Tag Archives: MHC

Transplantation, rejection and the genome

Image credit: Mark Gray, Spectrum Health
Image credit: Mark Gray, Spectrum Health

Transplantation is always a risky business when you use tissues and organs that are genetically dissimilar to those of your recipient, but sometimes it is the only option. With conditions such as Idiopathic Pulmonary Fibrosis (IPF) lung transplantation remains the only viable treatment that truly offers a chance at increased survival. Those people with end-stage heart disease or those with a failing liver, due to conditions such as Primary sclerosing cholangitis for example, can all require a transplant. Unfortunately not all of these people will get them. Qualifying for a transplant is no easy feat, the rest of the body must be in good condition, you must show that you have a willingness and the ability to improve your lifestyle, and of course you must be able to take the multitude of medications required to prepare for such an extreme operation. All in all those tasked with the difficult duty of deciding who gets a transplant, must get the rare and valuable transplant tissues to those who will benefit the most, and to those with the greatest need.

But I digress, it is the rejection of tissues, not the process and huge benefits of transplantation that will be the focus here…

Rejection of transplanted tissues is a complex and unique response mounted by the immune system. While I don’t plan on going into too much detail concerning the mechanisms behind transplant rejection, I will attempt to give a brief overview of some of the important points. It is the major histocompatibility (MHC) complexes that are responsible for advertising a cells identity as “self” or “non-self”. They essentially form a barcode that identifies a person’s cells as the cells of that person. If they are scanned and found to be incorrect they are identified as “non-self”. There are multiple pathways in acute rejection of transplanted tissues all involving these MHC molecules. The recipient’s immune system responds to recognising foreign MHC molecules in a variety of ways with the adaptive system charging ahead with cytotoxic T cells and/or antibody-producing B cells. This means that both chronic and acute rejection are associated with the death of cells from the donated tissue leading to eventual damage to the organ and finally necrosis, death of the tissue or organ. The MHC molecules are encoded by polymorphic alleles and expressed co-dominantly, which means that a person’s genome encodes 12 different MHC molecules and that even siblings still have only a 1 in 4 chance of having identical MHC complexes. Essentially, the take home message is that MHC matching is essential for clinical transplantation and has a clear beneficial effect on the outcome. Saying that, it is not necessary for MHC molecules to be matched perfectly, some mismatches are less disadvantageous than others due to the possibility of shared sections of structure between different MHC types. Of course it would be too simple if this was the only problem presented by transplantations, there are other factors to consider, for example, in some cases the recipient’s immune system may already have been exposed to, and so be ready to mount a fast and effective response against, a similar foreign molecule leading to what is called hyper-acute rejection that takes place within minutes of the donor organ being transplanted.

Acute rejection usually occurs within the first two weeks of the operation but rejection of transplanted organs can in fact be a long process spread over several months or even years. This chronic rejection is very understudied and not completely understood for most types of transplant. Chronic rejection of lung transplants is actually more common than with other solid organ transplants, and the survival rate is still only 50% after 5 years. Monitoring the acceptance of an organ such as this requires biopsies be done routinely to monitor the organs acceptance, or keep track of the rejection. With heart transplants this involves an endomyocardial biopsy that is expensive, painful and not without risk of complication. Another method to monitor this process, or to initially diagnose it, was developed by Stephen Quake from the Howard Hughes Medical Institute back in 2011. A non-invasive procedure to detect solid organ transplant rejection was proposed by Dr Quake and his colleagues. Other attempts have been made to produce a non-invasive procedure, such as the FDA-approved AlloMap molecular expression test, that focuses on monitoring the immune system of the patient by measuring the expression of certain genes. In contrast to this, the method developed by Stephen Quake relies upon the fact that, as I mentioned in the very first sentence, the genomes of transplanted tissues differ from those tissues and cells of the recipient. It also works from the fact that that cells die and are routinely shed, breaking apart to leave cell-free DNA in the bloodstream. The same team has used this knowledge to produce a diagnostic test that allows the determination of whether an unborn child has Down syndrome by analysing the blood of the expecting mother.

This latest technique, aimed at diagnosing transplant rejection, utilises high-throughput sequencing to identify the “signature” of cells from the donated tissue which can then be monitored over time. If there is a detectable change in the level of donor DNA in the bloodstream then it is implied that there is an increase in the rate of death of the donor cells that in turn implies the transplant is being rejected. The results of the study show that during organ rejection the levels of donor DNA can increase up to 4 times the level seen normally. Previous attempts to identify cell-free DNA in organ transplantation have been met with limited success, only managing to detect the Y chromosome present when a female has received a transplant from a male. However, Quake has managed to demonstrate that this technique is sex-independent and can be applied to any donor and recipient. While so far only demonstrated in patients who have undergone heart transplant, the theory behind the technique could be applied to the majority of transplantations if it can be shown that donor DNA is present in the bloodstream. Unlike the AlloMap this procedure has been shown to detect rejection before a biopsy and so could be a good diagnostic tool for allowing treatment to start early (with corticosteroids) before confirmation with a biopsy. If this, or a similar non-invasive technique were adopted, the savings made through reducing the number of biopsies could be up to $12 million in the United States. This is without even mentioning the cost of the impact on quality of life caused by routine biopsies.

While a good example of one way in which advances in sequencing and in genetic testing could have a positive impact on patients this could just be the start. Over the coming years further advances will be made in personalised medicine, for example Quake writes that taking a snapshot of the antibodies present in a person’s blood could be used outside of the field of transplantation and have an impact on monitoring the success of vaccinations, whether someone has an allergy or an autoimmune disease. He points out quite rightly that the immune system is not static, it fluctuates in its activity throughout a person’s lifetime and so testing could one day become a routine tool used for a huge variety of conditions at many points throughout the lifetime of a patient.

In the UK there always seems to be quite a lot of talk about waiting times with the National Health Service, while from the US I seem to hear complaints about pricing and inadequate insurers. With advances in sequencing (as well as the price coming down) and the development of techniques such as this, I can only hope that one day genetic and immunological testing has advanced to such a stage that a simple test, that can be done in my local practice, could diagnose any condition known to mankind simply and effectively. Of course we are a long way from that point but it does not feel impossible that we could see some significant steps towards this in the coming years.

 

Author: David Busse