Scientific Abstract

Hypertrophy of mesenteric adipose tissue is an early and defining feature of Crohn’s disease. It therefore seems possible that changes in adipose tissue may be central to development of disease activity. A major consequence of changed adipose tissue is an increase in the adipokine leptin within mesentery. Leptin is best known for its capacity to regulate food intake but more recently other wider activities of leptin have been described; amongst these activities is the capacity to regulate immunity. Dendritic antigen presenting cells (DC) are the primary antigen presenting cells implicated not only in the development of responsiveness to organisms invading via the intestine but also to tolerance of beneficial commensal bacteria. Inflammatory bowel disease is characterised by an abnormal response to changed gut microbiota and evidence from animal models and human studies indicates involvement of DC in initiation and persistence of intestinal inflammation. DC are known to express leptin receptors and are susceptible to the modulating effects of leptin. The purpose of this programme is to establish whether leptin activation of DC is a key contributor to the development of gut inflammation in Crohn’s disease and to identify possible routes via which effects of leptin are produced.

 

We will identify the expression of leptin receptors on DC and assess the potential of these cells to respond to leptin. Leptin activation is via specific receptors which include a long form which signals via the JAK/STAT pathway and is believed to be responsible for immune activation. In addition there are shorter forms of receptor, and also the leptin gene related protein which can regulate receptor expression. Expression of these receptors will be compared in DC from Crohn’s patients and controls. Expression will be measured on DC from the gut mucosa using biopsies from well characterised patients or blood DC which we have recently discovered express the gut homing marker beta7 indicating migration to the gut. 

 

The second aim will be to compare phenotypic and functional effects of leptin stimulation on DC from patients and controls. The effects studied will be those known to be affected by leptin in normal DC; thus leptin produces maturation of DC including upregulation of CD40, changes in migratory capacity including upregulation of CCR7 a marker associated with homing to lymph node, and up regulation of Toll like receptors. We will look for involvement and stimulation of these factors in Crohn’s disease. We will also study functional effects of exposure to leptin which will include changes in the expression of leptin receptor itself, production of pro and anti inflammatory cytokines and stimulation of T cells. Since leptin is known to influence utilisation of energy resources its effect on uptake of different fatty acids into DC will be studied by lipidomic analysis.

 

Our studies have already shown that different bacterial strains can change the function of DC so that LPS can promote IL12 production and reduce IL10 production in normal gut DC. Conversely a mixture of bifidobacterial strains does the reverse, reducing IL12 and promoting IL10 production. Such effects on human DC are seen not only on exposure to bacterial products in vitro but also in colonic DC following exposure to probiotic therapy in patients with ulcerative colitis. If leptin is a central plank of the proinflammatory profiling in DC then changes in this system would be expected on exposure to these bacterial products. Preliminary studies suggest that probiotic bacteria can reduce the relative expression of the long form of leptin receptor which would effectively reduce activation of DC. These potential interactions will be studied in detail.

 

In conclusion, this programme will identify whether effects of leptin through its leptin receptor is involved in the pro-inflammatory activity in Crohn’s disease and identify routes by which the inflammatory effects of leptin are produced in this disease. Such studies will raise the possibility of clinical interventions via direct modulation of leptin or its interaction with leptin receptors, an increasing possibility now that leptin receptor antagonists are being produced. Alternatively we may identify routes by which leptin acts such as CCR7 or TLR signalling which may provide alternative methods of counteracting pro-inflammatory effects in Crohn’s disease.