Extramedullary leukemia relapse after allogeneic stem cell transplantation: a novel mechanism of immune escape?

Background: Relapse is a significant cause of treatment failure after allogeneic stem cell transplantation. In many cases relapse occurs when leukemic cells escape from immune surveillance. Methods & results: In the setting of haploidentical transplantation, immune escape is usually the result of the loss of the mismatched haplotype from leukemic cells, while downregulation of HLA-expression has been postulated as a significant cause of immune escape after transplantation with the use of HLA-matched donors. We observed that patients with acute leukemia who relapse at the time of active graft-versus-host-disease, usually develop extramedullary leukemia while they remain free of leukemia in peripheral blood and bone marrow. Conclusion: Our observation points toward a novel mechanism of immune escape which is microenvironment-specific.

Direct evidence for normalization of platelet function resulting from platelet count reduction in essential thrombocythemia.

Essential thrombocythemia is characterized by persistent elevation and functional disturbances of platelets. Both the platelet function analyzer-100 (PFA-100) collagen-epinephrine (CEPI) cartridge and aggregometry with epinephrine are considered sensitive and valid methods in detecting abnormal platelet function in essential thrombocythemia. We attempted to confirm that restoration of abnormal platelet function results from platelet count reduction in essential thrombocythemia, by using these two methods. Thirty-nine essential thrombocythemia patients were divided into two groups on the basis of their platelet count. Group A participants (n = 20) exhibited platelet counts greater than 500 × 10/l, whereas group B participants (n = 19) had platelet counts below this limit. Hematological parameters, plasma von Willebrand factor (vWF) antigen and activity levels were assessed. Platelet function was analyzed by the PFA-100 and light transmission aggregometry with epinephrine, collagen, and ADP. The point mutation JAK2 V617F was identified and its effect on platelet function tests was also investigated. By using logistic regression analysis, white blood cell count, vWF activity level, and the measurements of aggregation in response to epinephrine were significantly different between the two groups. Epinephrine-induced aggregation retained the statistical significance in the multivariable procedure (P : 0.002). PFA-100 CEPI closure times were lower - but not statistically significant - in group B. Neither the JAK2 V617F positivity nor different cytoreductive treatments had any influence on ex-vivo platelet function tests. Our findings demonstrate normalization of platelet function resulting from platelet count reduction in essential thrombocythemia and reinforce the concept of lowering platelet counts in these patients.