Targeted individual prophylaxis offers superior risk stratification for cytomegalovirus reactivation after liver transplantation

Predicting Healthcare-Associated Infection in Patients with Pneumonia via QuantiFERON®-Monitoring

Objective

A functional immune system is essential for recovery from pneumonia; hence, measuring and monitoring immune-status indicators is clinically important. This study aimed to determine whether QuantiFERON monitoring (QMF) could predict healthcare-associated infection (HCAI) according to the immune-status of patients with pneumonia.

Methods

Prospective, observational, single-center study, patients ≥19 years hospitalized for pneumonia between October 2020 and November 2021. QFM was performed at hospital admission (D1) and seven days after (D2). Data from 90 patients in the D1 QFM group were analyzed, which was further divided into the non-healthcare-associated infection (non-HCAI, n = 41, 45.6%) and HCAI (n = 49, 54.4%) groups.

Results

The D1 and D2 QFM levels were both significantly higher in the non-HCAI group than in the HCAI group (D1 hCAI vs non-HCAI: 4.40 vs 5.75 IU/mL, D2 hCAI vs non-HCAI: 4.38 vs 6.10 IU/mL). Analysis of the change in D1 and D2 QFM levels by each group showed that D2 QFM levels increased over D1 QFM levels in the non-HCAI group (5.75 vs 6.10 IU/mL), while D2 QFM levels decreased over D1 QFM levels in the HCAI group (4.40 vs 4.38 IU/mL). D1 QFM was consistently negatively correlated with TNF-α and CRP. The integrated analysis of D1 QFM and CCI and D1 QFM and CURB-65 had fair to predict the occurrence of HCAI.

Conclusion

QFM can be used to predict the immune-status of patients in the context of healthcare-associated infections. These findings provide important insights into the current understanding of pneumonia treatment and recovery.

Immune Control of Human Cytomegalovirus (HCMV) Infection in HCMV-Seropositive Solid Organ Transplant Recipients: The Predictive Role of Different Immunological Assays

Human cytomegalovirus (HCMV) infection remains a major complication for solid organ transplant recipients (SOTRs). The aim of this study was to evaluate the role of HCMV-specific T cell immunity measured at the time of the HCMV-DNA peak in predicting the spontaneous clearance of infection. The performance of cytokine flow cytometry using infected dendritic cells (CFC-iDC), infected cell lysate (CFC-iCL) and pp65 peptide pool (CFC-pp65 pool) as stimuli, as well as ELISPOT assays using infected cell lysate (ELISPOT-iCL) and the pp65 peptide pool (ELISPOT-pp65 pool), was analysed. Among the 40 SOTRs enrolled, 16 patients (40%) required antiviral treatment for an HCMV infection (Non-Controllers), while the others spontaneously cleared the infection (Controllers). At the HCMV-DNA peak, the number of HCMV-specific CD4+ T cells detected by the CFC-iDC, CFC-iCL and CFC-pp65 pool assays in Controllers was higher than that detected in Non-Controllers, while no difference was observed in terms of HCMV-specific CD8+ T cell response. The same trend was observed when the HCMV-specific T cell response was measured by ELISPOT-iCL and ELISPOT-pp65 pool. We observed that the CD4+ CFC-pp65 pool assay was the best predictor of self-resolving HCMV infection at the time of the HCVM-DNA peak. The CFC-pp65 pool assay is able to discriminate between CD4+ and CD8+ T cell responses and could be used in daily clinical practice.

QuantiFERON-CMV as a Predictor of CMV Events During Preemptive Therapy in CMV-seropositive Kidney Transplant Recipients

BACKGROUND

Prevention of cytomegalovirus (CMV) infection after kidney transplantation is costly and burdensome.

METHODS

Given its promising utility in risk stratification, we evaluated the use of QuantiFERON-CMV (QFCMV) and additional clinical variables in this prospective cohort study to predict the first clinically significant CMV infection (CS-CMV, ranging from asymptomatic viremia requiring treatment to CMV disease) in the first posttransplant year. A cost-effectiveness analysis for guided prevention was done.

