The increased TREC levels in the intestinal mucosa could, theoretically, represent T lymphocytes that have matured in situ in the intestinal mucosa, as the intestinal mucosa

can act as a site for extrathymic maturation of both IEL and LPL T lymphocytes in human infants [17], and developing T cells that are rearranging their TCR genes are found in the small intestine in human adults [18]. In addition, immunocompromized mice, i.e. major histocompatibility complex (MHC) class I-deficient and TCR-αβ-deficient mice, of which the latter spontaneously develop colitis [5,29], also have evidence of extrathymic maturation. Thus, it is possible that T cell progenitors in the bone marrow receive signals from the inflamed intestine to go directly to the intestinal mucosa for further maturation. However, we employed flow Akt inhibitor cytometric analysis using previously established phenotypic characteristics LY2606368 clinical trial of T cell progenitors in the gut, identified as CD19-CD16-CD3-CD2+CD5+CD7+ lymphocytes [17,18][30], and found no differences in frequencies of this

population between IBD patients and non-inflamed controls. As only the LPL population was investigated, due to limited amounts of IEL, it could be argued that extrathymic maturation could be increased, specifically in the IEL compartment. However, as quantitative RT–PCR analysis of pre-TCR-α and RAG1 mRNA expression [18,30,31] was performed in mucosal biopsies containing both IEL and LPL, and revealed no increased expression in IBD patients compared to controls, this is highly unlikely. Corroborating our findings of significantly increased frequencies of mucosal T cells expressing

CD62L/L-selectin in UC but not CD patients is a report that HEV-like vessels expressing PNAd, one of the ligands for CD62L, were induced preferentially in active UC [32]. In addition, serum concentrations of soluble L-selectin have been shown to Elongation factor 2 kinase be correlated positively to disease activity in UC but not CD [33]. In mice, CD62L+ expressing CD4+ T cells [34], as well as CD4+CD45RBhi[1,2,35], can induce colitis upon transfer into immunodeficient recipient mice. However, in humans CD62L is expressed by both CD45RA+ and CD45RA- T lymphocytes, of which naive T cells express both, while the CD62L+CD45RA- T lymphocytes have been shown previously to be central memory T cells [36]. Although we did not analyse this population for expression of the chemokine receptor CCR7, this suggests that the increased frequency of CD4+CD62L+CD45RA- lymphocytes found in the intestinal mucosa of UC patients represents CD62L+CD45RA-CCR7+ central memory T lymphocytes, found predominantly in lymphoid tissue [37]. Although the present study investigated a limited number of patients, we demonstrate that UC patients, and not CD patients, display an increased recruitment of RTE to the colonic mucosa, possibly before acquiring immunoregulatory properties in the periphery.