Cutting Edge: Prolonged Antigen Presentation after Herpes Simplex Virus-1 Skin Infection

Nociceptive sensory neurons promote CD8 T cell responses to HSV-1 infection

Host protection against cutaneous herpes simplex virus 1 (HSV-1) infection relies on the induction of a robust adaptive immune response. Here, we show that Nav1.8+ sensory neurons, which are involved in pain perception, control the magnitude of CD8 T cell priming and expansion in HSV-1-infected mice. The ablation of Nav1.8-expressing sensory neurons is associated with extensive skin lesions characterized by enhanced inflammatory cytokine and chemokine production. Mechanistically, Nav1.8+ sensory neurons are required for the downregulation of neutrophil infiltration in the skin after viral clearance to limit the severity of tissue damage and restore skin homeostasis, as well as for eliciting robust CD8 T cell priming in skin-draining lymph nodes by controlling dendritic cell responses. Collectively, our data reveal an important role for the sensory nervous system in regulating both innate and adaptive immune responses to viral infection, thereby opening up possibilities for new therapeutic strategies.

Innate Immune Mechanisms and Herpes Simplex Virus Infection and Disease

Innate immune responses play a major role in the control of herpes simplex virus (HSV) infections, and a multiplicity of mechanisms have emerged as a result of human evolution to sense and respond to HSV infections. HSV in turn has evolved a number of ways to evade immune detection and to blunt human innate immune responses. In this review, we summarize the major host innate immune mechanisms and the HSV evasion mechanisms that have evolved. We further discuss how disease can result if this equilibrium between virus and host response is disrupted.

Spatiotemporally Distinct Interactions with Dendritic Cell Subsets Facilitates CD4+ and CD8+ T Cell Activation to Localized Viral Infection

The dynamics of when and where CD4(+) T cells provide help for CD8(+) T cell priming and which dendritic cells (DCs) activate CD4(+) T cells in vivo after localized infection are poorly understood. By using a cutaneous herpes simplex virus infection model combined with intravital 2-photon imaging of the draining lymph node (LN) to concurrently visualize pathogen-specific CD4(+) and CD8(+) T cells, we found that early priming of CD4(+) T cells involved clustering with migratory skin DCs. CD8(+) T cells did not interact with migratory DCs and their activation was delayed, requiring later clustering interactions with LN-resident XCR1(+) DCs. CD4(+) T cells interacted with these late CD8(+) T cell clusters on resident XCR1(+) DCs. Together, these data reveal asynchronous T cell activation by distinct DC subsets and highlight the key role of XCR1(+) DCs as the central platform for cytotoxic T lymphocyte activation and the delivery of CD4(+) T cell help.

Modeling the dynamics and migratory pathways of virus-specific antibody-secreting cell populations in primary influenza infection

The B cell response to influenza infection of the respiratory tract contributes to viral clearance and establishes profound resistance to reinfection by related viruses. Numerous studies have measured virus-specific antibody-secreting cell (ASC) frequencies in different anatomical compartments after influenza infection and provided a general picture of the kinetics of ASC formation and dispersion. However, the dynamics of ASC populations are difficult to determine experimentally and have received little attention. Here, we applied mathematical modeling to investigate the dynamics of ASC growth, death, and migration over the 2-week period following primary influenza infection in mice. Experimental data for model fitting came from high frequency measurements of virus-specific IgM, IgG, and IgA ASCs in the mediastinal lymph node (MLN), spleen, and lung. Model construction was based on a set of assumptions about ASC gain and loss from the sampled sites, and also on the directionality of ASC trafficking pathways. Most notably, modeling results suggest that differences in ASC fate and trafficking patterns reflect the site of formation and the expressed antibody class. Essentially all early IgA ASCs in the MLN migrated to spleen or lung, whereas cell death was likely the major reason for IgM and IgG ASC loss from the MLN. In contrast, the spleen contributed most of the IgM and IgG ASCs that migrated to the lung, but essentially none of the IgA ASCs. This finding points to a critical role for regional lymph nodes such as the MLN in the rapid generation of IgA ASCs that seed the lung. Results for the MLN also suggest that ASC death is a significant early feature of the B cell response. Overall, our analysis is consistent with accepted concepts in many regards, but it also indicates novel features of the B cell response to influenza that warrant further investigation.

A sharp T-cell antigen receptor signaling threshold for T-cell proliferation

T-cell antigen receptor (TCR) signaling is essential for activation, proliferation, and effector function of T cells. Modulation of both intensity and duration of TCR signaling can regulate these events. However, it remains unclear how individual T cells integrate such signals over time to make critical cell-fate decisions. We have previously developed an engineered mutant allele of the critical T-cell kinase zeta-chain-associated protein kinase 70 kDa (Zap70) that is catalytically inhibited by a small molecule inhibitor, thereby blocking TCR signaling specifically and efficiently. We have also characterized a fluorescent reporter Nur77-eGFP transgenic mouse line in which T cells up-regulate GFP uniquely in response to TCR stimulation. The combination of these technologies unmasked a sharp TCR signaling threshold for commitment to cell division both in vitro and in vivo. Further, we demonstrate that this threshold is independent of both the magnitude of the TCR stimulus and Interleukin 2. Similarly, we identify a temporal threshold of TCR signaling that is required for commitment to proliferation, after which T cells are able to proliferate in a Zap70 kinase-independent manner. Taken together, our studies reveal a sharp threshold for the magnitude and duration of TCR signaling required for commitment of T cells to proliferation. These results have important implications for understanding T-cell responses to infection and optimizing strategies for immunomodulatory drug delivery.

