Red Blood Cell (RBC) Survival Following Transfusion in Infants

Overview

OUR OVERALL HYPOTHESIS is that post-transfusion survival of allogeneic and autologous RBCs can be accurately quantified in anemic human infants using biotin-labeled RBCs combined with mathematical modeling that adjusts for confounding factors commonly encountered in neonates. These confounding factors include 1) dilution of labeled RBC as a result of growth stimulated erythropoiesis, anemia stimulated erythropoiesis, and blood transfusion; 2) loss of labeled RBC due to laboratory phlebotomy; and 3) variable RBC life spans resulting from RBCs having been produced at different developmental periods and under varying rates of erythropoiesis. In contrast to infants, adjustment for these factors is not necessary in healthy adults under conditions of steady state erythropoiesis. Instead in adults, RBC survival is typified by a linear decline in concentration of labeled RBCs over time. When this line is extrapolated to zero concentration, the intercept with the time axis represents the mean potential lifespan (MPL) of RBCs. (\<7 d) and stored (\>21 d) allogeneic adult RBCs transfused in the same infant.

Phase I: Includes only Aim #1 from Thrasher Foundation Grant in which adult subjects will be studied. SPECIFIC AIM #1 (Thrasher Foundation Grant): To develop in vitro and validate in vivo in adult humans and anemic infants the capability of biotinylating RBCs at up to 5 discrete densities for simultaneously determining RBC kinetics of multiple, distinct RBC populations. This requires expansion of the RBC biotin labeling technology we have previously developed. After refinement, the method will be applied in the subsequent aims conducted in anemic infants receiving clinically ordered RBC transfusions. As suggested by our previous RBC survival studies, we anticipate that the heaviest biotin labeling will alter the intrinsic RBC structural properties of RBCs, shortening their long term survival. The in vitro and in vivo validation studies we propose in adults are necessary because the conditions for reproducibly obtaining discrete RBC biotin densities at appropriate, closely spaced intervals have not been conclusively worked out, nor have we determined empirically which of the five densities will not artifactually shorten long-term RBC survival. We can use the lighter densities for RBC survival measurements and the heavier densities for the simultaneous RBC volume measurements needed to account for RBC volume increases caused by growth. It is important to perform these feasibility studies in both adults and infants because of the vastly different physiologic states, ie, normal, healthy adults are in steady state erythropoiesis while critically ill, anemic infants experience multiple clinical circumstances which perturb RBC survival, eg, growth, phlebotomy, intervening transfusion, etc. In addition to the biotin labeling of RBC method, the "differential agglutination, antigenic method" using flow cytometry will be applied for infants receiving allogeneic RBC transfusions. The results of the two methods will be compared with one another. The differential agglutination method utilizes differences in the RBC surface antigens between the donor and the recipient for determining short- and long-term RBC survival. Genotyping results of adult donor and the infant recipient RBC antigens (performed at the Mississippi Valley Regional Blood Center) permits identification of minor blood group differences between donor and recipient RBCs so that appropriately labeled minor RBC antibodies (available commercially and used by blood banks for minor blood group RBC typing) can be used in flow cytometric determination of RBC survival. RBC survival determined for each of the biotin labels and for the RBC antigenic differences will all be compared among one other. The differential agglutination/antigen method will considered the "gold standard" as the RBCs are labeled ex-vivo, AFTER the RBC transfusion, thus without modifying the RBC membrane surface proteins as biotinylation does. Our hypothesis is that the addition of too much biotin can lead to artifactually shortened in vivo RBC survival, but that lower doses of biotin to not. By also including the differential agglutination/antigen method RBC survival results, we will be able to better validate our multi-density biotin hypothesis. Our overall objective in this research is to improve red blood cell (RBC) transfusion practices for anemic, critically ill infants. This project has received support from two granting agencies: 1) The Thrasher Foundation, entitled, "Red Blood Cell Recovery and Survival Following Transfusion in Infants;" and 2) NIH PPG Grant P01 HL046925, Project 1 entitled, "Red Blood Cell Survival Following Transfusion In Infants." Abstracts for both are included below. This project will be completed in two phases: Phase I includes studies of autologous biotinylated RBCs transfused into normal, health adult human volunteers (see Specific Aim #1 in Thrasher Foundation Abstract below); and Phase II includes studies of infants requiring physician order red blood cell transfusions (see Specific Aims #2, #3, and #4 in Thrasher Foundation Abstract below; and see Specific