receptor-based uptake. Consistent with mannose receptor-based uptake, partial retargeting of the intracellular processing of RHDV VLP was observed, confirming that mannosylation of VLP provides both enhanced uptake and modified processing of associated antigens.

Priming of CD8+ T cells against cytomegalovirus-encoded antigens is dominated by cross-presentation

CMV can infect dendritic cells (DCs), and direct Ag presentation could, therefore, lead to the priming of CMV-specific CD8(+) T cells. However, CMV-encoded immune evasins severely impair Ag presentation in the MHC class I pathway; thus, it is widely assumed that cross-presentation drives the priming of antiviral T cells. We assessed the contribution of direct versus cross priming in mouse CMV (MCMV) infection using recombinant viruses. DCs infected with an MCMV strain encoding the gB498 epitope from HSV-1 were unable to stimulate in vitro naive gB498-specific CD8(+) T cells from TCR transgenic mice. Infection of C57BL/6 mice with this recombinant virus led, however, to the generation of abundant numbers of gB498-specific T cells in vivo. Of the DC subsets isolated from infected mice, only CD8α(+) DCs were able to stimulate naive T cells, suggesting that this DC subset cross-presents MCMV-encoded Ag in vivo. Upon infection of mice with MCMV mutants encoding Ag that can either be well or hardly cross-presented, mainly CD8(+) T cells specific for cross-presented epitopes were generated. Moreover, even in the absence of immune evasion genes interfering with MHC class I-mediated Ag presentation, priming of T cells to Ag that can only be presented directly was not observed. We conclude that the host uses mainly DCs capable of cross-presentation to induce the CMV-specific CD8(+) T cell response during primary, acute infection and discuss the implications for the development of a CMV vaccine.

Murine spleen contains a diversity of myeloid and dendritic cells distinct in antigen presenting function

The spleen contains multiple subsets of myeloid and dendritic cells (DC). DC are important antigen presenting cells (APC) which induce and control the adaptive immune response. They are cells specialized for antigen capture, processing and presentation to naïve T cells. However, DC are a heterogeneous population and each subset differs subtly in phenotype, function and location. Similarly, myeloid cell subsets can be distinguished which can also play an important role in the regulation of immunity. This review aims to characterize splenic subsets of DC and myeloid cells to better understand their individual roles in the immune response.

The orchestration of cellular and humoral responses is facilitated by divergent intracellular antigen trafficking in nanoparticle-based therapeutic vaccine

Therapeutic vaccination for the treatment of chronic hepatitis B is promising but has so far shown limited clinical efficacy. Herein, we employ polylactide nanoparticles (NPs) as the vaccine adjuvant and systematically explore their effect on activation of specific immunity and the underlying theoretical mechanisms. In vitro studies show that hepatitis B surface antigen (HBsAg) accumulates in antigen-presenting cells (APCs) to a larger content (270%) with the assistant of NP in comparison with the pure-antigen group. Besides the elevated costimulators (CD80/86) and increased major histocompatibility complex (MHC) II molecules, the MHC I molecules are also found upregulated. This result is mostly owing to the divergent antigen trafficking ways of NP-antigen in APCs, especially for the escape of exogenous HBsAg from the lysosomes to the cytosol. Interestingly, the MHC I level is downregulated in alum-antigen group, indicating a possible reason for its inefficiency in priming cellular response. Further in vivo experiments establish that NP-antigen group indeed enhances the CD8(+) CTL cytotoxicity and IFN-γ cytokine secretion. Meanwhile, specific antibody titer is also upregulated, and even surpasses that of the commercialized alum-antigen. All these results strongly support that NP-based antigen promotes an orchestration of cellular and humoral immune response, exhibiting favorable intrinsic properties to be applied in therapeutic vaccines.

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.

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.

