Utilization of Dried Blood Spot Citrulline Level as a Noninvasive Method for Monitoring Graft Function Following Intestinal Transplantation

Intestinal Transplant Immunology and Intestinal Graft Rejection: From Basic Mechanisms to Potential Biomarkers

Intestinal transplantation (ITx) remains a lifesaving option for patients suffering from irreversible intestinal failure and complications from total parenteral nutrition. Since its inception, it became obvious that intestinal grafts are highly immunogenic, due to their high lymphoid load, the abundance in epithelial cells and constant exposure to external antigens and microbiota. This combination of factors and several redundant effector pathways makes ITx immunobiology unique. To this complex immunologic situation, which leads to the highest rate of rejection among solid organs (>40%), there is added the lack of reliable non-invasive biomarkers, which would allow for frequent, convenient and reliable rejection surveillance. Numerous assays, of which several were previously used in inflammatory bowel disease, have been tested after ITx, but none have shown sufficient sensibility and/or specificity to be used alone for diagnosing acute rejection. Herein, we review and integrate the mechanistic aspects of graft rejection with the current knowledge of ITx immunobiology and summarize the quest for a noninvasive biomarker of rejection.

Serum citrulline as a diagnostic marker of sepsis-induced intestinal dysfunction

OBJECTIVE

To investigate the use of citrulline as an indicator for diagnosing septic acute intestinal dysfunction (SAID) in a rat model.

METHODS

SD rats were divided into three groups: a normal group (A), a model group (B), and a glutamine group (C). Group B was divided into a 36-h group (B1) and a 72-h feeding group (B2). The concentrations of serum citrulline, intestinal fatty acid-binding protein (I-FABP) and intestinal glutamine and histopathological changes were measured.

RESULTS

The lengths of the villus and thicknesses of the mucosal layer in groups B1, B2 and C were significantly different from those in group A. Citrulline concentrations in groups B1, B2 and C were lower than in group A; the serum concentrations in group C were significantly greater than in groups B1 and B2. The I-FABP levels of groups B1, B2 and C were higher than group A; I-FABP levels of groups B1 and B2 were higher than group C. Intestinal glutamine levels of groups B1 and B2 were lower than groups A and C. The serum citrulline of group C was negatively correlated with I-FABP and Chiu's score.

CONCLUSIONS

Serum citrulline could be used as the diagnostic indicator of SAID.

Reduced plasma citrulline levels in low birth weight infants with necrotizing enterocolitis

BACKGROUND

Citrulline, a nonprotein amino acid, is an intermediate of the urea cycle and synthesized in small intestine. Lower plasma citrulline levels were associated with reduced function of enterocytes. Necrotizing enterocolitis (NEC) causes high morbidity and mortality, and leads impaired intestinal functions.

METHODS

Plasma citrulline levels of neonates with a gestational age <32 weeks and ≤1,500 gm who developed NEC stage II/III were measured by high-performance liquid chromatography.

RESULTS

We enrolled 36 preterm infants including 20 with NEC and 16 controls. Median citrulline levels of NEC and control groups were 8.6 and 20.18 μmol/l (P < 0.05), and cut off level of citrulline was 13.15 μmol/l with a sensitivity of 80% and a specificity of 82%. Median arginine levels of NEC and control groups were 22.02 and 39.89 μmol/l (P < 0.05), and cut off level of arginine was 28.52 μmol/l with a sensitivity of 70% and a specificity of 75%. Blood sampling day, gender, parenteral, and enteral nutrition did not affect the amino acid levels.

CONCLUSION

We found lower plasma citrulline and arginine levels in preterm infants with NEC. Further studies are needed to determine most appropriate levels to predict recovery and prognosis of NEC, and treatment options with these amino acids in preterm infants.

A single center experience of abdominal wall graft rejection after combined intestinal and abdominal wall transplantation

We report our outcomes following combined intestinal and abdominal wall transplantation, focusing on the presentation and treatment of acute rejection of the abdominal wall vascularized composite allograft (VCA). Retrospective analysis of all patients with combined intestinal/VCA transplantation was undertaken. Graft abnormalities were documented photographically and biopsies taken, with histological classification of rejection according to Banff 2007 guidelines. We have performed five combined intestinal and abdominal wall transplants to date. Two patients developed erythematous, maculopapular to papular eruptions confined to the VCA, histologically confirmed as grade II/III rejection, yet with normal bowel on endoscopy. Both patients' rashes resolved within 72 h of increasing immunosuppressive treatment. One patient later developed a recurrence of the rash, confirmed as skin rejection, but did not immediately seek medical attention. Treatment was therefore delayed, and mild intestinal rejection developed. We describe the rash associated with VCA rejection, and propose that while the skin of an abdominal wall VCA may reject independently of the intestinal allograft, delay in treatment of rejection episodes may result in rejection of the intestinal graft.

