How the DM strain of MHV initially infects neurons requires further study. One possible explanation could be that traumatic disruption to the nerve endings in the brain allows access of the virus through the damaged axolemma; or the virus may be capable of directly infecting intact axons either in the brain, or after diffusing into the optic nerve in the cerebrospinal fluid. Alternatively, virus may be engulfed into the axons at synaptic terminals. Although direct infection after diffusion in cerebrospinal fluid could potentially explain the presence of viral antigen in optic nerves, the fact that antigen is detected in RGC bodies in the retina demonstrates that retrograde transport does occur. Although the presence of viral antigen in RGCs alone cannot exclude hematogenous spread as an alternate mechanism from retrograde transport, the timing of its spread (ie, detected only at day 6 pi in the retina versus day 3 pi within optic nerves and within liver via hematogenous spread), the lack of hematogenous spread to the CNS after systemic infection with equivalent doses of MHV-A59 (6) , and the presence of viral antigen in areas of the brain containing RGC projections all suggest that retrograde transport is the more likely mechanism of viral spread. Retrograde transport may follow fusion of the spike protein with axonal membranes followed by loss of most of the viral structural proteins and then binding of the nucleocapsid via one or more viral proteins to the retrograde molecular motors. Such a mechanism has been seen in other neurovirulent viruses that follow a retrograde direction of axonal transport (16, 17) . Although it is not common that a single virus can use both directional transport mechanisms, this is not the first virus to demonstrate such properties. For example, herpesviruses can be transported rapidly along microtubules in the retrograde direction from the axon terminus to the dorsal root ganglion and then anterograde in the opposite direction (18, 19) . Axonal transport is an important strategy used by several neurovirulent viruses, including herpes simplex, rabies, polio, influenza, and Borna disease viruses, which can be transported in axons either in anterograde or in retrograde direction (20Y22). In some instances, viruses can be transported within axons for a long distance, yet this journey occurs within the cell. Therefore, the virus cannot be inactivated by neutralizing antibody during its transit and may spread in the CNS without inducing an antivirus immune response while it is within the cell. When viruses can spread only within axons and/or via direct cell-tocell contact, they could potentially escape the attack of antiviral drugs or neutralizing antibodies. In the current studies, optic nerve inflammation induced by DM strain RSA59 consisted of predominantly macrophages/microglia, similar to immune responses in spinal cord (2) and without the marked T-cell infiltration seen in other demyelinating disease models (23Y26). Iba-1 staining was diffuse and was observed in cell bodies as well as cell processes, as has been seen previously with this macrophage/microglia marker (27, 28) . Although Iba-1 cannot distinguish between macrophages and microglia, we suspect most labeled cells represent infiltrating macrophages based on the overall increase in the number of cells observed. Macrophages have been shown to play both proinflammatory and anti-inflammatory roles in optic nerves (29) , and we suggest that the DM strain viral antigen might be moving within axons to avoid being cleared by infiltrating macrophages.