IL-2 suppression of IL-12p70 by a recombinant HSV-1 expressing IL-2 induces T-cell auto-reactivity and CNS demyelination

To evaluate the role of cellular infiltrates in CNS demyelination in immunocompetent mice, we have used a model of multiple sclerosis (MS) in which different strains of mice are infected with a recombinant HSV-1 expressing IL-2. Histologic examination of the mice infected with HSV-IL-2 demonstrates that natural killer cells, dendritic cells, B cells, and CD25 (IL-2rα) do not play any role in the HSV-IL-2-induced demyelination. T cell depletion, T cell knockout and T cell adoptive transfer experiments suggest that both CD8⁺ and CD4⁺ T cells contribute to HSV-IL-2-induced CNS demyelination with CD8⁺ T cells being the primary inducers. In the adoptive transfer studies, all of the transferred T cells irrespective of their CD25 status at the time of transfer were positive for expression of FoxP3 and depletion of FoxP3 blocked CNS demyelination by HSV-IL-2. The expression levels of IL-12p35 relative to IL-12p40 differed in BM-derived macrophages infected with HSV-IL-2 from those infected with wild-type HSV-1. HSV-IL-2-induced demyelination was blocked by injecting HSV-IL-2-infected mice with IL-12p70 DNA. This study demonstrates that suppression of the IL-12p70 function of macrophages by IL-2 causes T cells to become auto-aggressive. Interruption of this immunoregulatory axis results in demyelination of the optic nerve, the spinal cord and the brain by autoreactive T cells in the HSV-IL-2 mouse model of MS.

Cross-priming in health and disease

Cross-priming is an important mechanism to activate cytotoxic T lymphocytes (CTLs) for immune defence against viruses and tumours. Although it was discovered more than 25 years ago, we have only recently gained insight into the underlying cellular and molecular mechanisms, and we are just beginning to understand its physiological importance in health and disease. Here we summarize current concepts on the cross-talk between the immune cells involved in CTL cross-priming and on its role in antimicrobial and antitumour defence, as well as in immune-mediated diseases.

Identification of a novel antigen cross-presenting cell type in spleen

Antigen-presenting cells (APC), like dendritic cells (DC), are essential for T-cell activation, leading to immunity or tolerance. Multiple DC subsets each play a unique role in the immune response. Here, a novel splenic dendritic-like APC has been characterized in mice that has immune function and cell surface phenotype distinct from other, described DC subsets. These were identified as a cell type continuously produced in spleen long-term cultures (LTC) and have an in vivo equivalent cell type in mice, namely 'L-DC'. This study characterizes LTC-DC in terms of marker phenotype and function, and compares them with L-DC and other known splenic DC and myeloid subsets. L-DC display a myeloid dendritic-like phenotype equivalent to LTC-DC as CD11c(lo) CD11b(hi) MHC-II(-) CD8α(-) cells, distinct by high accessibility and endocytic capacity for blood-borne antigen. Both LTC-DC and L-DC have strong antigen cross-presentation ability leading to strong activation of CD8(+) T cells, particularly after exposure to lipopolysaccharide. However, they have weak ability to stimulate CD4(+) T cells in antigen-specific responses. Evidence is presented here for a novel DC type produced by in vitro haematopoiesis which has distinct antigen-presenting potential and reflects a DC subset present also in vivo in spleen.

Direct presentation is sufficient for an efficient anti-viral CD8+ T cell response

The extent to which direct- and cross-presentation (DP and CP) contribute to the priming of CD8⁺ T cell (T_CD8+) responses to viruses is unclear mainly because of the difficulty in separating the two processes. Hence, while CP in the absence of DP has been clearly demonstrated, induction of an anti-viral T_CD8+ response that excludes CP has never been purposely shown. Using vaccinia virus (VACV), which has been used as the vaccine to rid the world of smallpox and is proposed as a vector for many other vaccines, we show that DP is the main mechanism for the priming of an anti-viral T_CD8+ response. These findings provide important insights to our understanding of how one of the most effective anti-viral vaccines induces immunity and should contribute to the development of novel vaccines.

Professional antigen presenting cells fragment viral proteins and display some of the resulting peptides bound to MHC molecules at the cell surface. When virus-specific CD8⁺ T cells recognize these viral peptides they become activated, proliferate, and kill virus-infected cells to help rid the body of the virus. Two pathways have been described for the origin of the peptides presented by professional antigen presenting cells. In cross-presentation, the antigen presenting cells acquire the proteins from other cells which, in the case of a viral infection, must be infected. In direct presentation, the antigen presenting cells synthesize the proteins themselves and, therefore, during responses to viruses must be infected. However, the participation of direct presentation in anti-viral responses has never been deliberately demonstrated experimentally. In this paper we demonstrate that direct presentation occurs and is the main pathway to induce CD8⁺ T cells during infection with vaccinia virus. These findings provide important insights to our understanding of how one of the most effective anti-viral vaccines induces immunity and should contribute to the development of novel vaccines.

