New Test Gauges Danger of Rejection When Children Need Organ Transplants

By measuring the inflammatory marker CD154, Pleximmune promises to predict the risk of organ rejection in liver and small-bowel transplant procedures

This is a test: Write down the first thing that comes to your mind at the mention the word ‘transplant.’

I’ll bet no one reading this column thought about pediatric liver or pediatric small- bowel transplants. But these lifesaving procedures are being performed regularly in several very advanced settings in the United States. Some quick facts: About 2% of liver transplants nationally are in pediatric patients, and it is now thought that intestinal transplantation may one day replace total parenteral nutrition in children whose intestines have failed for a variety of reasons. Remember, there is no alternative if a liver transplant fails, and only lifelong parenteral nutrition if an intestinal transplant fails.

The most common indications for liver transplant in children are biliary atresia, hemochromatosis, Wilson disease, alpha-1 antitrypsin deficiency, and tyrosinemia. The main disorders leading to small-intestine transplant include short-gut syndrome as a result of Crohn’s disease, desmoid tumor, mid-gut volvulus, mesenteric vascular thrombosis, defective intestinal motility disorders, trauma, and post-radiation enteritis.

The major risk in these, as well as in all other transplants, is rejection of the new organ. Organ transplant rejection occurs when the transplant recipient activates both the cellular (lymphocyte-mediated) and the humoral (antibody-mediated) arms of the immune system. It is based on the donor organ being recognized as foreign. The CD154 antigen on the T-lymphocytes is one of the major mediators of the inflammatory response to the foreign antigens and the resultant rejection.

The earliest small-bowel transplants (eight total) were described in the 1960s, and all ended in death because of rejection. Although antirejection drugs including cyclosporine (discovered in the mid-1970s) and tacrolimus (approved in 1989) have revolutionized post-transplant care, preventing rejection requires constant vigilance.

A metaphor for post-organ transplant care is the Goldilocks approach. The immunosuppression should not be too much, nor too little, but just right. Under-immunosuppression can lead to rejection, while over-immunosuppression raises the risk of infections and malignancies. For liver transplants, immune tests, liver function tests, and radiologic findings, along with biopsies, are used to assess the rejection risk. For bowel transplants, biopsy findings along with clinical symptoms and abnormal laboratory values are employed.

Biopsy, a gold standard for rejection diagnosis, is an invasive tool and has severe risks associated with it, especially for small-bowel transplantation, in which a perforation may occur. Also, biopsies are not prognostic or forward-looking; they can only show rejection that has already started. Because rejection is discovered after it begins, large adjustments in medication are required to downregulate the immune system and prevent the rejection. What is needed is a way to predict rejection so that smaller preventive measures can be taken prophylactically.

Big change

Until now, no predictive tool existed for a risk-based customization of the immunosuppression therapy. That may now change.

A group from the University of Pittsburgh, one of the pioneer transplant sites, has come up with an FDA-approved predictive test that has been shown to help with this decision making. The test, called Pleximmune, is available from a small corporation called Plexision, also located in Pittsburgh. Pleximmune is an aid in the evaluation of the risk of acute cellular rejection and must be used in conjunction with biopsy, standard clinical assessment, and other laboratory information. It is intended for use over three time periods:

  • Pre-transplantation (predicts the risk of transplant rejection within 60 days post- transplant)
  • Early post-transplantation (within 60 days)
  • Late post-transplantation (200 days or more)

The Pleximmune test system uses in vitro cell culture to elicit the inflammatory immune response of the recipient to the donor. The test is based on the measure of the inflammatory marker CD154 on the T-cytotoxic memory cells of the recipient.

The test uses four cell culture reactions as its foundation:

  • The recipient peripheral blood lymphocytes (PBLs) are cultured alone in a culture medium as a negative control.
  • The recipient PBLs are cultured alone with a fluorochrome-labeled antibody to the inflammatory response marker CD154 to measure the background signals.
  • The recipient PBLs are cultured with donor or surrogate donor PBL in a culture with the same fluorochrome-labeled antibody.
  • And finally, the PBLs are cultured with a mismatched PBL in a culture medium to represent the immune reaction of the recipient to a mismatched PBL; this is used as a reference.

The FDA approved this whole blood-based test based on a clinical validation process that determined the test to be safe, of probable benefit, and with sufficient sensitivity, specificity, and both positive and negative predictive value for predicting rejection. In studies of 87 transplant recipients and 122 blood specimens from transplant patients, roughly one third were pre-transplant and two thirds were post-transplant, and 60% were male and 40% female. Median age was roughly 8.5 years with a range of less than six months to just over 20 years old. In the post-transplant cohorts, the test resulted in a predictive sensitivity of 57%, a specificity of 84%, a positive predictive value of 64%, and negative predictive value of 92%. In the pre-transplant cohorts, the results were 57%, 89%, 80%, and 74% respectively — all of which, in the world of predictive testing, are rather good.

The final test results are calculations based on the cell cultures, and although the methodology is too complex to explain in detail, suffice it to say that the results are presented as a simple number that leads to a conclusion of either increased or decreased risk of rejection.

The company points out, ‘Pleximmune is a prognostic test and is not indicated as a sole diagnostic tool in assessing the risk of liver or small-bowel transplant rejection in children.’

Inexpensive alternative

The price of the test is $825, making it a rather inexpensive alternative given the cost of a rejection. It is expected to be used about twice in the first year unless the patient has a complex clinical setting such as infection. It may actually take the place of some biopsies, which are roughly three times as expensive.

This niche of medical care has progressed from a certain death sentence just a few decades ago to one in which more than half of all recipients are still alive five years post-transplant, a remarkable achievement indeed. The Pleximmune test again proves that small discoveries over time help usher in Tomorrow’s Medicine.