repair. For example, in the same report, using cultured neurons, IFNγ-polarized M1-conditioned media was neurotoxic and prevented axon elongation after injury, while IL-4 treated, M2-conditioned media encouraged axon growth [63]. The beneficial functions of IL-4 appear to extend to endogenous IL-4, since IL-4−/−mice showed increased damage following spinal cord injury [66]. Other groups have observed similar polarization dynamics after spinal cord injury [67,68]. The positive effects of M2 cells on spinal cord injury can also be extended to the enzymatic products of Arg1 activity. For example, Cai et al. demonstrated that polyamines were sufficient to block the suppressive effects of myelin and myelin-associated glycoprotein on dorsal root ganglion neuron regeneration [69].