Aim #4 in NIH which is included in its entirety in the three Thrasher Grant specific aims). The Phase I studies are completed and we are no longer enrolling adults. We are enrolling infants for Phase II studies. We are adding these phases sequentially based on the recommendation of Martha Jones at the time our proposal was first submitted to the IRB. We have applied the knowledge we have gained from the Phase I studies in adult subject volunteers to make modifications of our study design for Phase II to be performed in infant study subjects receiving clinically ordered RBC transfusions. At the end of Phase II, justification for including a new, comparison "gold standard" method for determining RBC survival in infants (the "differential agglutination, antigenic method") is included. We have completed enrollment of all adult subjects for phase I. ABSTRACT I (FOR THRASHER FOUNDATION GRANT) Background: Anemic, critically ill newborn infants are among the most frequently transfused groups of patients in the US. An estimated 130,000 infants annually receive approximately 1,000,000 RBC transfusions. Unlike adults, important data are lacking regarding the transfusion product for optimal RBC survival in infants. Contributing to this lack are two major recent changes in neonatal blood banking and transfusion practices. These are 1) the use of transfused adult donor blood stored for up to the 42 day FDA limit instead of only using blood stored for less than 7 days as had been done previously; and 2) potential use of the infant's own blood harvested from the placenta to avoid the risks of viral infections and immune transfusion reactions from donor blood. Unfortunately, there are no definitive infant studies that address these changes by directly measuring RBC survival. Such infant studies have been hampered by technical problems. Specifically, RBC survival data must be adjusted for growth, laboratory phlebotomy loss, and intervening additional RBC transfusions but in practice have not. Moreover, safety issues have precluded many infant RBC survival studies (eg, exposure to 51Cr radioactivity and removal of too much blood from today's tiny, premature infants). Indeed, prior to 1970 (the end of using 51Cr as a RBC label in infant RBC survival studies), infants weighing less than 1,500 g at birth did not often survive, and RBC survival data do not exist for today's smallest, most frequently transfused infants whose birth weights are 500-1,000 g. Specific Aims: Specific Aim #1: To develop in vitro and validate in vivo in adults the capability for biotinylating RBCs at up to 5 discrete densities that are measurable by flow cytometry. These five RBC biotin density labels will be used in Specific Aims #2, 3, and 4 to simultaneously determine RBC survival of multiple, distinct populations of transfused RBCs in premature infants. Specific Aim #2. To determine whether RBC survival of donor and placental RBCs are significantly longer when adjusted by mathematical modeling. We anticipate that the unmodeled values substantially underestimate RBC survival. Specific Aim #3. To compare long-term RBC survival results for transfused adult donor and fetal/placental RBCs in anemic newborn infants. We anticipate that more rapid growth in fetuses will result in greater stress erythropoiesis leading to intrinsic RBC "defects" and shortened RBC survival compared to adult donor RBCs. Specific Aim #4. To quantify the effects of storage on model adjusted RBC survival of adult donor RBCs transfused into newborn infants. We anticipate that storage of donor RBCs will not alter RBC survival. Study Design: The studies proposed here build on biotinylation and mathematical modeling methods that our research group has developed for accurately measuring RBC survival without exposing the infant to radioactivity. The biotin RBC labeling method is well suited to newborn infants because it permits the simultaneous tracking of multiple RBC populations on \<10 µL blood. Survival of RBCs labeled at multiple biotin densities will be quantified using the standard RBC survival parameters, ie, post-transfusion short-term 24 h RBC recovery and long-term modeled RBC survival, ie, until 50 and 100% of biotin-labeled RBCs have disappeared from the circulation. The latter two measurements will be calculated using the required mathematical adjustments for growth, laboratory phlebotomy loss, and intervening RBC transfusions. Only Specific Aim #4 in the Thrasher Foundation Abstract involves human infant study subjects. Specific Aim #4) USE THE RBC BIOTINYLATION AND MATHEMATICAL MODELING METHODOLOGIES VALIDATED IN ADULT SHEEP AND NEWBORN LAMBS TO ACCURATELY MEASURE POST-TRANSFUSION RBC KINETICS IN ANEMIC NEWBORN INFANTS TRANSFUSED WITH FRESH AUTOLOGOUS, FRESH ALLOGENEIC AND STORED ALLOGENEIC RBCS. The use of biotin, a non-toxic, non-radioactive B vitamin, to distinguish among different RBC populations simultaneously by flow cytometry is critical for accomplishing our aims and holds clear advantages over other RBC labeling methods in both safety and accuracy. In utilizing the four Specific Aims to achieve our goal of establishing more effective transfusion practices by identifying the optimal RBC transfusion product for use in anemic INFANTS.