HLA testing and organ transplants

April 21, 2014 at 10:08 am

gary bike ride

Have you ever wondered how doctors determine whether a donated organ is the right fit for a patient in need of a transplant? Gary received a new liver in 2005 to overcome a rare autoimmune disease called Primary Sclerosing Cholangitis, but his surgeon couldn’t just put any old liver into his body — it had to meet a number of criteria.

There are three factors that play into matching an organ to a recipient: blood type, human leukocyte antigens (HLA), and antibodies. The donor’s and recipient’s blood type need to be compatible, though HLA and antibodies get tricky — you don’t need to have the exact same HLA, though the better the match the less likely you are to have harmful antibodies against your type.

How do antibodies work?

Most blood donors know their ABO blood type and understand how it works with blood transfusion: O can only receive type O blood; AB can receive blood from any other types; A can receive blood from an A or O donor; and B can receive blood from an O or B donor. If you receive the wrong blood type, a transfusion reaction can occur.

You may not know that this reaction is because of antibodies, proteins that attack specific particles, called antigens, in the body (in this case, on the red blood cells).

O red blood cells have no antigens, but that blood type has antibodies in the serum (the fluid in your blood) against antigens found on A and B red blood cells. AB red blood cells have both A and B antigens, but no antibodies in the serum (because these would attack the person’s own red blood cells). This is why O is the universal whole blood donor, while AB is the universal blood recipient.

HLA works in the same way, on a much larger scale — it’s the most polymorphic (variable) genetic system in humans. For example, there are only four or five alleles (genetic markers) that code for hair or eye color, but there are thousands of HLA alleles.  Each person only expresses a few alleles (inherited from their parents) so the chances of someone not related to you being an exact HLA match is essentially one in a million.

The immune system: HLA and antibodies

HLA and antibodies are part of our immune system: our body’s defense against invaders like bacteria, parasites, and viruses. According to Dr. Paul Warner in our HLA Lab,

The main function of the immune system is to tell what is ‘you’ from what is not ‘you’. Anything that’s not ‘you’ in a part of your body that should be sterile, like your blood, you need to get rid of, because it wants to use your body as a food source.  That’s basically what an infection is.  Because your specific HLA molecules are present on the surface of virtually every cell in your body, it’s a good way for your immune system to tell ‘friend’ from ‘foe’.

Unfortunately, this is a mixed blessing when we are trying to match a donor organ to a recipient. The immune system, designed to protect the body, will attack and reject an organ that it views as being foreign. So,

The main thing that we do is not so much [HLA] matching – we do mismatched transplants all the time. The main thing that we do is make sure that the person receiving that transplant does not have antibodies against the mismatched HLA of the donor, because those antibodies can cause graft rejection.

Detection and identification of antibodies against HLA in a transplant recipient is more important than HLA matching of the donor and recipient, by far. You can be a zero HLA match, as long as you don’t have antibodies against the mismatched HLA of the donor. However, the better the match, the better the long-term outcome – because you don’t make antibodies against your own HLA, so if someone has the same HLA you do, you won’t make antibodies against them, either.

So, what does the testing look like? Click through to see the steps.

After we receive samples of donor and patient blood, we will use these to determine whether the organ is a good fit for a patient.
First, we need to determine the HLA type of the donor and recipient; we use a machine called a thermal cycler and a process called polymerase chain reaction (PCR) which repeatedly heats and cools the samples to amplify specific sequences of DNA. The enzyme used to amplify the DNA has to be able to withstand the heat during this heating/cooling process, so we use an enzyme discovered in bacteria that live in the hot springs of Yellowstone Park. This whole process is quick: we can go from the initial whole blood sample to the person’s HLA type in approximately 2 hours. This testing needs to be done separately from the rest of the testing, as the DNA in the atmosphere (after the PCR amplification) can contaminate other samples!
Then we do a test to see if the recipient has antibodies against the donor HLA. This is called a crossmatch. We mix the serum from the recipient (the part of the blood where the antibodies are) with white blood cells from the donor (which have the donor HLA on them). After incubating the serum and cells to let any potential antibodies bind to their target cells, we wash the cells to get rid of any unbound antibodies (like antibodies against flu viruses, and anything else your body has made an immune response against. Remember, we are only looking for antibodies against the donor HLA, and everyone’s serum has lots of other antibodies present.) After adding an anti-antibody that has a fluorescent marker on it, any donor cells that have antibodies from the recipient’s serum bound to them will light up when exposed to an appropriate light source.
The patient samples are run through a flow cytometer which is a very complex machine that shoots a laser at the cells to detect antibodies. If antibodies bound to the cells are present, they can be detected because of the fluorescence, which the flow cytometer measures. The technologist running the test then compares the patient sample with several controls to determine whether the donor and recipient are compatible (the recipient has no antibodies against the donor), or incompatible (the recipient does have antibodies against the donor).
We also freeze samples of serum and cells from each donor and recipient. The frozen cells are stored in liquid nitrogen-chilled tanks (approximately -350 degrees F. Brrrrr!). This helps with research or if additional testing is needed down the road. For example, if a patient’s transplant starts to go bad five years after transplant, we can get a new serum sample from the patient, and test it against (thawed) donor cells to see if new antibodies have appeared. Then the patient can be treated to try to get rid of the antibodies, and hopefully prevent the organ from being totally rejected.

Because of these tests, Gary and thousands of other transplant recipients in the Pacific Northwest are able to enjoy many more years of doing what they love. In Gary’s case, it’s spending time with his family.