Human Memory B Cells Harbor Diverse Cross-neutralizing Antibodies Against BK and JC Polyomaviruses
Lindner JM, Cornacchione V, Sathe A, et al. Immunity. March 2019
Human polyomaviruses represent a unique challenge in transplantation.1 The polyomavirus BK virus (BKV) has a prevalence of up to 90% in adult populations, however its infection is generally asymptomatic–the virus lying dormant within the kidney and urinary tract. Immunosuppression, however, may allow BKV to reactivate and cause severe pathology. In renal transplant patients, BKV-associated nephropathy (BKVAN) is responsible for roughly 5% of graft failures, although this does vary between centers. One potential therapeutic approach against BKVAN is the administration of high-affinity, antigen-specific antibodies. Although production of these antibodies is possible, current methods such as B cell immortalization are time-consuming and inefficient.
Lindner et al2 developed a novel, highly efficient approach for recovery of antigen-specific immunoglobulin (Ig) from human B cells at any stage of maturity. Their B cell stimulation conditions efficiently expanded and stimulated Ig production from IgG+ and IgM+ B cells including all tested subsets: naïve, transitional, and memory B cells. This methodology also proved to be 5–10 times more efficient compared to immortalization with EBV, a common approach to B cell expansion. The strategy was then used to interrogate the humoral immune response to BKV. By isolating B cells that bound to either subtype 1 or 4 of BKV, the authors discovered a thoroughly diverse anti-BKV B cell receptor repertoire, containing >50 clonotypic sequences, with a predominance of neutralizing over binding-only antibodies. All identified IgG antibodies were capable of neutralizing at least one subtype of BKV, with some being able to neutralize additional subtypes, and the related polyomavirus JC virus. To explore the contribution of the Fc region of antibodies to their specificity, the investigators carried out synthetic class-switching on the anti-BKV specific antibodies by merging IgM heavy-chain variable domain sequences on an IgG1 or IgG3 backbone. The effect of this artificial class switching was unexpectedly variable, with certain antibodies losing their antigen binding capacity and others left largely unaffected. The authors explored this further by expressing these antibodies solely as IgM Fab fragments, finding that the binding profile of the fragments was broadling similar to when the Fab was expressed on an IgG1 or IgG3 backbone. Overall, these data suggest that the IgM FC domain has some role in reinforcing the specificity of IgM antibodies. Monomeric IgM displayed similar binding to wild-type pentameric and hexameric IgM, demonstrating that the impact of the FC region is inherent to the antibody isotype. The study reveals the power of this novel B cell screening technique and illustrates how BKV-specific antibodies are produced and function, with important implications for the production of therapeutic antibodies for the treatment of various polyomavirus strains.
1. Sawinski D, Goral S. BK virus infection: an update on diagnosis and treatment. Nephrol Dial Transplant. 2015;30:209–217.
2. Lindner JM, Cornacchione V, Sathe A, et al. Human memory B cells harbor diverse cross-neutralizing antibodies against BK and JC polyomaviruses. Immunity. 2019;50:668–676.e665.
A Urinary MicroRNA Panel That Is an Early Predictive Biomarker of Delayed Graft Function Following Kidney Transplantation
Khalid U, Newbury LJ, Simpson K, et al. Sci Rep. March 2019
Delayed graft function (DGF) is a significant challenge following transplantation, particularly when using marginal or expanded criteria organs. Notably, there is currently no objective diagnostic test beyond a biopsy. Noninvasive biomarkers that predict DGF development are therefore desirable as these may allow for individualized management of patients post-transplantation. MicroRNA (miRNA) is an abundantly expressed small noncoding RNA that functions in transcriptional regulation and may be readily measured in urine, where it has been shown to be associated with acute rejection.1 In preimplantation renal allograft biopsies, a panel of miRNAs has also been shown to assist in the prediction of DGF development.2
Bringing these observations together, Khalid et al3 investigated urinary miRNA as a means to predict DGF development. In a discovery cohort, miRNA was measured in urinary samples on postoperative days 1–5, with the first sample being first-pass urine obtained immediately posttransplant. Overall, samples from patients with DGF contained an increased amount of miRNA. Moreover, 6 specific miRNAs (miR-9, -10a, -21, 29a, -221, and -429) were significantly upregulated in DGF samples. Importantly, there was no difference in the expression of these miRNAs between recipients of living or deceased donor kidneys. The 6 miRNA panel provided a combined receiver operating characteristic-area under the curve (ROC-AUC) of 0.94 for discriminating DGF from the absence of DGF–superior to using any individual miRNA, and superior to using traditional clinical parameters of only donor and recipient clinical characteristics, which gave a ROC-AUC of 0.71. A second test cohort from a different transplant center was then used to confirm the utility of the newly defined miRNA panel and to validate the findings in the discovery cohort. Promisingly, miRNA in the test group showed an increase in patients developing DGF, with a ROC-AUC of 0.75. This reduction in discriminatory capacity may be expected given that the predictive value is always higher within the sample set from which it is defined. Although not necessarily a complete replacement for DGF scoring by clinical parameters, miRNA panels such as this may be useful when integrated with other available data. The ability to monitor miRNA noninvasively in the early postoperative period provides additional discriminatory information and is an attractive adjunct to current techniques. The small amount of urine required for the assay makes it a practical addition to the diagnostic toolkit posttransplantation, although anuric patients present an additional challenge. It will be very interesting to assess how many patients may avoid a biopsy using this type of integrated biomarker and clinical assessment and whether this results in an improvement in clinical outcomes.
1. van de Vrie M, Deegens JK, Eikmans M, et al. Urinary microRNA as biomarker in renal transplantation. Am J Transplant. 2017;17:1160–1166.
2. McGuinness D, Leierer J, Shapter O, et al. Identification of molecular markers of delayed graft function based on the regulation of biological ageing. PLoS One. 2016;11:e0146378.
3. Khalid U, Newbury LJ, Simpson K, et al. A urinary microRNA panel that is an early predictive biomarker of delayed graft function following kidney transplantation. Sci Rep. 2019;9:3584.