Distinct APC subtypes drive spatially segregated CD4+ and CD8+ T-cell effector activity during skin infection with HSV-1

Efficient infection control requires potent T-cell responses at sites of pathogen replication. However, the regulation of T-cell effector function in situ remains poorly understood. Here, we show key differences in the regulation of effector activity between CD4⁺ and CD8⁺ T-cells during skin infection with HSV-1. IFN-γ-producing CD4⁺ T cells disseminated widely throughout the skin and draining lymph nodes (LN), clearly exceeding the epithelial distribution of infectious virus. By contrast, IFN-γ-producing CD8⁺ T cells were only found within the infected epidermal layer of the skin and associated hair follicles. Mechanistically, while various subsets of lymphoid- and skin-derived dendritic cells (DC) elicited IFN-γ production by CD4⁺ T cells, CD8⁺ T cells responded exclusively to infected epidermal cells directly presenting viral antigen. Notably, uninfected cross-presenting DCs from both skin and LNs failed to trigger IFN-γ production by CD8⁺ T-cells. Thus, we describe a previously unappreciated complexity in the regulation of CD4⁺ and CD8⁺ T-cell effector activity that is subset-specific, microanatomically distinct and involves largely non-overlapping types of antigen-presenting cells (APC).

HSV-1 is a widely distributed pathogen causing a life-long latent infection associated with periodic bouts of reactivation and severe clinical complications. Adaptive immune responses encompassing CD4⁺ and CD8⁺ T-cell activities are key to both the clearance of infectious virus and the control of latent infection. However, precisely how such T-cell responses are regulated, particularly within acutely infected peripheral tissues, remains poorly understood. Using a mouse model of HSV-1 skin infection, we describe a complex regulation of T-cell responses at the site of acute infection. These responses were subset-specific and anatomically distinct, with CD4⁺ and CD8⁺ T-cell activities being directed to distinct anatomical compartments within the skin. While IFN-γ-producing CD4⁺ T cells were broadly distributed, including skin regions a considerable distance away from infected cells, CD8⁺ T-cell activity was strictly confined to directly infected epithelial compartments. This unexpected spatial segregation was a direct consequence of the involvement of largely non-overlapping types of antigen-presenting cells in driving CD4⁺ and CD8⁺ T-cell effector activity. Our results provide novel insights into the cellular regulation of T-cell immunity within peripheral tissues and have the potential to guide the development of T-cell subset-specific approaches for therapeutic and prophylactic intervention in antimicrobial immunity and autoimmunity.

Perforin deficiency impairs a critical immunoregulatory loop involving murine CD8(+) T cells and dendritic cells

Humans and mice with impaired perforin-dependent cytotoxic function may develop excessive T-cell activation and the fatal disorder hemophagocytic lymphohistiocytosis (HLH) after infection. Though cytotoxic lymphocytes can kill antigen-presenting cells, the physiological mechanism of perforin-mediated immune regulation has never been demonstrated in a disease-relevant context. We used a murine model of HLH to examine how perforin controls immune activation, and we have defined a feedback loop that is critical for immune homeostasis. This endogenous feedback loop involves perforin-dependent elimination of rare, antigen-presenting dendritic cells (DCs) by CD8(+) T cells and has a dominant influence on the magnitude of T-cell activation after viral infection. Antigen presentation by a minor fraction of DCs persisted in T-cell- or perforin-deficient animals and continued to drive T-cell activation well beyond initial priming in the latter animals. Depletion of DCs or transfer of perforin-sufficient T cells dampened endogenous DC antigen presentation and T-cell activation, demonstrating a reciprocal relationship between perforin in CD8(+) T cells and DC function. Thus, selective cytotoxic "pruning" of DC populations by CD8(+) T cells limits T-cell activation and protects against the development of HLH and potentially other immunopathological conditions.

TCR signaling requirements for activating T cells and for generating memory

Over the last two decades the molecular and cellular mechanisms underlying T cell activation, expansion, differentiation, and memory formation have been intensively investigated. These studies revealed that the generation of memory T cells is critically impacted by a number of factors, including the magnitude of the inflammatory response and cytokine production, the type of dendritic cell [DC] that presents the pathogen derived antigen, their maturation status, and the concomitant provision of costimulation. Nevertheless, the primary stimulus leading to T cell activation is generated through the T cell receptor [TCR] following its engagement with a peptide MHC ligand [pMHC]. The purpose of this review is to highlight classical and recent findings on how antigen recognition, the degree of TCR stimulation, and intracellular signal transduction pathways impact the formation of effector and memory T cells.

B cells are critical to T-cell-mediated antitumor immunity induced by a combined immune-stimulatory/conditionally cytotoxic therapy for glioblastoma

We have demonstrated that modifying the tumor microenvironment through intratumoral administration of adenoviral vectors (Ad) encoding the conditional cytotoxic molecule, i.e., HSV1-TK and the immune-stimulatory cytokine, i.e., fms-like tyrosine kinase 3 ligand (Flt3L) leads to T-cell-dependent tumor regression in rodent models of glioblastoma. We investigated the role of B cells during immune-mediated glioblastoma multiforme regression. Although treatment with Ad-TK+Ad-Flt3L induced tumor regression in 60% of wild-type (WT) mice, it completely failed in B-cell-deficient Igh6(-/-) mice. Tumor-specific T-cell precursors were detected in Ad-TK+Ad-Flt3L-treated WT mice but not in Igh6(-/-) mice. The treatment also failed in WT mice depleted of total B cells or marginal zone B cells. Because we could not detect circulating antibodies against tumor cells and the treatment was equally efficient in WT mice and in mice with B-cell-specific deletion of Prdm 1 (encoding Blimp-1), in which B cells are present but unable to fully differentiate into antibody-secreting plasma cells, tumor regression in this model is not dependent on B cells' production of tumor antigen-specific immunoglobulins. Instead, B cells seem to play a role as antigen-presenting cells (APCs). Treatment with Ad-TK+Ad-Flt3L led to an increase in the number of B cells in the cervical lymph nodes, which stimulated the proliferation of syngeneic T cells and induced clonal expansion of antitumor T cells. Our data show that B cells act as APCs, playing a critical role in clonal expansion of tumor antigen-specific T cells and brain tumor regression.