Citrulline levels in a paediatric age group: does measurement on dried blood spots have additional value?

BACKGROUND

Citrulline is considered to be a marker of absorptive enterocyte mass. Citrulline levels can be measured in plasma or dried blood spot (DBS) samples. The purpose of this study is to calculate reference intervals for plasma and DBS citrulline concentrations in children and to examine the effect of age and gender.

METHODS

In 151 healthy subjects ranging from 1 month to 20 years of age, plasma and DBS citrulline concentration were determined by using Liquid Chromatography-tandem Mass Spectrometry. Citrulline concentrations were examined in relation to age and gender. Reference values were calculated according to the guidelines of the International Federation of Clinical Chemistry and the National Committee on Clinical Laboratory Standards.

RESULTS

No significant influence of age and gender could be discerned on plasma or DBS citrulline concentration. In children, the reference intervals for citrulline bounded by the 2.5 and 97.5 percentiles are 13.31-69.05 μmol/L and 23.70-49.04 μmol/L for plasma and DBS samples respectively.

CONCLUSIONS

The reference intervals for citrulline levels in healthy children are widely dispersed. Measuring citrulline concentrations in dried blood spots delivers no additional value to plasma measurements for the calculation of reference intervals in children.

Citrulline evaluation in bowel transplantation

INTRODUCTION

The purpose of this study was to evaluate the correlation of plasma citrulline and rejection episodes in intestinal transplantation.

METHODS

From January 2007 until present, we performed citrulline assays on our small bowel patients. We investigated the correlation of these assays with the rejection status of the patients. The rejection status of the graft was defined based on graft biopsies.

RESULTS

Of 5195 citrulline samples, average serum citrulline levels decreased significantly when the patients presented a rejection episode. We found the following: no rejection, 17.38 microm/L; mild rejection, 13.05 microm/L; moderate rejection, 7.98 microm/L; and severe rejection, 6.05 microm/L. Our current emphasis is to determine the predictive power of citrulline with other biomarkers versus as a separate and isolated measurement.

CONCLUSIONS

In our study, citrulline levels correlated significantly with the rejection status of the graft. Serial follow-up of the patients using this assay may alert us to the possibility of increased alloreactivity and rejection episodes.

Intestinal transplantation: current outcomes and opportunities

PURPOSE OF REVIEW

This review highlights current outcomes in intestinal transplantation and summarizes advances that have recently occurred in five interrelated areas: progress in intestinal rehabilitation, immunologic and technical modifications, awareness of opportunities for improved allograft monitoring, and better assessment of long-term complications and morbidities.

RECENT FINDINGS

Improved long-term management of patients with intestinal failure as well as improved outcomes with intestine transplant are changing the previously established paradigms of timing for referral. For those requiring transplant, use of monoclonal and polyclonal antibody induction protocols have been associated with improved outcomes. Experience at centers of excellence demonstrates 1 and 5 year patient survival rates of 93 and 78%, respectively with ongoing investigations focusing on lowering long-term causes of graft loss such as chronic rejection or morbidities such as renal dysfunction. Descriptions of tissue, proteomic and genomic technologies to complement traditional methodologies to monitor graft function are emerging.

SUMMARY

Optimal timing for referral of children with intestinal failure and improved medical and surgical therapies increase the opportunity for intestinal adaptation without the need for transplant. For those undergoing transplant, technical and immunologic modifications, developments in graft monitoring, and reduction of long-term morbidities are leading to improved outcomes.

Current perspectives on pediatric intestinal transplantation

Irreversible intestinal failure in children is predominantly caused by surgical conditions such as volvulus, necrotizing enterocolitis, and gastroschisis. Functional intestinal failure from motility disorders such as intestinal pseudo-obstruction or enterocyte dysfunction with microvillus inclusion disease also may require intestine replacement. Approved indications for intestinal transplantation include liver dysfunction, loss of major venous access, frequent central line-related sepsis, and recurrent episodes of severe dehydration despite intravenous fluid management. Surgical options include transplantation of the isolated intestine, combined liver-intestine transplantation, or multivisceral transplantation of the stomach, duodenum, pancreas, and small bowel (with or without the liver). Immunosuppression for intestinal transplantation is based on tacrolimus therapy, often with induction immunosuppression using antilymphocyte antibodies (eg, antithymocyte antibody and alemtuzumab). Experience at centers of excellence demonstrates 1- and 5-year patient survival rates of 95% and 77%, respectively, with ongoing investigations focusing on lowering long-term causes of graft loss such as chronic rejection.

