The primary goal of this exploratory proposal is to develop a tool to determine if circulating autoantibodies directed at the M-type phospholipase receptor (PLA2R) from patients with idiopathic membranous nephropathy (MN) are pathogenic. We have established that about 75% of patients with idiopathic MN have circulating anti-PLA2R autoantibodies whereas none of the disease or normal controls has tested positive, thus indicating a high degree of specificity. The anti-PLA2R antibodies are IgG4, the subclass that predominates in idiopathic but not secondary MN, the antigen PLA2R is expressed on podocytes and colocalizes with IgG4 in the glomerular immune deposits of patients with MN, and the antibodies can be eluted from the glomerular immune deposits of MN renal biopsies. Moreover, the presence of circulating anti-PLA2R antibodies correlates with disease activity in MN. Although these findings strongly suggest that anti-PLA2R antibodies are responsible for, or at least contribute to disease development, proof of pathogenicity has yet to be established. Whereas simple transfer of the disease to laboratory animals with human anti-PLA2R would readily answer this question, human anti-PLA2R does not recognize PLA2R in the glomeruli of any small laboratory animal tested to date. Therefore, we plan to take advantage of both the lack of immunoreactive PLA2R in mouse glomeruli and the refinement of podocyte- specific transgenic techniques to develop a mouse model in which the human PLA2R is expressed in podocytes. To that end we propose the following specific aims:
Specific Aim 1 : Produce a transgenic mouse utilizing the NPHS1 (nephrin) promoter to drive expression of human PLA2R exclusively in mouse podocytes.
Specific Aim 2 : Verify that human PLA2R is expressed on mouse podocytes in vivo.
Specific Aim 3 : a) Determine if injected human anti-PLA2R autoantibodies bind to human PLA2R and form immune deposits typical of MN in the transgenic mouse glomeruli. b) Establish if injected human anti-PLA2R antibodies are able to cause proteinuria in the human PLA2R transgenic mouse.
Specific Aim 4 : Determine if the human PLA2R transgenic mice develop anti-PLA2R antibodies and MN when actively immunized with human PLA2R. Successful development of this model will pave the way for studies to both establish the pathogenic role of human anti-PLA2R autoantibodies and define the mechanisms of injury.
Having demonstrated that the M-type phospholipase receptor (PLA2R) is a specific target antigen of circulating autoantibodies in a high proportion of patients with idiopathic membranous nephropathy, we propose to develop a mouse model to determine if the antibodies are responsible for the clinical (proteinuria) and pathological manifestations of the disease. To that end, we will produce transgenic mice in which the human PLA2R is expressed on mouse podocytes, the location of the antigen in human kidneys.