QuantiFERON-CMV assay by chemiluminescence immunoassay: Is it more suitable for real-live monitoring of transplant patients?

BACKGROUND

The QuantiFERONCMV (QF-CMV) assay is an interferon-gamma release assay (IGRA) used to monitor CMV-specific cell-mediated immunity (CMV-CMI) by ELISA in transplant patients. However, a chemiluminescent immunoassay (CLIA) has been developed to quantify IFNG in the QuantiFERON-Tuberculosis (TB) to detect latent TB infection.

OBJECTIVES

The aim of this work is to compare the results of QF-CMV by ELISA with those obtained by CLIA in an automated Liaison XL analyzer using the QuantiFERON-TB Gold Plus reagents.

STUDY DESIGN

The QF-CMV assay had been performed by ELISA in kidney and lung transplant patients between July 2019-April 2023 at the IMIBIC/Reina Sofía Hospital (Cordoba, Spain). The remaining QF-CMV supernatants had been preserved at -80 ºC from then. Now, the IFNG levels in the same samples were determined by CLIA.

RESULTS

One hundred and three QF-CMV supernatants from kidney (n = 50) and lung (n = 53) transplant patients were selected. An agreement of 87.4 % (kappa coefficient 0.788) between CLIA and ELISA was observed. Thirteen (12.6 %) discrepant results were detected. Some Indeterminate results by ELISA converted to Non-reactive by CLIA (0.53-0.92 IU/mL for Mitogen-Nil values). Likewise, borderline Non-reactive results by ELISA were above the 0.2 IU/mL cut-off by CLIA and then were Reactive (0.21-0.31 for CMV-Nil values).

CONCLUSION

CLIA shows substantial concordance with ELISA and acceptable discrepancies. The possible higher sensitivity of CLIA returns a higher number of Reactive results, which entails potential clinical consequences. Therefore, a new threshold to confer protection against CMV infection after transplantation needs to be defined.

Memory SARS-CoV-2 T-cell response in convalescent COVID-19 patients with undetectable specific IgG antibodies: a comparative study

Background

During the COVID-19 pandemic, a variable percentage of patients with SARS-CoV-2 infection failed to elicit humoral response. This study investigates whether patients with undetectable SARS-CoV-2 IgG are able to generate SARS-CoV-2 memory T cells with proliferative capacity upon stimulation.

Methods

This cross-sectional study was conducted with convalescent COVID-19 patients, diagnosed with a positive real-time PCR (RT-PCR) from nasal and pharyngeal swab specimens. COVID-19 patients were enrolled ≥3 months after the last PCR positive. Proliferative T-cell response after whole blood stimulation was assessed using the FASCIA assay.

Results

A total of 119 participants (86 PCR-confirmed COVID-19 patients and 33 healthy controls) were randomly filtered from an initial cohort. Of these 86 patients, 59 had detectable (seropositive) and 27 had undetectable (seronegative) SARS-CoV-2 IgG. Seropositive patients were subclassified as asymptomatic/mild or severe according to the oxygen supplementation requirement. SARS-CoV-2 CD3+ and CD4+ T cells showed significantly lower proliferative response in seronegative than in seropositive patients. The ROC curve analysis indicated that ≥ 5 CD4+ blasts/μL of blood defined a "positive SARS-CoV-2 T cell response". According to this cut-off, 93.2% of seropositive patients had a positive T-cell response compared to 50% of seronegative patients and 20% of negative controls (chi-square; p < 0.001).

Conclusions

This proliferative assay is useful not only to discriminate convalescent patients from negative controls, but also to distinguish seropositive patients from those with undetectable SARS-CoV-2 IgG antibodies. Memory T cells in seronegative patients are able to respond to SARSCoV-2 peptides, although at a lower magnitude than seropositive patients.

Immunoguided Discontinuation of Prophylaxis for Cytomegalovirus Disease in Kidney Transplant Recipients Treated with Antithymocyte Globulin: A Randomized Clinical Trial

BACKGROUND

Antiviral prophylaxis is recommended in cytomegalovirus (CMV)-seropositive kidney transplant (KT) recipients receiving antithymocyte globulin (ATG) as induction. An alternative strategy of premature discontinuation of prophylaxis after CMV-specific cell-mediated immunity (CMV-CMI) recovery (immunoguided prevention) has not been studied. The aim of this study was to evaluate whether it is effective and safe to discontinue prophylaxis when CMV-CMI is detected and to continue with preemptive therapy.

