HIgM12 does not promote spinal cord remyelination but preserves spinal cord axons. (A) The same mice used to collect MR spectra longitudinally were sacrificed at 10 weeks post-treatment. Spinal cords were removed and processed for morphology analysis. Mice from all three treatment groups have similar levels of spinal cord inflammation, demyelination, and remyelination pathology. (B) When the total number of mid-thoracic level axons was compared across treatment groups, HIgM12-treated mice with improved NAA concentrations also contained more axons than the control IgM- and PBS-treated groups (P = 0.03 and P = 0.018 respectively, one-way ANOVA). (C) When axons of different calibers were analyzed, HIgM12-treated mice had more small-caliber (1 to 4 μm2, P = 0.039, one-way ANOVA) and medium-caliber (4 to 10 μm2, P = 0.037) axons than the PBS-treated mice. HIgM12-treated mice had more medium-caliber (4 to 10 μm2, P = 0.031) and large-caliber (>10 μm2, P = 0.028) axons than the control IgM-treated mice. Pathology analysis was performed blinded to the experimental groups.