Viral sequestration of antigen subverts cross presentation to CD8(+) T cells

Ebola Virus Infection of Flt3-Dependent, Conventional Dendritic Cells and Antigen Cross-presentation Leads to High Levels of T-Cell Activation

BACKGROUND

Previous studies have described that Ebola virus (EBOV) infection of human monocyte-derived dendritic cells (moDCs) inhibits dendritic cell (DC) maturation, resulting in poor T-cell activation. However, it is unknown how other DC subsets distinct from moDCs respond to EBOV infection.

METHODS

To better understand how DCs initiate T-cell activation during EBOV infection, we assessed the response of conventional mouse DCs (cDCs) to EBOV infection utilizing a recombinant EBOV expressing the model antigen ovalbumin.

RESULTS

In contrast to moDCs, mouse cDC2s and cDC1s were poorly infected with EBOV but were highly activated. DCs were able to prime CD8 T cells via cross-presentation of antigens obtained from cell debris of EBOV-infected cells. EBOV infection further enhanced DC cross-presentation.

CONCLUSIONS

Our findings indicate that EBOV infection of cDCs results in activation rather than inhibition, leading to high levels of T-cell activation. With that we propose a mechanistic explanation for the excess T-cell activation observed in human Ebola virus disease.

Direct Priming of CD8+ T Cells Persists in the Face of Cowpox Virus Inhibitors of Antigen Presentation

Vaccinia virus (VACV) was the vaccine used to eradicate smallpox and is being repurposed as a vaccine vector. CD8+ T cells are key anti-viral mediators, but require priming to become effector or memory cells. Priming requires an interaction with dendritic cells that are either infected (direct priming), or that have acquired virus proteins but remain uninfected (cross priming). To investigate CD8+ T cell priming pathways for VACV, we engineered the virus to express CPXV12 and CPXV203, two inhibitors of antigen presentation encoded by cowpox virus. These intracellular proteins would be expected to block direct but not cross priming. The inhibitors had diverse impacts on the size of anti-VACV CD8+ T cell responses across epitopes and by different infection routes in mice, superficially suggesting variable use of direct and cross priming. However, when we then tested a form of antigen that requires direct priming, we found surprisingly that CD8+ T cell responses were not diminished by co-expression with CPXV12 and CPXV203. We then directly quantified the impact of CPXV12 and CPXV203 on viral antigen presentation using mass spectrometry, which revealed strong, but incomplete inhibition of antigen presentation by the CPXV proteins. Therefore, direct priming of CD8+ T cells by poxviruses is robust enough to withstand highly potent viral inhibitors of antigen presentation. This is a reminder of the limits of viral immune evasion and shows that viral inhibitors of antigen presentation cannot be assumed to dissect cleanly direct and cross priming of anti-viral CD8+ T cells.ImportanceCD8+ T cells are key to anti-viral immunity, so it is important to understand how they are activated. Many viruses have proteins that protect infected cells from T cell attack by interfering with the process that allows virus infection to be recognised by CD8+ T cells. It is thought that these proteins would also stop infected cells from activating T cells in the first place. However, we show here that this is not the case for two very powerful inhibitory proteins from cowpox virus. This demonstrates the flexibility and robustness of immune processes that turn on the immune responses required to fight infection.

Surprisingly Effective Priming of CD8+ T Cells by Heat-Inactivated Vaccinia Virus Virions

Robust priming of CD8+ T cells by viruses is considered to require infection and de novo expression of viral antigens. A corollary of this is that inactivated viruses are thought of as being inevitably poor vaccines for eliciting these responses. In contrast to this dogma, we found that some antigens present in vaccinia virus (VACV) virions prime strong CD8+ T cell responses when the virus was rendered noninfectious by heat. More surprisingly, in some cases these responses were similar in magnitude to those primed by infectious virus administered at an equivalent dose. Next, we tested whether this was a special property of particular antigens and their epitopes and found that foreign epitopes tagged onto three different VACV virion proteins were able to elicit CD8+ T cell responses irrespective of whether the virus was viable or heat killed. Further, the polyfunctionality and cytotoxic ability of the CD8+ T cells primed by these VACVs was equivalent irrespective of whether they were administered to mice as inactivated or live viruses. Finally, we used these VACVs in prime-boost combinations of inactivated and live virus and found that priming with dead virus before a live booster was the most immunogenic regime. We conclude that VACV virions can be efficient vectors for targeting antigens to dendritic cells for effective priming of CD8+ T cells, even when rendered noninfectious and speculate that this might also be the case for other viruses.IMPORTANCE The design of viral vectored vaccines is often considered to require a trade-off between efficacy and safety. This is especially the case for vaccines that aim to induce killer (CD8+) T cells, where there is a well-established dogma that links infection in vaccinated individuals with effective induction of immunity. However, we found that some proteins of vaccinia virus generate strong CD8+ T cell responses even when the virus preparation was inactivated by heat prior to administration as a vaccine. We took advantage of this finding by engineering a new vaccine vector virus that could be used as an inactivated vaccine. These results suggest that vaccinia virus may be a more versatile vaccine vector than previously appreciated and that in some instances safety can be prioritized by the complete elimination of viral replication without a proportional loss of immunogenicity.

