G·A·B·B·A

The immune system is a complex organ that includes different types of cells, tissues and molecules. These components are coordinated very elegantly and efficiently, in order to maintain integrity and health of the human body, which is permanently surrounded by pathogens in the environment. The first line of attack to these invaders is mounted by the innate immune system that comprises both physical and chemical barriers and also specialized cells, which master antigen presentation, cytokine secretion and killing of unhealthy cells. Among these, dendritic cells (DCs) constitute a heterogeneous group of professional antigen presenting cells that belong to the innate immune arm of the immune system and are found in most tissues of the human body. In an immature stage, these cells patrol the body, searching for changes in organ homeostasis. Due to the expression of pathogen-recognition receptors, DCs can detect stress signals that might induce a phenotypical and functional maturation of these antigen-presenting cells. Mature DCs are able to migrate efficiently into secondary lymphoid tissues, like the lymph nodes or spleen where they can interact with cells of the adaptive immune system, like T cells, but also of the innate immune system, like NK cells. The bidirectional crosstalk between mature DCs and resting NK cells is carried out by the exchange of both soluble factors and interactions of surface receptors, and culminates in simultaneous activation of NK cells and in the survival of mature DCs, despite their interaction with activated cytotoxic lymphocytes. The formation of this regulatory immunological synapse between these two members of the innate immune system is important for the development of effective immune responses, supported by activated NK cells and primed by surviving mature DCs. Therefore, we aimed to characterize in this study the regulatory immunological synapse between human mature DCs and NK cells, which coordinates these two important innate leucocyte populations at the beginning of immune responses. Our results demonstrate that the synapse between mature conventional DCs and resting NK cells is formed very rapidly and matures with the polymerization of DC-derived filamentous actin at the cellular interface. Furthermore molecules involved in activating and inhibitory signaling are found clustered at the immunological synapse occupying, however, different spatial domains, respectively. This feature of the regulatory synapse might allow the parallel signaling of molecules involved in both NK cell activation and inhibition. Moreover, we observed that the dynamics of actin polymerization seems to be important for the maintenance of MHC class I polarization at the immunological synapse and for the effective delivery of inhibitory signals by these molecules via killer-cell immunoglobulin-like receptors to NK cells. This MHC class I/KIRs inhibitory signaling might be critical for the control of both cytokine production and cytotoxic activity of NK cells, when interacting with mature DCs. Moreover, our preliminary results on the crosstalk between plasmacytoid DC and NK cells points towards the formation of a cytotoxic-like immunological synapse, that should be further explored in order to dissect the role of it in the activation of NK cells by plasmacytoid DCs. Finally, we initiated a project that aims to visualize the interactions between DCs and NK cells in secondary lymphoid organs of humanized mice by two-photon microscopy. The results from this study could elucidate the duration of this crosstalk in an environment that resembles the sites of interaction between mature DCs and resting NK cells in the human body. The study of the immunological synapse between these two members of the innate immune system can provide the mechanistic understanding of how mature DCs can activate NK cells and survive to go on to activate adaptive immunity. This feature of DCs, to activate different waves of immune responses, could be harnessed for immunotherapies, including vaccinations.