Host strategies against virus entry via the olfactory system

In mammals, odorants are inhaled through the nose and inside the nasal cavity they trigger olfactory sensory neurons (OSN) that are located within the olfactory epithelium. OSN project their axons into glomerular structures of the olfactory bulb. There they synapse with dendrites of second-order neurons that project their axons to the olfactory cortex. Thus, olfaction is based on direct interaction of environmental matters with OSN. This poses the question of how neurotropic viruses are prevented from infecting OSN and entering the central nervous system. Recent evidence indicates that upon instillation of neurotropic virus OSN are readily infected. By axonal transport virus reaches the glomerular layer of the olfactory bulb where it is efficiently curbed by a type I IFNdependent mechanism. In this review local mechanisms limiting virus entry via the olfactory system and inhibiting virus spread within the CNS are recapitulated in the context of anatomical properties of the olfactory system.