PD-1 analysis on CD28(-) CD27(-) CD4 T cells allows stimulation-independent assessment of CMV viremic episodes in transplant recipients

SARS-CoV-2-specific cellular and humoral immunity after bivalent BA.4/5 COVID-19-vaccination in previously infected and non-infected individuals

Knowledge is limited as to how prior SARS-CoV-2 infection influences cellular and humoral immunity after booster-vaccination with bivalent BA.4/5-adapted mRNA-vaccines, and whether vaccine-induced immunity may indicate subsequent infection. In this observational study, individuals with prior infection (n = 64) showed higher vaccine-induced anti-spike IgG-antibodies and neutralizing titers, but the relative increase was significantly higher in non-infected individuals (n = 63). In general, both groups showed higher neutralizing activity towards the parental strain than towards Omicron-subvariants BA.1, BA.2 and BA.5. In contrast, CD4 or CD8 T cell levels towards spike from the parental strain and the Omicron-subvariants, and cytokine expression profiles were similar irrespective of prior infection. Breakthrough infections occurred more frequently among previously non-infected individuals, who had significantly lower vaccine-induced spike-specific neutralizing activity and CD4 T cell levels. In summary, we show that immunogenicity after BA.4/5-bivalent vaccination differs between individuals with and without prior infection. Moreover, our results may help to improve prediction of breakthrough infections.

NK cells enhance CAR-T cell antitumor efficacy by enhancing immune/tumor cells cluster formation and improving CAR-T cell fitness

BACKGROUND

Chimeric antigen receptor (CAR)-T cell immunotherapy has modified the concept of treatment in hematological malignancies. In comparison with pediatric patients, where responses are maintained over many years, older patients, such as those with non-Hodgkin's lymphoma (NHL) and multiple myeloma (MM), present lower persistence of CAR-T cells that might be due to decreased fitness of T cells acquired with aging. Moreover, cord blood derived-NK cells (CB-NKs) and CAR-NK cells derived from CB-NK can be used 'off-the-shelf' as immune cells with antitumor properties for the treatment of cancer patients. However, to date, clinical studies have only demonstrated the safety of these therapies but not optimal efficacy. To confront the shortcomings of each therapy, we devised a novel approach consisting of simultaneous (CAR-)NK cell and CAR-T cell administration. In this setting, NK cells demonstrate an important immunoregulation of T cells that could be exploited to enhance the efficacy of CAR-T cells.

METHODS

A combinatorial treatment based on either CAR-T and CAR-NK cells or CB-NK and CAR-T cells in two models of NHL and MM was performed. Antitumor efficacy was analyzed in vitro and in vivo, and parameters related to early activation, exhaustion and senescence of T cells were analyzed.

RESULTS

We show that CAR-NK cells derived from CB-NK are only effective at high doses (high E:T ratio) and that their activity rapidly decreases over time in comparison with CAR-T cells. In comparison and to exploit the potential of 'off-the-shelf' CB-NK, we demonstrate that a low number of CB-NK in the CAR-T cell product promotes an early activation of CAR-T cells and their migration to MM cells leading to enhanced anti-MM efficacy. Moreover, cytokines related to CRS development were not increased, and importantly, CB-NK enhanced the fitness of both CAR^pos and CAR^neg T cells, promoting lower levels of exhaustion and senescence.

CONCLUSION

This study demonstrates a relevant immunoregulatory role of CB-NK collaborating with CAR-T cells to enhance their antitumor activity. A novel and different approach to consider in CAR-T cell immunotherapy studies is presented here with the goal to enhance the efficacy of the treatment.

Insight for Immunotherapy of HCMV Infection

Human cytomegalovirus (HCMV), a ubiquitous in humans, has a high prevalence rate. Young people are susceptible to HCMV infection in developing countries, while older individuals are more susceptible in developed countries. Most patients have no obvious symptoms from the primary infection. Studies have indicated that the virus has gradually adapted to the host immune system. Therefore, the control of HCMV infection requires strong immune modulation. With the recent advances in immunotherapy, its application to HCMV infections is receiving increasing attention. Here, we discuss the immune response to HCMV infection, the immune escape mechanism, and the different roles that HCMV plays in various types of immunotherapy, including vaccines, adoptive cell therapy, checkpoint blockade therapy, and targeted antibodies.

