We performed clinical and pilot immunological studies evaluating non-specific and BKPyV-specific T-cell, NK-cell, and NKT-cell immune responses in KT recipients who developed BKPyVAN within the first year following KT. The present study identified individuals with BKPyV-specific innate immune responses prior to transplant, indicated by the presence of BKPyV-specific NK cells that secreted IFN-γ after stimulation with VP1 antigen along with the presence of diabetic kidney disease, as more likely to develop BKPyVAN within the first year post-KT. Additionally, the proportion of VP1-specific NK cells in KT recipients was observed to be slightly increased after transplant in accordance with BKPyVAN, despite post-transplant immunosuppression.
The demographic data of the participants in our study are comparable to those of previous studies, which reported the highest incidence of DNAemia as typically occurring within 3–6 months after transplantation.4, 10, 11 The previous two retrospective cohort studies conducted among Thai KT recipients demonstrated a BKPyVAN prevalence of 10–12%. However, this number could be an underestimate owing to the 50–70% compliance rate for post-transplant screening in our resource-limited setting.10, 11 The incidence of possible BKPyVAN in the present study was relatively high (one out of five patients); however, the incidence of proven BKPyVAN was notably lower than that of the previous retrospective studies in a similar setting (6.4–8.6%), likely owing to our active monitoring for and early detection of BKPyV DNAuria and the consequent optimization of immunosuppression.10, 11 Furthermore, underlying diabetes has been previously reported as a risk factor for BKPyVAN.3
Because an effective anti-BKPyV agent has not yet been established, vigilant immunosuppression adjustment in response to BKPyV-specific immune monitoring is crucial from the perspectives of both prevention and treatment.1, 7, 12 However, the mechanisms of injury during BKPyV infection in KT patients are not completely understood. To address this gap, we conducted a prospective cohort study in which we assessed the immunological factors that contribute to early BKPyV infection after transplant.13 Viral-specific T-cell immunity plays an essential role in controlling viral infections in recipients of solid organ or hematopoietic stem cell transplants.13 The decline in viral-specific T-cell immunity has been shown to increase the risk of various infections, such as adenovirus, BKPyV, and CMV infections, in transplant recipients.13–16 While most studies have investigated the role of CMV-specific T-cell immunity and applied their findings in clinical practice via a commercial test, such as the QuantiFERON-CMV assay, there are limited studies on BKPyV-specific immunity17, 18, which has left the mechanism of BKPyV persistence and reactivation unclear.8
To date, the cellular adaptive T-cell response has remained the most investigated and recognized aspect of controlling BKPyV infection; this response relies on the interplay between CD4+ and CD8+ T cells8. CD8+ T cells are predominantly specific to LT antigen, whereas CD4+ BKPyV-specific T cells mainly recognize VP18. According to DeWolfe et al.7, in KT recipients who underwent BKPyV-specific T-cell immune monitoring along with preemptive BKPyV monitoring, low CD4+ and high CD8+ T-cell proportions and increased amounts of effector CD8+ T cells were found prior to transplant in patients who later developed DNAemia. Furthermore, increases in both CD4+ and CD8+ T cells were observed in patients with DNAemia who were able to achieve viral clearance after diagnosis7. Our group reported a marginal trend of LT-specific CD4+ T-cell restoration in the KT recipients who developed BKPyVAN after adjustment to their level of immunosuppression.14 Thus, a measurement of BKPyV-specific cellular responses might serve as a guide for fine-tuning the magnitude of immunosuppression to prevent BKPyV reactivation.