RESULTS

One hundred adult kidney transplant recipients, CMV IgG + , were given basiliximab induction and maintained on steroid/mycophenolate/tacrolimus with weekly CMV monitoring. Thirty-nine patients developed CS-CMV infection (viral syndrome, n = 1; end-organ disease, n = 9; and asymptomatic viremia, n = 29). A nonreactive or indeterminate QFCMV result using the standard threshold around day 30 (but not before transplant) was associated with CS-CMV rates of 50% and 75%, respectively. A higher QFCMV threshold for reactivity (>1.0 IU interferon-γ/mL) outperformed the manufacturer's standard (>0.2 IU interferon-γ/mL) in predicting protection but still allowed a 16% incidence of CS-CMV. The combination of recipient age and type of donor, along with posttransplant QFCMV resulted in a prediction model that increased the negative predictive value from 84% (QFCMV alone) to 93%. QFCMV-guided preemptive therapy was of lower cost than preemptive therapy alone ( P  < 0.001, probabilistic sensitivity analysis) and was cost-effective (incremental net monetary benefit of 210 USD) assuming willingness-to-pay of 2000 USD to avoid 1 CMV disease.

CONCLUSIONS

Guided CMV prevention by the prediction model with QFCMV is cost-effective and would spare from CMV surveillance in 42% of patients with low risk for CS-CMV.

Cytomegalovirus-Specific T Cells in Pediatric Liver Transplant Recipients

Cytomegalovirus (CMV) infection is a major opportunistic infection after liver transplantation (LT) that necessitates monitoring. Because of the lack of studies in children, we aimed to investigate CMV-specific T cell immune reconstitution among pediatric LT recipients. The recipients were monitored for CMV infection and CMV-specific T cells from the start of immunosuppressive therapy until 48 weeks after LT. Clinically significant CMV viremia (csCMV) requiring preemptive therapy was defined as a CMV load of >2000 IU/mL. Peripheral blood CMV-specific T cells were analyzed by flow cytometry based on IFNγ secretion upon stimulation with CMV antigens including immediate early protein 1 (IE1) Ag, phosphoprotein 65 (pp65) Ag, and whole CMV lysate (wCMV). Of the 41 patients who underwent LT, 20 (48.8%) had csCMV. Most (17/20 patients) were asymptomatic and characterized as experiencing CMV reactivation. The onset of csCMV occurred approximately 7 weeks after LT (interquartile range: 4-12.9); csCMV rarely recurred after preemptive therapy. Lower pp65-specific CD8+ T cell response was associated with the occurrence of csCMV (p = 0.01) and correlated with increased viral load at the time of csCMV diagnosis (ρ = -0.553, p = 0.02). Moreover, those with csCMV had lower percentages of IE1-specific CD4+ and wCMV-reactive CD4+ T cells at 12 weeks after LT (p = 0.03 and p = 0.01, respectively). Despite intense immunosuppressive therapy, CMV-specific T cell immune reconstitution occurred in pediatric patients post-LT, which could confer protection against CMV reactivation.

Impact of high MELD scores on CMV viremia following liver transplantation

INTRODUCTION

Advanced liver disease or cirrhosis is associated with an increased risk of infections; however, the impact of high pretransplant model for end-stage liver disease (MELD) score on cytomegalovirus (CMV) viremia after liver transplantation is unknown.

METHODS

This single-center, retrospective, cohort study evaluated CMV high-risk (CMV immunoglobulin G D+/R-) liver transplant recipients who received valganciclovir prophylaxis for 3 months between 2009 and 2019. Patients were stratified by pretransplant MELD score of <35 (low MELD) and ≥35 (high MELD). The primary outcome was 12-month CMV viremia, and secondary outcomes included CMV resistance and tissue invasive disease, mortality, biopsy-proven acute rejection (BPAR), leukopenia, and thrombocytopenia. Multivariable Cox proportional-hazards modeling was used to assess the association of MELD score with the time to CMV viremia.

RESULTS

There were 162 and 79 patients in the low and high MELD groups, respectively. Pretransplant MELD score ≥35 was associated with an increased risk of CMV viremia (hazard ratio [HR] 1.73; confidence interval 1.06-2.82, p = .03). CMV viremia occurred at 162 ± 61 days in the low MELD group and 139 ± 62 days in the high MELD group. Although BPAR occurred early at 30 days (13-59) in the low-MELD group and at 18 days (11-66) in the high-MELD group (p = .56), BPAR was not associated with an increased risk of CMV viremia (HR 1.55 [0.93-2.60], p = .1).