Efficient Induction of Cytotoxic T Cells by Viral Vector Vaccination Requires STING-Dependent DC Functions

Modified Vaccinia virus Ankara (MVA) is an attenuated strain of vaccinia virus and currently under investigation as a promising vaccine vector against infectious diseases and cancer. MVA acquired mutations in host range and immunomodulatory genes, rendering the virus deficient for replication in most mammalian cells. MVA has a high safety profile and induces robust immune responses. However, the role of innate immune triggers for the induction of cytotoxic T cell responses after vaccination is incompletely understood. Stimulator of interferon genes (STING) is an adaptor protein which integrates signaling downstream of several DNA sensors and therefore mediates the induction of type I interferons and other cytokines or chemokines in response to various dsDNA viruses. Since the type I interferon response was entirely STING-dependent during MVA infection, we studied the effect of STING on primary and secondary cytotoxic T cell responses and memory T cell formation after MVA vaccination in STING KO mice. Moreover, we analyzed the impact of STING on the maturation of bone marrow-derived dendritic cells (BMDCs) and their functionality as antigen presenting cells for cytotoxic T cells during MVA infection in vitro. Our results show that STING has an impact on the antigen processing and presentation capacity of conventionel DCs and played a crucial role for DC maturation and type I interferon production. Importantly, STING was required for the induction of efficient cytotoxic T cell responses in vivo, since we observed significantly decreased short-lived effector and effector memory T cell responses after priming in STING KO mice. These findings indicate that STING probably integrates innate immune signaling downstream of different DNA sensors in DCs and shapes the cytotoxic T cell response via the DC maturation phenotype which strongly depends on type I interferons in this infection model. Understanding the detailed functions of innate immune triggers during MVA infection will contribute to the optimized design of MVA-based vaccines.

Nanoparticle-based immunotherapy: state of the art and future perspectives

INTRODUCTION

For several years now, medicine has been benefiting from the contribution of nanoparticles (NPs) technology for both diagnosis and therapy. They can be used as adjuvants, being capable per se of immune-modulating activity, or as carriers for molecules to be transported to a specific target, eventually loaded with specific ligands favoring specific uptake.

AREAS COVERED

The review focuses on experimental use of NPs as adjuvants/carriers for allergen immunotherapy (AIT). Human clinical trials conducted so far are discussed.

EXPERT OPINION

Results of experimental studies and recent clinical trials support the use of NPs as carrier/adjuvant in AIT. Comparisons between NP-based and classical AIT are needed, to show the usefulness of the NP-based approach. However, there are still unsolved problems: the persistence of non-degradable NPs with possible toxicological consequences, and the formation of the protein corona around the NPs, which could alter their activity and fate. Virus-like particles seem the most promising NPs for allergy treatment, as for other vaccines. Over the next decade, NP-based AIT will be largely used to treat allergic disorders.

Herpes Simplex Virus 1-Specific CD8+ T Cell Priming and Latent Ganglionic Retention Are Shaped by Viral Epitope Promoter Kinetics

