In clinical transplantation, an observed complication is that previous red blood cell (RBC) transfusions may reduce the risk of solid organ transplant rejection but increase the risk of bone marrow transplant (BMT) graft failure (Storb and Weiden, 1981). This statement is seemingly a contradiction. It is difficult to understand how RBC transfusions can both reduce the risk of solid organ transplant rejection and increase the risk of BMT rejection.
But this contradiction can be resolved by understanding three considerations. First, the immune system is extremely complex and attempts to control its effects routinely have both positive and negative effects (Vigano et al., 2012). As will be discussed fully below, the impact of RBC transfusions on transplantation is an example of such a situation. Second, scientists have made significant progress in explaining the negative effect of RBC transfusions, even overcoming BMT rejection in mouse models by administering a targeted immune-suppressing medicine at the time of RBC transfusion (Gilson, Patel, and Zimring, 2012). Applying this knowledge to humans provides a logical explanation for what is happening for the negative aspect of the contradiction.
Third, the positive benefit of RBC transfusion on solid organ transplants is a conclusion that doctors now question (Kochhar, Ghosh, and Singh, 2012). This is understandable as doctors originally made the observation of the reduction in risk for solid organ transplants at a time when the risk of rejection was very high. With advances in transplantation processes and drugs, the risk of rejection is now much lower. Any benefit that could be seen with RBC transfusion over and above these new techniques is extremely small (Kochhar, Ghosh, and Singh, 2012). As a result, there has been little recent experimentation into this effect, as it is no longer needed to do effective transplants. This essay will resolve the contradiction of the effect of RBC transfusions on transplantation by discussing the experimental progress made in understanding the negative effects on BMT, followed by a discussion of the current understanding of the reported positive effects on solid organ transplantation.
BMT is transplantation of hematopoietic, or blood-forming, stem cells in order to repopulate those cells within the patient’s bone marrow (National Cancer Institute, n.d.). BMT therapy in cancer begins with administration of powerful drugs that kill all the cancerous cells. These drugs also kill the stem cells located in the bone marrow. BMT is then used to re-establish the cells in the patient’s bone marrow after completion of the treatment cycle. Rejection of the graft is generally not an issue in cancer treatment, as the patent’s entire immune system has been killed. Without an immune system there is no immune response against the graft, so there is no rejection (Desmarets et al., 2009, p. 2315).
However, BMT is also used as a treatment for non-cancerous blood disorders such as sickle cell disease and thalassemia (Iannone et al., 2003). In that case, there are therapies that can treat these diseases. But these therapies do not offer the possible cure that BMT can provide. On the other hand, BMT has significant risks, including death. Furthermore, there is an increased risk of complications for patients where the treatments are not working well, the exact patient most likely to seek the BMT cure (Caocci, 2011). These factors result in ethical issues with providing BMT for patients with non-cancerous blood disorders. If BMT therapy is determined to be the right treatment course, it is not ethically justifiable to remove the patient’s entire immune system prior to transplantation just to reduce the chances of rejection (Caocci, 2011). So reduced intensity protocols have been developed. But not surprisingly, the reduced intensity protocols increase the possibility of graft rejection in as many as 15% of these patients (Desmarets et al., 2009, p. 2315; Patel et al., 2012, p. 1102). Treatment of patients with non-cancerous blood disorders is therefore a complex decision.
Further complicating this situation is the correlation between the number of RBC transfusions that the patient has had before BMT and the possibility of rejection of the graft (Desmarets et al., 2009, p. 2315). Given that the patients have a blood disease, prior treatment often includes numerous RBC transfusions. Understanding why RBC transfusions increase the risk of BMT graft rejection could provide methods of adjusting the way the transfusions are administered to avoid this problem. A first attempt at changing how the transfusions were done was removing as many of the white blood cells, or leukocytes, from the transfusions for these patients as possible, but this did not reduce the rate of later BMT rejection (Champlin et al., 2007). It is not even a way of knowing that leukocytes are not involved in the negative effect, because each and every white blood cell cannot be removed. Even a very small amount of contaminating cells could prove responsible (Desmarets et al., 2009).
A primary means of reducing the rejection of any transplant is “matching” the donor tissue with the recipient tissue. Tissue is said to “match” when they share human leukocyte antigens (HLA), which are proteins that are found on the surface of the tissue (American Cancer Society, n.d.). There are six major HLA (MHA) types and today they are routinely all matched before a BMT is approved. Although this match greatly reduces the possibility of rejection, there are many other proteins on the surface of tissues that are not routinely matched between donor and recipient. An example of a protein type that is not matched is the minor HLAs (mHA). Desmarets et al. theorized that it was an immune reaction to the mHA that caused the negative effect of RBC transfusions on BMT (2009, p. 2315). Specifically, they hypothesized that either the RBCs or platelets or leukocytes in the transfusion were the source of the mHA exposure to the patient’s immune system. The immune system of the patient is said to be “primed” by this first exposure. On second exposure to the mHA during the BMT, the immune system would see the transplanted cells as foreign, attack them, and cause the graft rejection (Desmarets et al., 2009).
Desmarets et al. tested this hypothesis by using a mouse model (2009). The donor and recipient mice were matched for the MHA but not the mHA. Using this basic mouse model they were able to transfuse the mice and then see later BMT rejection, thus replicating what occurs in humans. They also isolated a type of immune cell, CD8+ T-cells, that seemed responsible for the later graft rejection. The CD8+ cells were specific for a model mHA on the transfused RBC. These results supported their hypothesis that mHA antigens on the surface of the RBC were the trigger for the later graft rejection (Desmarets et al., 2009). But further experiments showed they were almost correct, but not quite.
