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| Year : 2006 | Volume
: 17
| Issue : 1 | Page : 1-4 |
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| Panel Reactive Antibody test (PRA) in renal transplantation |
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Ali H Hajeer
College of Medicine & Pathology Department, King Abdulaziz Medical City, Academy for Health Sciences, National Guard Health Affairs, Riyadh, Saudi Arabia
Click here for correspondence address and email
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How to cite this article:
Hajeer AH. Panel Reactive Antibody test (PRA) in renal transplantation. Saudi J Kidney Dis Transpl 2006;17:1-4 |
Our immune response has evolved to counteract foreign antigens, such as viruses and bacteria. Both cellular and humoral antibody responses can be triggered as a result of infection. However, the immune system may undesirably react to non-infectious agents such as food, pollen (allergies) allograft (transplant rejection) and respond to hidden or unhidden self-antigens (auto-immunity).
When exposed to tissues of another person, the immune system mounts a reaction, mainly to the polymorphic (variables or different) antigens of that individual. This is called priming of the immune system to the antigen(s). For example transfusion of platelets from an individual with a different class I antigens (constitutively expressed on platelets) will result in priming both arms of the immune response (cellular and humoral) against class I antigens. Organ transplanttation for such an individual may result in hyperacute rejection.
Priming of the immune response to HLA class I and / or II can occur as a result of pregnancy, blood transfusions or rejection of a previous transplant. [1] However, there are individuals who have HLA antibodies with no history of exposure to the above factors.
This can be explained by the presence of cross-reacting antibodies, usually of IgM class. Leukodepleted blood has been used to reduce the risk of all immunization. However, a randomized trial [2] found that filtration of leukocyte still results in post transfusion alloimmunization, merely because filtration cannot deplete all leucocytes.
Anti-HLA antibodies are usually referred to as panel reactive antibody (PRA) test that sometimes is referred to as percent reactive antibody, since the result is expressed as a percentage.
 How can anti-HLA antibodies cause rejection? | |  |
Hyper acute rejection
In case of a kidney for example, circulating anti-HLA class I antibodies bind to the expressed antigens on endothelial cells resulting in complement activation and cell death. This in turn attracts different immune cells (to the site) such as granulocytes and platelets and the formation of microthombi and necrosis. IgG anti - HLA and ABO antibodies have been shown to cause hyper acute rejection. [3],[4]
Acute Rejection
There is mounting evidence to suggest that acute rejection is caused by both cellular and humoral immune responses. HLA- specific lymphocytes, antibodies, [5] plasma cells [6] and C4d recovered from rejected allografts suggest that both humoral and cellular response act to induce rejection.
Anti HLA antibodies induce rejection not only by activating complement but also through antibody dependent cell-mediated cytotoxicity (ADCC) [5],[7]
| How can we identify anti-HLA antibodies? | | |
Complement dependent cytotoxicity (CDC)
CDC was the first technique to detect HLA specific antibodies. It employs the use of live lymphocytes to detect lymphocytespecific antibodies by activation of complement system and killing of lymphocytes. This is a widely used test, but has many drawbacks, (i) it is limited by the cell panel used, (ii) it depends on the quality of lymphocytes and rabbit complement, (iii) it detects non-HLA antigens and (iv) it only detects complement fixing antibodies. Therefore, patients cannot be tagged as sensitized on the basis of this test. Results are expressed as percentage of cells reacted with the tested serum.
| Elisa | | |
The enzyme-linked immunosorbant assay (ELISA) employs the use of HLA antigens, this removes the need for complement, live cells and excludes the non-HLA specific antibodies. [8],[9] There are two types of commercially available kits, one detects presence or absence of PRA, the other detects antibody specificity. PRA detected by ELISA is more sensitive than the CDC assay. [10],[11]
Flow Cytometry
This assay depends on the availability of a flow cytometer in the laboratory. Two different methods can be employed (i) an in house method in which whole lymphocytes are used as the antigens [12],[13],[14],[15],[16] and (ii) comercially available kits which employ beads coated with specific HLA- antigens. [17] HLA- A, B, Cw, DR, DQ, and DP coated beads have been used and shown to be much more sensitive than the CDC method. [10]
Advances in this methodology result in the introduction of single antigen coated beads. This method helps identifying the specificity of PRA in highly sensitized patients and improves the definition of acceptable mismatches. [18]
What method shall our lab use?
It is important not to rely on one method for the detection of PRA as each method has its own advantages and disadvantages.
One can use any of the three methods for screening, but a strategy for defining PRA specificity should be employed. It is advised to use more than one assay in the laboratory. But one has to keep in mind that the method used for PRA should match in sensitivity the method used for crossmatching.
| How often shall we screen for PRA? | | |
Pre transplant
Anti-HLA antibodies vary with time in one individual patient and may be influenced by many factors. Therefore, it is recommended to screen patients for PRA in specified intervals. This should not be more than 3 months. In case of blood transfusion, a sample should be sent to the laboratory between 14 to 28 days post transfusion.
If PRA and antibody specificity are determined on a series of patient sera, this can greatly help in predicting the crossmatch results. Even in patients who receive zero mismatches at HLA-A, B and DR benefit from regular screening and antibody characterization. Antibody specific to HLACw, DQ and even DP have been shown to influence graft outcome. [19],[20],[21]
Post Transplant
Even though transplant patients receive immunosuppressive drugs, their immune system is still capable of mounting immune responses. De novo antibodies produced post transplant are capable of rejection and chronic graft damage. Monitoring anti - HLA, antibodies post transplant, especially donor specific antibodies, can help in predicting graft outcome. It is recommended that serum samples should be tested for PRA monthly for the first 3 months and quarterly up to one year and annually thereafter. The development of anti - HLA antibodies post transplantation has been shown to be a strong predictor of graft failure.[22] It is therefore necessary to use assays that can detect those antibodies with high sensitivity and specificity such as ELISA and flow cytometry.
| Conclusion | | |
The PRA is a very useful marker in renal transplantation. Histocompatibility laboratories should have the sensitive and specific tools to allow for detecting the anti-HLA antibodies formed pre or post transplantation.
| References | | |
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| 22. | Worthington JE, Martin S, Al-Husseini DM, Dyer PA, Johnson RW. Posttransplantation production of donor HLA-specific antibodies as a predictor of renal transplant outcome. Transplantation 2003,75:1034-40. |
Correspondence Address:
Ali H Hajeer
College of Medicine Academy for Health Sciences King Abdulaziz Medical City P. O. Box 22490, Riyadh 11426
Saudi Arabia
 Source of Support: None, Conflict of Interest: None  | Check |
PMID: 17297529 
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