METHODS

In this open-label, non-inferiority clinical trial, patients were randomized 1:1 to follow immunoguided strategy, receiving prophylaxis (valganciclovir 900 mg daily) until CMV-CMI recovery or to receive fixed-duration prophylaxis until day +90. After prophylaxis, preemptive therapy (valganciclovir 900 mg twice daily) was indicated in both arms until month 6. The primary and secondary outcomes were incidence of CMV disease and replication, respectively, within the first 12 months. Desirability of outcome ranking (DOOR) assessed two deleterious events (CMV disease/replication and neutropenia).

RESULTS

A total of 150 CMV-seropositive KT recipients were randomly assigned. There was no difference in the incidence of CMV disease (0% vs. 2.7%; P = 0.149) and replication (17.1% vs. 13.5%; log-rank test, P = 0.422) between both arms. Incidence of neutropenia was lower in the immunoguided arm (9.2% vs. 37.8%; OR, 6.0; P < 0.001). A total of 66.1% of patients in the immunoguided arm showed a better DOOR, indicating a greater likelihood of a better outcome.

CONCLUSIONS

Prophylaxis can be prematurely discontinued in CMV-seropositive KT patients receiving ATG when CMV-CMI is recovered since no significant increase in the incidence of CMV replication or disease is observed.

Human cytomegalovirus (HCMV)-encoded microRNAs: potential biomarkers and clinical applications

HCMV-encoded microRNAs (hcmv-miRNAs) are non-coding and non-immunogenic molecules that target numerous cellular genes and allow the virus to modulate the host's signalling pathways, thus favouring viral survival and replication. Given their capacity to silence the human genes involved in various physiological processes, these hcmv-miRNAs have now emerged as a potential clinical biomarker in many human diseases. In this review, we summarize the evidence published on the diagnostic and prognostic value of hcmv-miRNAs in several human diseases and their clinical implications. Specifically, we discuss the role of hcmv-miRNAs in the development of cardiovascular diseases and cancer by silencing tumour suppressors. We also examine the current knowledge on the utility of some hcmv-miRNAs in predicting HCMV viraemia recurrence in transplant patients, as well as the interference of hcmv-miRNAs in the development of an appropriate immune response against other viral infections, which might have therapeutic implications.

Pretransplant CMV-Specific T-Cell Immunity But Not Dose of Antithymocyte Globulin Is Associated With Recovery of Specific Immunity After Kidney Transplantation

BACKGROUND

This is a prospective, multicenter, observational study in cytomegalovirus (CMV)-seropositive kidney transplant recipients with pretransplant CMV-specific cell-mediated immunity (CMV-CMI) receiving antithymocyte globulin (ATG). We aimed to investigate posttransplant CMV-CMI over time and the impact of the dose-dependent ATG.

METHODS

CMV-CMI was assessed at days +30, +45, +60, and +90 after transplantation with the QuantiFERON-CMV assay. A reactive result (interferon-γ [IFN-γ] ≥ 0.2 IU/mL) indicated a positive CMV-CMI.

RESULTS

A total of 78 positive CMV-CMI patients were enrolled in the study, of which 59.5% had a positive CMV-CMI at day +30 and 82.7% at day +90. Multivariate logistic regression analysis showed that ATG dose was not associated with positive CMV-CMI at any point. However, pretransplant IFN-γ level (>12 IU/mL vs ≤12 IU/mL) was associated with positive CMV-CMI at day +30 (odds ratio, 12.9; 95% confidence interval, 3.1-53.3; P < .001). In addition, all the patients who did not recover CMV-CMI at day +90 had a pretransplant IFN-γ level ≤12 IU/mL.

CONCLUSIONS

More than half of CMV-seropositive kidney transplant recipients receiving ATG recover (or maintain) CMV-CMI by the first month after transplantation. The pretransplant IFN-γ level, but not the ATG dose, shows a strong association with the kinetics of this recovery.