Desmarets et al. did two further experiments to get more details (2011). First, they tried to induce the BMT rejection in the mice with model mHA antigen on their RBC surfaces. Surprisingly, this was unsuccessful. To try to determine why it was unsuccessful, Desmarets et al. specifically tested the primed CD8+ T cells as to whether they could go on to function in the graft rejection process, a capability called “full effector function” (2011). These experiments found that these CD8+ T-cells lacked full effector function This result suggests that some other component of the transfusion other than a protein on the RBCs were responsible for the rejection effect. In fact, this lab had previously done experiments that showed that administration of platelets could also mimic the BMT rejection seen in humans (Patel et al., 2009).
Platelets are “small blood components that help the clotting process.” (American Red Cross, n.d.). They are found in RBC transfusions. Even when the process is done to remove white blood cells from RBCs to be transfused, some platelets remain (Glenister and Sparrow, 2010). Important for these experiments, mHA are expressed on the surface of platelet cells. Because of this, contaminating platelets could be the source of exposure of the patient’s immune system to these foreign antigens, and the cause of the BMT rejection.
This situation and all of the results described to this point suggest the cause of the negative effect on BMT to be contaminating platelets in the RBC transfusions (Patel et al, 2012). To be sure, Patel et al. tested the CD8+ T-cells produced from exposure to the platelets by the mouse. They found these CD8+ T-cells were both primed and had their effector functions activated at transplantation, resulting in graft rejection. This contrasts with the CD8+ T-cells from the RBC experiment of Desmarets et al. that could only be primed. This data supports the theory that the increased incidence of BMT graft rejection is due to the exposure of the patient’s immune system to mHA on the surface of contaminating platelets present in the RBC transfusions.
This theory was further supported by a later experiment. Gilson, Patel, and Zimring treated mice prior to platelet transfusion with an FDA approved biologic drug (Yervoy, CTLA4-Ig) that prevents T-cell based immune reactions (2012). This treatment eliminated the later BMT graft rejection in the mouse model. However, treatment with anti-CTLA4-Ig after transfusion did not stop the BMT rejection. Applying this result to humans, it is therefore logical to administer CTLA4-Ig prior to RBC transfusion of human patients if their condition suggests they may need a BMT in the future (Gilson, Patel, and Zimring, 2012, p. 2217). An important limitation to all of these experiments is that they were performed in various mouse models. Clinical experimentation in humans would have to occur to be certain about the conclusion. But in any case, the evidence supports a finding that contaminating platelets in the RBC transfusions are the cause of later BMT rejections.
Now that the negative effect is clearer, the positive effect of RBC transfusions on solid organ transplants needs to be examined. A first issue is the observation in a recent review by Kochar, Ghosh, and Singh that the benefits of prior transfusion on solid organ graft rejection is not consistent (2012). These authors state that there have been phases of transfusion policies “swinging from liberal transfusion to avoidance of transfusions and returning to abstinence” (Kochhar, Ghosh, and Singh, 2012). Thus it appears that Storb and Weiden’s observation in 1981 is an example of a study that found a benefit but that is not the case with all studies. But since the development of cyclosporine, a drug that inhibits lymphocytes and thus is immunosuppressive (Colombo and Ammirati, 2011), and the introduction of HLA matching, the relative benefit is questionable (Kochhar, Ghosh, and Singh, 2012). That is because cyclosporine and HLA matching so improved graft survival, that the relatively minor benefit provided by RBC transfusion was no longer needed.
A listed benefit in the Kochhar, Ghosh and Singh article for pre-transplantation RBC transfusion is the possibility of immunosuppression (2012, p. 248). This is likely the positive benefit reported by Storb and Weiden (1981) and referred to in the original contradiction. Two proposed mechanisms for this immunosuppression are either an enhancement of suppressor T-cell function or an immune tolerance effect achieved by unknown mechanisms (Kochhar, Ghosh, and Singh, 2012, p. 248). In particular, nonspecific immune suppression after transfusion has been reported but no recent experiments have been done to understand the precise mechanism. Further, Kochhar, Ghosh, and Singh concede that the evidence in the older experiments is not strong enough to support transfusions before solid organ transplantation as a routine approach (2012, p. 250). In summary, the experiments showing a benefit of transfusion to solid organ transplantation were done in a different era, pre-cyclosporine and pre-HLA matching. With the current immunosuppression and HLA typing technology available, the possible downsides of RBC transfusion before solid organ transplants likely outweigh the possibility of small benefits.
The contradiction posed in the opening question is therefore no longer a contradiction. It is the result of trying to control the immune system and both positive and negative results happen. Much work has been done to understand the negative results of RBC transfusion on BMT. This work supports a mechanism of exposure of the patient’s immune system to mHA on the surface of contaminating platelets in the transfusion as the cause of BMT graft rejection. Much less work has been done to examine the possible positive effects of RBC transfusion on solid organ transplants. This is because the effect was small to begin with, and with immunosuppressive drugs and HLA typing procedures, the need for this small benefit for successful transplants is gone. With these three understandings in mind, the contradiction between the positive and negative effects of RBC transfusion on transplantation that opened this essay has been explained.
References
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