Rituximab Treatment Prevents the Early Development of Proteinuria following Pig-to-Baboon Xeno-Kidney Transplantation

Abstract

We previously reported life-supporting a1,3-galactosyltransferase knockout (GalTKO) thymokidney xenograft survival of .2 months in baboons. However, despite otherwise normal renal function, recipients developed proteinuria with morphologic changes (podocyte effacement), a condition that presents a major obstacle to long-term studies in this model. A recent clinical study showed that rituximab therapy after allogeneic transplant prevented proteinuria possibly associated with loss of sphingomyelin phosphodiesterase acid-like 3b . Here, we demonstrate that rituximab prevents the disruption of pig podocytes in an SMPDL-3b-dependent manner in vitro and the early development of proteinuria after xenogeneic kidney transplantation in baboons. Immunofluorescence showed SMPDL-3b expression in pig glomerular epithelium; immunoprecipitation demonstrated rituximab binding to SMPDL-3b in glomeruli. Culture of isolated pig podocytes with naive baboon sera, which has preformed antipig natural antibodies, reduced SMPDL-3b expression, disrupted podocyte morphology, and decreased podocyte proliferation, whereas pretreatment with rituximab prevented these effects. Six baboons received rituximab before transplantation to deplete B cells and again in the peri-transplant period; 18 baboons treated only before transplantation served as historical controls. The onset of post-transplant proteinuria was significantly delayed in a B cell-independent manner in the animals that received peri-transplant rituximab treatment. Although further optimization of this protocol is required, these data provide intriguing clues to the mechanisms of post-transplant proteinuria in xenogeneic kidney transplantation and a potential strategy for its prevention.

Crystal Structure of the Acid Sphingomyelinase-like Phosphodiesterase SMPDL3B Provides Insights into Determinants of Substrate Specificity

“…Rituximab, a therapeutic anti-CD20 monoclonal antibody targeting B cells but also used in certain nephrological conditions, binds to SMPDL3B (13)(14)(15) and remediates these defects in a SMPDL3B-dependent manner (13). Rituximab similarly protects podocytes from morphological disruption, decreased proliferation, and SMPDL3B protein loss in xenotransplants (14).…”

“…Rituximab, a therapeutic anti-CD20 monoclonal antibody targeting B cells but also used in certain nephrological conditions, binds to SMPDL3B (13)(14)(15) and remediates these defects in a SMPDL3B-dependent manner (13). Rituximab similarly protects podocytes from morphological disruption, decreased proliferation, and SMPDL3B protein loss in xenotransplants (14). In contrast, this protein is elevated in glomeruli from patients with diabetic kidney disease where it binds to the soluble urokinase plasminogen activator receptor on podocytes; SMPDL3B knockdown protects podocytes from apoptosis in this condition (16).…”

New Biologics for Glomerular Disease on the Horizon

“…Of interest, some experimental data support the hypothesis that rituximab could also directly target the podocyte and reduce proteinuria, independent of the B-cell depletion induced by this immunosuppressive agent [33]. According to these preliminary data, rituximab could bind to CD20-negative glomerular epithelial cells, probably via the expression of a surface molecule called sphingomyelin phosphodiesterase acid-like 3b.…”

Rituximab

“…In this issue of JASN, Tasaki et al suggests that rituximab, an immunomodulator targeting CD20 on B lymphocytes, could delay onset of post-transplant proteinuria in baboons transplanted with kidneys from a-1,3-galactosyltransferase-deficient pigs when the drug was administered in the peritransplant period. 4 This discovery marks another important step toward a potential use of xenografts in nephrology.The possibility that rituximab reduces proteinuria has been reported on several occasions. Interestingly, the study of Tasaki et al demonstrates that protection from proteinuria was not associated with B-lymphocyte depletion and required kidney graft expression of sphingomyelin phosphodiesterase acid-like 3b (SMPDL-3b).…”

“…In this issue of JASN, Tasaki et al suggests that rituximab, an immunomodulator targeting CD20 on B lymphocytes, could delay onset of post-transplant proteinuria in baboons transplanted with kidneys from a-1,3-galactosyltransferase-deficient pigs when the drug was administered in the peritransplant period. 4 This discovery marks another important step toward a potential use of xenografts in nephrology.…”