Lack of evidence of association between IFNG and IL28B polymorphisms and QuantiFERON-CMV test results in seropositive transplant patients

The aim of this study was to analyze the relationship between the IFNG +874 T/A and IL28B (rs12979860) C/T polymorphisms and the secretion of IFNG by CD8+ T cells after stimulation with cytomegalovirus (CMV) peptides, measured using QuantiFERON-CMV (QF-CMV) assay. A total of 184 CMV-seropositive solid organ transplant patients (108 kidney, 68 liver and 8 lung) were recruited. Of them, 151 patients were QF-CMV Reactive (IFNG ≥ 0.2 UI/mL) and 33 were Non-reactive. Genotype frequencies in the study population were TT (26.6%), AT (50.0%) and AA (23.4%) for IFNG +874 and CC (52.7%), CT (39.1%) and TT (8.2%) for IL28B (rs12979860). These frequencies did not significantly differ between QF-CMV Reactive and Non-reactive patients. Nor were any significant differences observed in the quantitative IFNG level among the genotypes in either the IFNG or the IL28 genes. When we analyzed whether these polymorphisms had any impact on the risk of CMV replication after transplantation, the adjusted analysis showed no association. In summary, our results showed that IFNG +874 T/A and IL28B (rs12979860) C/T polymorphisms are not associated with the IFNG response to CMV measured by the QuantiFERON-CMV assay, although these results should be confirmed with a higher number of patients.

Regular monitoring of cytomegalovirus-specific cell-mediated immunity in intermediate-risk kidney transplant recipients: predictive value of the immediate post-transplant assessment

OBJECTIVE

Previous studies on monitoring of post-transplant cytomegalovirus (CMV)-specific cell-mediated immunity (CMI) are limited by single-centre designs and disparate risk categories. We aimed to assess the clinical value of a regular monitoring strategy in a large multicentre cohort of intermediate-risk kidney transplant (KT) recipients.

METHODS

We recruited 124 CMV-seropositive KT recipients with no T-cell-depleting induction pre-emptively managed at four Spanish institutions. CMV-specific interferon-γ-producing CD4⁺ and CD8⁺ T cells were counted through the first post-transplant year by intracellular cytokine staining after stimulation with pp65 and immediate early-1 peptides (mean of six measurements per patient). The primary outcome was the occurrence of any CMV event (asymptomatic infection and/or disease). Optimal cut-off values for CMV-specific T cells were calculated at baseline and day 15.

RESULTS

Twelve-month cumulative incidence of CMV infection and/or disease was 47.6%. Patients with pre-transplant CMV-specific CD8⁺ T-cell count <1.0 cells/μL had greater risk of CMV events (adjusted hazard ratio (aHR) 2.84; p 0.054). When the CMI assessment was performed in the immediate post-transplant period (day 15), the presence of <2.0 CD8⁺ T cells/μL (aHR 2.18; p 0.034) or <1.0 CD4⁺ T cells/μL (aHR 2.43; p 0.016) also predicted the subsequent development of a CMV event. In addition, lower counts of CMV-specific CD4⁺ (but not CD8⁺) T cells at days 60 and 180 were associated with a higher incidence of late-onset events.

CONCLUSIONS

Monitoring for CMV-specific CMI in intermediate-risk KT recipients must be regular to reflect dynamic changes in overall immunosuppression and individual susceptibility. The early assessment at post-transplant day 15 remains particularly informative.

Impact of age and cytomegalovirus on CD8+ T-cell compartment remodeling after solid organ transplantation: A one-year follow-up study

