UV exposure boosts transcutaneous immunization and improves tumor immunity: cytotoxic T-cell priming through the skin.

Immunologic approaches to combat cancer aim at the induction of tumor-reactive immune responses to achieve long-term protection. In this context, we recently developed a transcutaneous immunization (TCI) method using the Toll-like receptor (TLR) 7 agonist imiquimod and a peptide epitope. Application onto intact skin induces potent cytotoxic T lymphocyte (CTL) responses and protection against transplanted tumors. The purpose of this study was to explore the effects of UV irradiation on imiquimod-based TCI. Here we show that skin exposure to low-dose UV light before TCI with imiquimod strongly boosts specific CTL responses leading to memory formation and enhanced tumor protection. Toward the mechanisms, we show that the activation of bone-marrow-derived dermal dendritic cells (DCs), but not Langerin-expressing DCs, is responsible for enhanced CTL activation. We describe an optimized TCI method that mediates enhanced CTL and antitumor responses by a DC- and TLR-dependent mechanism. These data may provide the basis for the future development of advanced vaccination protocols against tumors and persistent virus infections.

Interferon-gamma induces chronic active myocarditis and cardiomyopathy in transgenic mice.

Chronic heart failure is associated with an activation of the immune system characterized among other factors by the cardiac synthesis and serum expression of proinflammatory cytokines. There is unequivocal clinical and experimental evidence that the cytokine tumor necrosis factor-alpha is involved in the development of chronic heart failure, but a putative cardiotoxic potential of the proinflammatory cytokine interferon (IFN)-gamma remains primarily unknown. To investigate this issue we analyzed the cardiac phenotype of SAP-IFN-gamma transgenic mice, which constitutively express IFN-gamma in their livers and hence exhibit high circulating serum levels of this cytokine. SAP-IFN-gamma mice spontaneously developed chronic active myocarditis, characterized by the infiltration of not only CD4(+) and CD8(+) T cells but also Mac2(+) (galectin 3(+)) macrophages and CD11c(+) dendritic cells, eventually culminating in cardiomyopathy. Echocardiographic analyses exhibited a left ventricular dilation and impaired systolic function induced by IFN-gamma overexpression. IFN-gamma-mediated cardiotoxicity was associated with high-level cardiac transcription of the proinflammatory cytokines tumor necrosis factor-alpha and interleukin-12 and the macrophage-attracting chemokines MCP1 and MIP1-alpha. Myotoxic IFN-gamma effects could not be detected in smooth or striated muscle tissue, suggesting cardiomyocellular specificity of the toxic IFN-gamma effect. The precise mechanism of IFN-gamma cardiotoxicity remains to be elucidated.

Transcutaneous immunization with imiquimod is amplified by CD40 ligation and results in sustained cytotoxic T-lymphocyte activation and tumor protection.

Transcutaneous immunization (TCI) using ligands of Toll-like receptors (TLRs) and cytotoxic T-lymphocyte (CTL) epitopes lead to the induction of potent T-cell responses. To characterize the efficacy of TCI-mediated CTL activation, we monitored the frequency and functional activity of specific CTL induced with TCI using the ovalbumin-derived epitope SIINFEKL composed in creme containing the synthetic TLR7 ligand R-837. We found that the frequency and activity decayed rapidly 10 d post-TCI. Consistently, no significant memory T-cell formation was detectable. In a prophylactic vaccination setting, TCI was protective against a lethal challenge with ovalbumin expressing EG.7 thymoma cells when the tumor cells were inoculated 5 d later. However, only a delay of tumor growth was observed when the tumor challenge was performed 55 d after immunization. Conversely, a single combined treatment with TCI and an agonist anti-CD40 (FGK-45) monoclonal antibody greatly enhanced the primary response, with up to 30% of peptide-specific CTL and the effective induction of memory cells. Consequently, mice treated with TCI/anti-CD40 were completely protected against a lethal tumor challenge with EG.7 tumor cells after 55 d. In this article, we demonstrate that transcutaneous immunization approaches using TLR ligands deliver sufficient amounts of antigen to mediate durable protection against tumors if adequate costimulation is provided. These results may contribute to the development of advanced vaccination protocols against malignancies and persistent virus infections.

Mast cells are crucial for early inflammation, migration of Langerhans cells, and CTL responses following topical application of TLR7 ligand in mice.

