Pituitary Adenylate Cyclase Activating Peptide Deficient Mice Exhibit Impaired Thymic and Extrathymic Regulatory T Cell Proliferation during EAE

We have shown that mice deficient in pituitary adenylate cyclase-activating polypeptide (PACAP, gene name ADCYAP1) manifest enhanced sensitivity to experimental autoimmune encephalomyelitis (EAE), supporting the anti-inflammatory actions described for this neuropeptide. In addition to an increased proinflammatory cytokine response in these mice, a reduction in regulatory T cell (Treg) abundance in the lymph nodes (LN) was observed, suggesting altered Treg kinetics. In the present study, we compared in PACAP deficient (KO) vs. wild type mice the abundances and rates of proliferation FoxP3(+) Tregs in three sites, the LN, central nervous system (CNS) and thymus and the relative proportions of Th1, Th2, and Th17 effector subsets in the LN and CNS. Flow cytometry analyses revealed a decrease in Treg proliferation and an increased T effector/Tregs ratio in the LN and CNS of PACAP KO mice. In the thymus, the primary site of do novo natural Treg production, the total numbers and proliferative rates of FoxP3(+) Tregs were significantly reduced. Moreover, the expression of IL-7, a cytokine implicated in thymic Treg expansion during EAE, failed to increase at the peak of the disease in the thymus and LN of PACAP KO mice. In addition to these Treg alterations, a specific reduction of Th2 cells (about 4-fold) was observed in the lymph nodes in PACAP KO mice, with no effects on Th1 and Th17 subsets, whereas in the CNS, Th1 and Th17 cells were increased and Th2 decreased. Our results suggest that endogenous production of the neuropeptide PACAP protects against EAE by modulating Treg expansion and Th subsets at multiple sites.