Inhibition of CD26 activity results in reduced T cell activity [9]. Interestingly, CD26 can increase T cell activation by ERK inhibitor increasing the co-stimulator CD86 on antigen-presenting cells in a process that requires enzymatic activity [10]. CD26 associates with other membrane proteins on T cells, including the tyrosine phosphatase CD45 and the ectoenzyme adenosine deaminase (ADA), which might be important

for the co-stimulatory activity of CD26 [8, 11]. However, inhibition of DPP-4 enzymatic activity may not block all these immune activities; the ability of soluble CD26 to bind ADA and enhancement of T cell proliferation can usually occur even when the active site of DPP-4 has been mutated [12, 13]. CD26 is also expressed on myeloid cells, and enzymatic inhibition decreased macrophage activation and migration into

adipose tissue [14]. In addition to GLP-1, DPP-4 also cleaves immune peptides, including neuropeptide Y (NPY) and chemokines such as interferon gamma-induced protein (IP)-10, stromal cell-derived factor (SDF)1-alpha and regulated upon activation normal T cell expressed and secreted (RANTES) [15]. DPP-4 cleavage can affect chemokine activity or receptor specificity; therefore, RXDX-106 purchase inhibition of DPP-4 could alter leucocyte chemotaxis [16]. In humanized mice, human haematopoetic stem cells show enhanced engraftment with DPP-4 inhibition, which may be due to altered migration of these cells [17]. Clinical trials are now under way

to test if sitagliptin can improve cord blood transplant outcomes (NCT00862719). In mouse models of T cell-mediated autoimmunity, inhibitors of DPP-4 can reduce disease severity and are associated with increases in transforming growth factor (TGF)-β levels and improvements in immune tolerance induction [18, 19]. Interestingly, in human autoimmune diseases such as multiple sclerosis and rheumatoid arthritis, increased Thiamet G CD26 levels on leucocytes are observed, yet there is decreased DPP-4-associated peptidase activity [20-22]. The reason for the discrepancy between activity and membrane CD26 levels is unclear, but this could be due to decreased shedding of CD26 from the membrane or decreased levels of other peptidases that cleave the same substrate. Despite evidence that sitagliptin might alter immune activity, few direct measurements of immune function after sitagliptin treatment in humans have been undertaken [23]. Therefore, we set up a double-blind clinical protocol in which healthy individuals were given either sitagliptin or placebo daily for 4 weeks. We chose to enrol healthy volunteers to separate effects of sitagliptin from disease effects on immune readouts (e.g. in type 2 diabetes).