DISCUSSION

MELD scores ≥35 were associated with an increased hazards of CMV viremia. In liver transplant recipients with MELD scores ≥35 who are CMV high-risk, additional CMV intervention may be warranted.

Role of cytomegalovirus specific cell-mediated immunity in the monitoring of cytomegalovirus infection among living donor liver transplantation adult recipients: A single-center experience

BACKGROUND

Cytomegalovirus (CMV) is one of the most common post-transplant viral infections causing significant morbidity and occasional mortality. Limited literature on the potential role of pre-transplant CMV-specific cell-mediated immunity (CMV-CMI) is available. This study aimed to evaluate the clinical utility of pre-transplant CMV-CMI monitoring in the occurrence of post-transplant CMV infection.

METHODS

This was a prospective, observational study where all adult CMV seropositive patients undergoing living donor liver transplantation at a tertiary care institute were enrolled. CMV-CMI was measured using QuantiFERON-CMV (Qiagen GmbH, Hilden, Germany) and interpreted as positive if the value was ≥0.2 IU/ml, 1-2 days prior to the transplant. Based on pre-transplant CMV-CMI, cases were classified into Group 1 (n = 13, 43.3%) (positive) and Group 2 (n = 17, 56.7%) (negative). CMV infection was defined as the detection of CMV-DNA > 2.7 log₁₀ IU/ml in plasma specimens.

RESULTS

The mean age was 43 years with male (n = 29, 96.9%) predominance. Overall 40% of recipients developed post-transplant CMV infection, two (15.4%) in group 1 and 10 (58.8%) in group 2 (p-value = 0.016). Recipients in group 2 had 87% higher odds (odds ratio 0.13, confidence interval [CI] 95) of developing post-transplant CMV infection compared to group 1. The overall median duration of occurrence of post-transplant CMV infection was 26 days with the median viral load being 2.8 log₁₀ IU/ml. The treatment duration was 13 days in group 1 and 28 days in group 2 (p = 0.003). Group 1 recipients showed rapid clearance of CMV-DNA within 7 days compared to group 2 in which it was 21 days (p = 0.004, CI 95).

CONCLUSION

Pre-transplant CMV-CMI may play a protective role against post-transplant CMV infection and can serve as an adjunct for pre-transplant risk stratification.

Comparison of Three Cellular Assays to Predict the Course of CMV Infection in Liver Transplant Recipients

To estimate protection from cytomegalovirus (CMV) replication after solid organ transplantation, CMV serology has been considered insufficient and thus CMV immunity is increasingly assessed by cellular in vitro methods. We compared two commercially available IFN-γ ELISpot assays (T-Track CMV and T-SPOT.CMV) and an IFN-γ ELISA (QuantiFERON-CMV). Currently, there is no study comparing these three assays. The assays were performed in 56 liver transplant recipients at the end of antiviral prophylaxis and one month thereafter. In CMV high- or intermediate-risk patients the two ELISpot assays showed significant correlation (p < 0.0001, r > 0.6) but the correlation of the ELISpot assays with QuantiFERON-CMV was weaker. Results of both ELISpot assays were similarly predictive of protection from CMV-DNAemia ≥500 copies/mL [CMV pp65 T-SPOT.CMV at the end of prophylaxis: area under curve (AUC) = 0.744, cut-off 142 spot forming units (SFU), sensitivity set to 100%, specificity 46%; CMV IE-1 T-Track CMV at month 1: AUC = 0.762, cut-off 3.5 SFU, sensitivity set to 100%, specificity 59%]. The QuantiFERON-CMV assay was inferior, reaching a specificity of 23% when setting the sensitivity to 100%. In conclusion, both CMV-specific ELISpot assays appear suitable to assess protection from CMV infection/reactivation in liver transplant recipients.

Immunosuppressive Agents and Infectious Risk in Transplantation: Managing the "Net State of Immunosuppression"

Successful solid organ transplantation reflects meticulous attention to the details of immunosuppression, balancing risks for graft rejection against risks for infection. The 'net state of immune suppression' is a conceptual framework of all factors contributing to infectious risk. Assays which measure immune function in the immunosuppressed transplant recipient relative to infectious risk and allograft function are lacking. The best measures of integrated immune function may be quantitative viral loads to assess the individual's ability to control latent viral infections. Few studies address adjustment of immunosuppression during active infections. Thus, confronted with infection in solid organ recipients, the management of immunosuppression is based largely on clinical experience. This review examines known measures of immune function and the immunologic effects of common immunosuppressive drugs and available studies reporting modification of drug regimens for specific infections. These data provide a conceptual framework for the management of immunosuppression during infection in organ recipients.