Effect of everolimus-based drug regimens on CMV-specific T-cell functionality after renal transplantation: 12-month ATHENA subcohort-study results

Post-transplant cytomegalovirus (CMV) infections and increased viral replication are associated with CMV-specific T-cell anergy. In the ATHENA-study, de-novo everolimus (EVR) with reduced-exposure tacrolimus (TAC) or cyclosporine (CyA) showed significant benefit in preventing CMV infections in renal transplant recipients as compared to standard TAC + mycophenolic acid (MPA). However, immunomodulatory mechanisms for this effect remain largely unknown. Ninety patients from the ATHENA-study completing the 12-month visit on-treatment (EVR + TAC n = 28; EVR + CyA n = 19; MPA + TAC n = 43) were included in a posthoc analysis. Total lymphocyte subpopulations were quantified. CMV-specific CD4 T cells were determined after stimulation with CMV-antigen, and cytokine-profiles and various T-cell anergy markers were analyzed using flow cytometry. While 25.6% of MPA + TAC-treated patients had CMV-infections, no such events were reported in EVR-treated patients. Absolute numbers of lymphocyte subpopulations were comparable between arms, whereas the percentage of regulatory T cells was significantly higher with EVR + CyA versus MPA + TAC (p = 0.019). Despite similar percentages of CMV-specific T cells, their median expression of CTLA-4 and PD-1 was lower with EVR + TAC (p < 0.05 for both) or EVR + CyA (p = 0.045 for CTLA-4) compared with MPA + TAC. Moreover, mean percentages of multifunctional CMV-specific T cells were higher with EVR + TAC (27.2%) and EVR + CyA (29.4%) than with MPA + TAC (19.0%). In conclusion, EVR-treated patients retained CMV-specific T-cell functionality, which may contribute to enhanced protection against CMV infections.

The CD4+ T Cell Response to Human Cytomegalovirus in Healthy and Immunocompromised People

While CD8+ T cells specific for human cytomegalovirus (HCMV) have been extensively studied in both healthy HCMV seropositive carriers and patients undergoing immunosuppression, studies on the CD4+ T cell response to HCMV had lagged behind. However, over the last few years there has been a significant advance in our understanding of the importance and contribution that CMV-specific CD4+ T cells make, not only to anti-viral immunity but also in the potential maintenance of latently infected cells. During primary infection with HCMV in adults, CD4+ T cells are important for the resolution of symptomatic disease, while persistent shedding of HCMV into urine and saliva is associated with a lack of HCMV specific CD4+ T cell response in young children. In immunosuppressed solid organ transplant recipients, a delayed appearance of HCMV-specific CD4+ T cells is associated with prolonged viremia and more severe clinical disease, while in haematopoietic stem cell transplant recipients, it has been suggested that HCMV-specific CD4+ T cells are required for HCMV-specific CD8+ T cells to exert their anti-viral effects. In addition, adoptive T-cell immunotherapy in transplant patients has shown that the presence of HCMV-specific CD4+ T cells is required for the maintenance of HCMV-specific CD8+ T cells. HCMV is a paradigm for immune evasion. The presence of viral genes that down-regulate MHC class II molecules and the expression of viral IL-10 both limit antigen presentation to CD4+ T cells, underlining the important role that this T cell subset has in antiviral immunity. This review will discuss the antigen specificity, effector function, phenotype and direct anti-viral properties of HCMV specific CD4+ T cells, as well as reviewing our understanding of the importance of this T cell subset in primary infection and long-term carriage in healthy individuals. In addition, their role and importance in congenital HCMV infection and during immunosuppression in both solid organ and haemopoietic stem cell transplantation is considered.