Uncovering the interplay between CD8, CD4 and antibody responses to complex pathogens

Vaccinia virus (VACV) was used as the vaccine strain to eradicate smallpox. VACV is still administered to healthcare workers or researchers who are at risk of contracting the virus, and to military personnel. Thus, VACV represents a weapon against outbreaks, both natural (e.g., monkeypox) or man-made (bioterror). This virus is also used as a vector for experimental vaccine development (cancer/infectious disease). As a prototypic poxvirus, VACV is a model system for studying host-pathogen interactions. Until recently, little was known about the targets of host immune responses, which was likely owing to VACVs large genome (>200 open reading frames). However, the last few years have witnessed an explosion of data, and VACV has quickly become a useful model to study adaptive immune responses. This review summarizes and highlights key findings based on identification of VACV antigens targeted by the immune system (CD4, CD8 and antibodies) and the complex interplay between responses.

Cross, but not direct, presentation of cell-associated virus antigens by spleen macrophages is influenced by their differentiation state

The initiation of T-cell immune responses requires professional antigen-presenting cells. Emerging data point towards an important role for macrophages (Mphi) in the priming of naïve T cells. In this study we analyzed the efficiency and the mechanisms by which Mphi derived from spleen (Sp-Mphi) or bone marrow (BM-Mphi) present Lymphocytic choriomeningitis virus (LCMV) antigens to epitope-specific T cells. We demonstrate that because of phagosomal maturation, Sp-Mphi downregulate their ability to cross-present cell-associated, but not soluble, antigens, as they are further differentiated in culture without altering their capacity to directly present virus antigens after infection. We propose that Sp-Mphi are extremely efficient at direct and cross-presentation. However, if these cells undergo further M-CSF-dependent maturation, they will adapt to be more scavenger and phagocytic and concurrently reduce their cross-presenting capacity. Accordingly, Sp-Mphi can have an important role in regulating T-cell responses through cross-presentation depending on their differentiation state.

The amino acid sequences flanking an antigenic determinant can strongly affect MHC class I cross-presentation without altering direct presentation

Direct presentation (DP) and cross presentation (CP) on MHC I by professional APCs are defined by the internal or external source of the Ag, respectively. Although some Ags are substrates for both DP and CP, others are only substrates for DP. The reasons for this difference remain largely unknown. In this study, we studied in tissue culture and also in vivo, the effects of altering the length and sequence of the amino acid chains flanking an MHC class I restricted determinant (the chicken OVA OVA(258-265), SIINFEKL) that is normally a good substrate for both DP and CP. We demonstrate that CP but not DP strictly requires flanking N and C-terminal extensions of minimal length. Furthermore, we show that removal but not replacement of just one amino acid 22 residues downstream from the determinant is sufficient to strongly affect CP without affecting either protein stability or DP. Thus, our work shows that the flanking residues of an antigenic determinant can differentially affect CP and DP, and that features of the Ag other than half-life can have a major impact in CP. Our studies may have implications for understanding CP in viral infections and possibly for the design of new vaccines.

Contribution of direct and cross-presentation to CTL immunity against herpes simplex virus 1

Dendritic cells (DC), which can be subdivided into different phenotypic and functional subsets, play a pivotal role in the generation of cytotoxic T cell immunity against viral infections. Understanding the modes of Ag acquisition, processing and presentation by DC is essential for the design of effective antiviral vaccines. We aimed to assess the contribution of direct vs cross-presentation for the induction of HSV1-specific CD8(+) T lymphocyte responses in mice. Using HSV1 strains expressing fluorescence proteins, we provide evidence for the ability of HSV1 to induce viral transcription. Using HSV1-wild-type as well as gB- or gH-deficient mutants to either directly inoculate DC or to infect target cells, which then were given to DC, we show that DC acquired viral Ag via phagocytosis of target cells and via direct inoculation of virus being released from target cells. Our study corroborates the function of the CD8(+) DC specialized in Ag cross-presentation and confirms this specific feature for Ags that these DC acquire directly from HSV1. However, although infection of cross-presenting DC amplified T cell responses, it was not a requirement for presentation of viral Ags, both in vitro and in vivo. Finally, we provide evidence that direct presentation did not contribute to the Ag presentation capacity of CD8(+) DC after phagocytosis of infected target cells. We conclude that cross-presentation is of major importance for the induction of CTL immunity in mice.