Reactivation of herpes simplex virus 1 (HSV-1) from neurons in sensory ganglia such as the trigeminal ganglia (TG) is influenced by virus-specific CD8⁺ T cells that infiltrate the ganglia at the onset of latency and contract to a stable activated tissue-resident memory population. In C57BL/6 mice, half of HSV-specific CD8⁺ T cells (gB-CD8s) recognize one dominant epitope (residues 498 to 505) on glycoprotein B (gB_498-505), while the remainder (non-gB-CD8s) recognize 19 subdominant epitopes from 12 viral proteins. To address how expression by HSV-1 influences the formation and ganglionic retention of CD8⁺ T cell populations, we developed recombinant HSV-1 with the native immunodominant gB epitope disrupted but then expressed ectopically from different viral promoters. In mice, the epitope expressed from the gB promoter restored full gB-CD8 immunodominance to 50%. Intriguingly, earlier expression from constitutive, immediate-early, and early promoters did not significantly increase immunodominance, indicating that these promoters cannot elicit more than half of the CD8 compartment. Epitope expressed from candidate viral promoters of "true late" HSV-1 genes either delayed or reduced the priming efficiency of gB-CD8s and their levels in the TG at early times. HSV expressing the epitope from the full latency-associated transcript promoter did not efficiently prime gB-CD8s; however, gB-CD8s primed by a concurrent wild-type flank infection infiltrated the TG and were retained long term, suggesting that latent epitope expression is sufficient to retain gB-CD8s. Taken together, the data indicate that viral promoters shape latent HSV-1-specific CD8⁺ T cell populations and should be an important consideration in future vaccine design.IMPORTANCE Latency of HSV-1 in host neurons enables long-term persistence from which reactivation may occur to cause recurrent diseases, such as blinding herpetic stromal keratitis. Latency is not antigenically silent, and viral proteins are sporadically expressed at low levels without full virion production. This protein expression is recognized by ganglion-resident HSV-1-specific CD8⁺ T cells that maintain a protective resident population. Since these T cells can influence lytic/latent decisions in reactivating neurons, we argue that improving their ganglionic retention and function may offer a strategy in vaccine design to reduce reactivation and recurrent disease. To understand factors driving the infiltration and retention of ganglionic CD8s, we examined several HSV recombinants that have different viral promoters driving expression of the immunodominant gB epitope. We show that the selection of epitope promoter influences CD8⁺ T cell population hierarchies and their function.

Sequestration of Late Antigens Within Viral Factories Impairs MVA Vector-Induced Protective Memory CTL Responses

Cytotoxic CD8+ T cell (CTL) responses play an essential role in antiviral immunity. Here, we focused on the activation of CTL which recognize epitopes derived from viral or recombinant antigens with either early or late expression kinetics after infection with Modified Vaccinia Virus Ankara (MVA). Late antigens but not early antigens failed to efficiently stimulate murine CTL lines in vitro and were unable to activate and expand protective memory T cell responses in mice in vivo. The reduced or absent presentation of late antigens was not due to impaired antigen presentation or delayed protein synthesis, but was caused by sequestration of late antigens within viral factories (VFs). Additionally, the trapping of late antigens in VFs conflicts with antigen processing and presentation as proteasomal activity was strongly reduced or absent in VFs, suggesting inefficient antigen degradation. This study gives for the first time a mechanistic explanation for the weak immunogenicity of late viral antigens for memory CTL activation. Since MVA is preferentially used as a boost vector in heterologous prime/boost vaccinations, this is an important information for future vaccine design.

Virus-like particles (VLP) in prophylaxis and immunotherapy of allergic diseases

BACKGROUND

Apart from active allergen avoidance, immunotherapy is regarded as the most effective form of treatment available for type I allergies. Such treatments involve the administration of allergen preparations in various forms and by various routes. Virus-like particles (VLPs) offer a very effective platform for immunization with the allergen and are characterized by high immunogenicity, low allergenicity and high clinical efficacy. Formulations that include Toll-like receptor ligands, T cell stimulatory epitopes and/or depot-forming adjuvants appear to enhance activation of the relevant immune cells. Short nucleotide sequences including CpG motifs have also been intensively explored as potent stimulators of dendritic cells and B cells.

METHODS

The present paper is based on a systematic literature search in PubMed and MEDLINE, and focuses on the pertinent immunological processes and on clinical data relating to use of VLPs and CpG motifs for the treatment of allergic rhinitis (AR).

RESULTS

Many published studies have reported positive clinical results following administration of VLPs, either alone or in combination with CpG motifs and, in some cases, even in the absence of the allergen-specific allergen.

CONCLUSIONS

These results indicate that VLPs modulate immune responses in ways which underline their exceptional promise as a platform for the immunotherapy of allergic disorders. However, clinical evaluations remain limited, and further large-scale and longer-term studies will be necessary to substantiate the efficacy and safety of these novel therapies.

ZFP36 RNA-binding proteins restrain T cell activation and anti-viral immunity

Dynamic post-transcriptional control of RNA expression by RNA-binding proteins (RBPs) is critical during immune response. ZFP36 RBPs are prominent inflammatory regulators linked to autoimmunity and cancer, but functions in adaptive immunity are less clear. We used HITS-CLIP to define ZFP36 targets in mouse T cells, revealing unanticipated actions in regulating T-cell activation, proliferation, and effector functions. Transcriptome and ribosome profiling showed that ZFP36 represses mRNA target abundance and translation, notably through novel AU-rich sites in coding sequence. Functional studies revealed that ZFP36 regulates early T-cell activation kinetics cell autonomously, by attenuating activation marker expression, limiting T cell expansion, and promoting apoptosis. Strikingly, loss of ZFP36 in vivo accelerated T cell responses to acute viral infection and enhanced anti-viral immunity. These findings uncover a critical role for ZFP36 RBPs in restraining T cell expansion and effector functions, and suggest ZFP36 inhibition as a strategy to enhance immune-based therapies.

