Biosensors for MRT in Immunology

In medicine, Magnetic Resonance Tomography (MRT) is an established procedure for non-invasive diagnostics. Due to the intrinsically low detection sensitivity, the high water content of the soft tissues is routinely used for morphologic recordings. The desirable spatially resolved imaging of biomolecular processes in cells and tissues requires - due to the low concentration of the substances involved - a fundamentally new approach where sensitive, specifically binding molecular probes – so-called biosensors – are used. In this respect, the isotope ¹²⁹Xe offers an enormous potential for MRT: The magnetic nuclear polarization which determines the MRT signal strength can - by means of optical procedures - be increased by up to five orders of magnitude compared to the usual thermal polarization in magnetic fields with a field strength of a few tesla. Thus, the problem of the low sensitivity of the MR measurement can be circumvented. The resonance frequency of ¹²⁹Xe is strongly characterized by the chemical neighbourhood, and signals without background which are less prone to misinterpretation are obtained. In addition, xenon is compatible for humans and animals. To be able to use these advantageous properties of the chemically inert rare gas isotope ¹²⁹Xe for a biosensor, molecular cages accessible for xenon are functionalized by linking them with highly affine binding partners of selected target molecules [1].

Now, in a cooperation of the three Berlin Institutes PTB, Leibniz Institute for Molecular Pharmacology and Max Delbrück Centre for Molecular Medicine, a biosensor based on ¹²⁹Xe has been developed to deal with questions from the field of immunology [2]. The investigations are focused on the presentation of exogenous antigens by the molecule complex MHC II (main histocompatibility complex II), a process which induces the specific immune reaction of the organism to the foreign matter. For the experiments, a molecular cage accessible for xenon was linked with the peptide haemagglutinin (HA), a fragment of the influenza virus and a substrate of the allele HLA DR1 of MHC II, via a polyethylene glycol molecule and a quadripeptide (GEEG) connected upstream which improves the solubility (Fig. 1a). In an aqueous solution with ¹²⁹Xe, this complex formation can bind to an HLA DR1 whereas xenon atoms permanently change between the solution and the molecular cage. Three different ¹²⁹Xe resonances can be attributed to these states: a) free atoms in solution, b) atoms in the cage of the biosensor without an HLA DR1 bond, and c) atoms in the cage of the biosensor bound to the HLA DR, so that the position of the ¹²⁹Xe signal is sensitive to the bonding event in a nuclear magnetic resonance spectrum (Fig. 1b). By using optically polarised ¹²⁹Xe, the signal intensity can be increased in such a way that the complex formation can be detected at a concentration of only 5 µM. This method is to be further developed for investigating immunological issues such as research on the causes of autoimmune diseases, in-vitro as well as in-vivo, by means of spectroscopic and imaging methods of nuclear magnetic resonance.