Role of dendritic cells in enhancement of herpes simplex virus type 1 latency and reactivation in vaccinated mice

Ocular infection with herpes simplex virus type 1 (HSV-1) frequently leads to recurrent infection, which is a major cause of corneal scarring. Thus, the prevention of the establishment of latency should be a primary goal of vaccination against HSV-1. To this end, we have examined the contribution of dendritic cells (DCs) to the efficacy of a vaccine against ocular HSV-1 infection. Transgenic mice (expressing a CD11c-diphtheria toxin receptor-green fluorescent protein construct) with a BALB/c background were immunized with a vaccine consisting of DNA that encodes five HSV-1 glycoproteins or were immunized with vector control DNA. The vaccinated mice were then depleted of their DCs through the injection of diphtheria toxin before and after ocular challenge with HSV-1. Analyses of HSV-1 replication in the eye, blepharitis, corneal scarring, and the survival of the infected mice upon primary infection indicated that DC depletion neither promoted nor compromised the efficacy of the vaccine. In contrast, DC depletion was associated with an approximately fivefold reduction in the level of latent virus in the trigeminal ganglia (TGs) of latently infected mice, as well as a significant reduction in the reactivation rate of latent virus. The possibility that DCs enhance the latency of HSV-1 in the TGs of ocularly infected mice suggests for the first time that DCs, rather than acting as "immune saviors," can exacerbate disease and compromise vaccine efficacy by enhancing viral latency and reactivation.

An efficient culture method for generating large quantities of mature mouse splenic macrophages

In this study, we established an efficient in vitro culture method for generating mature splenic macrophages (Sp-Mphi). Splenocytes were cultured in the presence of conditioned medium containing macrophage colony-stimulating factor (M-CSF) for 7 days post post-isolation and the generated Sp-Mphi were characterized phenotypically and functionally. Through this method, 9 x 10(6)/mouse Sp-Mphi were obtained in comparison to 2 x 10(5)/mouse when Mphi were cultured in regular medium. In addition, the purity of these cells was as high as 80% by day 5 and >90% by day 7 of culturing, confirmed with Mphi-specific markers. The increased Sp-Mphi yields, in the presence of M-CSF, point towards the existence of a precursor population in the spleen that can be influenced to differentiate into Sp-Mphi. Moreover, we compared the maturation of generated Sp-Mphi to conventional bone marrow-derived Mphi (BM-Mphi) in vitro. Interestingly, Sp-Mphi exhibited lower capacity to phagocytose dead cells after 3 days of maturation, but showed similar internalizing capacity after 5 and 7 of maturation to BM-Mphi cultured for the same time period. Importantly, Sp-Mphi upregulated the expression of several surface markers such as MOMA-2 and CD68 while downregulating SIGN-R1 after 7 days, indicating that these Sp-Mphi undergo further maturation in vitro due to culturing in M-CSF. Taken together, we describe and validate a method for generating Sp-Mphi in large quantities and high purity. These data should prove valuable in future studies characterizing the functions and maturation of Sp-Mphi.

