Research Highlights : Transplantation

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Research Highlights

Schroder, Paul M. MD, PhD1; Luo, Xunrong MD, PhD2

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Transplantation 106(1):p 4-5, January 2022. | DOI: 10.1097/TP.0000000000004023
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Evolution of Cytomegalovirus-responsive T-cell Clonality Following Solid Organ Transplantation

Higdon LE, Schaffert S, Huang H, et al. J Immunol. 2021;207:2077–2085.

Cytomegalovirus (CMV) is an opportunistic infection in transplant recipients that carries significant morbidity and mortality. CMV viral latency develops following primary viral infection. The prevalence of CMV seropositivity (hence latency) in adult transplant recipients in the United States averages ~58.9% and increases significantly with age such that it reaches 90.8% in patients older than 80.1 Organ transplantation may significantly alter CMV immunity via several possible means1: T-cell depletion is commonly practiced as an induction therapy for most organ transplantation, which may alter the host T-cell repertoire against CMV2; an organ carrying latent CMV infection from a CMV-seropositive donor may be transplanted into a CMV-seronegative recipient in the context of immunosuppression and impaired CMV immune control. How CMV cellular immunity evolves under these circumstances is not known.

The current study by Higdon et al2 used single-cell T-cell receptor alpha and beta sequencing to determine the clonality and diversity of CMV-responsive CD8 and CD4 T-cell repertoire during the first year following transplantation in human recipients. The study enrolled a total of 6 heart or kidney transplant recipients, all of whom exhibited CMV seropositivity at the time of transplantation. Peripheral blood mononuclear cells from 3 time points were collected and examined: pretransplant baseline and approximately 3 and 12 mo posttransplant. Peripheral blood mononuclear cells were stimulated with a peptide library consisting of 15 amino acid peptides with sequential 11 amino acid overlaps that span the entire length of the CMV immediate early 1 (IE-1) immunodominant polypeptide, and those responding to this peptide library by producing IFN-interferon-gamma were sorted and sequenced for T-cell receptor alpha and beta at the single-cell resolution. In doing so, they found that the repertoire of CD8 T cells responding to IE-1 stimulation was oligoclonal throughout the 3 time points examined; although depending on whether lymphodepleting induction was used, the dominating clone may differ pre- and posttransplantation. The expansion of IE-1–responsive CD8 T-cell clones was driven by the dominant clones. However, regardless of the degree of oligoclonality, clonal diversity was stably maintained throughout the studied period, underscoring that during the immediate posttransplant period, CMV-specificity was not impacted by memory inflation of CMV-responsive clones. In contrast, CMV-responsive CD4 T cells remained highly polyclonal at all time points and there was no evidence of clonal expansion among any clones.

Collectively, this work provided a molecular understanding of the CMV-specific T-cell repertoire posttransplantation in previously CMV-seropositive recipients and underscored the stability of this repertoire despite memory inflation. This work further provides useful insights to potential therapeutic strategies and their ideal timing to modulate this repertoire early posttransplantation while emphasizing on diagnostic strategies that determine CMV immunity for personalizing immunosuppression. Several aspects of this study are directly relevant to CMV risk-stratification and management posttransplantation and warrant further studies. First, whether an active CMV infection, namely with measurable CMV viremia, may alter the clonality and clonal stability in posttransplant recipients should be examined in the context of common induction and immunosuppression. In the current study, all recipients remained CMV-free based on conventional clinical diagnostic tests. Second, in recipients with previous CMV seronegativity but who were receiving a CMV-seropositive organ, how the CMV-specific repertoire evolves even with subclinical reactivation from the donor organ (as a primary infection in the recipient) would also be interesting to study and would have implications to optimal management of this patient population. The current study suggested that the emergence of CMV-specific clones due solely to donor CMV positivity is likely a late event, although definitive conclusions would be hard to reach given the small number of subjects and the context in which the current study was conducted. Finally, a clear understanding of whether findings in the CMV-specific repertoire can be generalized to other viruses that also establish latency (eg, hepatitis B virus) and to cellular immunity initially established by vaccination would have a significant impact on future therapeutic and management strategies of transplant recipients in the rapidly evolving modern landscape of pathogens.

Overall, the current study by Higdon et al has taken an important first step for future studies in this area.