The immune system must quickly respond to anything that may cause disease – from cancerous cells to viruses. For instance, a type of white blood cell called a T cell patrols the body, looking for potential threats. If a T cell identifies such a threat, it “activates” and undergoes various changes so that it can help to eliminate the problem.

One way that T cells change is by switching on different genes to make specific proteins. The information in the genes is first used as a template to produce a molecule called a messenger RNA (mRNA), which is then translated to build proteins. So-called RNA-binding proteins help control events before, during and after the translation stage in the process. Previous studies have shown that one particular RNA-binding protein, called ZFP36, controls the translation of proteins that are important for how the immune system recognizes the body’s own tissue and deals with cancer cells. However, it was less clear if it also helped T cells to activate and defeat viruses.

Now, using cutting-edge technology, Moore et al. have identified thousands of new mRNAs controlled by ZFP36 in mice, many of which did indeed make proteins that help T cells activate and spread throughout the body. Further experiments showed that mice that lack ZFP36 in the T cells were much quicker at responding to viruses than other mice. This suggests that ZFP36 actually restrains T cells and slows down the body’s immune system.

Knowing more about how T cells work could lead to new treatments for diseases; it may, for example, allow scientists to engineer T cells to better attack cancer cells, However, other studies have shown that mice without ZFP36 often go on to develop autoimmune diseases, which result from the immune system attacking healthy cells by mistake. As such, it seems that there is a fine line between improving the body’s immune system and increasing the risk of autoimmune diseases, and that RNA-binding proteins play an important role in managing this delicate balance.

Roles of lymphatic endothelial cells expressing peripheral tissue antigens in CD4 T-cell tolerance induction

Lymphatic endothelial cells (LECs) directly express peripheral tissue antigens and induce CD8 T-cell deletional tolerance. LECs express MHC-II molecules, suggesting they might also tolerize CD4 T cells. We demonstrate that when β-galactosidase (β-gal) is expressed in LECs, β-gal-specific CD8 T cells undergo deletion via the PD-1/PD-L1 and LAG-3/MHC-II pathways. In contrast, LECs do not present endogenous β-gal in the context of MHC-II molecules to β-gal-specific CD4 T cells. Lack of presentation is independent of antigen localization, as membrane-bound haemagglutinin and I-Eα are also not presented by MHC-II molecules. LECs express invariant chain and cathepsin L, but not H2-M, suggesting that they cannot load endogenous antigenic peptides onto MHC-II molecules. Importantly, LECs transfer β-gal to dendritic cells, which subsequently present it to induce CD4 T-cell anergy. Therefore, LECs serve as an antigen reservoir for CD4 T-cell tolerance, and MHC-II molecules on LECs are used to induce CD8 T-cell tolerance via LAG-3.

Lymphatic endothelial cells (LECs) induce peripheral tolerance of CD8 T cells. Here the authors show that LECs cannot directly tolerize CD4 T cells as they lack the machinery for loading the antigenic peptide to MHC-II; instead, LECs pass these antigens to dendritic cells that induce CD4 tolerance.

T cells targeting a neuronal paraneoplastic antigen mediate tumor rejection and trigger CNS autoimmunity with humoral activation

Paraneoplastic neurologic diseases (PND) involving immune responses directed toward intracellular antigens are poorly understood. Here, we examine immunity to the PND antigen Nova2, which is expressed exclusively in central nervous system (CNS) neurons. We hypothesized that ectopic expression of neuronal antigen in the periphery could incite PND. In our C57BL/6 mouse model, CNS antigen expression limits antigen-specific CD4+ and CD8+ T-cell expansion. Chimera experiments demonstrate that this tolerance is mediated by antigen expression in nonhematopoietic cells. CNS antigen expression does not limit tumor rejection by adoptively transferred transgenic T cells but does limit the generation of a memory population that can be expanded upon secondary challenge in vivo. Despite mediating cancer rejection, adoptively transferred transgenic T cells do not lead to paraneoplastic neuronal targeting. Preliminary experiments suggest an additional requirement for humoral activation to induce CNS autoimmunity. This work provides evidence that the requirements for cancer immunity and neuronal autoimmunity are uncoupled. Since humoral immunity was not required for tumor rejection, B-cell targeting therapy, such as rituximab, may be a rational treatment option for PND that does not hamper tumor immunity.

