Our results highlight the loss of BKv-specific T-cell responses during the course of BKv replication, with an exhausted-like phenotype of BKv-specific CD4 and CD8 T-cells in KTRs with BKvAN.
BKv infection remains a major challenge in kidney transplantation, because it can lead to BKvAN, which causes graft loss in 50% of cases in this context12–14. There is currently no specific antiviral treatment for BKvAN, and decreasing the intensity of immunosuppression is the only treatment shown to date to be effective15,16. We show here that decreasing immunosuppression does not prevent high rates of graft loss in cases of established BKvAN (histologically proven), probably because of the delayed restoration of BKv T-cell responses and irreversible tissue lesions. Immunosuppressive regimens should therefore be adapted sooner after the detection of BKv in plasma, but achieving the balance between the control of BKv replication and the risk of acute allogeneic graft rejection when lowering immunosuppressive treatments remains a major challenge17. Indeed, after immunosuppression reduction for BKv replication, more than 20% of patients experience acute rejection, with lower graft function and survival18.
The use of intensive immunosuppressive strategies has been identified as a key factor in BKv reactivation19,20. We found that thymoglobulin, a polyclonal antibody induction therapy that causes profound T-cell depletion, was a risk factor for BKv-viremia, consistent with current knowledge21, but not for BKvAN. This result implies that thymoglobulin could be a trigger for the blood replication of BKv, but not sufficient for the occurrence of BKvAN. Other authors have also reported a higher risk of BKvAN with certain immunosuppressive regimens, including high levels of exposure to steroids or tacrolimus/mycophenolate mofetil combinations22–24. The choice of immunosuppressive drugs plays a role in the risk of BKvAN, as the use of tacrolimus was associated with a higher risk of developing BKvAN than the use of cyclosporine A (CsA)25. The effect of tacrolimus may be explained by an activation of BKv replication, possibly in relation to its interaction with FKBP-12, whereas CsA reduces BKv replication in vitro25–27. We also found that mycophenolate mofetil exposure was associated with a higher rate of BKvAN, but this factor was not identified as an independent risk factor in our model. Nevertheless, precise AUC determinations may be useful for monitoring the inhibition of T-cell activation and proliferation in kidney transplant patients28,29.
Despite stepwise immunosuppression reduction, some patients clear BKv, whereas others develop persistent BKv replication, highlighting the role of individual mechanisms in viral control. In solid organ transplantation, the individual risk of opportunistic infection is largely determined by the “net state of immunosuppression”. Many factors contribute to this net state, including the immunosuppressive regimen and other individual predisposing factors, such as immune-aging, concomitant viral infection, and antiviral T-cell efficacy30.
The BKv-specific T-cell response plays a crucial role in controlling BKv replication, although the precise pathophysiological mechanisms involved remain unknown8,31,32. To date, there is several evidence to underline that the presence of BKv-specific CD8 T cells is predictive of the successful control of BKv reactivation after transplantation33–35. We provide here an exhaustive characterization of BKv-specific T cells, in terms of their phenotype and functional assessment. We describe the changes in the BKv-specific T-cell response occurring during the course of BKv reactivation, with the progressive loss of BKv-specific T-cell responses. We observed changes in T-cell functionality that appeared to be limited to BKv-specific T cells, as none of the other antiviral responses tested were affected in patients with BKvAN.
Viruses during chronic infection can make use of the negative regulatory signal mediated by the immune checkpoint pathway to escape immune system control36. In our work, the inverse correlation between BKv viral load and BKv-specific T-cell responses, and the higher levels of PD1 inhibitory receptor expression suggest an exhaustion of BKv-specific T cells in patients with BKvAN, as observed in other chronic viral infections37–39.
HLA matching is a major challenge for successful transplantation, as HLA mismatches have a significant negative impact on allograft survival in kidney transplant patients40–42. However, the impact of HLA matching on the development and progression of BKvAN remains a matter of debate43. HLA is important for viral peptide presentation to both CD4 and CD8 T cells. In this point of view, BKv-specific CD8 T cells directed against the LPLMRKAYL 9-mer epitope are associated with the clearance of BKv44,45, which binds to several HLA-B molecules44,46,47.
In transplant recipients, antiviral T cells must operate under suboptimal (immunosuppressed) conditions, with differential individual effects. In addition, allogeneic kidney transplantation constitutes a complex immunological situation in which viral peptides may be presented by the donor or recipient HLA. In mismatch situations, the recognition, by recipient BKv-specific memory T cells, of the BKv peptides presented by donor HLA molecules on kidney epithelial cells may be altered, leading to BKv replication. Conversely, allogeneic responses may provide a stimulatory environment to BKv-specific T cells and induce additional help to restore or elaborate a functional BKv-specific CD8 T-cell response. We showed that BKvAN was mostly observed in cases of HLA-DR compatibility and was associated with an impairment of BKv-specific T-cell functions. This is consistent with previous reports showing that BKvAN occurs even in the context of good HLA matching13,48, with a negative impact of HLA matching in the outcome of BKvAN49. Conversely, in some cases, HLA mismatching may promote the development of BKvAN21,50,51 because of the use of heavier immunosuppression52. In such cases, a similar inhibition of viral responses, not restricted to BKv, would be attempted.
Interestingly, we found that class II HLA-DR mismatches played an important role in BKv control, suggesting the contribution of CD4 T cells. CD4 T cells play a crucial role for the generation, maintenance and reactivation of memory CD8 T-cell responses53–55. We hypothesized that mismatched CD4 T cells receiving allogeneic stimuli would be able to provide the necessary help to rescue BKv-memory CD8 T-cell responses (Fig. 6). This hypothesis was confirmed by replacing in vitro autologous CD4 T cells with allogeneic CD4 T cells and evaluating the improvement in the BKv-specific CD8 T-cell response. We previously reported that functional CD4 T cells generated in response to a different antigen (heterospecific helpers) can provide effective help to memory CD8 T cells in experimental mouse models11. A higher degree of HLA mismatching can provide heterospecific CD4 help to BKv-specific memory CD8 T cells. This heterospecific help may increase the ability of the host to maintain an effective BKv-specific memory CD8 T-cell response that can protect against BKvAN. Along similar lines, Zeng et al. showed that kidney biopsy specimens from patients with BKv viremia contained more abundant alloreactive T-cell clones than virus-reactive clones, with a cell ratio of up to 7.756. In addition, Sawinski et al. showed that persistent BKv-viremia does not increase graft loss but was associated with the development of class II de novo donor-specific antibodies57. Furthermore, we cannot rule out the possibility that, in the context of allogeneic transplantation, heterospecific immunity involves a potential cross-reactivity of virus-specific T cells with multiple HLA complexes58.
In summary, we show that BKvAN is associated with a progressive severe loss of BKv-specific T-cell responses and an exhausted-like phenotype. These BKv-specific T-cell responses are crucial for the control of viral replication in the kidney. Our results suggest that a higher degree of HLA mismatching provides more effective control of BKv replication, possibly due to the involvement of heterospecific CD4 help processes. Thus, although a high degree of HLA matching may protect against graft rejection, it may, paradoxically, also be a significant risk factor for BKvAN. This finding should be taken into account in strategies aiming to mitigate the risk of BKvAN.