CD8+ T-cell expansion and maintenance after recombinant adenovirus immunization rely upon cooperation between hematopoietic and nonhematopoietic antigen-presenting cells

We have recently reported that CD8(+) T-cell memory maintenance after immunization with recombinant human adenovirus type 5 (rHuAd5) is dependent upon persistent transgene expression beyond the peak of the response. In this report, we have further investigated the location and nature of the cell populations responsible for this sustained response. The draining lymph nodes were found to be important for primary expansion but not for memory maintenance, suggesting that antigen presentation through a nonlymphoid source was required. Using bone marrow chimeric mice, we determined that antigen presentation by nonhematopoietic antigen-presenting cells (APCs) was sufficient for maintenance of CD8(+) T-cell numbers. However, antigen presentation by this mechanism alone yielded a memory population that displayed alterations in phenotype, cytokine production and protective capacity, indicating that antigen presentation through both hematopoietic and nonhematopoietic APCs ultimately defines the memory CD8(+) T-cell response produced by rHuAd5. These results shed new light on the immunobiology of rHuAd5 vectors and provide evidence for a mechanism of CD8(+) T-cell expansion and memory maintenance that relies upon both hematopoietic and nonhematopoietic APCs.

Direct presentation regulates the magnitude of the CD8+ T cell response to cell-associated antigen through prolonged T cell proliferation

The magnitude and complexity of Ag-specific CD8(+) T cell responses is determined by intrinsic properties of the immune system and extrinsic factors, such as vaccination. We evaluated mechanisms that regulate the CD8(+) T cell response to two distinct determinants derived from the same protein Ag, SV40 T Ag (T Ag), following immunization of C57BL/6 mice with T Ag-transformed cells. The results show that direct presentation of T cell determinants by T Ag-transformed cells regulates the magnitude of the CD8(+) T cell response in vivo but not the immunodominance hierarchy. The immunodominance hierarchy was reversed in a dose-dependent manner by addition of excess naive T cells targeting the subdominant determinant. However, T cell competition played only a minor role in limiting T cell accumulation under physiological conditions. We found that the magnitude of the T cell response was regulated by the ability of T Ag-transformed cells to directly present the T Ag determinants. The hierarchy of the CD8(+) T cell response was maintained when Ag presentation in vivo was restricted to cross-presentation, but the presence of T Ag-transformed cells capable of direct presentation dramatically enhanced T cell accumulation at the peak of the response. This enhancement was due to a prolonged period of T cell proliferation, resulting in a delay in T cell contraction. Our findings reveal that direct presentation by nonprofessional APCs can dramatically enhance accumulation of CD8(+) T cells during the primary response, revealing a potential strategy to enhance vaccination approaches.

Rapid recruitment and activation of CD8+ T cells after herpes simplex virus type 1 skin infection

After localized infection, naive antigen-specific T cells must localize to those lymph nodes (LNs) draining the site of infection before engaging antigen-bearing dendritic cells. Given that naive precursors are initially distributed randomly throughout the secondary lymphoid compartment, it is unclear how long it takes most antigen-specific precursors to mobilize to draining LNs and become recruited into the primary T cell response. Here, we have examined the kinetics of these events, measuring the period over which naive precursors are recruited into the primary T cell response after cutaneous infection with herpes simplex virus type 1 (HSV-1). We show that despite prolonged MHC class-I-restricted antigen presentation, most naive HSV-specific precursors were recruited from the circulation in the first 4 days after inoculation. Furthermore, this prolonged presentation was also not essential for memory development, as truncating the period of antigen presentation to around 4 days did not affect the level of contraction, or long-term stability of the HSV-specific CD8(+) memory T cell pool. Thus, despite initially being dispersed throughout the entire circulation, the recruitment of naive precursors is achieved quite quickly, even when priming is restricted to a small number of draining LNs.

Plasticity in programming of effector and memory CD8 T-cell formation

CD8(+) T cells (also called cytotoxic T lymphocytes) play a major role in protective immunity against many infectious pathogens and can eradicate malignant cells. The path from naive precursor to effector and memory CD8(+) T-cell development begins with interactions between matured antigen-bearing dendritic cells (DCs) and antigen-specific naive T-cell clonal precursors. By integrating differences in antigenic, costimulatory, and inflammatory signals, a developmental program is established that governs many key parameters associated with the ensuing response, including the extent and magnitude of clonal expansion, the functional capacities of the effector cells, and the size of the memory pool that survives after the contraction phase. In this review, we discuss the multitude of signals that drive effector and memory CD8(+) T-cell differentiation and how the differences in the nature of these signals contribute to the diversity of CD8(+) T-cell responses.

A detailed characterisation of the distribution and presentation of DNA vaccine encoded antigen

The association between plasmid DNA distribution, the amount of Ag produced, Ag persistence and the identity and localisation of cells presenting DNA-encoded Ag all have important consequences for both quantitative and qualitative aspects of T cell responses induced by DNA vaccines. Using a variety of approaches to detect and quantify the uptake of injected DNA, and the production and presentation of DNA-encoded antigen, we report that injected DNA vaccines rapidly enter the peripheral blood from the injection site and also reach muscle-draining lymph nodes directly as free DNA. 24 h after plasmid injection, MHCII⁺CD11b⁺B220⁻CD11c^low/− cells in the draining and distal LNs and spleen contain pDNA. Interestingly, we also observed pDNA⁺MHCII^low/−CD11b⁺ within the bone marrow. Concomitantly, we detected Ag-containing/expressing cells at both the injection site and in draining lymph nodes. Three days after plasmid injection we detected rare pMHC⁺CD11c⁺ cells within secondary lymphoid tissue and simultaneously observed Ag-specific CD4⁺ T cell accumulation and blastogenesis in these tissues. Our results show that the events that determine the induction of DNA vaccine immune responses occur within days of DNA injection and that the response becomes systemic very rapidly, possibly with involvement from resident BM cells.