Retinal antigen-specific regulatory T cells protect against spontaneous and induced autoimmunity and require local dendritic cells

BACKGROUND

We previously reported that the peripheral regulatory T cells (pTregs) generated 'on-demand' in the retina were crucial to retinal immune privilege, and in vitro analysis of retinal dendritic cells (DC) showed they possessed antigen presenting cell (APC) activity that promoted development of the Tregs and effector T cells (Teffs). Here, we expanded these findings by examining whether locally generated, locally acting pTregs were protective against spontaneous autoimmunity and autoimmunity mediated by interphotoreceptor retinoid-binding protein (IRBP). We also examined the APC capacity of retinal DC in vivo.

METHODS

Transgenic (Tg) mice expressing diphtheria toxin receptor (DTR) and/or green fluorescent protein (GFP) under control of the endogenous FoxP3 promoter (GFP only in FG mice, GFP and DTR in FDG mice) or the CD11c promoter (GFP and DTR in CDG mice) were used in conjunction with Tg mice expressing beta-galactosidase (βgal) as retinal neo-self antigen and βgal-specific TCR Tg mice (BG2). Retinal T cell responses were assayed by flow cytometry and retinal autoimmune disease assessed by histological examination.

RESULTS

Local depletion of the Tregs enhanced actively induced experimental autoimmune uveoretinitis to the highly expressed retinal self-antigen IRBP in FDG mice and spontaneous autoimmunity in βgal-FDG-BG2 mice, but not in mice lacking autoreactive T cells or their target antigen in the retina. The presence of retinal βgal downregulated the generation of antigen-specific Teffs and pTregs within the retina in response to local βgal challenge. Retinal DC depletion prevented generation of Tregs and Teffs within retina after βgal injection. Microglia remaining after DC depletion did not make up for loss of DC-dependent antigen presentation.

CONCLUSIONS

Our results suggest that local retinal Tregs protect against spontaneous organ-specific autoimmunity and that T cell responses within the retina require the presence of local DC.

Is The Allergen Really Needed in Allergy Immunotherapy?

Immunotherapy for type I allergies is well established and is regarded to be the most efficient treatment option besides allergen avoidance. As of today, different forms of allergen preparations are used in this regard, as well as different routes of application. Virus-like particles (VLPs) represent a potent vaccine platform with proven immunogenicity and clinical efficacy. The addition of toll-like receptor ligands and/or depot-forming adjuvants further enhances activation of innate as well as adaptive immune responses. CpG motifs represent intensively investigated and potent direct stimulators of plasmacytoid dendritic cells and B cells, while T cell responses are enhanced indirectly through increased antigen presentation and cytokine release. This article will focus on the function of VLPs loaded with DNA rich in nonmethylated CG motifs (CpGs) and the clinical experience gained in the treatment of allergic rhinitis, demonstrating clinical efficacy also if administered without allergens. Several published studies have demonstrated a beneficial impact on allergic symptoms by treatment with CpG-loaded VLPs. Subcutaneous injection of VLPs loaded with CpGs was tested with or without the adjuvant alum in the presence or absence of an allergen. The results encourage further investigation of VLPs and CpG motifs in immunotherapy, either as a stand-alone product or as adjuvants for allergen-specific immunotherapy.

Immunotherapy of type-1 allergies with virus-like particles and CpG-motifs

Immunotherapy of type-I-allergies is regarded as the most efficient treatment option besides allergen avoidance. Different forms of allergen preparations are used as well as different routes of application. Virus-like particles represent a potent vaccine platform with proven immunogenicity and clinical efficacy. The addition of toll-like receptor ligands and/or depot-forming adjuvants further enhances immune cell activation. This article will focus on the function of virus-like particles loaded with DNA rich in CpG-motifs and discuss clinical experience in treatment of allergic rhinitis. Evidence will be presented that clinically effective treatment can be obtained even in the absence of allergens. Results encourage further investigation of virus-like particles and CpG-motifs in immunotherapy, either as a stand alone product, or as adjuvants for allergen-specific immunotherapy.

Immunotherapy of allergic rhinitis: new therapeutic opportunities with virus-like particles filled with CpG motifs

BACKGROUND

The incidence of allergic rhinitis (AR) has increased constantly over the last decades. The disease can significantly lower quality of life and subsequently might progress to allergic asthma. Allergen-specific immunotherapy is mostly used to cope with the cause of the disease. However, incidence of systemic reactions or limited compliance hampers the widespread use of this therapeutic approach. Therefore, new candidates are examined to improve immunotherapy of allergies. Recently, a new technology was developed with the aim to positively influence the immune system of allergic patients. Virus-like particles (VLPs) represent a potent vaccine platform that has been proven to be immunogenic and clinically effective. To enhance immune cell activation, addition of Toll-like receptor ligands and/or depot-forming adjuvants seems to be helpful. In this context, CpG motifs represent intensive investigated and potent stimulators of T cells. This article focuses on the function of VLPs and CpG motifs and their clinical experience for treatment of AR.

