Immunoreactive vasoactive intestinal peptide contributes to the immunosuppressive activity of normal aqueous humor

Suppression of immune-mediated inflammation within the normal anterior chamber (AC) of the eye is in part the result of active suppression of effector T cell activities by immunosuppressive cytokines found in aqueous humor (AqH), the fluid filling the AC. There are immunosuppressive factors found in the low m.w. fraction (< 5 kDa) of AqH, including the neuropeptide alpha-melanocyte-stimulating hormone (alpha-MSH). In seeking other factors, we now report that the neuropeptide vasoactive intestinal peptide (VIP) is also present in normal AqH. VIP immunoreactivity was found in normal rabbit eyes at a concentration of 12 +/- 1 nM. At this intraocular concentration, VIP suppressed Ag-stimulated lymph node cell (LNC) proliferation and IFN-gamma production in vitro. Although suppression of LNC proliferation was not neutralized by absorption of VIP from the low m.w. fraction of AqH, removal of VIP did neutralize suppression of IFN-gamma production by this fraction of AqH. Absorption of both VIP and alpha-MSH from this fraction of AqH permitted production of IFN-gamma by Ag-stimulated LNC that was no different than absorbing VIP alone. The low m.w. fraction of AqH absorbed of either alpha-MSH and VIP lost its ability to suppress local adoptive transfer of delayed-type hypersensitivity. The results suggest that VIP is an important immunosuppressive neuropeptide in AqH. Neuropeptides play an important role in ocular immune privilege and creation of an intraocular immunosuppressive microenvironment.