In addition to detrimental
inflammation, widespread axon degeneration is an important feature of
multiple sclerosis (MS) pathology and a major correlate for permanent clinical deficits. Thus, treatments that combine immunomodulatory and
neuroprotective effects are beneficial for MS. Using
myelin oligodendrocyte glycoprotein peptide 35-55 (MOG)-induced
experimental autoimmune encephalomyelitis (EAE) as a model of MS, we recently showed that daily treatment with the
phosphodiesterase 5 (
PDE5) inhibitor sildenafil at peak disease rapidly ameliorates clinical symptoms and neuropathology (Pifarre et al., 2011). We have now investigated the immunomodulatory and neuroprotective actions of
sildenafil treatment from the onset of EAE when the immune response prevails and show that early administration of the drug prevents
disease progression. Ultrastructural analysis of spinal cord evidenced that
sildenafil treatment preserves axons and myelin and increases the number of remyelinating axons. Immunostaining of oligodendrocytes at different stages of differentiation showed that
sildenafil protects immature and mature myelinating oligodendrocytes.
Brain-derived neurotrophic factor (
BDNF), a recognized
neuroprotectant in EAE, was up-regulated by
sildenafil in immune and neural cells suggesting its implication in the beneficial effects of the drug.
RNA microarray analysis of spinal cord revealed that
sildenafil up-regulates
YM-1, a marker of the alternative macrophage/microglial M2 phenotype that has neuroprotective and regenerative properties. Immunostaining confirmed up-regulation of
YM-1 while the classical macrophage/microglial activation marker Iba-1 was down-regulated. Microarray analysis also showed a notable up-regulation of several members of the
granzyme B cluster (GrBs). Immunostaining revealed expression of GrBs in Foxp3+-T regulatory cells (Tregs) suggesting a role for these
proteases in
sildenafil-induced suppression of T effector cells (Teffs). In vitro analysis of splenocytes from
sildenafil-treated animals showed down-regulation of Th1/Th2/Th17 responses while Tregs were up-regulated. Additionally,
sildenafil treatment prevented MOG-specific
IgG2b accumulation in serum. Taken together these data demonstrates that daily
sildenafil treatment from the initiation of EAE symptoms prevents further
clinical deterioration by stimulating immunomodulatory and neuroprotective mechanisms. Importantly, we also show here that
sildenafil enhances the ability of human Tregs from healthy donors to down-regulate the proliferation of Teffs in vitro, strongly supporting the potential of
sildenafil for therapeutic intervention in MS.