CD26 or dipeptidyl peptidase IV (DP IV) is expressed on various cell types, including T cells. Although T cells can receive activating signals via CD26, the physiological role of CD26/DP IV is largely unknown. We used the reversible DP IV inhibitor Lys[Z(NO2)]-pyrrolidide (I40) to dissect the role of DP IV in experimental autoimmune encephalomyelitis (EAE) and to explore the therapeutic potential of DP IV inhibition for autoimmunity. I40 administration in vivo decreased and delayed clinical and neuropathological signs of adoptive transfer EAE. I40 blocked DP IV activity in vivo and increased the secretion of the immunosuppressive cytokine TGF-β1 in spinal cord tissue and plasma during acute EAE. In vitro, while suppressing autoreactive T cell proliferation and TNF-α production, I40 consistently up-regulated TGF-β1 secretion. A neutralizing anti-TGF-β1 Ab blocked the inhibitory effect of I40 on T cell proliferation to myelin Ag. DP IV inhibition in vivo was not generally immunosuppressive, neither eliminating encephalitogenic T cells nor inhibiting T cell priming. These data suggest that DP IV inhibition represents a novel and specific therapeutic approach protecting from autoimmune disease by a mechanism that includes an active TGF-β1-mediated antiinflammatory effect at the site of pathology.

Various studies have shown that the membrane ectoenzyme dipeptidyl peptidase IV (DPIV; CD26), expressed on T, natural killer (NK) and B cells in the immune system, is involved in the regulation of DNA synthesis and cytokine production. We show that the specific DP IV inhibitors Lys[ Z(NO2)]‐thiazolidide, Lys[Z(NO2)]‐piperidide, and Lys[Z(NO2)]‐pyrrolidide inhibit DNA synthesis as well as production of interleukin‐2 (IL‐2), IL‐10, IL‐12, and interferon‐γ (IFN‐γ) of pokeweed mitogen (PWM)‐stimulated purified T cells. Most importantly, these inhibitors induce a three‐ to fourfold increased secretion of latent transforming growth factor‐β1 (TGF‐β1) by PWM‐stimulated peripheral blood mononuclear cells (PBMC) and T cells, as measured with a specific TGF‐β1 enzyme‐linked immunosorbent assay and in the Mv1Lu bioassay. As we could demonstrate previously, TGF‐β1 exhibits the same inhibitory effects as DP IV inhibitors on DNA synthesis and cytokine production (Cytokine 1994, 6, 382–8; J Interferon Cytokine Res 1995, 15, 685–90). A neutralizing chicken anti‐TGF‐β1 antibody was capable of abolishing the DP IV inhibitor‐induced suppression of DNA synthesis of PWM‐stimulated PBMC and T cells. These data suggest that TGF‐β1 might have key functions in the molecular action of DP IV/CD26 in regulation of DNA synthesis and cytokine production.

The T cell marker CD26/dipeptidyl peptidase (DP) IV is associated with an effector phenotype and markedly elevated in the human CNS disorder multiple sclerosis. However, little is known about the in vivo role of CD26/DP IV in health and disease, and the underlying mechanism of its function in CNS inflammation. To directly address the role of CD26/DP IV in vivo, we examined Th1 immune responses and susceptibility to experimental autoimmune encephalomyelitis in CD26−/− mice. We show that gene deletion of CD26 in mice leads to deregulation of Th1 immune responses. Although production of IFN-γ and TNF-α by pathogenic T cells in response to myelin Ag was enhanced in CD26−/− mice, production of the immunosuppressive cytokine TGF-β1 was diminished in vivo and in vitro. In contrast to the reduction in TGF-β1 production, responsiveness to external TGF-β1 was normal in T cells from CD26−/− mice, excluding alterations in TGF-β1 sensitivity as a mechanism causing the loss of immune regulation. Natural ligands of CD26/DP IV induced TGF-β1 production in T cells from wild-type mice. However, natural ligands of CD26/DP IV failed to elicit TGF-β1 production in T cells from CD26−/− mice. The striking functional deregulation of Th1 immunity was also seen in vivo. Thus, clinical experimental autoimmune encephalomyelitis scores were significantly increased in CD26−/− mice immunized with peptide from myelin oligodendrocyte glycoprotein. These results identify CD26/DP IV as a nonredundant inhibitory receptor controlling T cell activation and Th1-mediated autoimmunity, and may have important therapeutic implications for the treatment of autoimmune CNS disease.

Acne is a chronic disease hallmarked by sebaceous hyperplasia, follicular hyperkeratosis, and inflammation. Parallel targeting of these factors is required to treat acne effectively. Inhibitors of dipeptidyl peptidase IV (DP IV) and aminopeptidase N (APN) show strong anti-inflammatory effects on immune cells and therapeutic efficacy in autoimmune disorders. Our investigation focused on the expression and functional relevance of these ectopeptidases in three cell types which exhibit an altered phenotype in early acne lesions. We showed for the first time expression of DP IV and APN on human sebocytes. In the SZ95 sebocyte cell line, the DP IV inhibitors Lys[Z(NO2)]-thiazolidide and Lys[Z(NO2)]-pyrrolidide and the APN inhibitors actinonin and bestatin suppressed proliferation, enhanced terminal differentiation, and slightly decreased total neutral lipid production. The anti-inflammatory and differentiation-restoring cytokine IL-1 receptor antagonist was significantly upregulated in SZ95 sebocytes and the HaCaT keratinocyte cell line in the presence of inhibitors. Furthermore, the inhibitors suppressed proliferation and IL-2 production of Propionibacterium acnes-stimulated T cells ex vivo and enhanced the expression of the immunosuppressive cytokine transforming growth factor-beta1. Our data provide first evidence for a functional role of DP IV and APN in the sebaceous gland apparatus and for their inhibitors, used alone or in combination, as completely new substances possibly affecting acne pathogenesis in a therapeutic manner.

Dipeptidyl peptidase IV (DP IV, CD26) plays an essential role in the activation and proliferation of lymphocytes, which is shown by the immunosuppressive effects of synthetic DP IV inhibitors. Similarly, both human immunodeficiency virus‐1 (HIV‐1) Tat protein and the N‐terminal peptide Tat(1–9) inhibit DP IV activity and T cell proliferation. Therefore, the N‐terminal amino acid sequence of HIV‐1 Tat is important for the inhibition of DP IV. Recently, we characterized the thromboxane A2 receptor peptide TXA2‐R(1–9), bearing the N‐terminal MWP sequence motif, as a potent DP IV inhibitor possibly playing a functional role during antigen presentation by inhibiting T cell‐expressed DP IV [Wrenger, S., Faust, J., Mrestani‐Klaus, C., Fengler, A., Stöckel‐Maschek, A., Lorey, S., Kähne, T., Brandt, W., Neubert, K., Ansorge, S. & Reinhold, D. (2000) J. Biol. Chem.275, 22180–22186]. Here, we demonstrate that amino acid substitutions at different positions of Tat(1–9) can result in a change of the inhibition type. Certain Tat(1–9)‐related peptides are found to be competitive, and others linear mixed‐type or parabolic mixed‐type inhibitors indicating different inhibitor binding sites on DP IV, at the active site and out of the active site. The parabolic mixed‐type mechanism, attributed to both non‐mutually exclusive inhibitor binding sites of the enzyme, is described in detail. From the kinetic investigations and molecular modeling experiments, possible interactions of the oligopeptides with specified amino acids of DP IV are suggested. These findings give new insights for the development of more potent and specific peptide‐based DP IV inhibitors. Such inhibitors could be useful for the treatment of autoimmune and inflammatory diseases.