Cytomegalovirus (CMV), a member of the β-herpesvirus family, is a major complicating infection in transplant patients. CMV latency has a long-term impact on CD8⁺ T-cell differentiation. It is unclear, however, whether this effect can be detected in one-year period. To investigate this, we analyzed the remodeling of the CD8⁺ T-cell compartment during the first year after solid organ transplantation. A total of 55 kidney or lung transplant patients were recruited. CD8⁺ T-cell subsets were prospectively analyzed at pretransplant, at 3 or 6months and 12months after transplantation (mo post-Tx). A significant increase in the frequency of CD27^-CD28^-CD8⁺ T cells (from 32.8% to 42.3%; p=0.014) was observed from pretransplant to 12mo post-Tx. Further analysis, however, showed that the largest expansion was observed from 3/6 to 12mo post-Tx whereas small non-significant variations were observed from pretransplant to 3/6mo post-Tx. The adjusted analysis showed that age and CMV seropositivity were statistically associated with the baseline frequency of CD27^-CD28^-CD8⁺ T cells. Additionally, CMV replication was related to the posttransplant expansion of this subpopulation, since it was not observed in patients without CMV viremia (24% vs. 4.2%). The results indicate that the expanded frequency associated with late CMV replication is additive to the baseline frequency related to aging and CMV seropositivity. If the expanded frequency remains at this high level for a long period it might have clinical consequences related to the control of future reactivations of CMV or of other related viruses.

Rural habitat as risk factor for hepatitis E virus seroconversion in HIV-infected patients: A prospective longitudinal study

Our objective was to determine the incidence and clinical manifestations of acute hepatitis E virus (HEV) in HIV-infected patients. A prospective longitudinal study including HIV-infected HEV-seronegative patients was conducted; HEV seroconversion (to IgG and/or IgM) was the main outcome variable. All patients were tested for HEV antibodies every 3-6 months. For patients who developed HEV seroconversion, a data collection protocol was followed to identify associated clinical manifestations and analytical alterations. A total of 627 patients (89.9%) were followed during a median of 11.96 months (IQR: 8.52-14.52 months) and formed the study population. Forty-one patients developed detectable anti-HEV antibodies (7.2 cases per 100 patients/year). Our study found a high incidence of HEV in HIV-infected patients in southern Spain strongly associated with a rural habitat.

Risk of tuberculosis after lung transplantation: the value of pretransplant chest computed tomography and the impact of mTOR inhibitors and azathioprine use

BACKGROUND

It is necessary to determine the incidence and risk factors for tuberculosis (TB), as well as strategies to assess and treat latent tuberculosis infection (LTBI) in lung transplant recipients.

METHODS

A retrospective cohort study of 398 lung transplant recipients was performed. Episodes of TB were studied and the incidence rate was calculated. Logistic regression analysis was used to analyze specific variables as potential risk factors for TB.

RESULTS

Median follow-up was 558 days (range 1-6636). Six cases (1.5%) of TB were documented in 398 transplant patients. The incidence density of TB was 406.3 cases/10(5) patient-years (95% confidence interval [CI] 164.7-845), which is higher than in the general population (13.10 cases/10(5) person-years). All cases occurred in the period 1993-2006, when the tuberculin skin test (TST) and treatment of LTBI in positive TST patients were not part of the protocol. Pretransplant computed tomography (CT) showed residual lesions in 50% of patients who developed TB, although the TST was negative and the chest radiograph was inconclusive. Multivariate analysis identified the presence of residual lesions in the pretransplant chest CT (odds ratio [OR] 11.5, 95% CI 1.9-69.1, P = 0.008), use of azathioprine (OR 10.6, 95% CI 1.1-99.1, P = 0.038), and use of everolimus (OR 6.7, 95% CI 1.1-39.8, P = 0.036) as independent risk factors for TB.

CONCLUSIONS

Residual lesions in the pretransplant chest CTs and the use of azathioprine and mTOR inhibitors are associated with the risk of TB.

Prevention strategies differentially modulate the impact of cytomegalovirus replication on CD8(+) T-cell differentiation in high-risk solid organ transplant patients

