In 2013, the New England Journal of Medicine published a widely discussed article by Yu et al that observed podocyte B7-1 (CD80) immunostaining in one patient with primary focal segmental glomerulosclerosis, and 4 patients with recurrent focal segmental glomerulosclerosis after kidney transplantation. This staining was interpreted as disease-related B7-1 induction and treated with abatacept, a CTLA4-immunoglobulin fusion protein that targets B7-1 and acts as a co-stimulatory inhibitor. Abatacept is currently approved for the treatment of rheumatoid arthritis, and this article raised hopes of its potential utility in treating proteinuric patients with B7-1 positive glomerular disease.
However, in a recent article published in AJKD, Larson et al report that Yu et al did not include a proper negative control, and that the figures in that paper may have demonstrated false positive staining related to the secondary antibody. In a series of 60 biopsies with focal segmental glomerulosclerosis, not a single case of true B7 staining was observed. This carefully performed study evaluated formalin fixed as well as frozen tissue of each biopsy.
The false positive staining in the original study resulted from a donkey anti-goat antibody reacting with human IgG that is taken up by podocytes in a non-specific manner in any disease associated with heavy proteinuria. These non-specific deposits have long been recognized by renal pathologists as protein reabsorption droplets.
The paper by Yu et al gives only a very cursory description of the immunohistochemical staining procedure. Another problem was the use of a very small cohort of patients to draw a generalized conclusion that led to a lot of investigators spending needless effort trying to reproduce an observation that was not very robust to begin with.
It is important to note that the current publication by Larsen et al does not address the issue of whether abatacept is an appropriate treatment for patients with focal segmental glomerulosclerosis with refractory proteinuria. It simply shows that staining of biopsies for B7-1 cannot be used as an appropriate method to select patients likely to respond to this therapy.
Parmjeet Randhawa, MD