Persistence of transgene expression influences CD8+ T-cell expansion and maintenance following immunization with recombinant adenovirus

Previous studies determined that the CD8(+) T-cell response elicited by recombinant adenovirus exhibited a protracted contraction phase that was associated with long-term presentation of antigen. To gain further insight into this process, a doxycycline-regulated adenovirus was constructed to enable controlled extinction of transgene expression in vivo. We investigated the impact of premature termination of transgene expression at various time points (day 3 to day 60) following immunization. When transgene expression was terminated before the maximum response had been attained, overall expansion was attenuated, yielding a small memory population. When transgene expression was terminated between day 13 and day 30, the memory population was not sustained, demonstrating that the early memory population was antigen dependent. Extinction of transgene expression at day 60 had no obvious impact on memory maintenance, indicating that maintenance of the memory population may ultimately become independent of transgene expression. Premature termination of antigen expression had significant but modest effects on the phenotype and cytokine profile of the memory population. These results offer new insights into the mechanisms of memory CD8(+) T-cell maintenance following immunization with a recombinant adenovirus.

Induction of protective cytotoxic T-cell responses by a B-cell-based cellular vaccine requires stable expression of antigen

B cell-based cellular vaccines represent a promising approach to active immunotherapy of cancer complementing the use of dendritic cells, especially in pediatric patients and patients with low bone marrow reserves. B cells can be easily prepared in large numbers and readily home to secondary lymphoid organs, the primary site of induction of cytotoxic T lymphocyte (CTL) responses. However, most B cell-based vaccines tested so far failed to induce functional and protective CTLs in in vivo models. Here we demonstrate that B cells activated via the Toll like receptor-9 (TLR-9) and CD40 up-regulate surface expression of MHC and costimulatory molecules, produce IL-12, and exhibit potent antigen-presenting properties in vitro. Importantly, while administration of peptide-coated or transiently transfected B cells fails to induce immune responses, therapeutic immunization with low numbers of genetically modified B cells stably expressing antigen results in an induction of functional CTLs and protection against the growth of tumor in an animal model. Following activation, B cells partially loose their ability to home to organized lymphoid tissue due to the shedding of CD62L; however, this property can be restored by expression of protease-resistant mutant of CD62L. In summary, the data presented in this report suggest that genetically modified activated B cells represent a promising candidate for a cancer vaccine eliciting functional systemic CTLs.

A matter of timing: unsynchronized antigen expression and antigen presentation diminish secondary T cell responses

Despite the low and short expression of secondary Ag, prime-boost immunizations using homologous or heterologous vectors are capable of amplifying memory CD8(+) T cells. This is mainly attributed to the rapid presentation of Ag by APCs and the high proliferative capacity of memory CD8(+) T cells. Nevertheless, certain viruses and vectors often require prolonged Ag presentation for optimal T cell priming, and the influence of such a prolonged presentation during secondary immune induction is not clear. To address this issue, we primed and boosted mice intradermally (i.d.) with plasmid DNA that was recently reported to require prolonged Ag presentation for maximal CD8(+) T cell priming. Although functional memory CD8(+) T cells were present in the mice after i.d. priming, the secondary CD8(+) T cell response elicited was limited and reached a similar level of that observed during priming. The initial levels of secondary Ag expressed in the boosted mice were sufficient to prime CD8(+) T cell response in naive hosts, suggesting that lower Ag load alone does not explain the limited secondary immune responses observed. Removal of the injection site 5 or 10 days after i.d. boosting immunization resulted in diminished Ag presentation and no expansion of memory CD8(+) T cells. In fact, Ag-presenting activity following boost occurred mainly two weeks postimmunization, a time when the Ag was no longer expressed in situ. These findings suggest that when the boosting vector triggers prolonged Ag presentation, the lack of synchronicity between Ag accessibility and Ag presentation limits secondary immune responses.

Cross-presentation of viral and self antigens by skin-derived CD103+ dendritic cells

Skin-derived dendritic cells (DCs) include Langerhans cells, classical dermal DCs and a langerin-positive CD103(+) dermal subset. We examined their involvement in the presentation of skin-associated viral and self antigens. Only the CD103(+) subset efficiently presented antigens of herpes simplex virus type 1 to naive CD8(+) T cells, although all subsets presented these antigens to CD4(+) T cells. This showed that CD103(+) DCs were the migratory subset most efficient at processing viral antigens into the major histocompatibility complex class I pathway, potentially through cross-presentation. This was supported by data showing only CD103(+) DCs efficiently cross-presented skin-derived self antigens. This indicates CD103(+) DCs are the main migratory subtype able to cross-present viral and self antigens, which identifies another level of specialization for skin DCs.

Kinetics of major histocompatibility class I antigen presentation in acute infection

Ag presentation within the regional lymph node is crucial for the initiation of CD8+ T cell responses following viral infection. The magnitude and quality of the CD8+ T cell response are regulated by the interplay between the size of the APC population and duration of Ag presentation. To understand how these parameters are finely regulated during an immune response, we have investigated the dynamics of Ag presentation in influenza A virus and HSV-1 infection. In both infections, APC production was calculated to occur over the first few days of infection, after which there was slow exponential decay over a period of up to 2 wk. This production rate is most likely determined by the Ag availability and recruitment and/or maturation rate of dendritic cells. APC production was found to closely parallel lymph node cell recruitment in both infections. This was greatest in the first 6 h of infection for HSV and over the second and third day for influenza. In HSV infection, the peak production also coincides with peak viral levels. By contrast, in influenza infection, APC production ceased between the third and fourth day despite the presence of high levels of virus until 5 days after infection. These analyses demonstrate that two quite different self-limiting infections generate the APC necessary to drive T cell responses early in infection at different rates. Understanding how such contrasting kinetics of Ag presentation impacts on the growth and size of developing protective T cell populations has important implications for the design of vaccines and immunotherapies.

