Abatacept for Glomerular Diseases: A New Era of Intelligent immunosuppression?

At ASN Kidney Week there was some interest in abatacept as a targeted therapy for glomerular diseases. T-cell activation requires 2 signals; (i) binding of the T-cell receptor to the antigen-MHC complex on the antigen-presenting cell and (ii) a co-stimulatory signal involving CTLA-4 on the T-cell and B7-1 on the antigen presenting cell. Abatacept is a fusion protein composed of the Fc region of IgG1 fused to the extracellular domain of CTLA-4 which inhibits the T-cell co-stimulatory pathway via B7-1 binding. 

          FSGS

The headlines must go to the small case series of abatacept in FSGS published in NEJM. The rationale for its use was the observation that B7-1 expression is not apparent in normal human podocytes but is found in certain diseased podocytes including a subset of FSGS patients. The series included 4 patients with recurrent FSGS post-transplantation (rituximab-resistant) and one with glucocorticoid-resistant primary FSGS. All patients achieved either partial or complete remission.

In vitro studies demonstrated that α3-Integrin knockout mice constitutively expressed B7-1 in podocytes and abatacept blocked B7-1 mediated podocyte migration in these cells. The molecular mechanism of B7-1-induced podocyte dysfunction was shown to be disruption of activation of the glomerular protein β1-integrin. The authors conclude that B7-1 immunostaining of biopsies may identify a subgroup of patients who would benefit from treatment with abatacept.

2       Lupus Nephritis

The late-breaking session included a randomized controlled trial of Euro-lupus regime cyclophosphamide (i.e. low dose IV) with or without abatacept for proliferative lupus nephritis [Access Trial]. Azathioprine was introduced at 3 months and stopped at 6 months in the abatacept group if they had achieved a remission. Overall, there was no difference in remission rate between the groups. Despite the neutral outcome, 2 points should be taken from the study: (i) The Euro-lupus regime appeared to work in a US cohort of patients where almost 80% were either Hispanic or African American. (ii) Abatacept patients who achieved remission maintained this at 1 year despite coming off immunosuppression at 6 months. However, with the growing confidence in Mycophenolate-based therapy for lupus nephritis and the lack of improved remission with Abatacept in this study, its place in the treatment of proliferative lupus nephritis remains uncertain.

3       Diabetic Nephropathy

An oral presentation on abatacept in Diabetic Nephropathy [FR-OR010] reported increased B7-1 expression in both murine podocytes cultured in high-glucose and on human glomerular podocytes from biopsy specimens. The use of Abatacept in diabetic mice prevented an increase in albuminuria.

Bottom Line: The FSGS case series beautifully illustrates how targeted therapies may be applied to immune-mediated renal diseases. While this case series is very small, it demonstrates the potential for reclassifying disease based on pathogenesis (i.e. B7-1-mediated) rather than crude pathological patterns (focal segmental sclerosis). This is similar to the recent re-classification of MPGN into complement or immune complex-mediated forms. With new targeted therapies like abatacept (and eculizumab for complement mediated glomerulopathies), we may be entering an era of intelligent immunosuppression based on molecular pathogenic signals rather than crude histological patterns.

suPARBaby?

The American Journal of Kidney Diseases published a fascinating letter this month from Jochen Reiser's group concerning the transmission of the suPAR from a mother to her newborn infant. The patient in question had a history of primary FSGS and after her baby was born, the infant had proteinuria which eventually resolved. At the time, this was published as a letter in the NEJM and was used as evidence for the existence of a soluble FSGS "factor" which had yet to be identified.

Now, more than 10 years later, they got their hands on blood from the mother and the infant. The level of suPAR in the mother was 4635pg/ml and in the child was 5225 pg/ml. This compares with a mean of 2884 pg/ml in controls. Unfortunately, the family moved away from the area so there were no follow-up samples. However, the patient's pediatrician reported that the baby's urine was negative for albumin at one year of age.

Although this does not definitively prove that suPAR is the causative agent in primary FSGS, and there is some debate about the true pathogenicity, this, along with recently published data in JASN, adds to the weight of evidence suggesting that this may be the elusive factor we were looking for. Interesting times indeed.

Still mysterious: the elusive circulating factor for FSGS

Important new findings were recently published in relation to proteinuria and FSGS, which are definitely of interest to our community.

First, the punch line:

There is new evidence for a “circulating factor” in recurrent FSGS in a fascinating case of a re-transplanted kidney (here)

BUT

There is growing evidence that suPAR is a non-specific marker of kidney disease and therefore not likely to be the “circulating factor.”(here)

In fact, it appears that it is non-specifically found in CKD, and correlates with a declining GFR.

Now for some details:

The re-transplanted kidney

A letter to the NEJM editor (here) describes an amazing case of resolution of recurrent FSGS after re-transplantation. 

A 27 year old patient with primary FSGS receiving a kidney from his healthy 24 year old sister developed proteinuria in the nephrotic range (up to 25 g/day!) within 2 days of transplantation, and had no improvement after plasmapheresis and standard immunosuppressive treatment. A renal biopsy confirmed foot process effacement, the first hallmark of recurrent podocyte damage heralding recurrent FSGS. Incredibly, with all appropriate consents and institutional approval, the transplant team removed the allograft from Patient 1 and re-transplanted it into another patient who had ESRD due to diabetes. Within 3-4 days, the proteinuria resolved and a repeat biopsy showed resolution of foot process effacement and re-establishment of a normal podocyte architecture. Eight months later, Patient 2 is reported to be doing very well, with good allograft function and no proteinuria.

This case demonstrates in a remarkable way that recurrent FSGS results from an elusive “factor” rapidly produced by the recipient (with primary FSGS), and that the allograft itself can remain fully functional if removed from the influence of this “factor” and placed in another patient.

suPAR is not suPER specific

What may have seemed to be exciting news in 2011, namely the notion that soluble uPAR may be predictive of recurrent FSGS (here), appears to be unfortunately evolving into yet another unsuccessful attempt to identify the ever elusive circulating factor.

Recent work published in Kidney International by Maas et al. (here) confirms that suPAR is not able to distinguish between idiopathic FSGS, secondary FSGS or minimal change disease. 

This is actually not surprising, because a closer look at the clinical data in Wei et al. (here) reveals that the admittedly arbitrary cut-off for separating primary FSGS from all other glomerular disease (3000 pg/ml) did not hold up when tested among their patient cohorts with idiopathic, recurrent versus non-recurrent FSGS (all had suPAR> 3000 pg/ml, thus suPAR could not predict the recurrent from the non-recurrent cases). 

The second figure in the Maas et al. paper may help explain this conundrum: they show a negative correlation between suPAR and eGFR, meaning that as GFR drops, suPAR levels rise, which essentially means that suPAR is simply a marker of CKD.

Future work will no doubt continue to address these issues, but the apparent lack of specificity of suPAR for FSGS casts serious doubt on its proposed role as the circulating factor.

So, the search is still on!!!