Blog entry by Arif Khwaja
High impact journals tend to focus on clinical trial data that are likely to impact on practice. So it was nice to see a very simple but elegant piece of ‘bench to bedside’ work published in this weeks’ NEJM from Canaud and colleagues from INSERM, Paris looking at the role of mammalian target of rapamycin (mTOR)signalling in the pathogenesis of proliferative vascular lesions and fibrosis in patients with in patients with Anti-Phospholipid Syndrome (APS). APS is characterised not only recurrent thrombosis but also by proliferation of vascular smooth muscle cells and fibrosis of the vascular intima and media. i.e. vasculopathy exists in APS even in the absence of significant thrombosis.
mTOR is a serine/threonine protein kinase that belongs to the PI3-Kinase family that integrates a variety of intracellular and extracellular signalling pathways that control cell metabolism, growth, proliferation and survival. mTOR is the catalytic subunit of 2 distinct complexes mTORC1 and mTORC2 that localise to distinct subcellular compartments and have differing biological function. Inhibitors of mTOR such as sirolimus or everolimus are widely used in kidney transplantation and there is increasing interest in their use in polycystic kidney disease and general organ fibrosis. Of course their anti-proliferative, anti-fibrotic effects in kidney transplantation manifest clinically as impaired wound healing but potentially beneficial effects on tumours such as skin cancer. Similarly sirolimus eluting coronary stents have been shown to inhibit recurrent vascular stenosis and therefore the group hypothesized that mTORC pathway may be involved in the lesions in APS.
They looked at 35 patients with APS (25 of whom had lupus) who had undergone kidney transplantation and compared with 74 case-controls. Immunohistochemistry and immunofluorescence for phosphorylated S6 ribosomal protein (S6RP – a marker of mTORC1 activity) and phosphorylated Akt (a marker of mTORC2 activity) provided an assay of mTORC activity in kidney biopsies. Colocalisation experiments were also performed using markers of endothelial cells and vascular smooth muscle cells.
The key findings were:
i) significantly increased activation of both mTORCs in endothelial cells and vascular smooth muscle cells from patients who had APS- nephropathy – there was also increased proliferation of both endothelial and vascular smooth muscle cells in kidney biopsies
ii) Anti-phospholipid antibodies isolated from serum from patients with APS markedly upregulated mTORC1 and mTORC2 in a human endothelial cell line (HMEC-1 cells) and this activation was abolished by preincubation of cells with mTOR inhibitors including sirolimus. This upregulation did not appear to be due to anti-HLA antibodies as these antibodies were equally prevalent in the control patients who did not have evidence of increased mTORC activity. Control human IgG did not upregulate mTORC activity.
iii)Biopsies performed at 3 and 12 months after transplantation revealed markedly increased phosphorylation of S6RP and AKT (i.e. increased mTORC activity) in APL patients compared to controls. Furthermore APL patients on Sirolimus had significantly reduced evidence of mTORC activation compared to those APL patients not on sirolimus and this was associated with a significant reduction in the development of vascular lesions on biopsy (as characterised and quantified by fibrous intimal hyperplasia)
iv) The improved vascular histology in APS patients on sirolimus was associated with better graft survival and eGFR post-transplantation compared to those APS patients not on sirolimus – although the numbers are very small. All patients with APS were anticoagulated
The biopsy staining as well as the in vitro data with anti-phospholipid antibodies from patient sera suggest that the vascular lesions in APS post-transplantation are mediated by activation of mTORC by antiphospholipid antibodies. One has to be careful about making clinical recommendations on the basis of this kind of work but the data does seem to suggest that sirolimus maybe a better agent than a CNI for APS patients post-transplant. Whilst there are clearly lots of questions about delineating the precise mechanism of how anti-phospholipid antibodies activate mTORC and how this translates into a particular histological phenotype, the work presented is a very elegant example of applying basic science to better understand a clinico-pathological phenotype.