Rheumatic diseases encompass a diverse group of chronic disorders that commonly affect musculoskeletal structures. Osteoarthritis (OA) and rheumatoid arthritis (RA) are the two most common, leading to considerable functional limitations and irreversible disability when patients are unsuccessfully treated. Although the specific causes of many rheumatic conditions remain unknown, it is generally accepted that immune mechanisms and/or uncontrolled inflammatory responses are involved in their etiology and symptomatology. In this regard, the bidirectional communication between neuroendocrine and immune system has been demonstrated to provide a homeostatic network that is involved in several pathological conditions. Adipokines represent a wide variety of bioactive, immune and inflammatory mediators mainly released by adipocytes that act as signal molecules in the neuroendocrine-immune interactions. Adipokines can also be synthesized by synoviocytes, osteoclasts, osteoblasts, chondrocytes and inflammatory cells in the joint microenvironment, showing potent modulatory properties on different effector cells in OA and RA pathogenesis. Effects of adiponectin, leptin, resistin and visfatin on local and systemic inflammation are broadly described. However, more recently, other adipokines, such as progranulin, chemerin, lipocalin-2, vaspin, omentin-1 and nesfatin, have been recognized to display immunomodulatory actions in rheumatic diseases. This review highlights the latest relevant findings on the role of the adipokine network in the pathophysiology of OA and RA.

Objective. Vasoactive intestinal peptide (VIP) hasshown potent antiinflammatory effects in murine arthritis and ex vivo in human rheumatoid arthritis (RA) synovial cells. To investigate the potential endogenous participation of this system in the pathogenesis of RA, we analyzed the expression and regulation of VIP and its functional receptors in human fibroblast-like synoviocytes (FLS) from patients with osteoarthritis (OA) and patients with RA.Methods. The expression of VIP was studied by reverse transcription-polymerase chain reaction (RT-PCR), enzyme immunoassay, and immunofluorescence in cultured FLS, and by immunohistochemical analysis in synovial tissue. The expression and function of the potential VIP receptors in FLS were studied by RT-PCR, determination of intracellular cAMP production, cell membrane adenylate cyclase (AC) activity, and interleukin-6, CCL2, and CXCL8 production in response to VIP or specific agonists and antagonists.Results. VIP expression was detected in human FLS at the messenger RNA and protein levels, and it was significantly decreased in RA FLS compared with OA FLS. VIP receptor type 1 (VPAC 1 ) was the dominant AC-coupled receptor in OA FLS, in contrast with RA FLS, in which VPAC 2 was dominant. Tumor necrosis factor ␣-treated OA FLS reproduced the VIP and VPAC receptor expression pattern of RA FLS. The antagonistic effects of VIP on FLS proinflammatory factor production were reproduced by VPAC 1 -and VPAC 2 -specific agonists in OA FLS and RA FLS, respectively.Conclusion. VIP expression is down-regulated in RA and in tumor necrosis factor ␣-treated FLS, suggesting that down-regulation of this endogenous antiinflammatory factor may contribute to the pathogenesis of RA. In RA FLS, VPAC 2 mediates the antiinflammatory effects of VIP, suggesting that VPAC 2 agonists may be an alternative to VIP as antiinflammatory agents.The vasoactive intestinal peptide (VIP)/pituitary adenylate cyclase-activating peptide (PACAP) system consists of 2 peptides and 3 receptors: VIP receptor type 1 (VPAC 1 ), VPAC 2 , and PACAP type 1 (PAC 1 ) receptor. These receptors belong to family 2 of the G proteincoupled receptors, which in recent years have shown remarkable antiinflammatory and immunomodulatory properties (1-3). VPAC 1 is constitutively expressed in macrophages and lymphocytes and binds VIP and PACAP with equal affinity (1,3). VPAC 2 has also been described in lymphocytes and macrophages as an inducible receptor after T cell receptor triggering or lipopolysaccharide (LPS) stimulation (1,3). The bestcharacterized effects of VIP/PACAP on immune cells are mediated by the adenylate cyclase (AC) pathway coupled to these receptors. Finally, the PAC 1 receptor is the PACAP-specific receptor, although at high concentrations VIP is a heterologous ligand (4). Endogenous VIP and PACAP can be produced by both neural and immune cells, but their regulation and participation in the pathogenesis of human inflammation are not known.

Conclusion. The results of this study revealed a prominent function of FLS in the recognition of both dsRNA and ssRNA, which may be present in the joint microenvironment. This study also advances the healing function of the endogenous neuroimmune peptide VIP, which inhibited TLR-3-, RIG-I-, MDA5-, and TLR-7-mediated stimulation of antiviral, proinflammatory, and joint destruction mediators.

Genetic background, epigenetic modifications, and environmental factors trigger autoimmune response in rheumatoid arthritis (RA). Several pathogenic infections have been related to the onset of RA and may cause an inadequate immunological tolerance towards critical self-antigens leading to chronic joint inflammation and an imbalance between different T helper (Th) subsets. Vasoactive intestinal peptide (VIP) is a mediator that modulates all the stages comprised between the arrival of pathogens and Th cell differentiation in RA through its known anti-inflammatory and immunomodulatory actions. This “neuroimmunopeptide” modulates the pathogenic activity of diverse cell subpopulations involved in RA as lymphocytes, fibroblast-like synoviocytes (FLS), or macrophages. In addition, VIP decreases the expression of pattern recognition receptor (PRR) such as toll-like receptors (TLRs) in FLS from RA patients. These receptors act as sensors of pathogen-associated molecular pattern (PAMP) and damage-associated molecular pattern (DAMP) connecting the innate and adaptive immune system. Moreover, VIP modulates the imbalance between Th subsets in RA, decreasing pathogenic Th1 and Th17 subsets and favoring Th2 or Treg profile during the differentiation/polarization of naïve or memory Th cells. Finally, VIP regulates the plasticity between theses subsets. In this review, we provide an overview of VIP effects on the aforementioned features of RA pathology.

Therapies based on the pharmacological disruption signalling of IL-22 could be beneficial for the treatment of rheumatic diseases. The restricted expression of IL-22R1 to non-lymphoid cells could lead to a reduction of side effects mediated by immune responses.