Deciphering mechanisms shaping Natural Killer cell responses to Trypanosoma cruzi-infected dermal fibroblasts

The potentially life-threatening Chagas disease is caused by the protozoan parasite Trypanosoma cruzi (T. cruzi). It is mainly prevalent in rural areas in Latin America and is classified as one of the 20 neglected tropical diseases by the WHO. Whereas the acute stage of infection is usually mild or asymptomatic, about 30% of infected individuals develop cardiac or gastrointestinal symptoms during the chronic stage. The approved drugs can entirely eliminate the parasites exclusively during the acute stage, with treatment often accompanied by severe side effects. Since T. cruzi parasites are mainly transmitted to humans by triatomine bugs, the skin is the main entry route for the parasites. Thus, it is likely that the early immune response against parasitized skin fibroblasts influences the parasite's spread to other tissues and organs. Considering the obligatory intracellular life cycle of T. cruzi, natural killer (NK) cells are expected to substantially contribute to the innate immune response. They can sense their target cells through a variety of different mechanisms, resulting in direct cytotoxic effector functions and secretion of cytokines such as IFN-γ. The present study aimed to investigate the response of human NK cells to T. cruzi-infected dermal fibroblasts as well as the underlying mechanisms. Initially, in vitro cocultures were established for this purpose, demonstrating that T. cruzi-infected primary human dermal fibroblasts as well as infected cells of the foreskin fibroblast cell line BJ promote NK cell degranulation. Since transcriptome analysis demonstrated a comparable response of primary and BJ fibroblasts to T. cruzi infection, BJ fibroblasts were used for subsequent mechanistic analyses. Here, NK cell activation by T. cruzi infected BJ fibroblasts was shown to be strongly dependent on the engagement of the stimulatory NK cell receptors NKp46 and CD160. T. cruzi infection of BJ fibroblasts resulted in the increased presence of the respective ligands vimentin and HVEM on the fibroblast surface as well as an increased binding of NKp46 and CD160. Finally, NKp46 and CD160 receptor blockade on NK cells led to significantly reduced NK cell effector functions. In addition to these cell-cell contact-dependent mechanisms, contact-independent activation of NK cells by supernatants of T. cruzi-infected fibroblasts was observed. IFN-β neutralization experiments demonstrated that IFN-β secreted by T. cruzi infected BJ fibroblasts directly promoted NK cell activity. However, secreted IFN-β also increased MHC-I expression on fibroblasts, leading to the reduction of the NK cell response through interaction with inhibitory NK cell receptors. Results of coculture experiments indicate that activating stimuli predominate in the first few hours, but these could be compensated by inhibitory signals in the later course of infection. Finally, the role of the MHC-I molecule HLA-C was investigated, which showed increased expression on fibroblasts in response to T. cruzi infection and which has been described to interact with both the activating CD160 as well as activating and inhibitory KIRs on NK cells. NK cell cocultures with HLA-C knockout BJ fibroblasts generated in this study showed that HLA-C inhibited NK cell activity of most individuals, probably to avoid an exaggerated response. In conclusion, the in vitro model established in this study enables the investigation of regulatory mechanisms of the NK cell response in T. cruzi-parasitized dermis. Characterization of these mechanisms revealed that the NK cell response here is controlled by a precisely tuned balance of diverse stimulatory and inhibitory signals. In the future, the better understanding of these mechanisms may help to develop efficient and tolerable therapy options.