The present study aimed to determine whether antiviral prevention strategies against cytomegalovirus (CMV) infection used in high-risk D+R- solid organ transplanted patients can modulate the impact of CMV replication on CD8(+) T-cell differentiation. The different CD8(+) T-cell subpopulations were measured at a single point when at least one year had elapsed since transplantation. A total of 68 D+R- patients were included, of which 33 underwent pre-emptive therapy and 35 received prophylaxis. Multivariate analysis showed that CMV replication was associated with the expansion of CD28־ EMRA CD8(+) T cells in patients managed pre-emptively but not in patients under prophylaxis (21.4% vs. 3.6%). This finding is likely related to the higher frequency of CMV recurrence observed in patients under pre-emptive therapy compared to those under prophylaxis (75% vs. 14.3%; p < 0.001). In fact, multivariate analysis showed that having more than one replication episode was associated with a 17.2% increase (p = 0.001) in the percentage of CD28־ EMRA CD8(+) T cells compared to "no episode" and with a 10.9% increase with respect to "single episodes" (p = 0.025). Additionally, patients with IFNγ response to CMV (QuantiFERON-CMV Reactive) had a higher percentage of late-differentiated CD8(+) T cells than patients lacking this response. In summary, recurrent CMV replication in D+R- patients under pre-emptive therapy was associated with the expansion of CD28־ EMRA CD8(+) T cells, which might have a short-term beneficial effect related to the high functionality of this T-cell subpopulation. Nevertheless, we cannot rule out that this accumulation might have a long-term detrimental effect related to immunosenescence and inflammation.

Impact of the PROVAUR stewardship programme on linezolid resistance in a tertiary university hospital: a before-and-after interventional study

BACKGROUND

There is little evidence of the impact of antimicrobial stewardship programmes on antimicrobial resistance.

OBJECTIVES

To study the efficacy and safety of a package of educational and interventional measures to optimize linezolid use and its impact on bacterial resistance.

METHODS

A quasi-experimental study was designed and carried out before and after implementation of a stewardship programme in hospitalized patients with Gram-positive infections treated with linezolid.

RESULTS

The intervention reduced linezolid consumption by 76%. The risk of linezolid-resistant CoNS isolates (OR = 0.37; 95% CI = 0.27-0.49; P < 0.001) and Enterococcus faecalis (OR = 0.44; 95% CI = 0.21-0.90; P = 0.03) during the intervention period was lower than in the pre-intervention period.

CONCLUSIONS

A programme to optimize linezolid use can contribute to reducing the resistance rate of CoNS and E. faecalis to this antibiotic.

Interferon-γ production by CMV-specific CD8+ T lymphocytes provides protection against cytomegalovirus reactivation in critically ill patients

PURPOSE

To evaluate the usefulness of the secretion of interferon-γ (IFNγ) by cytomegalovirus (CMV)-specific CD8+ T cells to determine the risk of CMV reactivation in critically ill non-immunosuppressed patients.

METHODS

Two-center prospective cohort study including critically ill non-immunosuppressed CMV-seropositive patients admitted between December 2012 and March 2013. The incidence of CMV reactivation by polymerase chain reaction (real-time PCR) in plasma was investigated. IFNγ secretion by CMV-specific CD8+ T lymphocytes was determined at the time of admission to the intensive care unit (ICU) by means of the QuantiFERON(®)-CMV (QF-CMV) test. Cox regression analyses were performed to investigate CMV reactivation risk factors.

RESULTS

Fifty-three patients were included, of whom 13 (24.5%) presented CMV reactivation. Twenty-six patients (49.1%) were QF-CMV "reactive" (QF-CMV(R)). Of the 26 QF-CMV(R) patients, 11.5% (3/26) had CMV reactivation, whereas 37% (10/27) of QF-CMV "non reactive" patients (QF-CMV(NR)) presented reactivation (p = 0.03). By Cox regression, the presence of QF-CMV(R) at ICU admission (HR 0.09, 95% CI 0.02-0.44; p = 0.003) was associated with a decreased risk of CMV reactivation. The sensitivity, specificity, positive predictive value, and negative predictive value of QF-CMV were 77, 57, 37, and 88%, respectively. Eleven of the 53 patients (20.7%) died during the follow-up period. Mortality was more frequent in patients with CMV reactivation (6/13, 46.1 vs. 5/40, 12.5%; p = 0.015).

CONCLUSIONS

In critically ill non-immunosuppressed patients, the presence of functional CMV-specific CD8+ T lymphocyte response at intensive care unit admission provides protection against CMV reactivation.