METHODS

A literature review was performed.

RESULTS

Several published studies showed a beneficial impact of the treatment on allergic symptoms. They tested VLPs filled with or without CpG motifs in combination with or without allergen.

CONCLUSION

Results encourage further investigations of VLPs and CpG motifs as adjuncts to or even alternative candidates for immunotherapy of allergic disorders.

A comparison of immunotherapy delivery methods for allergen immunotherapy

BACKGROUND

Allergic diseases are among the most common diseases in humans. Besides allergen avoidance, allergen-specific immunotherapy is the only causative treatment option. During recent years, many innovations of this therapy have emerged.

METHODS

Selective literature research in Medline and PubMed, under the inclusion of national and international guidelines and Cochrane meta analyses.

RESULTS

In several meta-analyses, the clinical efficacy of subcutaneous immunotherapy (SCIT) has been largely demonstrated. Recently, major research activities in mucosal immunotherapies focused on the sublingual application route. There are well-documented clinical data on the efficacy and safety of this form of immunotherapy. New application routes as well as new immune-modifying agents such as virus-like particles or CpG-motifs have also been investigated.

CONCLUSION

SIT is accepted to be the only causative treatment option for allergies. New application routes and new immune-modifying agents will allow for different delivery methods in the future.

Virus-encoded ectopic CD74 enhances poxvirus vaccine efficacy

Vaccinia virus (VV) has been used globally as a vaccine to eradicate smallpox. Widespread use of this viral vaccine has been tempered in recent years because of its immuno-evasive properties, with restrictions prohibiting VV inoculation of individuals with immune deficiencies or atopic skin diseases. VV infection is known to perturb several pathways for immune recognition including MHC class II (MHCII) and CD1d-restricted antigen presentation. MHCII and CD1d molecules associate with a conserved intracellular chaperone, CD74, also known as invariant chain. Upon VV infection, cellular CD74 levels are significantly reduced in antigen-presenting cells, consistent with the observed destabilization of MHCII molecules. In the current study, the ability of sustained CD74 expression to overcome VV-induced suppression of antigen presentation was investigated. Viral inhibition of MHCII antigen presentation could be partially ameliorated by ectopic expression of CD74 or by infection of cells with a recombinant VV encoding murine CD74 (mCD74-VV). In contrast, virus-induced disruptions in CD1d-mediated antigen presentation persisted even with sustained CD74 expression. Mice immunized with the recombinant mCD74-VV displayed greater protection during VV challenge and more robust anti-VV antibody responses. Together, these observations suggest that recombinant VV vaccines encoding CD74 may be useful tools to improve CD4⁺ T-cell responses to viral and tumour antigens.

Systemic minor histocompatibility antigen expression in blood endothelial cells prevents T cell-mediated vascular immunopathology

Attenuation of T cell-mediated damage of blood endothelial cells (BECs) in transplanted organs is important to prevent transplant vasculopathy (TV) and chronic rejection. Here, we assessed the importance of minor histocompatibility antigen (mHA) distribution and different coinhibitory molecules for T cell-BEC interaction. A transgenic mHA was directed specifically to BECs using the Tie2 promoter and cellular interactions were assessed in graft-versus-host disease-like and heterotopic heart transplantation settings. We found that cognate CD4(+) T-cell help was critical for the activation of BEC-specific CD8(+) T cells. However, systemic mHA expression on BECs efficiently attenuated adoptively transferred, BEC-specific CD4(+) and CD8(+) T cells and hence prevented tissue damage, whereas restriction of mHA expression to heart BECs precipitated the development of TV. Importantly, the lack of the coinhibitory molecules programmed death-1 (PD-1) and B and T lymphocyte attenuator fostered the initial activation of BEC-specific CD4(+) T cells, but did not affect development of TV. In contrast, TV was significantly augmented in the absence of PD-1 on BEC-specific CD8(+) T cells. Taken together, these results indicate that antigen distribution in the vascular bed determines the impact of coinhibition and, as a consequence, critically impinges on T cell-mediated vascular immunopathology.

