Vasoactive intestinal peptide (VIP) induces regulatory dendritic cells (DC) in vitro that inhibit cellular immune responses. We tested the role of physiological levels of VIP on immune responses to murine cytomegalovirus (mCMV) using VIP-knockout (VIP-KO) mice and radiation chimeras engrafted with syngenic VIP-KO hematopoietic cells. VIP-KO mice and had less weight loss and better survival following mCMV infection compared with wild-type littermates (WT). MCMV-infected VIP-KO mice had lower viral loads, faster clearance of virus, with increased numbers of IFN-γ+ NK and NKT cells, and enhanced cytolytic activity of NK cells. Adaptive anti-viral cellular immunity was increased in mCMV-infected VIP-KO mice compared with WT mice, with more Th1/Tc1 polarized T-cells, fewer IL-10+ T-cells, and more mCMV-M45 epitope peptide-MHC class I-tetramer+ CD8+ T-cells (tetramer+ CD8 T-cells). MCMV-immune VIP-KO mice had enhanced ability to clear mCMV-peptide pulsed target cells in vivo. Enhanced anti-viral immunity was also seen in WT transplant recipients engrafted with VIP-KO hematopoietic cells, indicating that VIP synthesized by neuronal cells did not suppress immune responses. Following mCMV infection there was a marked up-regulation of MHC class II (MHC-II) and CD80 co-stimulatory molecule expression on DC from VIP-KO mice compared with DC from WT mice, while PD-1 and PD-L1 expression were up-regulated in activated CD8+ T-cells and DC, respectively, in WT mice but not in VIP-KO mice. Since the absence of VIP in immune cells increased innate and adaptive anti-viral immunity by altering co-stimulatory and co-inhibitory pathways, selective targeting of VIP-signaling represents an attractive therapeutic target to enhance anti-viral immunity.

Vasoactive intestinal peptide (VIP) is a neuropeptide hormone that suppresses Th1-mediated cellular immunity. We previously reported that VIP-knockout (VIP-KO) mice have enhanced cellular immune responses and increased survival following murine cytomegalovirus (mCMV) infection in C57BL/6 mice. In this study, we tested whether treatment with a VIP receptor antagonistic peptide protects C57BL/6 and BALB/c mice from mCMV-infection. One week of daily subcutaneous injections of VIPhyb was non-toxic and did not alter frequencies of immune cell subsets in non-infected mice. VIPhyb administration to mCMV-infected C57BL/6 and BALB/c mice markedly enhanced survival, viral clearance, and reduced liver and lung pathology compared with saline-treated controls. The numbers of effector/memory CD8+ T-cells and mature NK cells were increased in VIPhyb-treated mice compared with PBS-treated groups. Pharmacological blockade of VIP-receptor binding or genetic blockade of VIP-signaling prevented the up-regulation of PD-L1 and PD-1 expression on DC and activated CD8+ T-cells, respectively, in mCMV-infected mice, and enhanced CD80, CD86, and MHC-II expression on conventional and plasmacytoid DC. VIPhyb-treatment increased type-I IFN synthesis, numbers of IFN-γ- and TNF-α-expressing NK cells and T-cells, and the numbers of mCMV-M45 epitope-peptide-MHC-I tetramer CD8+ T-cells following mCMV infection. VIP-treatment lowered the percentage of Treg cells in spleens compared with PBS-treated WT mice following mCMV infection, while significantly decreasing levels of serum VEGF induced by mCMV-infection. The mice in all treated groups exhibited similar levels of anti-mCMV antibody titers. Short-term administration of a VIP-receptor antagonist represents a novel approach to enhance innate and adaptive cellular immunity in a murine model of CMV infection.