Factors related to the development of CMV-specific CD8+ T cell response in CMV-seropositive solid organ transplant candidates

This cross-sectional study analyzes factors associated with the development of CMV-specific CD8+ response, measured by IFNg production after cytomegalovirus (CMV) peptide stimulation, in CMV-seropositive solid organ transplantation candidates. A total of 114 candidates were enrolled, of whom 22.8% (26/114) were nonreactive (IFNγ < 0.2 IU/mL). Multivariate logistic regression analysis showed that age, HLA alleles and organ to be transplanted were associated with developing CMV-specific CD8+ immunity (reactive; IFNγ ≥ 0.2 IU/mL). The probability of being reactive was higher in candidates over 50 than in those under 50 (OR 6.33, 95%CI 1.93-20.74). Candidates with HLA-A1 and/or HLA-A2 alleles had a higher probability of being reactive than those with non-HLA-A1/non-HLA-A2 alleles (OR 10.97, 95%CI 3.36-35.83). Renal candidates had a higher probability of being reactive than lung (adjusted OR 8.85, 95%CI 2.24-34.92) and liver candidates (OR 4.87, 95%CI 1.12-21.19). The AUC of this model was 0.84 (p < 0.001). Positive and negative predictive values were 84.8% and 76.9%, respectively. In renal candidates longer dialysis was associated with an increased frequency of reactive individuals (p = 0.040). Therefore, although the assessment of CMV-specific CD8+ response is recommended in all R+ candidates, it is essential in those with a lower probability of being reactive, such as non-renal candidates, candidates under 50 or those with non-HLA-A1/non-HLA-A2 alleles.

Pretransplant interferon-γ secretion by CMV-specific CD8+ T cells informs the risk of CMV replication after transplantation

In this prospective study we analyzed pretransplant interferon-γ secretion by cytomegalovirus (CMV)-specific CD8+ T cells to assess its possible utility in determining the risk of CMV replication after solid organ transplantation. A total of 113 lung and kidney transplant patients were enrolled in the study but only 55 were evaluable. All CMV-seronegative recipients were pretransplant "nonreactive" (IFNγ <0.2 IU/mL) (11/11), whereas 30/44 (68.2%) CMV-seropositive (R+) recipients were "reactive" (IFNγ ≥0.2 IU/mL) and 14/44 (31.8%) were "nonreactive". In the R(+) "nonreactive" group, 7/14 (50%) developed posttransplant CMV replication, whereas the virus replicated only in 4/30 (13.3%) of the R(+) "reactive" patients (p = 0.021). According to the best multivariate model, pretransplant "nonreactive" recipients receiving an organ from a CMV-seropositive donor had a 10-fold increased risk of CMV replication compared to pretransplant "reactive" recipients (adjusted OR 10.49, 95% CI 1.88-58.46). This model displayed good discrimination ability (AUC 0.80) and calibration (Hosmer-Lemeshow test, p = 0.92). Negative and positive predictive values were 83.7% and 75%, respectively. The accuracy of the model was 82%. Therefore, assessment of interferon-γ secretion by cytomegalovirus (CMV)-specific CD8+ T cells prior to transplantation is useful in informing the risk of posttransplant CMV replication in solid organ transplant patients.

CMV and Immunosenescence: from basics to clinics

Alone among herpesviruses, persistent Cytomegalovirus (CMV) markedly alters the numbers and proportions of peripheral immune cells in infected-vs-uninfected people. Because the rate of CMV infection increases with age in most countries, it has been suggested that it drives or at least exacerbates “immunosenescence”. This contention remains controversial and was the primary subject of the Third International Workshop on CMV & Immunosenescence which was held in Cordoba, Spain, 15-16^th March, 2012. Discussions focused on several main themes including the effects of CMV on adaptive immunity and immunosenescence, characterization of CMV-specific T cells, impact of CMV infection and ageing on innate immunity, and finally, most important, the clinical implications of immunosenescence and CMV infection. Here we summarize the major findings of this workshop.