Recombinant lentivector as a genetic immunization vehicle for antitumor immunity

Encouraged by remarkable successes in preventing infectious diseases and by the well-established potential of the immune system for controlling tumor growth, active therapeutic immunization approaches hold great promise for treating malignant tumors. In recent years, engineered recombinant viral vectors have been carefully examined as genetic-immunization vehicles and have been demonstrated to induce potent T-cell-mediated immune responses that can control tumor growth. Very recent efforts suggest that lentivectors possess important advantages over other candidate recombinant viral vectors for genetic immunization. Here, we review the development of recombinant lentivectors and the characteristics of T-cell immune responses elicited by lentivector immunization, including the mechanism of T-cell priming with a focus on the role of skin dendritic cells and potential applications for tumor immunotherapy.

Superior induction of anti-tumor CTL immunity by extended peptide vaccines involves prolonged, DC-focused antigen presentation

Anti-tumor vaccines consisting of extended CTL peptides in combination with CpG-ODN were shown to be superior to those comprising minimal CTL epitopes and CpG-ODN, in that they elicit stronger effector CTL responses with greater tumoricidal potential. We now demonstrate that this improved performance is primarily due to the focusing of CTL epitope presentation onto activated DC in the inflamed lymph nodes draining the vaccination site. In the case of vaccination with minimal peptides, additional APC including T and B cells are also loaded with CTL epitopes. Our data suggest that circulation of these peptide-loaded lymphocytes leads to epitope presentation in non-inflamed lymphoid organs distal from the vaccination site, in the absence of potent costimulatory signals required for efficient CTL priming. The resulting blend of pro-immunogenic and tolerogenic signals, which results in suboptimal activation of the CTL response, is avoided by vaccinating with extended CTL peptides. An additional advantage of extended CTL peptide vaccines is an increased duration of in vivo epitope presentation.

Dendritic cells: driving the differentiation programme of T cells in viral infections

Protective immunity against viral pathogens depends on the generation and maintenance of a small population of memory CD8(+) T cells. Successful memory cell generation begins with early interactions between naïve T cell and dendritic cells (DCs) within the inflammatory milieu of the secondary lymphoid tissues. Recent insights into the role of different populations of DCs, and kinetics of antigen presentation, during viral infections have helped to understand how DCs can shape the immune response. Here, we review the recent progress that has been made towards defining how specific DC subsets drive effector CD8(+) T-cell expansion and differentiation into memory cells. Further, we endeavour to examine how the molecular signals imparted by DCs coordinate to generate protective CD8(+) T-cell immunity.

Infection with Trypanosoma cruzi restricts the repertoire of parasite-specific CD8+ T cells leading to immunodominance

Interference or competition between CD8(+) T cells restricted by distinct MHC-I molecules can be a powerful means to establish an immunodominant response. However, its importance during infections is still questionable. In this study, we describe that following infection of mice with the human pathogen Trypanosoma cruzi, an immunodominant CD8(+) T cell immune response is developed directed to an H-2K(b)-restricted epitope expressed by members of the trans-sialidase family of surface proteins. To determine whether this immunodominance was exerted over other non-H-2K(b)-restricted epitopes, we measured during infection of heterozygote mice, immune responses to three distinct epitopes, all expressed by members of the trans-sialidase family, recognized by H-2K(b)-, H-2K(k)-, or H-2K(d)-restricted CD8(+) T cells. Infected heterozygote or homozygote mice displayed comparably strong immune responses to the H-2K(b)-restricted immunodominant epitope. In contrast, H-2K(k)- or H-2K(d)-restricted immune responses were significantly impaired in heterozygote infected mice when compared with homozygote ones. This interference was not dependent on the dose of parasite or the timing of infection. Also, it was not seen in heterozygote mice immunized with recombinant adenoviruses expressing T. cruzi Ags. Finally, we observed that the immunodominance was circumvented by concomitant infection with two T. cruzi strains containing distinct immunodominant epitopes, suggesting that the operating mechanism most likely involves competition of T cells for limiting APCs. This type of interference never described during infection with a human parasite may represent a sophisticated strategy to restrict priming of CD8(+) T cells of distinct specificities, avoiding complete pathogen elimination by host effector cells, and thus favoring host parasitism.

Duration of antigen expression in vivo following DNA immunization modifies the magnitude, contraction, and secondary responses of CD8+ T lymphocytes

The duration of Ag expression in vivo has been reported to have a minimal impact on both the magnitude and kinetics of contraction of a pathogen-induced CD8(+) T cell response. In this study, we controlled the duration of Ag expression by excising the ear pinnae following intradermal ear pinnae DNA immunization. This resulted in decreased magnitude, accelerated contraction and differentiation, and surprisingly greater secondary CD8(+) T cell responses. Furthermore, we found delayed and prolonged Ag presentation in the immunized mice; however, this presentation was considerably decreased when the depot Ag was eliminated. These findings suggest that the magnitude and the contraction phase of the CD8(+) T cell response following intradermal DNA immunization is regulated by the duration rather than the initial exposure to Ag.

Increasing the survival of dendritic cells in vivo does not replace the requirement for CD4+ T cell help during primary CD8+ T cell responses

The survival of dendritic cells (DC) in vivo determines the duration of Ag presentation and is critical in determining the strength and magnitude of the resulting T cell response. We used a mouse model to show that Ag-loaded C57BL/6 DC (MHC class II(+/+) (MHC II(+/+))) that reach the lymph node survived longer than Ag-loaded MHC II(-/-) DC, with the numbers of C57BL/6 DC being approximately 2.5-fold the number of the MHC II(-/-) DC by day 4 and approximately 5-fold by day 7. The differential survival of DC in vivo was not affected by low doses of LPS, but in vitro pretreatment with CD40L or with high doses of LPS increased the numbers of MHC II(-/-) DC to levels approaching those of C57BL/6 DC. Regardless of their numbers and relative survival in lymph nodes, MHC II(-/-) DC were profoundly defective in their ability to induce CTL responses against the gp33 peptide epitope, and were unable to induce expansion and optimal cytotoxic activity of CD8(+) T cells specific for the male Ag UTY. We conclude that CD4(+) T cell help for CD8(+) responses involves mechanisms other than the increased survival of Ag-presenting DC in the lymph node.