Bioanalytical hydrophilic interaction chromatography: recent challenges, solutions and applications

Hydrophilic interaction chromatography (HILIC) has, in recent years, been shown to be an important supplement to reversed-phase liquid chromatography for polar analytes. HILIC, in conjunction with tandem mass spectrometry (MS/MS), has been steadily gaining acceptance in the analysis of polar compounds from complex biological matrices. This hyphenated technique offers the advantages of improved sensitivity by employing high organic content in the mobile phase, shortened sample preparation time with direct injection of the organic-solvent extracts of biological samples and the potential for ultra-fast analysis because of low-column backpressure. This article reviews recent challenges presented by HILIC, advancements in the better understanding of retention characteristics of analytes with different mobile- and stationary-phase compositions and solutions to ion suppression and interference problems encountered in HILIC–MS/MS assays. Applications of HILIC–MS/MS are summarized, including those for pharmacokinetic studies, metabolic studies, therapeutic drug monitoring and clinical diagnostics.

Plasma citrulline measurement using UPLC tandem mass-spectrometry to determine small intestinal enterocyte pathology

Citrulline is a nonessential free amino acid, detectable in various biological fluids such as plasma, urine and cerebrospinal fluid. The plasma citrulline concentration is increasingly considered to be a reliable biomarker of enterocyte function. Current analysis usually involves lengthy HPLC separations as a part of classical amino acid profiling, or mass spectrometry usually in combination with derivatization. We employed UPLC-HILIC-tandem mass-spectrometry (MS/MS) of acetonitrile-derived supernatants from plasma samples of control subjects and of patients who had received myeloablative chemotherapy. Detection was achieved by the selected reaction monitoring of transitions: m/z 176-->70 and 180-->74 (for the deuterated standard), respectively. The method was precise and accurate with inter-day CV<3.9% (n=30), recoveries ranging from 98.0 to 100.3% and high linearity from 0.3 to at least 2,000 micromol/L. The results for 202 plasma samples agreed well with those obtained by the classical HPLC-fluorescence method. By a simple protein precipitation/extraction step and the UPLC separation the result can be available within 30 min of receipt with a capacity of at least 12 assays per hour. Citrulline in blood and plasma or serum was stable for at least 2 days at room temperature which would permit postal transport to the laboratory. The UPLC-MS/MS method for measuring plasma citrulline concentrations is fast and robust and is therefore an ideal tool for monitoring the intestinal enterocyte capacity of patients with various pathological conditions.

Current status of small bowel and multivisceral transplantation

Intestinal transplantation has shown exceptional growth over the past 20 years with remarkable progress. As with other solid organ transplants, intestinal transplantation has moved out of the experimental realm to become the stan dard of care for many patients with intestinal failure. Intestinal transplantation may soon be extended routinely to patients who, although not strictly meeting the criteria for intestinal failure, may benefit from intestinal transplantation, such as patients who have nonresectable indolent tumors or diffuse thrombosis of the portomesenteric system. As clinical experience has increased with intestinal transplantation, outcomes have improved. The currently reported 1-year graft and patient survival rate is 80%, which approaches that for other solid abdominal organs. Unfortunately, most of the gains in survival are seen in the first postoperative year, with long-term survival remaining basically unchanged since the early 1990s. With improved outcomes, more centers have entered into the intestinal transplant arena. In the United States alone, 20 centers performed at least one intestinal transplant in 2007. Increase in access to intestinal transplantation and more widespread awareness of this option likely will result in a consistent increase in the number of yearly transplants for the foreseeable future. Immunosuppressive regimens continue to evolve, with induction therapy being the major change in the past 5 years. Although rejection rates in the first year after transplant have been reduced by induction therapy, long-term side effects of heavy immunosuppression continue to weigh negatively on transplant outcomes. The future for immunosuppression lies in two areas: (1) individual monitoring of the immunosuppression level for each individual patient and (2) development of serum and tissue markers for the early identification of rejection. It is likely that a combination of technologies will allow immunosuppression to be tailored to each recipient. Development of these approaches to immunosuppression is necessary to predict graft dysfunction ahead of irreversible graft injury and allows adjustments in immunosuppression before the onset of rejection. Intestinal transplantation continues to be performed only in situations in which all other therapeutic modalities have failed. No randomized trials compare intestinal transplantation to long-term PN to establish guidelines for a timely referral for this treatment option. Late referral remains a crippling problem in the field of intestinal transplantation, with a great number of patients in need of simultaneous liver transplantation at the time of listing for intestinal transplantation. Early referral for isolated intestinal transplant will reduce the need for simultaneous multiorgan transplants and increase the residual organs available for patients in need of (primarily) liver transplantation.