Elite athletes on regular training show more pronounced induction of vaccine-specific T-cells and antibodies after tetravalent influenza vaccination than controls

Compliance of elite athletes with vaccination recommendations is low mainly based on concerns about side-effects and perceived poor vaccine efficacy due to continued physical training. We therefore employed seasonal influenza vaccination to investigate the effect of regular physical training on vaccine-induced cellular and humoral immunity in elite athletes and controls. Lymphocyte subpopulations and vaccine-specific T-cells were quantified and functionally characterized from 45 athletes and 25 controls before, and 1, 2 and 26 weeks after vaccination. Moreover, influenza-specific antibodies and their neutralizing function were quantified. Both groups showed a significant increase in vaccine-reactive CD4 T-cell levels which peaked one week after vaccination (p < 0.0001). The increase was significantly more pronounced in athletes (4.1-fold) compared to controls (2.3-fold; p = 0.0007). The cytokine profile changed from multifunctional T-cells co-producing IFNγ, IL-2 and TNFα to cells with restricted cytokine expression. This change in functionality was associated with a significant increase in CTLA-4 expression (p < 0.0001), which again was more pronounced in athletes. Likewise, the increase in neutralizing antibodies was stronger in athletes (p = 0.004 for H1N1; p = 0.032 for H3N2). In conclusion, both groups mounted a strong vaccine-specific cellular and humoral immunity after standard vaccination. The more pronounced increase in specific T-cells and neutralizing antibodies indicates that high frequency and intensity of training enhance vaccine-responses in elite athletes.

Impaired polyfunctionality of CD8+ T cells in severe sepsis patients with human cytomegalovirus reactivation

Human cytomegalovirus (HCMV) establishes a lifelong chronic latent infection and often reactivates in immunocompromised patients. In addition, HCMV reactivates in patients with sepsis or other critical illnesses, particularly in patients with poor prognoses. However, the immunological characteristics of sepsis patients with HCMV reactivation have not been elucidated. In the present study, we examined T-cell responses in severe sepsis patients with and without HCMV reactivation. First, HCMV pp65-specific T-cell functions were assessed by intracellular cytokine staining (ICS) for IFN-γ, TNF-α, and MIP-1β and by CD107a staining. We analyzed the ICS data for each function individually and found no difference between the patient groups. However, the relative frequency of polyfunctional CD8⁺ T cells was significantly decreased in sepsis patients with HCMV reactivation. Next, we examined programmed cell death protein 1 (PD-1) expression. It was significantly increased in the CD8⁺ T-cell population in severe sepsis patients with HCMV reactivation, indicating CD8⁺ T-cell exhaustion. Interestingly, the frequency of PD-1⁺ cells in the CD8⁺ T-cell population was inversely correlated with the relative frequency of polyfunctional CD8⁺ T cells. Herein, we demonstrate that HCMV reactivation in severe sepsis patients is associated with PD-1 expression and impaired polyfunctionality of CD8⁺ T cells.

CTLA-4-expression on VZV-specific T cells in CSF and blood is specifically increased in patients with VZV related central nervous system infections

VZV-reactivation may lead to symptomatic central nervous system (CNS) diseases, but identification of VZV as causative pathogen of CNS-diseases is challenging. This study was performed to characterize VZV-specific T cells from cerebrospinal fluid (CSF) and blood of patients with active CNS-disease and to determine whether this may improve differential diagnosis. 27 patients with pleocytosis in the CSF were recruited and classified into three groups (10 VZV-related, 10 non-VZV-related, 7 unclear). VZV-specific CD4⁺ T cells were quantified in CSF and blood after simultaneous stimulation with a VZV-antigen lysate and detection of cytokines (IFN-γ, IL-2, TNF-α) and CTLA-4. Polyclonal stimulation served as positive control. VZV-specific CD4⁺ T-cell frequencies were highest in both CSF (p = 0.0001) and blood (p = 0.011) of patients with VZV-infection, and were enriched at the site of infection (p = 0.002). While cytokine-expression profiles only showed minor differences between the groups, CTLA-4-expression levels on VZV-specific T cells from CSF and blood were significantly increased in VZV-related CNS-infections (p = 0.0002 and p<0.0001) and clearly identified VZV-related CNS-diseases (100% sensitivity and 100% specificity). Polyclonally stimulated T cells did not show any quantitative and phenotypical differences between the groups. Increased frequency and CTLA-4-expression of VZV-specific T cells from CSF or blood are specifically found in patients with VZV-related CNS-infection.

