Clinically Relevant Reactivation of Polyomavirus BK (BKPyV) in HLA-A02-Positive Renal Transplant Recipients Is Associated with Impaired Effector-Memory Differentiation of BKPyV-Specific CD8 ⁺ T Cells

Abstract

Polyomavirus BK (BKPyV) frequently reactivates in immunosuppressed renal transplant recipients (RTRs) and may lead to graft loss due to BKPyV-induced interstitial nephritis (BKVN). Little is known on the differentiation of CD8 ⁺ T cells targeting BKPyV in RTRs. Here we investigated whether BKPyV-specific CD8 ⁺ T cell differentiation differs in RTRs with varying degrees of BKPyV reactivation and/or BKVN.

Using combinatorial encoding with tetramers carrying BKPyV major capsid protein (VP1) and large T antigen protein (LTAG) epitopes, we investigated CD8 ⁺ T cell responses to BKPyV in longitudinally obtained PBMC samples from 46 HLA-A02-positive RTRs and 20 healthy adults. We were also able to isolate BKPyV-specific CD8 ⁺ T cells from five renal allografts, two of which were affected by BKVN.

Before transplantation, BKPyV-specific CD8 ⁺ T cells targeting VP1 and LTAG epitopes appeared predominantly as central-memory and CD27 ⁺/CD28 ⁺ effector-memory (T _EM), and naïve-like PD-1-expressing cells, respectively. After viral reactivation, BKPyV-specific CD8 ⁺ T cells assumed CD28 ⁻ T _EM and T _EMRA states in patients who were able to control BKPyV, whereas differentiation lagged behind in patients with severe viral reactivation or BKVN. Furthermore, VP1-specific CD69 ⁺/CD103 ⁺ tissue-resident memory (T _RM) cells accumulated in BKVN-affected allografts but lacked signs of effector differentiation. In contrast, granzyme B-expressing effector cells were detected in allografts not affected by BKVN.

In conclusion, effector-memory differentiation of BKPyV-specific CD8 ⁺ T cells in patients with high viral load or BKVN is impaired. Further characterization of the specific mechanisms behind this altered cellular differentiation is necessary to develop therapies that can prevent the emergence of BKVN.

Author Summary

In immunosuppressed renal transplant recipients (RTRs), BKPyV frequently reactivates from latency and may cause severe interstitial nephritis in the allograft (BKVN). Not only is there no effective treatment, it also not understood why BKVN arises in some RTRs but not in all. In the current study we investigated populations of CD8 ⁺ T cells targeting epitopes from structural and non-structural BKPyV proteins in RTRs over the course of transplantation. In contrast to RTRs who suffered from self-limiting reactivation of BKPyV, patients who developed severe viral reactivation and BKVN were found to have BKPyV-specific CD8 ⁺ T cells which did not, or less often differentiate into CD28 ⁻ effector-memory cells during viral reactivation. Moreover, virus-specific CD8 ⁺ T cell activation and differentiation was not only impaired in the circulation, but possibly also in BKVN-affected renal allografts. In contrast to the CD8 ⁺ T cells in kidneys from three patients who did not develop BKVN, T cells in two BKVN-affected kidneys did not display typical cytotoxic effector traits. These findings suggest that impaired BKPyV-specific CD8 ⁺ T cell maturation in response to viral reactivation, possibly owing to inter-individual differences in sensitivity to immunosuppressive medication or to certain viral quasispecies, underlies the emergence of severe viral reactivation and BKVN.