Citrulline as a biomarker of intestinal failure due to enterocyte mass reduction

BACKGROUND & AIMS

In human, citrulline (plasma concentration about 40 micromol/L) is an amino acid involved in intermediary metabolism and that is not incorporated in proteins. Circulating citrulline is mainly produced by enterocytes of the small bowel. For this reason plasma or serum citrulline concentration has been proposed as a biomarker of remnant small bowel mass and function. This article reviews this concept and its metabolic basis.

METHODS

Conditions in which there is a significantly reduced small bowel enterocyte mass and function and a plasma or serum citrulline were measured in adults and children. These studies included patients with a short bowel syndrome, villous atrophy states, Crohn's disease, during monitoring of digestive toxicity of chemotherapy and radiotherapy or follow-up of patients after small bowel transplantation.

RESULTS

In all these situations, with more than 500 studied patients a decreased level of plasma citrulline correlated with the reduced enterocyte mass independently of nutritional and inflammatory status. A close correlation between small bowel remnant length and citrullinemia was found. In addition, diagnosis of intestinal failure was assessed through plasma citrulline levels in severe small bowel diseases in which there is a marked enterocyte mass reduction.

DISCUSSION

The threshold for establishing a diagnosis of intestinal failure is lower in villous atrophy disease (10mumol/L) than in short bowel syndrome (20mumol/L). Compromised renal function is an important factor when considering plasma citrulline levels as a marker of intestinal failure as this potentially can increase circulating citrulline values.

CONCLUSIONS

Reduced plasma citrulline levels are an innovative quantitative biomarker of significantly reduced enterocyte mass and function in different disease states in humans.

Therapeutic efficacy of intestinal and multivisceral transplantation: survival and nutrition outcome

The clinical introduction of intestinal transplantation has added a new dimension and offered a valid therapeutic option for patients with irreversible intestinal failure. In the year 2000, the Center for Medicare & Medicaid Services (CMS) recognized intestinal, combined liver-intestinal, and multivisceral transplantation as the standard of care for patients with irreversible intestinal and parenteral nutrition (PN) failure. Accordingly, the indications for the procedure are currently limited to those who develop life-threatening PN complications. However, a recent improvement in survival similar to other solid organ transplant recipients should justify lifting the current restricted criteria, and the procedure should be considered before the development of PN failure. Equally important is the awareness of the recent evolution in nutrition management and outcome after transplantation. Early and progressive enteral feeding using a complex polymeric formula is safe and effective after successful transplantation. Full nutrition autonomy is universally achievable among most intestinal and multivisceral recipients, with enjoyment of unrestricted oral diet. Such a therapeutic benefit is commonly maintained among long-term survivors, with full rehabilitation and restoration of quality of life.

Critical sample pretreatment in monitoring dried blood spot citrulline

BACKGROUND

Plasma Citrulline concentration has been correlated to functional enterocyte mass. Dried blood spot (DBS) analysis using LC-MSMS reduces sample amount needed. We optimized DBS elution to increase precision and accuracy of DBS LC-MSMS analysis.

METHOD

DBS control samples were eluted in varying pH (2.2-7.0) and for varying times (15-75 min) and Cit, Arg and Orn were analyzed using LC-MSMS, with and without derivatization. In 20 volunteers, the DBS LC-MSMS assay was correlated with a plasma ion exchange HPLC method.

RESULTS

For Citrulline an extraction optimum was obtained at pH 2.6, whereas lower Arginine concentrations were found using low extraction pH. Increasing elution times lead to increased concentrations. Within-run CV was higher with, compared to without derivatization. No close association could be found between plasma HPLC and DBS LC-MSMS concentrations.

CONCLUSION

Analysis of amino acids on DBS using LC-MSMS should be optimized regarding the purpose of the assay. In our study, most optimal results were obtained without derivatization and elution in pH 2.6 for 45 min. Cellular amino acids in DBS might influence the correlation of Cit with severity of enteral disorders. Therefore, further evaluation of DBS Cit as a marker for enteral disorders is warranted.

Subclinical Rejection in the Initial Postoperative Period in Small Intestinal Transplantation: A Negative Influence on Graft Survival

BACKGROUND

Subclinical rejection (SCR) is a known entity in various solid organ transplants but not in intestinal transplantation.