Use of Viral Load as a Surrogate Marker in Clinical Studies of Cytomegalovirus in Solid Organ Transplantation: A Systematic Review and Meta-analysis

Symptomatic cytomegalovirus (CMV) disease has been the standard endpoint for clinical trials in organ transplant recipients. Viral load may be a more relevant endpoint due to low frequency of disease. We performed a meta-analysis and systematic review of the literature. We found several lines of evidence to support the validity of viral load as an appropriate surrogate end-point, including the following: (1) viral loads in CMV disease are significantly greater than in asymptomatic viremia (odds ratio, 9.3 95% confidence interval, 4.6-19.3); (2) kinetics of viral replication are strongly associated with progression to disease; (3) pooled incidence of CMV viremia and disease is significantly lower during prophylaxis compared with the full patient follow-up period (viremia incidence: 3.2% vs 34.3%; P < .001) (disease incidence: 1.1% vs 13.0%; P < .001); (4) treatment of viremia prevented disease; and (5) viral load decline correlated with symptom resolution. Based on the analysis, we conclude that CMV load is an appropriate surrogate endpoint for CMV trials in organ transplant recipients.

CMV specific T cell immunity predicts early viremia after liver transplantation

BACKGROUND

Cytomegalovirus (CMV) infection is one of the most important factors affecting liver transplant with direct and indirect effects. However, CMV disease after transplant remains poorly predicted.

OBJECTIVE

In this study, preoperative CMV-specific cell-mediated immunity was evaluated in recipients of liver transplant in Korea, where most people are seropositive.

METHODS

A total of 32 patients were enrolled in a prospective study, and blood samples were collected before liver transplant to determine CMV-specific cell-mediated immunity. Testing using ELiSpot IFN-γ (CMVspot) and CMV serology were performed simultaneously.

RESULTS

CMVspot results showed that 30 recipients had CMV-specific cell-mediated immunity, of which 29 were positive for phosphoprotein 65 and 14 for immediate early protein 1 (IE-1). All patients were positive for CMV IgG before transplantation, and 17 patients had a CMV viremia episode after transplantation. CMVspot showed 100% specificity and positive predictive value, and 11.76% sensitivity to predict CMV viremia. Patients with positive or borderline results for IE-1 did not show viremia two months after transplantation (p = .041).

CONCLUSION

CMVspot may be helpful in establishing a treatment strategy that includes regular monitoring for risk stratification of CMV reactivation.

Analysis of the QuantiFERON-CMV assay, CMV viraemia and antiviral treatment following solid organ transplantation in Western Australia

Prevention of cytomegalovirus (CMV) infection remains an important aspect of improving long term outcomes of solid organ transplantation and currently relies on prophylactic antiviral medication and early detection of viraemia or disease. Uptake of diagnostic tools to personalise assessment of CMV immunity and guide interpretation of viral testing remains low. We assessed the QuantiFERON-CMV assay in 54 Western Australian recipients of renal, heart, lung or liver allografts to determine the relationship between CMV-specific immunity, viraemia and disease following cessation of antiviral prophylaxis. We carried out an initial validation study which demonstrated that the QuantiFERON-CMV assay is highly precise and strongly correlated with CMV-specific antibodies in 30 healthy blood donors (sensitivity 82%, specificity 95%). In the solid organ transplant recipients we examined, the prevalence of asymptomatic CMV viraemia was high at 61% but only two patients ultimately developed CMV disease, both of whom had negative QuantiFERON-CMV responses, indicating lack of CMV T-cell immunity. The vast majority (94%) of patients who had spontaneous resolution or stability of asymptomatic CMV viraemia without any antiviral treatment had positive QuantiFERON-CMV responses. Positive QuantiFERON-CMV responses at cessation of antiviral prophylaxis were significantly associated with pre-transplant CMV seropositivity and the development of asymptomatic viraemia post-transplantation. Overall, 27% of patients were recommenced on antiviral therapy because of asymptomatic CMV viraemia. Patients with non-reactive QuantiFERON-CMV responses had earlier onset, higher level CMV viraemia compared to those with positive QuantiFERON-CMV responses, although the difference did not reach statistical significance. QuantiFERON-CMV results may contribute to decision making in concert with the serological risk profile, net state of immunosuppression and CMV viral load.

