Tubulointerstitial nephritis antigen (TIN antigen) is a basement membrane component which is recognized by human autoantibodies in TIN and has been shown to induce TIN in Brown Norway (BN) rats. Detectable by immunofluorescent microscopy, TIN antigen reacts with monoclonal, polyclonal, and human autoantibodies in basement membranes of kidney cortex, small intestine, skin and cornea. Specific sites of TIN antigen within kidney cortex include basement membranes of proximal tubules, distal tubules, Bowman's capsule and peritubular capillaries, with highest concentration in proximal tubular basement membrane (TBM). TIN antigen is also present in interstitium between tubules and in the periarterial sheath, but not in glomerular basement membrane or mesangial matrix. Immunoblotting of TIN antigen isolated from rabbit TBM reveals a major 58 kDa component with minor components of 300 kDa, 175 kDa, 160 kDa, 100 kDa and 50 kDa. Partial protein sequence analysis indicates that 58 kDa TIN antigen represents a newly defined glycoprotein. The structural relationships between various molecular weight forms are currently being investigated. High molecular weight (HMW) forms of TIN antigen, consisting of a mixture of 300 kDa, 175 kDa and 160 kDa forms, are more efficient than low molecular weight (LMW) forms (58 kDa and 50 kDa forms) in inducing TIN in BN rats. The resultant antibody specificity of rats injected with either HMW TIN antigen or LMW TIN antigen is identical as determined by immunofluorescent microscopy and Western analysis. Higher antibody titers and greater amounts of kidney-bound IgG are found in the HMW TIN antigen-immunized animals. TIN antigen is the primary target of anti-TBM antibodies in human and experimental immunologically-mediated anti-TBM nephritis.