Plasmacytoid dendritic cells efficiently cross-prime naive T cells in vivo after TLR activation

Cross-presentation is a crucial mechanism in tumoral and microbial immunity because it allows internalized cell associated or exogenous antigens (Ags) to be delivered into the major histocompatibility complex I pathway. This pathway is important for the development of CD8(+) T-cell responses and for the induction of tolerance. In mice, cross-presentation is considered to be a unique property of CD8alpha+ conventional dendritic cells (DCs). Here we show that splenic plasmacytoid DCs (pDCs) efficiently capture exogenous Ags in vivo but are not able to cross-present these Ags at steady state. However, in vitro and in vivo stimulation by Toll-like receptor-7, or -9 or viruses licenses pDCs to cross-present soluble or particulate Ags by a transporter associated with antigen processing-dependent mechanism. Induction of cross-presentation confers to pDCs the ability to generate efficient effector CD8+ T-cell responses against exogenous Ags in vivo, showing that pDCs may play a crucial role in induction of adaptive immune responses against pathogens that do not infect tissues of hemopoietic origin. This study provides the first evidence for an in vivo role of splenic pDCs in Ag cross-presentation and T-cell cross-priming and suggests that pDCs may constitute an attractive target to boost the efficacy of vaccines based on cytotoxic T lymphocyte induction.

Phagocytosis and antigen presentation in dendritic cells

Like macrophages and neutrophils, dendritic cells (DCs) are considered professional phagocytes. Even if the three cell types phagocytose parasites, bacteria, cell debris, or even intact cells very efficiently, the functional outcomes of the phagocytic event are quite different. Macrophages and neutrophils scavenge and destroy phagocytosed particles, a critical step in innate immunity. DCs, in contrast, have developed means to 'preserve' useful information from the ingested particles that serve to initiate adaptive immune responses. Thus, both phagosomal degradation and acidification are much lower in DCs than in macrophages or neutrophils. Reduced degradation results in the conservation of antigenic peptides and in their increased presentation on major histocompatibility complex class I and II molecules. In this article, we review the mechanisms that control this delicate equilibrium between phagosomal degradation/cytotoxicity and antigen presentation in the different families of phagocytes.

Key differences in TLR3/poly I:C signaling and cytokine induction by human primary cells: a phenomenon absent from murine cell systems

TLR3 recognizes double-stranded RNA, a product associated with viral infections. Many details of TLR3-induced mechanisms have emerged from gene-targeted mice or inhibition studies in transformed cell lines. However, the pathways activated in human immune cells or cells from disease tissue are less well understood. We have investigated TLR3-induced mechanisms of human primary cells of the innate immune system, including dendritic cells (DCs), macrophages (MØs), endothelial cells (ECs), and synovial fibroblasts isolated from rheumatoid arthritis joint tissue (RA-SFs). Here, we report that while these cells all express TLR3, they differ substantially in their response to TLR3 stimulation. The key antiviral response chemokine IP-10 was produced by all cell types, while DCs and MØs failed to produce the proinflammatory cytokines TNFalpha and IL-6. Unexpectedly, TNFalpha was found secreted by TLR3-stimulated RA-SF. Furthermore, TLR3 stimulation did not activate NFkappaB, MAPKs, or IRF-3 in DCs and MØs, but was able to do so in ECs and RA-SF. These findings were specific for human cells, thereby revealing a complexity not previously expected. This is the first report of such cell type- and species-specific response for any TLR stimulation and helps to explain important difficulties in correlating murine models of inflammatory diseases and human inflammation.

Phage display of a CTL epitope elicits a long-term in vivo cytotoxic response

The Ovalbumin(257-264) CTL epitope on the major coat protein of the filamentous bacteriophage in different antigen formulations was displayed and the immune response in C57BL6/J mice studied. The display of single cytotoxic epitope on the surface of the virion is sufficient to induce priming and sustain long-term major histocompatibility complex class I restricted cytotoxic T lymphocytes response in vivo. The filamentous bacteriophage is a versatile carrier able to display simultaneously either single or multiple epitopes and can elicit a cellular response carrying very little peptide (<1.5 microg).

Distinct pathways for signaling maturation in macrophages and dendritic cells after infection with paramyxovirus simian virus 5

Professional antigen-presenting cells are critical components of both the innate and adaptive immune responses. Although dendritic cells (DCs) are generally thought to be the primary activators of naive T cells, macrophages have also been shown to fulfill this role. As with DCs, the capacity to induce optimal activation of T cells requires that macrophages undergo a process that results in the increased expression of costimulatory molecules, such as CD40, CD80, and CD86, and the production of cytokines. In this study we analyzed the effect of infection of macrophages generated from BALB/c mice with the paramyxovirus simian virus 5 (SV5). Here we have shown that bone marrow-derived macrophages (BMMs) are not productively infected at any multiplicity of infection tested. Analysis of activation markers revealed that SV5-infected BMMs robustly upregulated CD40 and modestly upregulated CD86, but did not upregulate the expression of CD80. Further, SV5-infected BMMs secreted low levels of interferon-beta and interleukin (IL)-12p40, but high levels of tumor necrosis factor-alpha and IL-6. Intriguingly, upregulation of these molecules on BMMs, unlike our previous results using bone marrow-derived dendritic cells, was not dependent on live virus. These findings provide evidence that different professional antigen-presenting cells can detect and respond to virus via distinct mechanisms.