Early viral-specific T-cell testing predicts late cytomegalovirus reactivation following liver transplantation

INTRODUCTION

Although antiviral prophylaxis is effective in preventing early cytomegalovirus (CMV) reactivation following liver transplantation (OLT), it predisposes patients to late CMV after prophylaxis has ceased. QuantiFERON-CMV (QFN-CMV, Qiagen, The Netherlands) measures an individual's viral-specific immune response.

METHODS

Fifty-nine OLT recipients were prospectively monitored post-OLT in an observational cohort study. QFN-CMV was performed at regular time-points. An absolute QFN-CMV <0.1 IU/mL was considered non-reactive.

RESULTS

50/59 (84.7%) had a reactive QFN-CMV by M6. 38/59 (64.4%) had antiviral prophylaxis or treatment before M6, with 31/38 (81.6%) developing a reactive QFN-CMV by 6 months. Over 90% already had a reactive result as early as 3 months post transplant, 3 patients (5.08%) developed late CMV between 6-12 months (median 251 days)-all had a non-reactive M6 QFN-CMV. And 2/3 experienced CMV disease. Non-reactive M6 QFN-CMV was significantly associated with late CMV (OR = 54.4, PPV = 0.33, NPV = 1.00, P = .003).

CONCLUSION

Although only 5% of recipients developed late CMV, 2/3 suffered CMV disease. M6 QFN-CMV has an excellent NPV for late CMV, suggesting patients who exhibit a robust ex vivo immune response at M6 can safely cease CMV monitoring. Furthermore, >90% already express viral-specific immunity as early as 3 months. Conceivably, antiviral prophylaxis could be discontinued early in these patients.

Low-dose valganciclovir for cytomegalovirus prophylaxis in intermediate-risk liver transplantation recipients

Liver transplantation recipients (LTRs) who are seropositive for cytomegalovirus (CMV) (recipient seropositive [R+]) are at intermediate risk for CMV disease. A preventative strategy following transplant is considered standard of care. Current guidelines recommend high-dose valganciclovir (VGCV; 900 mg/day adjusted for renal function) for prophylaxis given limited data on the efficacy and safety of low-dose VGCV (450 mg/day adjusted for renal function). We describe our experience using low-dose VGCV prophylaxis for R+ LTRs at our institution. A single-center, retrospective study was conducted using a database of 364 LTRs over a 4-year period (2011-2014). Adult first-time R+ LTRs receiving low-dose VGCV prophylaxis were included. The primary endpoint was CMV disease at 1 year after transplant. Patients were compared with historical controls receiving high-dose VGCV prophylaxis. Secondary endpoints were biopsy-proven rejection and leukopenia on VGCV. With respect to leukopenia, patients receiving low-dose VGCV were compared with a group of D+R- patients from the database receiving high-dose VGCV. Univariate analyses were performed using chi-squared, Fisher's exact, and Wilcoxon rank sum tests. A total of 200 R+ LTRs met inclusion criteria. Median age was 60 years (interquartile range [IQR], 54-66 years), and 129 (65%) LTRs were male. Median Model for End-Stage Liver Disease score was 22 (IQR, 14-31), and 178 (89%) patients received deceased donor transplants. CMV disease occurred in only 9 (5%) patients, similar to rates in previous studies of LTRs receiving high-dose VGCV. Biopsy-proven rejection occurred in 18 (9%) patients. Patients received VGCV prophylaxis for a median of 3.4 (IQR, 3.1-4.3) months; 151 (76%) R+ LTRs receiving low-dose VGCV developed leukopenia. Premature VGCV discontinuation and granulocyte-colony stimulating factor use were infrequent and not significantly different between the 2 groups. In conclusion, low-dose VGCV was safe and effective for prevention of CMV disease in our cohort of 200 R+ LTR and should be considered as an option in future guidelines. Liver Transplantation 24 616-622 2018 AASLD.

