Mast cells are immune cells that produce and secrete a variety of mediators and cytokines that influence various inflammatory and immune processes. Leptin is a cytokine regulating metabolic, endocrine as well as immune functions via the leptin receptor which is expressed by many immune cells. However, there are no data about leptin receptor expression in mast cells. Immunohistochemical and immunofluorescent double stainings showed the expression of leptin and leptin receptors in mast cells in human skin and several parts of the respiratory, gastrointestinal and urogenital tract. Leptin was expressed in mast cells expressing the classification marker chymase, whereas a variable expression was observed in tryptase positive mast cells. For leptin receptors, the expression pattern was tissue dependent and not related to tryptase or chymase expression. Our results demonstrate the expression of leptin and leptin receptors on mast cells, suggesting paracrine and/or autocrine immunomodulatory effects of leptin on mast cells.

The engagement of inhibitory receptors specific for major histocompatibility complex class I (MHC-I) molecules educates natural killer (NK) cells, meaning the improvement of the response of activation receptors to subsequent stimulation. It is not known whether inhibitory MHC-I receptors educate only NK cells or whether they improve the responsiveness of all cell types, which express them. To address this issue, we analyzed the expression of inhibitory MHC-I receptors on intestinal intraepithelial lymphocytes (iIELs) and show that T-cell receptor (TCR)-CD8 iIELs express multiple inhibitory receptors specific for MHC-I molecules, including CD94/NKG2A, Ly49A, and Ly49G2. However, the presence of MHC-I ligand for these receptors did not improve the response of iIELs to activation via the TCR. The absence of iIEL education by MHC-I receptors was not related to a lack of inhibitory function of these receptors in iIELs and a failure of these receptors to couple to the TCR. Thus, unlike NK cells, iIELs do not undergo an MHC-I-guided education process. These data suggest that education is an NK cell-specific function of inhibitory MHC-I receptors. (Blood. 2011; 118(2):339-347)

Natural killer (NK) cells are part of the first line defense against tumors, parasites and virus-infected cells. Therefore, factors that control NK-cell numbers and their function are important. CD27 is constitutively expressed on NK cells and its expression correlates with sequential phases in NK-cell development, discriminating phenotypically and functionally different subsets within the NK-cell population. Although CD27 has been described to have an important regulatory role in effector and memory T and B lymphocytes, its role in NK-cell biology remains to be addressed. In this study, we used CD27 À/À mice to investigate the role of CD27 in NK-cell development and function, both during the resting state and upon stimulation. The results show that NK-cell numbers are not impaired in CD27 À/À mice. Moreover, CD27 À/À NK cells reach full phenotypic maturity, evidenced by normal expression of CD49b, CD43 and CD11b. Expression of activating receptors is unaltered, whereas expression of several inhibitory receptors is increased. Cytotoxicity and interferon-c production by NK cells from CD27 À/À mice in the resting state are normal. However, upon in vivo anti-CD40-or poly-I:C-mediated activation, or in vitro interleukin-15 priming plus anti-NKp46 stimulation, the absence of CD27 results in decreased cytolytic activity and cytokine production by spleen and liver NK cells. In conclusion, this study demonstrates that CD27 is dispensable for the development of functional NK cells. However, upon stimulation of NK cells, CD27 displays an important role in their activation and functionality.

NK cells are important mediators of the early defense. In mice, immature and mature NK (mNK) cells constitutively express the TNF receptor family member CD27; however, mNK cells eventually lose CD27 expression and become resting NK cells. Interaction of CD27 with its ligand, CD70, enhances proliferation and effector functions of NK cells. We used mice that constitutively express CD70 on B cells (CD70-Tg) to study the in vivo effects of continuous triggering of CD27 on NK cells. Continuous CD70-CD27 interaction resulted in strongly down-modulated CD27 expression on NK cells and gradually reduced absolute NK cell numbers. This reduction was most prominent in the mNK cell subpopulation and was at least partially due to increased apoptosis. Residual NK cells showed lower expression of activating Ly49 receptors and normal (liver) or decreased (spleen) IFN-c production. Nevertheless, NK cells from CD70-Tg mice displayed higher YAC-1 killing capacities. CD70-Tg NK cells exhibited up-regulated expression of NKG2D, which is in accordance with the increased YAC-1 lysis, as this is mainly NKG2D-dependent. Taken together, this study is the first to demonstrate that continuous CD70 triggering of CD27 on NK cells in vivo results in a severe reduction of NK cells. On a single cell basis, however, residual NK cells display enhanced cytotoxicity.Key words: CD70-Tg mice . Cytotoxicity . Differentiation Supporting Information available online Introduction NK cells are large granular lymphocytes of the innate immune system that play a crucial role in the early host defense [1,2]. Upon activation, they directly eliminate target cells through exocytosis of perforin-and granzyme-containing granules, or by Fas ligand (CD178) or TRAIL pathways [3][4][5][6][7]. NK cells also produce cytokines and chemokines, which enable them to recruit non-specific haematopoetic cells, activate dendritic cells and prime adaptive lymphocytes [8][9][10][11]. As such, NK cells bridge between innate and adaptive immunity. The functional behaviour of NK cells is regulated by the engagement of a broad array of activating and inhibitory cell membrane receptors (reviewed in Lanier [12]).The BM is considered to be the main site for NK cell development [13][14][15][16]. Here, multipotent haematopoietic precursors generate NK cell precursors (NKP). Murine NKP are lineage(lin) À CD122 1 NK1.1 À CD49b À . NKP differentiate into immature NK (iNK) cells, which exhibit a lin À CD122 1 NK1. [20,21]. Interestingly, Hayakawa and Smyth [22]showed that within the TCR b À NK1.1 1 gated NK cell pool there is a CD11b low subpopulation, including both iNK and early mNK cells, which is homogenously CD27 high (referred to as subset 1), whereas the CD11b high population of late mNK cells consists of two functionally distinct subsets: i.e. CD27 high (referred to as subset 2) and CD27 low (referred to as subset 3). NK cells from subset 1 are the first NK cell population detected after BM transplantation and they give rise to subset 2 after adoptive transfer. Subset 2 consists of fu...