Validation of T-Track® CMV to assess the functionality of cytomegalovirus-reactive cell-mediated immunity in hemodialysis patients

Background

Uncontrolled cytomegalovirus (CMV) replication in immunocompromised solid-organ transplant recipients is a clinically relevant issue and an indication of impaired CMV-specific cell-mediated immunity (CMI). Primary aim of this study was to assess the suitability of the immune monitoring tool T-Track® CMV to determine CMV-reactive CMI in a cohort of hemodialysis patients representative of patients eligible for renal transplantation. Positive and negative agreement of T-Track® CMV with CMV serology was examined in 124 hemodialysis patients, of whom 67 (54%) revealed a positive CMV serostatus. Secondary aim of the study was to evaluate T-Track® CMV performance against two unrelated CMV-specific CMI monitoring assays, QuantiFERON®-CMV and a cocktail of six class I iTAg™ MHC Tetramers.

Results

Positive T-Track® CMV results were obtained in 90% (60/67) of CMV-seropositive hemodialysis patients. In comparison, 73% (45/62) and 77% (40/52) positive agreement with CMV serology was achieved using QuantiFERON®-CMV and iTAg™ MHC Tetramer. Positive T-Track® CMV responses in CMV-seropositive patients were dominated by pp65-reactive cells (58/67 [87%]), while IE-1-responsive cells contributed to an improved (87% to 90%) positive agreement of T-Track® CMV with CMV serology. Interestingly, T-Track® CMV, QuantiFERON®-CMV and iTAg™ MHC Tetramers showed 79% (45/57), 87% (48/55) and 93% (42/45) negative agreement with serology, respectively, and a strong inter-assay variability. Notably, T-Track® CMV was able to detect IE-1-reactive cells in blood samples of patients with a negative CMV serology, suggesting either a previous exposure to CMV that yielded a cellular but no humoral immune response, or TCR cross-reactivity with foreign antigens, both suggesting a possible protective immunity against CMV in these patients.

Conclusion

T-Track® CMV is a highly sensitive assay, enabling the functional assessment of CMV-responsive cells in hemodialysis patients prior to renal transplantation. T-Track® CMV thus represents a valuable immune monitoring tool to identify candidate transplant recipients potentially at increased risk for CMV-related clinical complications.

Electronic supplementary material

The online version of this article (doi:10.1186/s12865-017-0194-z) contains supplementary material, which is available to authorized users.

Human Cytomegalovirus (HCMV)-Specific CD4+ T Cells Are Polyfunctional and Can Respond to HCMV-Infected Dendritic Cells In Vitro

