Classic major histocompatibility complex class I molecules: new actors at the neuromuscular junction.

The presence and function of immune molecules in the central nervous system (CNS) have been under debate for a long time. There is mounting evidence that molecules fundamental for immune function are indeed expressed by both neurons and glia and that such molecules may have important nonimmunological function for the organization and stability of synaptic connections. Here, we present data showing that the classic form of major histocompatibility complex (MHC) class I molecules is expressed in spinal motoneurons, in particular in their axons and presynaptically at their synapses with skeletal muscles, the neuromuscular junctions (NMJs). The expression is strongly increased after axon lesion in the peripheral nerve. In the absence of classic MHC I, the organization of NMJs is disturbed with NMJs in higher numbers than normal, thereby equipping single muscle fibers with multiple NMJs. It is suggested that these effects are mediated by the classic MHC class I in the motor axons, possibly through effects mediated by the peripherally myelinating Schwann cells, which express receptors for classic MHC class I. The presence of immune molecules normally used by other cells for antigen presentation in peripheral motor axons may have implications for the onset of specific motoneuron disease.