Although virus-specific T cells play a large role in the elimination of reactivated BKPyV, NK cells are early responders in the antiviral response. Our study demonstrated that the elevated pre-transplant BKPyV-specific innate immunity represented by a higher percentage of NK- and VP1-specific NK cells was independently associated with the development of BKPyVAN after transplant. The innate immune system consists of numerous cells and soluble molecules, and it plays essential roles in both suppressing viral replication and activating adaptive immunity to eradicate viruses.19 Inflammatory NK-cell antiviral responses involve interactions between activation receptors, such as killer-cell immunoglobulin-like receptors (KIRs), and host-cell MHC class I molecules, which alters cell sensitivity to lysis by NK cells.20 After KT, the ischemic injury may reactivate BKPyV from the allograft kidney and trigger the innate immune system shortly after allograft reception. NK cells recognize virus-infected cells through the downregulation of MHC class I and inhibitory receptors as well as via the upregulation of activator molecules19. Patients with a small number of KIRs are at higher risk of developing BKPyVAN. 8, 19 A detailed analysis of all receptors revealed significantly lower frequencies of the activating KIR in patients with BKVPyAN as compared with the controls21. Our findings support the potential role of NK cells in the pathogenesis of BKPyVAN development. Specifically, they reveal a correlation between the %VP1-specific NK cells and BKPyV infection. We demonstrated that patients with BKPyVAN had higher pre-KT %VP1-specific NK and %NKT cells as compared with patients without high-level BKPyV DNAuria. Thus, innate immunity is important as the first line of defense against BKPyV infection. However, further studies and longer study periods are needed to address the associated mechanism.
We also investigated a cell type that reflects a bridge between the innate and adaptive immune systems, NKT cells. Unlike conventional T cells, which recognize peptide antigens presented by MHC molecules, NKT cells recognize glycolipid antigens presented by CD1d. Once activated, these cells can perform both functions commonly ascribed to T-helper cells and those characteristic of cytotoxic T cells. NKT cells have been reported to recognize and eliminate herpes viruses-infected cells.22 Gaya et al. showed that NKT cells also promote antibody responses during viral infection and can be detected up to three days before the formation of germinal centers in a mouse model. This NKT-cell role is believed to enhance B-cell antibody responses after viral infection.23
There are limitations in this study. Higher anti-BKPyV titers have been reported to be related with post-transplant BKPyVAN because high donor BKPyV-specific antibody titers can be interpreted as a marker of recent viral exposure and a potentially higher BKPyV viral load in the allograft.24, 25 Unfortunately, we were unable to control for this because we did not have donor BKPyV serology. Additionally, we were able to measure BKPyV-specific immunity in only approximately half of the patients; the relatively small size of this group prevented us from exploring other associations. Larger studies regarding BKPyV-specific innate immunity is needed to confirm these data and their clinical implications. Because of our short follow-up period, we could not demonstrate long-term post-transplant outcomes and T-cell immunity. There were only a few patients in this study who developed BKPyV DNAemia and BKPyVAN, so the association between VP1-specific NK and NKT cells and presumptive and proven BKPyVAN may need a longer amount of time to be observed. Our team has continued to monitor the patients from this study for up to 2 years post-KT; this data collection is ongoing, and the result is eagerly anticipated. Last, the lack of a commercial assay for quantifying BKPyV-specific immunity could limit its use in some settings.
Notably, this study is the first work to demonstrate the clinical and immunological factors associated with BKPyV infection. Its findings emphasize the importance of a post-KT BKPyV screening surveillance system for KT recipients in light of the limited standardized treatment options. The 2019 guidelines from the AST-IDCOP suggest conducting screenings of the plasma BKPyV load or urine BKPyV viral load (optional) monthly until month 9 post-transplantation, followed by every 3 months until 2 years post-transplantation.3 Although urine monitoring is presented as optional in these newly updated guidelines, our study supports the use of such preemptive monitoring to prevent unfavorable consequences.
Since we included early stage of BKPyVAN (possible BKPyVAN), this allow us to explore BKPyV DNAuria which is a marker of early reactivation of BKPyV infection. Therefore, we were able to demonstrate early immune response such as innate immune response presented by BKPyV-specific NK cell responses. Our study also addresses the importance of both innate and adaptive BKPyV-specific immunity in KT recipients. Even the guideline from the AST-IDCOP does not recommend measuring either quantification or qualification of BKPyV-specific immunity prior to KT at the present time based on the fact that solid evidence is still lacking to support the utilization. We believe our data would at least fulfill knowledge gap in this field3. Since there is a lack of evidence-based treatment options3, it is essential to closely monitor replication of BKPyV in KT recipients and promptly modify immunosuppressants in order to contain the virus to prevent further damage. The potential role of BKPyV‐specific T‐cell and antibody responses as adjunct markers to predict onset and clearance of BKPyV DNAemia needs to be defined, since this information can guide the reduction in immunosuppressants and the need for re‐increase after clearance. Further research is needed to establish mechanisms that contribute to BKPyV infection and the affected pathways which are critical for immune defense against other pathogens.