Until recently, IgE-activated mast cells have been regarded merely as effector cells of adaptive immune responses, involved in allergic reactions and mucosal immunity to parasites. Herein, we report that murine dermal mast cells, activated by local administration of a cream containing the synthetic TLR7 ligand imiquimod, are essential to initiate an early inflammatory reaction. The mast-cell-derived cytokines TNF-alpha and IL-1beta play an important role in this process. Furthermore, TLR7-activated mast cells are also able to promote the emigration of Langerhans cells, which partly depends on the expression of mast-cell-derived IL-1beta. We have previously shown that TLR7 ligation enhances transcutaneous immunization evoked by topical application of vaccine antigens to the skin, a procedure that directly targets skin-resident antigen-presenting cells. Consequently, we now demonstrate here that the capacity to mount a peptide-specific cytotoxic T-lymphocyte response following transcutaneous immunization using imiquimod as adjuvant is severely impaired in mast-cell-deficient mice. Thus, these findings demonstrate the potent versatility of alternatively activated mast cells at the interface of innate and adaptive immunity.

Initiation of adaptive immune responses by transcutaneous immunization.

The development of new, effective, easy-to-use and lower-cost vaccination approaches for the combat against malignant and infectious diseases is a pre-eminent need: cancer is a leading cause of morbidity in the Western World; there are numerous pathogenic diseases for which we still have no protective or therapeutic cure; and the financial limitations of developing countries to fight these diseases. In this mini-review we focus on transcutaneous immunization (TCI), a relatively new route for antigen delivery. TCI protocols appear to be particularly promising by gaining access to skin resident APC, which are highly efficient for the initiation of humoral and/or cellular immune responses. Consisting of an adjuvant as a stimulus in combination with an antigen which defines the target, TCI offers a most attractive immunization strategy to mount highly specific full-blown adaptive immune responses. As a topically applicable cell-free adjuvant/antigen mixture, TCI might be suitable to improve patient compliance, as well as feasible economically for the use in Third World countries. In addition, this non-invasive procedure might increase the safety of vaccinations by eliminating the risk of infections related to the recycling and improper disposal of needles. The dissection of antigen and adjuvant is important because it allows "free" combinations in contrast to classical immunizations which are based on application of the pathogen of interest. The most relevant ways and means to find new, effective pathogenic target antigens are "reverse vaccinology" and the direct peptide-epitope identification from MHC molecules with mass-spectrometry. Due to these efficient approaches the variety of antigenic epitopes for potential protective/therapeutic use is perpetually expanding. The most studied adjuvants in TCI approaches are cholera toxin (CT) and its less toxic relative, the heat-labile enterotoxin (LT). Both CT and LT can serve as antigen as well. In contrast to these large proteins, which can only penetrate "pre-treated" skin barrier, the immune response modifier, TLR7 agonist R-837 (Imiquimod) is a small compound adjuvant that easily passages non-disrupted epidermis. It remains currently elusive which cells of the complex-structured "skin-associated lymphoid tissue" (SALT) respond to the adjuvant and which APC carries the antigen to the draining lymphnodes for subsequent initiation of adaptive immune responses.

Precursor frequency can compensate for lower TCR expression in T cell competition during priming in vivo.

The factors controlling clonal dominance of cytotoxic T lymphocyte (CTL) responses are currently not well understood. To study the functional impact of the strength of the interaction of a T cell with an antigen-presenting cell in this context, we established a new mouse model comprised of two T cell receptor (TCR)-transgenic strains expressing the identical TCR in differing amounts, hence providing two CTL clones with different avidities but identical specificity and affinity. Utilizing this new model, we show that upon antigen challenge higher-avidity CTL expand at the expense of moderate-avidity CTL in vivo if present in equal numbers. Beyond this, moderate-avidity T cells can also contribute to a CTL response when present in excess. These results suggest that in addition to a proposed affinity/avidity threshold, the precursor frequency is important in defining clonal dominance. A new model in which TCR density and precursor frequency define the outcome of a CTL response is discussed.

Interaction of TLR2 and TLR4 ligands with the N-terminal domain of Gp96 amplifies innate and adaptive immune responses.

Activation of dendritic cells by ligands for Toll-like receptors (TLR) is a crucial event in the initiation of innate and adaptive immune responses. Several classes of TLR ligands have been identified that interact with distinct members of the TLR-family. TLR4 ligands include lipopolysaccharide derived from different Gram-negative bacteria and viral proteins. Recent reports have demonstrated the TLR-mediated activation of dendritic cells by heat shock proteins (HSPs). However, doubts were raised as to what extent this effect was due to lipopolysaccharide contaminations of the HSP preparations. We re-examined this phenomenon using Gp96 or its N-terminal domain, nominally endotoxin-free (<0.5 enzyme units/mg). As described previously, innate immune cells are activated by Gp96 at high concentrations (> or =50 microg/ml) but not at lower concentrations. However, preincubation of low amounts of Gp96 with TLR2 and TLR4 ligands at concentrations unable to activate dendritic cells by themselves results in the production of high levels of proinflammatory cytokines, up-regulation of activation markers, and amplification of T cell activation. Our results provide significant new insights into the mechanism of HSP-mediated dendritic cell activation and present a new function of HSPs in the amplification of dendritic cell activation by bacterial products and induction of adaptive immune responses.