Local "on-demand" generation and function of antigen-specific Foxp3+ regulatory T cells

Extrathymically derived regulatory T cells (iTregs) protect against autoimmunity to tissue-specific Ags. However, whether Ag-specific iTreg generation and function is limited to secondary lymphoid tissue or whether it can occur within the tissue-specific local environment of the cognate Ag remains unresolved. Mice expressing β-galactosidase (βgal) on a retina-specific promoter (βgal mice) in conjunction with mice expressing GFP and diphtheria toxin (DTx) receptor (DTR) under control of the Foxp3 promoter, and βgal-specific TCR transgenic (BG2) mice were used to examine this question. Local depletion (ocular DTx), but not systemic depletion (i.p. DTx), of βgal-specific iTregs enhanced experimental autoimmune uveoretinitis induced by activated βgal-specific effector T cells. Injections of small amounts of βgal into the anterior chamber of the eye produced similar numbers of βgal-specific iTregs in the retina whether the mouse was depleted of pre-existing, circulating Tregs. Taken together, these results suggest that protection from tissue-specific autoimmunity depends on the function of local Ag-specific iTregs and that the retina is capable of local, "on-demand" iTreg generation that is independent of circulating Tregs.

Regulation of CD8(+) T Cell Responses to Retinal Antigen by Local FoxP3(+) Regulatory T Cells

While pathogenic CD4 T cells are well known mediators of autoimmune uveoretinitis, CD8 T cells can also be uveitogenic. Since preliminary studies indicated that C57BL/6 mice were minimally susceptible to autoimmune uveoretinitis induction by CD8 T cells, the basis of the retinal disease resistance was sought. Mice that express β-galactosidase (βgal) on a retina-specific promoter (arrβgal mice) were backcrossed to mice expressing green fluorescent protein (GFP) and diphtheria toxin (DTx) receptor (DTR) under control of the Foxp3 promoter (Foxp3-DTR/GFP mice), and to T cell receptor transgenic mice that produce βgal-specific CD8 T cells (BG1 mice). These mice were used to explore the role of regulatory T cells in the resistance to retinal autoimmune disease. Experiments with T cells from double transgenic BG1 × Foxp3-DTR/GFP mice transferred into Foxp3-DTR/GFP × arrβgal mice confirmed that the retina was well protected from attempts to induce disease by adoptive transfer of activated BG1 T cells. The successful induction of retinal disease following unilateral intraocular administration of DTx to deplete regulatory T cells showed that the protective activity was dependent on local, toxin-sensitive regulatory T cells; the opposite, untreated eye remained disease-free. Although there were very few Foxp3(+) regulatory T cells in the parenchyma of quiescent retina, and they did not accumulate in retina, their depletion by local toxin administration led to disease susceptibility. We propose that these regulatory T cells modulate the pathogenic activity of βgal-specific CD8 T cells in the retinas of arrβgal mice on a local basis, allowing immuno regulation to be responsive to local conditions.

Local activation of dendritic cells alters the pathogenesis of autoimmune disease in the retina

Interest in the identities, properties, functions, and origins of local APC in CNS tissues is growing. We recently reported that dendritic cells (DC) distinct from microglia were present in quiescent retina and rapidly responded to injured neurons. In this study, the disease-promoting and regulatory contributions of these APC in experimental autoimmune uveoretinitis (EAU) were examined. Local delivery of purified, exogenous DC or monocytes from bone marrow substantially increased the incidence and severity of EAU induced by adoptive transfer of activated, autoreactive CD4 or CD8 T cells that was limited to the manipulated eye. In vitro assays of APC activity of DC from quiescent retina showed that they promoted generation of Foxp3(+) T cells and inhibited activation of naive T cells by splenic DC and Ag. Conversely, in vitro assays of DC purified from injured retina demonstrated an enhanced ability to activate T cells and reduced induction of Foxp3(+) T cells. These findings were supported by the observation that in situ activation of DC before adoptive transfer of β-galactosidase-specific T cells dramatically increased severity and incidence of EAU. Recruitment of T cells into retina by local delivery of Ag in vivo showed that quiescent retina promoted development of parenchymal Foxp3(+) T cells, but assays of preinjured retina did not. Together, these results demonstrated that local conditions in the retina determined APC function and affected the pathogenesis of EAU by both CD4 and CD8 T cells.

