Mechanisms of immune surveillance in the skin

Dendritic cells (DC) are specialized antigen presenting cells (APC) believed to be crucial in modulating the immune system and, therefore, in immune surveillance. Under pathological conditions, immune surveillance plays an important role in the defense against microorganisms as well as in keeping newly arising tumors under control. However, immune surveillance has never been proven to be operative in vivo under homeostatic conditions. In addition, depending on the cellular and molecular microenvironment, immune responses can lead not only to protective immunity against pathogens, but also to tolerance. It is the goal of this proposal to understand how skin immune responses are regulated under homeostatic and pathological conditions. For this purpose we have generated transgenic mouse models in which Ova antigen expression can be induced by Cre/loxP mediated transgene recombination in a tissue-specific and time-controllable manner either in the periphery (epidermis) or directly in antigen presenting cells (DC). These mice will allow us to analyze whether priming of T-cells occurs under homeostatic conditions when innocuous antigens are expressed in the periphery or whether additional danger signals are required to get an antigen-specific immune response. Comparison to immune responses raised against the same antigen expressed directly on DC will give more insight in the importance of antigen uptake and cross- presentation in eliciting immune responses. We will utilize this model to investigate how antigen-specific immune responses in the skin can be modulated by concomitant stimulation of the innate immune system, e.g Toll Like Receptor 7 (TLR7) by the agonist Imiquimod. We will furthermore focus on the mechanisms underlying the immunostimulatory effect of the synthetic compound Imiquimod in the skin. Finally we want to establish a tumor model to study antigen specific immune responses against defined tumor antigens. Understanding how these immune responses are regulated could ultimately lead to the development of therapeutic strategies aimed either at maintaining cutaneous homeostasis or at potentiating immune surveillance against otherwise progressively growing tumors.

Dendritic cells (DC) are specialized antigen presenting cells (APC) believed to be crucial in modulating the immune system. Under pathological conditions, immune surveillance plays an important role in the defense against microorganisms as well as in keeping newly arising tumors under control. However, immune surveillance has never been proven to be operative in vivo under homeostatic conditions. In addition, depending on the cellular and molecular microenvironment, immune responses can lead not only to protective immunity against pathogens, but also to tolerance. It was the goal of this proposal to understand how immune responses are regulated in healthy and diseased skin and if priming of T-cells occurs under homeostatic conditions when innocuous antigens are expressed in the epidermis. For this purpose we have generated transgenic mice in which Ova antigen expression can be induced by Cre/loxP mediated transgene recombination in a tissue-specific and time-controllable manner in the epidermis. Although Ova was efficiently expressed by epidermal cells and presented by DC, adult transgenic mice did not develop any obvious autoimmune disease symptoms like hair or weight loss. In skin-draining lymph nodes T-cells specifically recognizing Ova were activated and proliferated transiently. However, these T-cells were not able to fully respond to Ova suggesting that tolerance was induced. It is believed, that the conditions at the time of first antigen encounter may influence the outcome of the immune response. We therefore tested if Ova-specific immune responses in the skin can be modulated by concomitant application of pro-inflammatory stimuli such as Toll Like Receptor agonists at the time of Ova induction. None of stimuli applied was able to break tolerance. However, when we directly injected fully mature DCs presenting high amounts of Ova peptide on their surface, tolerance was no longer induced and antigen-specific T-cells proliferated like in wild-type controls. Our data suggest that low amount of antigen expressed in the induction phase of the immune response results in tolerance even in the presence of danger signals. This could be a protective mechanism to avoid induction of autoimmune disease during inflammatory or infectious diseases in the skin.