The Cross-priming Pathway: A Portrait of an Intricate Immune System

Antigen presentation by professional antigen-presenting cells (pAPCs) to cytotoxic CD8(+) T cells can occur via two processing routes - the direct and cross-presentation pathways. Cross-presentation of exogenous antigens in the context of major histocompatibility complex (MHC) class I molecules has recently attracted a lot of research interest because it may prove crucial for vaccine development. This alternative pathway has been implicated in priming CD8(+) T-cell responses to pathogens as well as tumours in vivo (cross-priming). In cross-presentation, the internalized antigens can be processed through diverse intracellular routes. As many unresolved questions regarding the molecular basis that controls the cross-priming process still exist, it is essential to explore the various elements involved therein, to better elucidate this pathway. In this review, we summarize current data that explore how the source and nature of antigens could affect their cross-presentation. Moreover, we will discuss and outline how recent advances regarding pAPCs' properties have increased our appreciation of the complex nature of the cross-priming pathway in vivo. In conclusion, we contemplate how the direct and cross-presentation pathways can function to allow the immune system to deal efficiently with diverse pathogens.

Understanding orthopoxvirus interference with host immune responses to inform novel vaccine design

Jenner's original vaccine used cowpox virus. Cowpox virus and, subsequently, vaccinia virus, a closely related Orthopoxvirus, provided the means to eradicate smallpox. This history and the unique properties of the virus suggest that vaccinia virus will continue to provide a useful vaccine platform. Yet, surprisingly, it has become apparent that much of the virus genome encodes accessory proteins that interfere with host immune responses to infection. Manipulation of these genes offers the potential for new generations of orthopoxvirus vaccines in which we will have far greater control over key features of the vaccination, including the sites of virus infection, the degree of virus replication, the pathogenicity of the virus and, most importantly, the suppression or induction of immune responses of specific types.

Polymorphonuclear neutrophils deliver activation signals and antigenic molecules to dendritic cells: a new link between leukocytes upstream of T lymphocytes

Polymorphonuclear neutrophils (PMNs) are rapidly recruited to tissues upon injury or infection. There, they can encounter local and/or recruited immature dendritic cells (iDCs), a colocalization that could promote at least transient interactions and mutually influence the two leukocyte populations. Using human live blood PMNs and monocyte-derived iDCs, we examined if these leukocytes actually interacted and whether this influenced DC function. Indeed, coculture with live but not apoptotic PMNs led to up-regulation of membrane CD40, CD86, and human leukocyte antigen (HLA)-DR on DCs. Whereas CD40 up-regulation was dependent on soluble factors released by PMNs, as determined in cultures conducted in different chambers, cell contact was necessary for CD86 and HLA-DR up-regulation, a process that was inhibited by anti-CD18 antibodies, indicating that CD18 ligation was required. We also found that via a cell contact-dependent mechanism, DCs acquired Candida albicans-derived antigens from live as well as from apoptotic PMNs and could thus elicit antigen-specific T lymphocyte responses. Altogether, our data demonstrate the occurrence of cross-talk between human PMNs and DCs and provide new insights into the immune processes occurring upstream of the interactions between DCs and T lymphocytes.