METHODS

The purpose of this study is to characterize the presence and effect of SCR in small intestinal transplantation (Itx). A total of 151 patients who underwent Itx and maintained a functioning graft for at least 3 months after Itx were investigated. The clinicopathological characteristics associated with a SCR episode within 3 months after Itx were analyzed. Cox regression with the landmark method (the landmark time being 3 months after Itx) was used for the analyses of overall graft survival and cause-specific hazard rate of SCR.

RESULTS

A total of 2744 small intestinal transplant biopsies within 3 months after Itx were available for retrospective evaluation; 171 cases (6.2%) were determined as SCR and 78 patients (51.7%) experienced SCR episode within 3 months after Itx. Adult patients were associated with a significantly higher occurrence of a SCR episode (P=0.001). Overall graft survival at 5 years posttransplant for patients experiencing SCR within 3 months posttransplant and for patients without SCR was 37.2% and 60.2%, respectively (P=0.009). Cause-specific hazard rate analysis showed that a SCR episode was associated with a significantly higher hazard rate of death due to infection (P=0.005).

CONCLUSIONS

A SCR episode in the initial postoperative period of Itx is a significant factor for unfavorable graft prognosis, likely representing alloimmune injury ultimately resulting in patient morbidity due to infection.

Citrulline and the gut

PURPOSE OF REVIEW

Citrulline, a nonprotein amino acid, is an important source of endogenous arginine. The gut is the main source of citrulline in humans. Hence, citrulline is a potential biomarker of short bowel function. Conversely, citrulline uptake by the gut is important for an oral supply of this amino acid as an alternative to arginine. This review discusses these two aspects of citrulline, as well as the recent developments in the understanding of its metabolism.

RECENT FINDINGS

Citrullinemia is such an efficient marker when the active mass of the bowel is affected that it can be used as a prognostic marker for parenteral nutrition weaning (if citrullinemia is >20 micromol/l) and as a factor for deciding between parenteral and enteral nutrition (as long as the pathology is considered). Citrullinemia should be used with care as a marker either of the intestinal absorption or following small bowel transplantation.

SUMMARY

Citrulline is easily taken up by the gut, with a broad set of transporters that can remove it from the lumen in the enterocytes. This is confirmed by pharmacokinetic studies and the efficacy is so great that oral complementation with citrulline seems more efficient than complementation with arginine to provide arginine.

Citrulline: a new player in the control of nitrogen homeostasis

Citrulline (CIT) is an amino acid that is not involved in protein synthesis but that is tightly linked to arginine (ARG) metabolism. CIT displays a very specific metabolism: In the 1980s, Windmuller demonstrated that the small intestine releases CIT, which is mainly taken up by the kidney and metabolized into ARG. Because CIT is not taken up by the liver, this ARG-CIT-ARG cycle can be seen as a means of protecting dietary ARG from liver degradation and of sustaining protein homeostasis. These observations have led to the concept that plasma CIT concentration would be a good marker of intestinal failure in short bowel syndrome. Hence, in massive intestinal resection, citrullinemia is greatly reduced, and this is proportional to the severity of the intestinal disease. This concept was then extended to other situations in which the intestinal function is compromised. The data strongly suggest that CIT may be a conditionally essential amino acid in situations where the intestinal function is compromised. Recent data support this idea. Thus, CIT supplementation is able to restore nitrogen balance, generate large amounts of ARG in rats with short bowel syndrome, and increase muscle protein content (+20%) as well as muscle protein synthesis (+90%) in elderly malnourished rats. Finally, recent data indicate that CIT per se could be able to stimulate muscle protein synthesis. Hence, CIT could play a pivotal role in maintaining protein homeostasis, and the determination of the underlying mechanisms involved in its action should be important for the development of new nutritional strategies in malnourished patients with compromised intestinal functions.

Hydrophilic interaction chromatography

Separation of polar compounds on polar stationary phases with partly aqueous eluents is by no means a new separation mode in LC. The first HPLC applications were published more than 30 years ago, and were for a long time mostly confined to carbohydrate analysis. In the early 1990s new phases started to emerge, and the practice was given a name, hydrophilic interaction chromatography (HILIC). Although the use of this separation mode has been relatively limited, we have seen a sudden increase in popularity over the last few years, promoted by the need to analyze polar compounds in increasingly complex mixtures. Another reason for the increase in popularity is the widespread use of MS coupled to LC. The partly aqueous eluents high in ACN with a limited need of adding salt is almost ideal for ESI. The applications now encompass most categories of polar compounds, charged as well as uncharged, although HILIC is particularly well suited for solutes lacking charge where coulombic interactions cannot be used to mediate retention. The review attempts to summarize the ongoing discussion on the separation mechanism and gives an overview of the stationary phases used and the applications addressed with this separation mode in LC.