A functional role for spleen Tyrosine Kinase in acute renal disease

Thesis Summary End-stage renal disease (ESRD) remains an important cause of morbidity and mortality worldwide. ESRD can develop when there is inappropriate activation of either the adaptive or innate immune systems, leading to fibrosis and scarring as the final common endpoint. At present therapy for acute antibody-dependent renal injury such as glomerulonephritis is limited to the use of toxic, largely non-selective immunosuppressive drugs or non-immunosuppressive treatments such as anti-hypertensive agents that have limited efficacy. Likewise therapy for ischaemia reperfusion injury (which involves a number of innate or antibody-independent immune mechanisms) is limited to being supportive only. Clearly, novel more specific therapies are required. Spleen tyrosine kinase (Syk) is a non-receptor tyrosine kinase that is best known for its role in B cell receptor and Fc-γ receptor mediated immune signalling in B cells, neutrophils and macrophages. In addition there is increasing evidence that Syk plays a role in innate immunity. Selective Syk inhibitors are being developed to treat a range of autoimmune diseases such as allergic rhinitis, asthma and rheumatoid arthritis. However the role of Syk signalling in both antibody-dependent and antibody-independent acute kidney disease remains largely unknown. To explore the role of Syk in acute renal disease a number of studies were undertaken. Firstly I examined Syk activation in a range of human renal biopsies and identified prominent Syk activation in infiltrating leukocytes (mainly neutrophils and a subset of macrophages) in proliferative glomerulonephritis. In particular, larger numbers of Syk positive leucocytes was associated with those diseases causing rapidly progressive glomerulonephritis. There was a modest correlation with renal dysfunction and systemic inflammation. The second study involved Syk blockade using a small molecule inhibitor in a rat model of nephrotoxic nephritis. Prominent Syk activation was also demonstrated in infiltrating glomerular neutrophils in untreated animals. Syk blockade was protective against glomerular thrombosis, platelet activation, proteinuria and inflammation. The role of Syk in myeloid cells was further explored in a third study where the outcome of myeloid-specific deletion of the Syk gene using the Cre-LoxP system was examined in a mouse model of nephrotoxic nephritis. Prominent Syk activation was again seen in a subset of infiltrating glomerular neutrophils in disease controls. Conditional gene deletion of Syk from myeloid cells was sufficient to provide substantial protection against crescent formation and thrombosis, tubular injury, renal dysfunction, inflammation and fibrosis. Lastly Syk blockade and conditional gene deletion of Syk in myeloid cells was explored in a mouse model of ischaemia reperfusion injury. Both systemic Syk inhibition and conditional gene deletion of Syk in myeloid cells gave significant protection against renal dysfunction, interstitial neutrophil influx, platelet accumulation, macrophage infiltration and inflammation. This identified a specific role for Syk in myeloid cells in this model and are the first demonstration that Syk plays a functional role in acute antibody-independent kidney damage. Taken together, these studies demonstrate the therapeutic potential for Syk inhibitors in human RPGN and as a preventative therapy for IR injury in the transplantation and cardiac surgery setting.