Impact of cytomegalovirus on early immunosenescence of CD8+ T lymphocytes after solid organ transplantation

The increasing number of elderly people eligible for solid organ transplants has made it necessary to reevaluate how the decline in immune function associated to ageing (immunosenescence) affects solid organ transplants. Some immunosenescence biomarkers, such as the expansion of CD28(-)CD8+ T lymphocytes, have been associated to cytomegalovirus infection and are related to a form of accelerated immune senescence in transplant recipients. However, the impact of cytomegalovirus replication on downregulation of CD28 on total CD8+ T cells is independent of patients' age, whereas downregulation on cytomegalovirus-specific CD8+ T cells depends on patients' age, inducing early immunosenescence of cytomegalovirus-specific CD8+ T cells in young but not elderly solid organ transplants recipients. Although immunosenescence in transplant recipients should be considered a two-edged sword as it is a risk factor for the development of tumors after transplantation, it has a beneficial effect in attenuating acute allograft rejection and correlates with better clinical outcomes.

Acyl-acyl carrier protein thioesterase activity from sunflower (Helianthus annuus L.) seeds

During sunflower (Helianthus annuus L.) seed formation there was an active period of lipid biosynthesis between 12 and 28 days after flowering (DAF). The maximum in-vitro acyl-acyl carrier protein (ACP) thioesterase activities (EC 3.1.2.14) were found at 15 DAF, preceding the largest accumulation of lipid in the seed. Data from the apparent kinetic parameters, Vmax and Km, from seeds of 15 and 30 DAF, showed that changes in acyl-ACP thioesterase activity are not only quantitative, but also qualitative, since, although the preferred substrate was always oleoyl-ACP, the affinity for palmitoyl-ACP decreased, whereas that for stearoyl-ACP increased with seed maturation. Bisubstrate assays carried out at 30 DAF seemed to indicate that the total activity found in mature seeds is due to a single enzyme with 100/75/15 affinity for oleoyl-ACP/stearoyl-ACP/ palmitoyl-ACP. In contrast, at 15 DAF, enzymatic data together with partial sequences from cDNAs indicated the presence of at least two enzymes with different properties, a FatA-like thioesterase, with a high affinity for oleoyl-ACP, plus a FatB-like enzyme, with preference for long-chain saturated fatty acids, both being expressed during the active lipid biosynthesis period. Competition assays carried out with CAS-5, a mutant with a higher content of palmitic acid in the seed oil, indicated that a modified FatA-type thioesterase is involved in the mutant phenotype.

Lipid characterization in vegetative tissues of high saturated fatty acid sunflower mutants

Modifications of the fatty acid composition of plant vegetative tissues produce deficient plant growth. To determine the expression of the seed high-saturated sunflower (Helianthus annuus L.) mutant character during the vegetative cycle, five sunflower mutant lines (three high-stearic and two high-palmitic) have been studied during their germination and vegetative cycle. No significant variations with regard to the control lines were observed in the mutant vegetative tissue lipids; however, during seed germination important differences between lines were found. Although in the early steps of germination the palmitic and stearic acid levels in the respective mutants seedling cotyledons continued being higher than those of the control lines, they decreased and reached values similar to the controls, except in CAS-3. Variations in the cotyledon palmitic acid content with regard to the control line were also observed in high-stearic mutants, suggesting the expression of a modified acyl-ACP thioesterase or recycling of seed fatty acids during seedling development.

Characterization of polar and nonpolar seed lipid classes from highly saturated fatty acid sunflower mutants

The seed lipids from five sunflower mutants, two with high palmitic acid contents, one of them in high oleic background, and three with high stearic acid contents, have been characterized. All lipid classes of these mutant seeds have increased saturated fatty acid content although triacylglycerols had the highest levels. The increase in saturated fatty acids was mainly at the expense of oleic acid while linoleic acid levels remained unchanged. No difference between mutants and standard sunflower lines used as controls was found in minor fatty acids: linolenic, arachidic, and behenic. In the high-palmitic mutants palmitoleic acid (16:1n-7) and some palmitolinoleic acid (16:2n-7, 16:2n-4) also appeared. Phosphatidylinositol, the lipid with the highest palmitic acid content in controls, also had the highest content of palmitic or stearic acids, depending on the mutant type, suggesting that saturated fatty acids are needed for its physiological function. Positional analysis showed that mutant oils have very low content of saturated fatty acids in the sn-2 position of triacylglycerols, between the content of olive oil and cocoa butter.