Expression of stimulatory and inhibitory molecules in interactions between natural killer cells neurons

Abstract

Natural killer (NK) cells are important effector cells of the innate and the early induced immune defense. In the nervous system they appear early during inflammation and have been ascribed a role in clearance of certain viral infections from the brain. NK cells are regulated by multiple signals transmitted via inhibitory and stimulatory receptors. In mice, they are inhibited by major histocompatibility complex (MHC) class I molecules and activated by retinoic acid early inducible gene-1 (RAE1), minor histocompatibility antigen H60 and the murine ULBP-like transcript 1 (MULT1) molecules, that are ligands for the stimulatory NKG2D receptor. The present thesis project was undertaken with the aim of elucidating whether neurons could be targets for an NK cell-mediated attack and the molecular mechanisms involved in an interaction between NK cells and neurons.

Initial experiments were performed on embryonic mouse dorsal root ganglia (DRG) neurons in culture. Neurons exposed to syngeneic IL-2-activated NK cells were totally destroyed, leaving only remnants of neurons. The killing was cell contact- and perfor-independent and the neurons showed signs of apoptosis. Furthermore, the surrounding glial cells constituted protection against the NK cell-mediated degeneration of DRG neurons that was independent of MHC class I expression. In contrast to the complete destruction of the DRG neurons of the peripheral nervous system ventral spinal cord neurons and hippocampal neurons of the central nervous system were resistant to NK cell-mediated killing.

The resistance to NK cell-mediated lysis of the latter neurons was not related to protection by MHC class I molecules, since MHC class I deficient neurons were equally resistant to lysis. Analysis of the expression of ligands for the activating receptor NKG2D revealed differential expression of RAE1 in the different populations of neurons, with high levels of both RAE1 transcripts and surface expression of NKG2D-binding proteins in the susceptible DRG neurons, while the expression of these molecules in the resistant hippocampal neurons was low. Furthermore, blocking antibodies against NKG2D inhibited the NK cell-mediated killing of DRG neurons.

Expression of RAE1 has so far only been described in embryonic tissues. The results of our in vivo study showed that, RAE1 transcripts are expressed also in postnatal and young adult mice. Moreover, a peripheral nerve lesion of the sciatic nerve, resulted in an early reduction in expression of stimulatory RAE1 and MULT1 transcripts, followed by an increase in the expression of classical MHC class I transcripts compared to sham-operated control mice. hi addition, murine cytomegalovirus and influenza A virus infections of DRG neuronal cultures caused increased expression of transcripts encoding RAE1, MULT1 and classical MHC class I molecules.

Interferon-gamma treatment of DRG neuronal cultures resulted in an increased expression of classical and non-classical MHC class I encoding transcripts. Treatment of the DRG neuronal cultures with retinoic acid receptor agonists did not affect the expression of transcripts of any of the investigated molecules. A two-compartment in vitro model was used to study whether interferon-gamma and influenza A virus added to the axonal compartment could affect the expression of molecules in the nerve cell body compartment. Exposure to both agents resulted in an increase in MHC class I expression in the nerve cell body compartment whereas the expression of RAE I and MULT1 was unaffected.

In conclusion, this thesis provides further insight into the potential role of NK cells in diseases of the nervous system. The results show; that NK cells may induce degeneration of neurons that express ligands for the NK cell stimulatory receptor NKG2D, that transcripts for these ligands are expressed in the adult nervous system and that a peripheral nerve lesion, a viral infection and treatment with interferon-gamma can modify the expression of these ligands.