Design and development of synthetic peptide vaccines: past, present and future

Synthetic peptide vaccines aiming at the induction of a protective CD8(+) T-cell response against infectious or malignant diseases are widely used in the clinic but, despite their success in animal models, they do not yet live up to their promise in humans. This review assesses the development of synthetic peptide vaccines, weighs it against the immunological concepts that have emerged, and identifies the key issues that play a role in the failure or success of a synthetic peptide vaccine. The current state-of-the-art peptide vaccine is a complete synthetic inflammatory product that is ingested by professional antigen-presenting cells and stimulates both CD4(+) and CD8(+) T cells.

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.

Killer T cells regulate antigen presentation for early expansion of memory, but not naive, CD8+ T cell

Antigen presentation within the lymph node draining a site of infection is crucial for initiation of cytotoxic T cell responses. Precisely how this antigen presentation regulates T cell expansion in vivo is unclear. Here, we show that, in primary infection, antigen presentation peaks approximately 3 days postinfection and then slowly decays until day 12. This prolonged antigen presentation is required for optimal expansion of naive CD8(+) T cells, because early ablation of dendritic cells reduces the later CD8(+) T cell response. Antigen presentation during secondary infection was 10-fold lower in magnitude and largely terminated by day 4 postinfection. Expansion of memory, but not naive, antigen-specific T cells was tightly controlled by perforin-dependent cytolysis of antigen-presenting cells. The ability of the memory T cells to remove antigen-presenting cells provides a negative-feedback loop to directly limit the duration of antigen presentation in vivo.

CTL response compensation for the loss of an immunodominant class I-restricted HSV-1 determinant

The T-cell response to even complex pathogens is often focused on only a handful of immunodominant determinants. Such narrow responses provoke a selective pressure that can drive the emergence of CTL escape variants, raising the question of whether a broader response, targeting multiple non-dominant peptides may be more beneficial. To examine the ability of the T-cell repertoire to respond to non-dominant determinants, we have investigated how mutating the dominant peptide in HSV affects the magnitude of the CD8+ T-cell response. We found that the CTL response to HSV lacking the dominant peptide was only modestly reduced compared with the wild-type virus and, surprisingly, this compensation occurred without any enhancement in the response to an established minor epitope. These findings are supportive of a malleable T-cell repertoire that can elicit strong responses to alternate, unknown determinants in the absence of the dominant response.

Comparative study of macrophage response in mice after DNA immunization and infection with herpes simplex virus type 1

Functional activity of macrophages and intensity of T cell immune response in mice were studied after intravaginal and intraperitoneal infection with herpes simplex virus type 1 and DNA vaccination in combination with adjuvant treatment (recombinant granulocyte-macrophage colony-stimulating factor and glucosaminylmuramyl dipeptide). DNA vaccination induced a virus-specific T cell immune response with no macrophagic inflammatory reaction. Infection with herpes simplex virus type 1 was accompanied by sustained inflammation, but not by the T cell immune response.

Skin-derived dendritic cells induce potent CD8(+) T cell immunity in recombinant lentivector-mediated genetic immunization

The skin contains readily accessible dendritic cells (DCs) with potent antigen presentation function and functional plasticity enabling the integration of antigen specificity with environmentally responsive immune control. Recent studies challenge the established paradigm of cutaneous immune function by suggesting that lymph node-resident DCs, rather than skin-derived DCs (sDCs), are responsible for eliciting T cell immunity against cutaneous pathogens including viral vectors. We show that cutaneous delivery of lentivirus results in direct transfection of sDCs and potent and prolonged antigen presentation. Further, sDCs are the predominant antigen-presenting cells for the induction of potent and durable CD8(+) T cell immunity. These results support the classical paradigm of cutaneous immune function and suggest that antigen presentation by sDCs contributes to the high potency of lentivector-mediated genetic immunization.

Migratory Dendritic Cells Transfer Antigen to a Lymph Node-Resident Dendritic Cell Population for Efficient CTL Priming

Skin dendritic cells (DCs) are thought to act as key initiators of local T cell immunity. Here we show that after skin infection with herpes simplex virus (HSV), cytotoxic T lymphocyte (CTL) activation required MHC class I-restricted presentation by nonmigratory CD8(+) DCs rather than skin-derived DCs. Despite a lack of direct presentation by migratory DCs, blocking their egress from infected skin substantially inhibited class I-restricted presentation and HSV-specific CTL responses. These results support the argument for initial transport of antigen by migrating DCs, followed by its transfer to the lymphoid-resident DCs for presentation and CTL priming. Given that relatively robust CTL responses were seen with small numbers of skin-emigrant DCs, we propose that this inter-DC antigen transfer functions to amplify presentation across a larger network of lymphoid-resident DCs for efficient T cell activation.

Programming, demarcating, and manipulating CD8 + T‐cell memory

In response to infection, antigen-specific CD8+ T cells undergo massive expansion in numbers, acquire effector mechanisms, and disseminate throughout the body. The expansion phase is followed by a contraction (death) phase, where 90-95% of antigen-specific CD8+ T cells are eliminated. The remaining antigen-specific CD8+ T cells form the initial memory pool, which can be stably maintained for life. In this review, we discuss evidence that early events after infection 'program' CD8+ T cells to expand, contract, and generate memory in a fashion that is largely insensitive to the duration of infection or antigen display. Recent data demonstrate, despite numerical stability, that memory CD8+ T-cell populations undergo phenotypic and functional changes with time after immunization. However, the early suggestion that specific markers can be used to identify memory CD8+ T cells has not been supported by recent studies. Thus, we argue that specific functional characteristics, such as the ability to persist and undergo vigorous secondary expansion leading to elevated memory cell numbers, remain the best markers of 'good' memory cells. Finally, we discuss experimental approaches to manipulate and accelerate generation of CD8+ T cells with memory characteristics, and how these systems can inform both basic and applied immunology.

