Stability of antigens on leukocytes in banked platelet concentrates: decline in HLA-DR antigen expression and mixed lymphocyte culture stimulating capacity following storage

Repeatedly transfused thrombocytopenic patients frequently form antibodies directed against human leukocyte antigens (HLA) and become unresponsive to random donor platelet transfusions. Although exposure to foreign antigens borne on donor leukocytes appears necessary to provoke primary sensitization, the stability of leukocyte antigens during routine platelet storage is largely unknown. Accordingly, we serially measured the expression of surface markers on leukocytes derived from platelet concentrates during storage using immunofluorescence and flow cytometry. Our results indicate that the expression of class I HLA antigens, Leu-4 (T cell), and HLe-1 (pan leukocyte) remained stable on lymphocytes under standard platelet storage conditions, but that the percentage of lymphocytes bearing class II HLA antigens declined significantly over time. This decline in lymphocyte HLA class II expression was associated with a significantly diminished ability of stored leukocytes to stimulate blastogenesis in mixed lymphocyte culture. However, leukocytes retained the ability to respond in mixed lymphocyte culture (MLC) following storage. We also performed studies on lymphocytes cultured in the presence of cyclohexamide, which suggested that the expression of class I HLA antigens and B2 microglobulin are highly sensitive to the inhibition of protein synthesis, whereas the expression of class II HLA antigens, Leu- 4, and HLe-1 are not. Our results may prove useful in understanding the mechanisms that lead to platelet refractoriness and in designing strategies to prevent HLA alloimmunization.