Diagnostic utility of monitoring cytomegalovirus-specific immunity by QuantiFERON-cytomegalovirus assay in kidney transplant recipients

Background

Despite universal prophylaxis, late cytomegalovirus (CMV) infection occurs in a high proportion of kidney transplant recipients. We evaluated whether a specific viral T-cell response allows for the better identification of recipients who are at high risk of CMV infection after prophylaxis withdrawal.

Methods

We conducted a prospective study in 19 pretransplant anti-CMV seronegative kidney graft recipients R- (18 from seropositive donors [D+] and one from a seronegative donor [D-]) and 67 seropositive recipients R(+) (59 from seropositive donors and eight from seronegative donors) who received antiviral prophylaxis with valganciclovir. The QuantiFERON-CMV (QF-CMV) assay was performed within the first and third months after transplantation. Blood samples were monitored for CMV DNAemia using a commercial quantitative nucleic acid amplification test (QNAT) that was calibrated to the World Health Organization International Standard.

Results

Twenty-one of the 86 patients (24%) developed CMV viremia after prophylaxis withdrawal within 12 months posttransplantation. In the CMV R(+) group, the QF-CMV assay yielded reactive results (QF-CMV[+]) in 51 of 67 patients (76%) compared with 7 of 19 patients (37%) in the CMV R(−) group (p = 0.001). In the CMV R(+) group, infection occurred in seven of 16 recipients (44%) who were QF-CMV(−) and eight of 51 recipients (16%) who were QF-CMV(+). In the CMV R(−) group, infection evolved in five of 12 recipients (42%) who were QF-CMV(−) and one of 7 recipients (14%) who were QF-CMV(+). No difference was found in the incidence of CMV infection stratified according to the QF-CMV results with regard to the recipients’ pretransplant CMV IgG serology (p = 0.985). Cytomegalovirus infection occurred in 15 of 36 patients (42%) with hypogammaglobulinemia (HGG) 90 days posttransplantation compared with two of 34 patients (6%) without HGG (p = 0.0004). Cytomegalovirus infection occurred in seven of 13 patients (54%) with lymphocytopenia compared with 14 of 70 patients (20%) without lymphocytopenia (p = 0.015). The multivariate analysis revealed that the nonreactive QuantiFERON-CMV assay was an independent risk factor for postprophylaxis CMV infection.

Conclusions

In kidney transplant recipients who received posttransplantation prophylaxis, negative QF-CMV results better defined the risk of CMV infection than initial CMV IgG status after prophylaxis withdrawal. Hypogammaglobulinemia and lymphocytopenia were risk factors for CMV infection.

Immune Monitoring of Infectious Complications in Transplant Patients: an Important Step towards Improved Clinical Management

Immune reconstitution following organ transplantation is absolutely critical in preventing infectious complications. However, understanding the kinetics of immune reconstitution and its potential impact on the clinical management of transplant patients remains a significant challenge. Over the last decade, various platform technologies have emerged which have provided important insights into the immune reconstitution kinetics in transplant patients. However, many of these technologies are too complicated and cumbersome to implement in a clinical setting. In this issue of the Journal of Clinical Microbiology, Chiereghin et al. (J. Clin. Microbiol. 56:e01040-17, 2018, https://doi.org/10.1128/JCM.01040-17) report the results of their evaluation of the QuantiFERON-CMV (QFN-CMV) assay to assess human cytomegalovirus (CMV)-specific CD8+ T-cell immunity in heart transplant recipients as a prognostic tool. These studies showed that patients with absence of global immune reactivity in the QFN-CMV assay were at a higher risk of developing CMV after discontinuing antiviral prophylaxis. Furthermore, failure to reconstitute CMV-specific immunity after resolution of the first episode of viremia was associated with viral relapse. These observations, along with other recent clinical studies utilizing the QFN-CMV assay, demonstrate that systematic monitoring of antiviral immunity can be successfully used as a prognostic tool and also to guide changes to the clinical management of transplant patients.

Infectious Complications and Vaccinations in the Posttransplant Population

Infections remain a major cause of mortality and morbidity after both kidney and liver transplantation, and internists increasingly play a major role in diagnosing and treating these infections. Because of immunosuppression, solid organ transplant recipients do not often demonstrate classic signs and symptoms of infection and have a broader variety of common and opportunistic infections, many of which are generally more difficult to diagnose and treat. Although these patients have many risk factors for infection, a major determinant is the time after transplant as it relates to levels of immunosuppression, healing, and hospital or environmental exposures.