Rituximab: a boot to protect the foot.

Given the shortage of organs available for transplantation and the rising prevalence of ESRD,1 new avenues including xeno-kidney transplantation need to be explored. Despite tremendous advances in achieving immunologic hyporesponsiveness in xeno-kidney transplantation, long-term organ survival remains amajor challenge.2 Although dramatic improvements have recently been achieved by genetic deletion of a major xenoantigen causing hyperacute rejection in experimental donors (a-1,3-galactosyltransferase),3 most recipients still experience additional barriers limiting graft survival, such as acute vascular rejection and severe post-transplant proteinuria. 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). Rituximab was able to bind directly to CD20-negative glomerular epithelial cells and to immunoprecipitate the 50-kD isoform of SMPDL-3b from pig glomerular lysates. Rituximab prevented both pig podocyte injury and the suppression of SMPDL-3b expression caused by exposure to naive baboon sera. This was associated with preservation of cell viability. Additional key novel experiments demonstrated that preformed natural antibodies are necessary in combination with complement for the baboon sera to reduce podocyte viability. In particular, exposure of cultured podocytes to untreated serum compromised podocyte viability only in the presence of complement, and this phenomenon was prevented when serum was preabsorbed. The protection of podocyte function in xenotransplantation by rituximab does not come as a total surprise. We have previously demonstrated that rituximab binds SMPDL3b, a proteinwith a rituximab-binding amino acid sequence that is expressed in podocyte lipid rafts.5 Rituximab protects podocytes through the preservation of SMPDL-3b and acid sphingomyelinase expression and activity from sera-induced injury when the sera of patients with FSGS are used.5 However, the fraction of the sera responsible for downregulation of SMPDL-3b was hitherto unknown; Tasaki et al. have now shown that preformed natural antibodies are indeed responsible for xenotransplant-related proteinuria. If, and how, antibodies also play a role in the downregulation of SMPDL-3b that we have observed in FSGS remains to be established. Tasaki et al. demonstrated that peritransplant administration of rituximab delayed, but did not prevent, the post-transplant occurrence of proteinuria. The authors interpreted this observation as resulting from persistent preformed antibodies once the rituximab levels fell over time. In prior studies, when a single dose of rituximab was administered to patients with FSGS in the peritransplant period, 26% of these patients still experienced nephrotic-range proteinuria in the first 30 days after transplant,5 suggesting that additional mechanisms may be involved or work in parallel. First, rituximab may be binding to SMPDL-3b on the surface of podocytes, which probably interferes with activation of podocyte aVb3 integrin by circulating factors, such as the soluble urokinase plasminogen activator receptor.6,7 The role of these circulating factors should also be explored in xenograft-related post-transplant proteinuria. Second, upregulation of additional molecules that may serve as danger signals in podocytes may occur, as the authors described preliminary upregulation of CD80 in their transplant model, similar to podocyte CD80 expression under disease conditions.8 Third, unique to the transplant models used by Tasaki et al. is the fact that recipient baboons underwent splenectomy and thymectomy before transplant. Because a high incidence of anti-nephrin antibodies have been reported in experimental models of graft-versus-host disease9 (which may cause nephrotic-range proteinuria10), and nephrin is expressed in the thymus (where it likely represents an antigen to induce tolerance11), it would be interesting to know Published online ahead of print. Publication date available at www.jasn.org.