Human cytomegalovirus (HCMV) infection and periodic reactivation are generally well controlled by the HCMV-specific T cell response in healthy people. While the CD8⁺ T cell response to HCMV has been extensively studied, the HCMV-specific CD4⁺ T cell effector response is not as well understood, especially in the context of direct interactions with HCMV-infected cells. We screened the gamma interferon (IFN-γ) and interleukin-10 (IL-10) responses to 6 HCMV peptide pools (pp65, pp71, IE1, IE2, gB, and US3, selected because they were the peptides most frequently responded to in our previous studies) in 84 donors aged 23 to 74 years. The HCMV-specific CD4⁺ T cell response to pp65, IE1, IE2, and gB was predominantly Th1 biased, with neither the loss nor the accumulation of these responses occurring with increasing age. A larger proportion of donors produced an IL-10 response to pp71 and US3, but the IFN-γ response was still dominant. CD4⁺ T cells specific to the HCMV proteins studied were predominantly effector memory cells and produced both cytotoxic (CD107a expression) and cytokine (macrophage inflammatory protein 1β secretion) effector responses. Importantly, when we measured the CD4⁺ T cell response to cytomegalovirus (CMV)-infected dendritic cells in vitro, we observed that the CD4⁺ T cells produced a range of cytotoxic and secretory effector functions, despite the presence of CMV-encoded immune evasion molecules. CD4⁺ T cell responses to HCMV-infected dendritic cells were sufficient to control the dissemination of virus in an in vitro assay. Together, the results show that HCMV-specific CD4⁺ T cell responses, even those from elderly individuals, are highly functional and are directly antiviral.IMPORTANCE Human cytomegalovirus (HCMV) infection is carried for a lifetime and in healthy people is kept under control by the immune system. HCMV has evolved many mechanisms to evade the immune response, possibly explaining why the virus is never eliminated during the host's lifetime. The dysfunction of immune cells associated with the long-term carriage of HCMV has been linked with poor responses to new pathogens and vaccines when people are older. In this study, we investigated the response of a subset of immune cells (CD4⁺ T cells) to HCMV proteins in healthy donors of all ages, and we demonstrate that the functionality of CD4⁺ T cells is maintained. We also show that CD4⁺ T cells produce effector functions in response to HCMV-infected cells and can prevent virus spread. Our work demonstrates that these HCMV-specific immune cells retain many important functions and help to prevent deleterious HCMV disease in healthy older people.

Anti-PD-1/PD-L1 therapy for infectious diseases: learning from the cancer paradigm

OBJECTIVES

Immune checkpoint pathways regulate optimal host immune responses against transformed cells, induce immunological memory, and limit tissue pathology. Conversely, aberrant immune checkpoint activity signifies a poor prognosis in cancer and infectious diseases. Host-directed therapy (HDT) via immune checkpoint blockade has revolutionized cancer treatment with therapeutic implications for chronic infections, thus laying the foundation for this review.

METHODS

Online literature searches were performed via PubMed, PubMed Central, and Google using the keywords "immune checkpoint inhibition"; "host-directed therapy"; "T cell exhaustion"; "cancer immunotherapy"; "anti-PD-1 therapy"; "anti-PD-L1 therapy"; "chronic infections"; "antigen-specific cells"; "tuberculosis"; "malaria"; "viral infections"; "human immunodeficiency virus"; "hepatitis B virus"; "hepatitis C virus"; "cytomegalovirus" and "Epstein-Barr virus". Search results were filtered based on relevance to the topics covered in this review.

RESULTS

The use of monoclonal antibodies directed against the antigen-experienced T-cell marker programmed cell death 1 (PD-1) and its ligand PD-L1 in the context of chronic infectious diseases is reviewed. The potential pitfalls and precautions, based on clinical experience from treating patients with cancer with PD-1/PD-L1 pathway inhibitors, are also described.

CONCLUSIONS

Anti-PD-1/PD-L1 therapy holds promise as adjunctive therapy for chronic infectious diseases such as tuberculosis and HIV, and must therefore be tested in randomized clinical trials.

The "ABC" of Virus-Specific T Cell Immunity in Solid Organ Transplantation

Transplant patients are at increased risk of viral complications due to impaired control of viral replication, resulting from HLA mismatching between graft and host and the immunosuppression needed to avert alloimmune reactions. In the past decade, quantitative viral load measurements have become widely available to identify patients at risk and to inform treatment decisions with respect to immunosuppressive drugs and antiviral therapies. Because viral loads are viewed as the result of viral replication and virus-specific immune control, virus-specific T cell monitoring has been explored to optimize management of adenovirus, BK polyomavirus and cytomegalovirus ("ABC") in transplant patients. Although most studies are descriptive using different technologies, the overall results show that the quantity and quality of virus-specific T cells inversely correlate with viral replication, whereby strong cellular immune responses are associated with containment of viral replication. The key obstacles to the introduction of assays for virus-specific T cells into clinical practice is the definition of reliable cutoffs for clinical decision making, the poor negative predictive value of some assays, and the absence of interventional trials justifying changes of antiviral treatment or immunosuppression. More clinical research is needed using optimized assays and targets before standardization and commutability can be envisaged as achieved for viral load testing.