Functional Consequences of Memory Inflation After Solid Organ Transplantation

Higdon LE, Schaffert S, Cohen RH, et al. J Immunol. 2021;207:2086–2095.

Memory inflation is a phenomenon in which T cells responding to persistent antigen leads to an expansion of memory cells that plateu and persist even after the initial priming event has subsided. This phenomenon was originally described in the context of the antiviral CD8 T-cell response to murine cytomegalovirus (CMV).1 In contrast to exhaustion, in which T cells become unresponsive and ultimately deleted, inflation results in continued expansion of T cells that retain their function and responsiveness to antigen over time. Memory inflation has also been observed in other types of responses including hepatits C virus, parvovirus, norovirus, and adenovirus, and the cells demonstrate a polyfunctional phenotype.2 This expansion and maintenance of polyfunctional cells has been suggested to be important for maintaining latency of viral infections such as CMV, particularly in the context of immunosuppression and organ transplantation.3

In a recent article in Journal of Immunology, Higdon et al described this process of memory inflation and its effects on the functions of CMV-responsive T cells after organ transplantation.4 The authors obtained blood samples from 6 solid organ transplant recipients that were CMV seropositive and stimulated the cells in vitro with CMV-specific peptide library at different time points before and after transplantation. Using a combined single-cell–targeted sequencing approach, the authors first identified 6 different clusters of CD8 T cells exhibiting different states of differentiation and functions. Then, by analyzing the T-cell receptor alpha and beta gene sequencing, they segregated the populations into rare versus expanded (those that represented at least 2% of the total population) clones and found that the expanded clone populations had a higher incidence of polyfunctionality. Further longitudinal analysis of samples by 3 and 12 mo posttransplant demonstrated that the clones that expanded in size from pretransplant to 3 mo posttransplant exhibited increased polyfunctionality. There were few changes in the clonally expanded populations in the 3-mo posttransplant time frame compared with the 12-mo posttransplant time frame.

These results provide a valuable insight into the CMV-responsive CD8 T-cell population and their expansion and functions over time after solid organ transplantation. Interestingly, the greatest change in functionality of these cells occurred during the early posttrasnplant period when patients were subjected to surgery, induction therapy, and new maintenance immunosuppression regimen. In contrast, after the first 3 mo, the CD8 T-cell populations seemed to maintain a steady state in their clusters of functionality. This leads to several other questions for the future. For instance, the specific induction therapy regimen may influence the memory inflation of CMV and other viral responsive T-cell populations. Identifying these signatures of polyfunctionality at this early time after transplant may help to guide surveillance and prophylactic medication regimens in the transplant population by helping to predict those that may be more susceptible to viral reactivation.

This study largely focuses on the CD8 T-cell population, but by using similar methods with a different set of functional markers, the CD4 T-cell population may be used to provide even greater insight into the inflationary T-cell mileu. Longer-term follow-up in these and other transplant subjects may also be useful to see if patterns among these T-cell populations can be used to predict chronic allograft injury or help guide changes in immunosuppressive regimens to prevent viral reactivation or rejection.

In summary, a broader application of these combined single-cell–targeted sequencing methods may be one of the keys to understanding the complex immune environment after solid organ transplantation.


1. Bate SL, Dollard SC, Cannon MJ. Cytomegalovirus seroprevalence in the United States: the national health and nutrition examination surveys, 1988-2004. Clin Infect Dis. 2010;50:1439–1447.
2. Higdon LE, Schaffert S, Huang H, et al. Evolution of cytomegalovirus-responsive T cell clonality following solid organ transplantation. J Immunol. 2021;207:2077–2085.


1. Karrer U, Sierro S, Wagner M, et al. Memory inflation: continuous accumulation of antiviral CD8+ T cells over time. J Immunol. 2003;170:2022–2029.
2. Klenerman P. The (gradual) rise of memory inflation. Immunol Rev. 2018;283:99–112.
3. Snyder LD, Chan C, Kwon D, et al. Polyfunctional T-cell signatures to predict protection from cytomegalovirus after lung transplantation. Am J Respir Crit Care Med. 2016;193:78–85.
4. Higdon LE, Schaffert S, Cohen RH, et al. Functional consequences of memory inflation after solid organ transplantation. J Immunol. 2021;207:2086–2095.
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