Synergistic activation of dendritic cells by combined Toll-like receptor ligation induces superior CTL responses in vivo.

Toll-like receptors (TLRs) are able to interact with pathogen-derived products and their signals induce the coordinated activation of innate and adaptive immune mechanisms. Dendritic cells (DCs) play a central role in these events. As the different TLRs are able to trigger MyD88/TRIF-dependent and -independent signaling pathways, we wondered if the simultaneous activation of these signaling cascades would synergize with respect to DC activation and induce superior cytotoxic T-lymphocyte (CTL) activity in vivo. We observed that indeed the combined activation of MyD88-dependent and -independent signaling induced by TLR7 and TLR3 ligands provoked a more rapid and more sustained bone marrow-derived DC (BMDC) activation with regard to the secretion of proinflammatory cytokines, like IL-6 and IL-12p70, and the expression of costimulatory molecules like CD40, CD70, and CD86. Furthermore, in the presence of combined TLR ligand-stimulated DCs, CD4(+) and CD8(+) T cells were insensitive toward the inhibitory effects of regulatory T cells. Most importantly, peptide-loaded BMDCs stimulated by TLR ligand combinations resulted in a marked increase of CTL effector functions in wild-type mice in vivo. Thus, our results provide evidence that unlocking the full potential of DCs by advanced activation protocols will boost their immunogenic potential and improve DC-based vaccination strategies.

Dendritic cell aggresome-like-induced structure formation and delayed antigen presentation coincide in influenza virus-infected dendritic cells.

Influenza virus infection induces maturation of murine dendritic cells (DCs), which is most important for the initiation of an immune response. However, in contrast to EL-4 and MC57 cells, DCs present viral CTL epitopes with a delay of up to 10 h. This delay in Ag presentation coincides with the up-regulation of MHC class I molecules as well as costimulatory molecules on the cell surface and the accumulation of newly synthesized ubiquitinated proteins in large cytosolic structures, called DC aggresome-like-induced structures (DALIS). These structures were observed previously after LPS-induced maturation of DCs, and it was speculated that they play a role in the regulation of MHC class I Ag presentation. Our findings provide the first evidence for a connection between DC maturation, MHC class I-restricted Ag presentation, and DALIS formation, which is further supported by the observation that DALIS contain ubiquitinated influenza nucleoprotein.

Cutting edge: priming of CTL by transcutaneous peptide immunization with imiquimod.

CTL are important in combating cancer and viruses. Therefore, triggering the complete potential of CTL effector functions by new vaccination strategies will not only improve prophylaxis of tumor or virus-related diseases, but also open opportunities for effective therapeutic immunizations. Using transcutaneous immunization, we show that epicutaneous (e.c.)(4) application of an ointment containing a CTL epitope and the TLR7 ligand imiquimod is highly effective in activating T cells in mice using TCR-transgenic CTL or in wild-type mice. Transcutaneous immunization-activated CTL mount a full-blown immune response against the target epitope characterized by proliferation, cytolytic activity, and the production of IFN-gamma that is completely restricted to the epitope used for vaccination. Our results obtained by simple e.c. application of an ointment, without further skin irritating procedures, provide the basis for the development of new, easy to use vaccines against cancer or virus-associated diseases.

T cell avidity determines the level of CTL activation.

To investigate the influence of avidity on T cell activation in vitro and in vivo, we analyzed T cells from St40 and St42 mice, which express the same transgenic TCR specific for an E1a-derived epitope of adenovirus type 5 with different expression levels and therefore different avidities. Splenocytes from both strains showed comparable cytolytic activities and required identical peptide concentrations for efficient target cell lysis and up-regulation of activation markers. However, the kinetics of CD25 up-regulation were strikingly different: whereas the majority of the high-avidity St42 T cells up-regulated the IL-2Ralpha chain within a few hours, low-avidity St40 T cells expressed only 50% of the CD25 of high-avidity T cells after 2 days. In addition, low-avidity T cells proliferated poorly and displayed impaired secretion of IL-2 and IFN-gamma. Similar results were seen with high-avidity St42 T cells stimulated with a partial agonistic peptide. Upon adoptive transfer and subsequent immunization with adenovirus, both high- and low-avidity T cells expanded, but St40 T cells were undetectable 10 days after immunization. Our model system now allows analysis of whether T cells with identical specificities but different avidities influence each other during activation and homeostatic proliferation.