Plasma IL-10 Levels to Guide Antiviral Prophylaxis Prevention of Late-Onset Cytomegalovirus Disease, in High Risk Solid Kidney and Liver Transplant Recipients

BACKGROUND

Immune measurements that distinguish solid organ transplantation (SOT) recipients who control cytomegalovirus (CMV) infection from those who progress to CMV-disease (CMV-dz) may be clinically useful in guiding tailored prevention strategies. We previously reported that elevated plasma levels of the immune-modulator IL-10 are associated with late CMV-dz. Here we evaluate whether IL-10 levels measured soon after prophylaxis discontinuation are predictive of CMV-dz risk.

METHODS

Plasma IL-10 levels were quantitatively measured by ELISA kit in 40 D/R SOT patients. All 40 D/R high-risk patients were prospectively followed for at least 12 months post-SOT: 13 subjects developed CMV-dz, all within 6 months of prophylaxis discontinuation.

RESULTS

IL-10 was detectable at the first post-prophylaxis measurement for 11 of 13 subjects who developed CMV-dz. In contrast, IL-10 was detectable in only 6 of 27 CMV asymptomatic patients. Monitoring IL-10 plasma levels within 1 month prophylaxis suspension appeared to have clinically useful level of 85% sensitivity and 78% specificity.

CONCLUSIONS

The exact role of IL-10 with its multiple immunoregulatory effects during CMV infection is not clear. Moreover, IL-10 production can be influenced by pathological and infectious contexts, and/or anti-rejection immunosuppressant therapy. Despite mechanisms of IL-10 dysregulation may substantially differ among SOT patients, our findings suggest that measurable plasma IL-10 soon after prophylaxis discontinuation may be an adequate indicator of subsequent CMV-dz. If a similar prognostic performance is confirmed in a larger D/R cohort, IL-10 plasma levels could be used to guide the length of prophylaxis, providing a clinically useful means to reduce the incidence of CMV-dz in high risk patients.

Loss of CD28 on Peripheral T Cells Decreases the Risk for Early Acute Rejection after Kidney Transplantation

Background

End-stage renal disease patients have a dysfunctional, prematurely aged peripheral T-cell system. Here we hypothesized that the degree of premature T-cell ageing before kidney transplantation predicts the risk for early acute allograft rejection (EAR).

Methods

222 living donor kidney transplant recipients were prospectively analyzed. EAR was defined as biopsy proven acute allograft rejection within 3 months after kidney transplantation. The differentiation status of circulating T cells, the relative telomere length and the number of CD31⁺ naive T cells were determined as T-cell ageing parameters.

Results

Of the 222 patients analyzed, 30 (14%) developed an EAR. The donor age and the historical panel reactive antibody score were significantly higher (p = 0.024 and p = 0.039 respectively) and the number of related donor kidney transplantation was significantly lower (p = 0.018) in the EAR group. EAR-patients showed lower CD4⁺CD28null T-cell numbers (p<0.01) and the same trend was observed for CD8⁺CD28null T-cell numbers (p = 0.08). No differences regarding the other ageing parameters were found. A multivariate Cox regression analysis showed that higher CD4⁺CD28null T-cell numbers was associated with a lower risk for EAR (HR: 0.65, p = 0.028). In vitro, a significant lower percentage of alloreactive T cells was observed within CD28null T cells (p<0.001).

Conclusion

Immunological ageing-related expansion of highly differentiated CD28null T cells is associated with a lower risk for EAR.

