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Platelet activation by HLA antibodies
Figure 7. Proposed mechanism of platelet activation. When an activating HLA antibody (ab) binds to HLA on the platelet surface it crosslinks with FcγRIIa. This induces FcγRIIa-dependent signaling leading to the activation of Syk via the ITAM motif on FcγRIIa. Downstream signaling leads to platelet activation: α-granules are released (followed by e.g. CD62P exposure), integrin αIIbb3 is activated and platelets start to aggregate. This activation pathway can be inhibited by IV.3 (which blocks crosslinking with FcγRIIa) or Syk inhibitor IV (which blocks signaling via Syk). HLA antibodies that bind to an epitope on HLA preventing interaction with FcγRIIa do not induce platelet activation. FcγRIIa-dependent signaling also leads to phosphatidylserine exposure and induces enhanced phagocytosis by macrophages.
the release of neuraminidase which cleaves sialic acid from platelet surface receptors thereby enhancing clear- ance of platelets through the Ashwell Morell receptors expressed by hepatocytes.42 We show that HLA antibodies activate platelets in an FcγRIIa-dependent manner. Whether this also results in release of sialidase and subse- quent clearance of platelets in patients with HLA antibod- ies has not been studied. However, our macrophage inter- nalization experiments show that platelet activation induced by HLA alloantibodies may have an impact on platelet survival.
Incubation of healthy donor platelets with patients’ sera containing HLA antibodies revealed that FcγRIIa-depen- dent platelet activation could be induced by polyclonal HLA antibodies from approximately one third of the test- ed sera from refractory patients. Some sera, such as sera 7 and 8, only induced activation of platelets in one of the two donors. This suggests that only a subset of antibodies present in the polyclonal sera is responsible for FcγRIIa- dependent platelet activation. Apparently, these antibod- ies only matched HLA antigens present on one of the two donors. HLA antibodies present in sera which did not induce FcγRIIa-dependent activation with the donor platelets tested might potentially have a different effect when tested with platelets derived from a larger panel of donors. As observed for the panel of monoclonal HLA
antibodies, the results obtained for the patients’ sera indi- cate that only a subset of HLA antibodies is capable of inducing FcγRIIa-dependent platelet activation.
Prediction models based on the three-dimensional struc- ture of HLA epitopes (eplets), rather than the HLA anti- gens itself, have been developed as a basis for matched platelet transfusions.34,43 If the eplets targeted by platelet- activating HLA antibodies were known, it is possible that incorporation of this knowledge in the selection of HLA- matching donor platelets could help to further reduce side effects of platelet transfusions in refractory patients.
In conclusion, we have shown that platelets are activat- ed in an FcγRIIa-dependent manner by a subset of HLA antibodies (Figure 7). This mechanism may contribute to the enhanced clearance of platelets in refractory patients, as suggested by the increased phagocytosis of platelets opsonized by a subset of activating HLA antibodies. This suggests that testing the capacity of patients’ sera to induce platelet activation could be used to further stratify patients with HLA antibodies who need platelet transfu- sions.
Acknowledgments
Supported by grants PPOC-2013-019 and PPOC-2015- 024P (Netherlands Ministry of Health). AJGJ is supported by a Clinical Fellowship of the European Hematology Association.
References
1. Hickey MJ, Valenzuela NM, Reed EF, Reed EF. Alloantibody generation and effector func- tion following sensitization to human leuko- cyte antigen. Front Immunol. 2016;7:1-13.
2. Stanworth SJ, Navarrete C, Estcourt L, Marsh J. Platelet refractoriness – practical
approaches and ongoing dilemmas in patient management. Br J Haematol. 2015;171(3):297-305.
3. Pavenski K, Freedman J, Semple JW. HLA alloimmunization against platelet transfu- sions: pathophysiology, significance, pre- vention and management. Tissue Antigens. 2012;79(4):237-245.
4. De Clippel D, Baeten M, Torfs A, et al.
Screening for HLA antibodies in platelet- pheresis donors with a history of transfu- sion or pregnancy. Transfusion. 2014;54(12): 3036-3042.
5. Triulzi DJ, Kleinman S, Kakaiya RM, et al. The effect of previous pregnancy and trans- fusion on HLA alloimmunization in blood donors: implications for a transfusion-relat- ed acute lung injury risk reduction strategy.
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