Developing and maintaining protective CD8 + memory T cells

A critical aim of vaccine-related research is to identify the mechanisms by which memory T cells are formed and maintained over long periods of time. In recent years, we have designed experiments aimed at addressing two key questions: (i) what are the factors that maintain functionally responsive CD8+ memory cells over long periods of time, and (ii) what are the signals during the early stages of infection that drive the differentiation of long-lived CD8+ memory T cells? We have identified a role for CD4+ T cells in the generation of CD8+ T-cell-mediated protection from secondary challenge. While CD4+ T cells appear to play a role in the programme of CD8 memory, we find that they are also required for the long-term maintenance of CD8+ memory T-cell numbers and function. This property is independent of CD40-CD40L interactions, and we propose a role for CD4+ T cells in maintaining the ability of CD8+ memory T cells to respond to interleukin-7 (IL-7) and IL-15. By manipulating both the time course of infection and the timing of antigen presentation to newly recruited CD8+ T cells, we also demonstrate that the programming of effector and memory potential are at least partially distinct processes.

CD4+ T Cells Can Protect APC from CTL-Mediated Elimination

Professional APC play a central role in generating antiviral CD8(+) CTL immunity. However, the fate of such APC following interaction with these same CTL remains poorly understood. We have shown previously that prolonged Ag presentation persists in the presence of a strong CTL response following HSV infection. In this study, we examined the mechanism of survival of APC in vivo when presenting an immunodominant determinant from HSV. We show that transferred peptide-labeled dendritic cells were eliminated from draining lymph nodes in the presence of HSV-specific CTL. Maturation of dendritic cells with LPS or anti-CD40 before injection protected against CTL lysis in vivo. Furthermore, endogenous APC could be eliminated from draining lymph nodes early after HSV infection by adoptive transfer of HSV-specific CTL, yet the cotransfer of significant virus-specific CD4(+) T cell help promoted prolonged Ag presentation. This suggests that Th cells may assist in prolonging class I-restricted Ag presentation, potentially enhancing CTL recruitment and allowing more efficient T cell priming.

Transgenic reporter mouse for bioluminescence imaging of herpes simplex virus 1 infection in living mice

Bioluminescence imaging allows spatial and temporal progression of viral infection to be detected and quantified in living mice, thereby providing a new approach for studies of viral-host pathogenesis. It has been necessary to construct and validate recombinant reporter viruses that express firefly luciferase to investigate viral replication and spread with this imaging technology. This strategy greatly limits the ability to analyze multiple strains of virus and/or existing viral mutants, and reporter viruses also may be attenuated relative to the respective parental viruses. To facilitate bioluminescence imaging of herpes simplex virus type 1 (HSV-1), we developed a transgenic reporter mouse that uses the promoter from HSV-1 thymidine kinase to control expression of firefly luciferase. Infection with HSV-1 activated expression of firefly luciferase in corneal and flank models of infection, and amounts of bioluminescence increased in proportion to increasing input titers of virus. Imaging could detect infection with three different strains of HSV-1 with the following relative rank order of bioluminescence produced at the site of infection: McKrae > 17 > KOS. Corneal infection with as few as 1 x 10(3) pfu strain McKrae was detectable above background levels. By comparison, infection with vaccinia virus did not affect bioluminescence in the reporter mouse. Collectively, these data establish a new transgenic reporter mouse for infection with HSV-1, thereby enabling in vivo bioluminescence imaging studies of HSV-1 pathogenesis without constructing new reporter viruses.

Sculpting the immunological response to dengue fever by polytopic vaccination

The twin challenges of immunodominance and heterologous immunity have hampered discovery of an effective vaccine against all four dengue viruses. Here, we explore how the T cell competition and selection underlying these asymmetrical properties impede effective T cell vaccine design. The theory we develop predicts dengue vaccine clinical trial data well. From the insights that we gain by this theory, we propose two new ideas for design of epitope-based T cell vaccines against dengue: polytopic injection and subdominant epitope priming.

The onset of CD8+-T-cell contraction is influenced by the peak of Listeria monocytogenes infection and antigen display

The CD8+-T-cell response to infection with Listeria monocytogenes consists of expansion, contraction, and memory phases. The transition between expansion and contraction is reported to occur on different days postinfection with virulent (8 to 9 days) and attenuated (DeltaactA) (7 days) L. monocytogenes strains. We hypothesized that differences in the infectious courses, and therefore antigen (Ag) display, determine the precise time of the expansion/contraction transition in response to these infections. To test this, we infected BALB/c mice with 0.1 50% lethal dose of DeltaactA or virulent L. monocytogenes and measured bacterial numbers, Ag display, and Ag-specific CD8+-T-cell responses on various days after infection. We found that bacterial numbers and Ag display peaked between 12 and 36 h and between 36 and 60 h after infection with DeltaactA and virulent L. monocytogenes strains, respectively. Infection with DeltaactA L. monocytogenes resulted in a sharp peak in the Ag-specific CD8+-T-cell response on day 7, while infection with virulent L. monocytogenes yielded a prolonged peak with equivalent numbers of Ag-specific CD8+ T cells on days 6, 7, and 8 after infection. Truncating virulent infection with antibiotics on day 1 or 2 after infection resulted in a shift in the expansion/contraction transition from day 8 to day 7 after infection. However, antibiotic treatment beginning on day 3, after the peak of virulent L. monocytogenes infection and Ag display, had no effect upon the magnitude or timing of the CD8+-T-cell response. These results demonstrate a direct relationship between the course of infection and Ag display and that the timing of these events is important in shaping the T-cell response to infection.