The Immunology of Posttransplant CMV Infection: Potential Effect of CMV Immunoglobulins on Distinct Components of the Immune Response to CMV

The immune response to cytomegalovirus (CMV) infection is highly complex, including humoral, cellular, innate, and adaptive immune responses. Detection of CMV by the innate immune system triggers production of type I IFNs and inflammatory cytokines which initiate cellular and humoral responses that are critical during the early viremic phase of CMV infection. Sustained control of CMV infection is largely accounted for by cellular immunity, involving various T-cell and B-cell subsets. In solid organ transplant patients, global suppression of innate and adaptive immunities by immunosuppressive agents limits immunological defense, including inhibition of natural killer cell activity with ongoing lowering of Ig levels and CMV-specific antibody titers. This is coupled with a short-term suppression of CMV-specific T cells, the extent and duration of which can predict risk of progression to CMV viremia. CMV immunoglobulin (CMVIG) preparations have the potential to exert immunomodulatory effects as well as providing passive immunization. Specific CMVIG antibodies and virus neutralization might be enhanced by modulation of dendritic cell activity and by a decrease in T-cell activation, effects which are of importance during the initial phase of infection. In summary, the role of CMVIG in reconstituting specific anti-CMV antibodies may be enhanced by some degree of modulation of the innate and adaptive immune responses, which could help to control some of the direct and indirect effects of CMV infection.

BK polyomavirus-specific cellular immune responses are age-dependent and strongly correlate with phases of virus replication

BK polyomavirus (BKPyV) infection is widespread and typically asymptomatic during childhood, but may cause nephropathy in kidney transplant recipients. However, there is only limited knowledge on BKPyV-specific immunity in children and adults, and its role in BKPyV-replication and disease posttransplant. We therefore characterized BKPyV-specific immunity from 122 immunocompetent individuals (1-84 years), 38 adult kidney recipients with (n = 14) and without BKPyV-associated complications (n = 24), and 25 hemodialysis (HD) patients. Blood samples were stimulated with overlapping peptides of BKPyV large-T antigen and VP1 followed by flow-cytometric analysis of activated CD4 T cells expressing interferon-γ, IL-2 and tumor necrosis factor-α. Antibody-levels were determined using enzyme-linked immunosorbent assay. Both BKPyV-IgG levels and BKPyV-specific CD4 T cell frequencies were age-dependent (p = 0.0059) with maximum levels between 20 and 30 years (0.042%, interquartile range 0.05%). Transplant recipients showed a significantly higher BKPyV-specific T cell prevalence (57.9%) compared to age-matched controls (21.7%) or HD patients (28%, p = 0.017). Clinically relevant BKPyV-replication was associated with elevated frequencies of BKPyV-specific T cells (p = 0.0002), but decreased percentage of cells expressing multiple cytokines (p = 0.009). In conclusion, BKPyV-specific cellular immunity reflects phases of active BKPyV-replication either after primary infection in childhood or during reactivation after transplantation. Combined analysis of BKPyV-specific T cell functionality and viral loads may improve individual risk assessment.

Clinical immune-monitoring strategies for predicting infection risk in solid organ transplantation

Infectious complications remain a leading cause of morbidity and mortality after solid organ transplantation (SOT), and largely depend on the net state of immunosuppression achieved with current regimens. Cytomegalovirus (CMV) is a major opportunistic viral pathogen in this setting. The application of strategies of immunological monitoring in SOT recipients would allow tailoring of immunosuppression and prophylaxis practices according to the individual's actual risk of infection. Immune monitoring may be pathogen-specific or nonspecific. Nonspecific immune monitoring may rely on either the quantification of peripheral blood biomarkers that reflect the status of a given arm of the immune response (serum immunoglobulins and complement factors, lymphocyte sub-populations, soluble form of CD30), or on the functional assessment of T-cell responsiveness (release of intracellular adenosine triphosphate following a mitogenic stimulus). In addition, various methods are currently available for monitoring pathogen-specific responses, such as CMV-specific T-cell-mediated immune response, based on interferon-γ release assays, intracellular cytokine staining or main histocompatibility complex-tetramer technology. This review summarizes the clinical evidence to date supporting the use of these approaches to the post-transplant immune status, as well as their potential limitations. Intervention studies based on validated strategies for immune monitoring still need to be performed.