Immunerkennung endogener RNA-Liganden

Innate immunity has evolved different pattern recognition receptors to detect pathogen associated molecular patterns (PAMPs) and to elicit an immune response. Whereas double stranded RNA in the cytoplasm is detected by RIG I and MDA5, double- and single stranded RNA in the endosome is recognized by Toll like Receptors (TLRs) 3, 7 and 8. Under physiological conditions, these receptors discriminate between foreign and self RNA based on certain structures and modification patters. Under pathological conditons, these receptors can also recognize self RNA. The major part of this thesis deals with the detection of self RNA as a potential danger signal in the cytoplasm. In the case of viral infection, in addition to the activation of RIG I and MDA5, double stranded RNA also leads to the activation of RNase L. Subsequently, cellular RNA is digested and fragments bound to pattern recognition receptors, which results in an amplification of the danger signal. In this thesis, it could be shown that the fragments of self-RNA are exclusively recognized by RIG I. It was proven that this RIG I activation is not a common effect of digested RNA but caused by specific RNA structures. The sequences of these molecules were obtained by Next Generation Sequencing. The endogenous RIG I ligands are derived from expansion segments in the 28S rRNA and from ITS2, a processing product of the 45S rRNA. Their specific RIG I activating effect could be shown in different immunostimulation experiments. Unlike the common model of an ultimate RIG I ligand, they do not bear a 5’ triphosphate but a 3’ monophosphate and are not rich in uridine but in guanosine and cytosine. Instead, the endogenous RIG I ligands seem to be detected due to their double-strandness and a certain loop structure. Involvement of ITS2 indicates a further link not only between antiviral response and RNase L but also between RNase L, rRNA biogenesis and cancer development. Generation of immunostimulatory RNA species is not only limited to virus induced RNase L but can also be obtained by artificial transfection of RNase A into a cell. Consequently, RNase A might be established as a novel species of adjuvant. In the second part of this thesis, the impact of modifications on the TLR7 and 8 mediated recognition of 18S rRNA derived RNA63 was analysed. Unmodified RNA63 and RNA63 bearing a 2’-O-deoxy or 2’-O-fluor modification all activate TLR7 and TLR8 while 2’-O-methylation inhibits TLR7 depended signaling and is solely detected by TLR8. This is why TLR8 activation is more resistent to RNA-modifications than TLR7activation. Consequently, 2’-O-methylated RNA63 is a specific TLR8 ligand.