The CD8+T Cell Population Elicited by Recombinant Adenovirus Displays a Novel Partially Exhausted Phenotype Associated with Prolonged Antigen Presentation That Nonetheless Provides Long-Term Immunity

We have previously reported that the CD8+ T cell response elicited by recombinant adenovirus vaccination displayed a delayed contraction in the spleen. In our current study, we demonstrate that this unusual kinetic is a general phenomenon observed in multiple tissues. Phenotypic analysis of transgene-specific CD8+ T cells present 30 days postimmunization with recombinant adenovirus revealed a population with evidence of partial exhaustion, suggesting that the cells had been chronically exposed to Ag. Although Ag expression could no longer be detected 3 wk after immunization, examination of Ag presentation within the draining lymph nodes demonstrated that APCs were loaded with Ag peptide for at least 40 days postimmunization, suggesting that Ag remains available to the system for a prolonged period, although the exact source of this Ag remains to be determined. At 60 days postimmunization, the CD8+ T cell population continued to exhibit a phenotype consistent with partially exhausted effector memory cells. Nonetheless, these CD8+ T cells conferred sterilizing immunity against virus challenge 7-12 wk postimmunization, suggesting that robust protective immunity can be provided by CD8+ T cells with an exhausted phenotype. These data demonstrate that prolonged exposure to Ag may not necessarily impair protective immunity and prompt a re-evaluation of the impact of persistent exposure to Ag on T cell function.

Distinct regulation of H2-M3-restricted memory T cell responses in lymph node and spleen

CD8 T cell populations restricted by H2-M3 MHC class Ib molecules expand rapidly during primary Listeria monocytogenes infection but only minimally upon reinfection. In contrast, CD8 T cells restricted by MHC class Ia molecules undergo more delayed expansion during primary infection but rapid and robust expansion following reinfection. In this study we demonstrate that primary H2-M3-restricted CD8 T cell responses are unaffected by the frequency of naive MHC class Ia-restricted T cells during L. monocytogenes infection. The magnitude of H2-M3-restricted memory responses, in contrast, is down-modulated by increasing frequencies of MHC class Ia-restricted effector T cells following secondary systemic infection. Suppression by MHC class Ia-restricted T cells, however, is not a universal feature of MHC class Ib-restricted memory responses. Primary systemic L. monocytogenes infection followed by secondary tissue infection, for example, results in robust expansion of H2-M3-restricted memory T cells in draining lymph nodes, despite the activation of MHC class Ia-restricted memory T cell responses. Thus, whereas MHC class Ia-restricted memory T cell populations predominate in spleens following systemic reinfection, H2-M3-restricted memory T cell responses remain prominent in lymph nodes draining localized infections. Our studies demonstrate that interactions between CD8 T cell populations can differ, depending on the status of the responding T cells (naive vs memory) and the route of reinfection. These results may have important implications for prime-boost vaccination strategies.

Immunization with lentiviral vector-transduced dendritic cells induces strong and long-lasting T cell responses and therapeutic immunity

Dendritic cell (DC) therapies are currently being evaluated for the treatment of cancer. The majority of ongoing clinical trials use DCs loaded with defined antigenic peptides or proteins, or tumor-derived products, such as lysates or apoptotic cells, as sources of Ag. Although several theoretical considerations suggest that DCs expressing transgenic protein Ags may be more effective immunogens than protein-loaded cells, methods for efficiently transfecting DCs are only now being developed. In this study we directly compare the immunogenicity of peptide/protein-pulsed DCs with lentiviral vector-transduced DCs, and their comparative efficacy in tumor immunotherapy. Maturing, bone marrow-derived DCs can be efficiently transduced with lentiviral vectors, and transduction does not affect DC maturation, plasticity, or Ag presentation function. Transduced DCs efficiently process and present both MHC class I- and II-restricted epitopes from the expressed transgenic Ag OVA. Compared with peptide- or protein-pulsed DCs, lentiviral vector-transduced DCs elicit stronger and longer-lasting T cell responses in vivo, as measured by both in vivo killing assays and intracellular production of IFN-gamma by Ag-specific T cells. In the B16-OVA tumor therapy model, the growth of established tumors was significantly inhibited by a single immunization using lentiviral vector-transduced DCs, resulting in significantly longer survival of immunized animals. These results suggest that compared with Ag-pulsed DCs, vaccination with lentiviral vector-transduced DCs may achieve more potent antitumor immunity. These data support the further development of lentiviral vectors to transduce DCs with genes encoding Ags or immunomodulatory adjuvants to generate and control systemic immune responses.

CD8 T-cell ability to exert immunodomination correlates with T-cell receptor: Epitope association rate

When presented alone, H7 a and HY antigens elicit CD8 T-cell responses of similar amplitude, but H7 a totally abrogates the response to HY when both antigens are presented on the same antigen-presenting cell. We found that H7a- and HY-specific T-cell precursors had similar frequencies in nonimmune mice and expressed similar levels of CD5. The H7a -specific CD8 T-cell repertoire harvested at the time of primary response showed highly restricted T-cell receptor (TCR) diversity. Furthermore, T cells specific for H7a and HY expressed equivalent levels of CD8 and TCR and displayed similar tetramer decay rates. The key difference was that anti-H7a T cells exhibited a much more rapid TCR:epitope on-rate than anti-HY T cells. Coupled with evidence that primed CD8 T cells limit the duration of antigen presentation by killing or inactivating antigen-presenting cells, our data support a novel and simple model for immunodomination: the main feature of T cells that exert immunodomination is that, compared with other T cells, they are functionally primed after a shorter duration of antigen presentation.