A novel bacterial artificial chromosome-transgenic podoplanin-cre mouse targets lymphoid organ stromal cells in vivo

Stromal cells provide the structural foundation of secondary lymphoid organs (SLOs), and regulate leukocyte access and cell migration within the different compartments of spleen and lymph nodes (LNs). Furthermore, several stromal cell subsets have been implied in shaping of T cell responses through direct presentation of antigen. Despite significant gain of knowledge on the biology of different SLO-resident stromal cell subsets, their molecular and functional characterization has remained incomplete. To address this need, we have generated a bacterial artificial chromosome-transgenic mouse model that utilizes the podoplanin (pdpn) promoter to express the Cre-recombinase exclusively in stromal cells of SLOs. The characterization of the Pdpn–Cre mouse revealed transgene expression in subsets of fibroblastic reticular cells and lymphatic endothelial cells in LNs. Furthermore, the transgene facilitated the identification of a novel splenic perivascular stromal cell subpopulation that forms web-like structures around central arterioles. Assessment of the in vivo antigen expression in the genetically tagged stromal cells in Pdpn–Cre mice revealed activation of both MHC I and II-restricted TCR transgenic T cells. Taken together, stromal pdpn–Cre expression is well-suited to characterize the phenotype and to dissect the function of lymphoid organ stromal cells.

Characterization of recombinant raccoonpox vaccine vectors in chickens

Raccoonpox virus (RCN) has been used as a recombinant vector against several mammalian pathogens but has not been tested in birds. The replication of RCN in chick embryo fibroblasts (CEFs) and chickens was studied with the use of highly pathogenic avian influenza virus H5N1 hemagglutinin (HA) as a model antigen and luciferase (luc) as a reporter gene. Although RCN replicated to low levels in CEFs, it efficiently expressed recombinant proteins and, in vivo, elicited anti-HA immunoglobulin yolk (IgY) antibody responses comparable to inactivated influenza virus. Biophotonic in vivo imaging of 1-wk-old chicks with RCN-luc showed strong expression of the luc reporter gene lasting up to 3 days postinfection. These studies demonstrate the potential of RCN as a vaccine vector for avian influenza and other poultry pathogens.

Exosome-driven antigen transfer for MHC class II presentation facilitated by the receptor binding activity of influenza hemagglutinin

The mechanisms underlying MHC class I-restricted cross-presentation, the transfer of Ag from an infected cell to a professional APC, have been studied in great detail. Much less is known about the equivalent process for MHC class II-restricted presentation. After infection or transfection of class II-negative donor cells, we observed minimal transfer of a proteasome-dependent "class I-like" epitope within the influenza neuraminidase glycoprotein but potent transfer of a classical, H-2M-dependent epitope within the hemagglutinin (HA) glycoprotein. Additional experiments determined transfer to be exosome-mediated and substantially enhanced by the receptor binding activity of incorporated HA. Furthermore, a carrier effect was observed in that incorporated HA improved exosome-mediated transfer of a second membrane protein. This route of Ag presentation should be relevant to other enveloped viruses, may skew CD4(+) responses toward exosome-incorporated glycoproteins, and points toward novel vaccine strategies.

Topological journey of parasite-derived antigens for presentation by MHC class I molecules

Within cells of their host, many bacteria and parasites inhabit specialized compartments, such as a modified phagosome for Mycobacterium tuberculosis or a parasitophorous vacuole for Toxoplasma gondii. These locations could exclude microbial material from entry into the MHC class I surveillance pathway. Remarkably, however, under these circumstances, cells can still signal the presence of invading pathogens to circulating CD8(+) T cells, which typically play a key role in protection against such intracellular organisms. Here, we review MHC I presentation pathways in various contexts, ranging from model antigens in non-infectious settings to pathogen-infected cells. We suggest that presentation of intracellular pathogens can be described as not just one, but several distinct pathways; perhaps because diverse pathogens have evolved different strategies to interact with host cells.

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.

Designing CD8+ T cell vaccines: it's not rocket science (yet)

CD8+ T cells play important roles in clearing viral infections and eradicating tumors. Designing vaccines that elicit effective CD8+ T cell responses requires a thorough knowledge of the pathways of antigen presentation in vivo. Here, I review recent progress in understanding the activation of naïve CD8+ T cells in vivo, with particular emphasis on cross-priming, the presentation of protein antigens acquired by dendritic cells from their environment. With the rapid advances in this area of research, the dawn of rational vaccine design is at hand.

Viruses as vaccine vectors for infectious diseases and cancer

Recent developments in the use of viruses as vaccine vectors have been facilitated by a better understanding of viral biology. Advances occur as we gain greater insight into the interrelationship of viruses and the immune system. Viral-vector vaccines remain the best means to induce cellular immunity and are now showing promise for the induction of strong humoral responses. The potential benefits for global health that are offered by this field reflect the scope and utility of viruses as vaccine vectors for human and veterinary applications, with targets ranging from certain types of cancer to a vast array of infectious diseases.