Elicitation of immunity to HIV type 1 Gag is determined by Gag structure

The gag gene of the human immunodeficiency virus type 1 (HIV-1) encodes for viral proteins that self-assemble into viral particles. The primary Gag gene products (capsid, matrix, and nucleocapsid) elicit humoral and cellular immune responses during natural infection, and these proteins are included in many preclinical and clinical HIV/AIDS vaccines. However, the structure (particulate or soluble) of these proteins may influence the immunity elicited during vaccination. In this study, mice were inoculated with four different HIV-1 Gag vaccines to compare the elicitation of immune responses by the same Gag immunogen presented to the immune system in different forms. The immunity elicited by particles produced in vivo by DNA plasmid (pGag) was compared to these same proteins retained intracellularly (pGag(DMyr)). In addition, the elicitation of anti- Gag immunity by Gag(p55) virus-like particles (VLPs) or soluble, nonparticulate Gag(p55) proteins was compared. Enhanced cellular responses, but almost no anti-Gag antibodies, were elicited with intracellularly retained Gag proteins. In contrast, DNA vaccines expressing VLPs elicited both anti-Gag antibodies and cellular responses. Mice vaccinated with purified Gag(p55) VLPs elicited robust humoral and cellular immune responses, which were significantly higher than the immunity elicited by soluble, nonparticulate Gag(p55) protein. Overall, purified particles of Gag effectively elicited the broadest and highest titers of anti-Gag immunity. The structural form of Gag influences the elicited immune responses and should be considered in the design of HIV/AIDS vaccines.

Induction of T cell immunity by cutaneous genetic immunization with recombinant lentivector

Recent studies suggest that T cell-based cellular immunity plays an important role in preventing and delaying progression of infectious and neoplastic diseases. Based on these findings, recent efforts in vaccine research are giving rise to a new generation of "T cell" vaccines. The development of T cell vaccines has been problematic. Current investigations are focusing on gene-based immunization strategies, including the development of non-viral "naked" plasmid DNA and recombinant viral vector-based genetic immunization approaches. Here, we briefly review recent progress in the development of recombinant viral vectors for genetic immunization and our own recent studies elucidating differences in mechanisms of genetic immunization. We propose that the mechanism of immune induction depends in part on unique features of specific viral vectors, and that a comparison of representative vectors mechanistically will enable a more informed understanding of the determining parameters of immune induction. Our initial studies have focused on the identification of antigen-presenting-cell subsets important for priming CD8+ T cell immunity, the effects of antigen persistence on immune responses, and the unique immunogenicity of skin as a target tissue for vaccine delivery. We review data suggesting that the unique properties of recombinant lentivectors make them appealing candidates as genetic immunization vehicles for eliciting T cell immune responses.

Cross-presentation of the long-lived lymphocytic choriomeningitis virus nucleoprotein does not require neosynthesis and is enhanced via heat shock proteins

Many viral proteins that contain MHC class I-restricted peptides are long-lived, and it is elusive how they can give rise to class I epitopes. Recently, we showed that direct presentation of an epitope of the long-lived lymphocytic choriomeningitis virus nucleoprotein (LCMV-NP) required neosynthesis in accordance with the defective ribosomal products hypothesis. In this study, we report that LCMV-NP can be cross-primed in mice using either LCMV-NP-transfected human HEK293 or BALB/c-derived B8 cells as Ag donor cells. In addition, we establish that contrary to direct presentation, cross-presentation required accumulation of the mature LCMV-NP and could not be sustained by the newly synthesized LCMV-NP protein, intermediate proteasomal degradation products, or the minimal NP396 epitope. Nevertheless, NP cross-presentation was enhanced by heat shock and was blunted by inhibitors of heat shock protein 90 and gp96. We propose that cross-presentation has evolved to sustain the presentation of stable viral proteins when their neosynthesis has ceased in infected donor cells.

Apoptotic cells at the crossroads of tolerance and immunity

Clearance of apoptotic cells by phagocytes can result in either anti-inflammatory and immunosuppressive effects or prostimulatory consequences through presentation of cell-associated antigens to T cells. The differences in outcome are due to the conditions under which apoptosis is induced, the type of phagocytic cell, the nature of the receptors involved in apoptotic cell capture, and the milieu in which phagocytosis of apoptotic cells takes place. Preferential ligation of specific receptors on professional antigen-presenting cells (dendritic cells) has been proposed to induce potentially tolerogenic signals. On the other hand, dendritic cells can efficiently process and present antigens from pathogen-infected apoptotic cells to T cells. In this review, we discuss how apoptotic cells manipulate immunity through interactions with dendritic cells.