Measurement of leukocyte-platelet aggregates (LPA) by FACS: a comparative analysis

In addition to their role in hemostasis and coagulation platelets bear critical roles in modulation of the innate and adaptive immune system. Upon platelet activation in response to tissue injury, bacterial or viral infections, they secrete many soluble factors or directly interact with leukocytes. An increase of leukocyte-platelet aggregates (LPA) has been described for many pathological conditions. Nevertheless, a standardized method for the reliable measurement of PLAs is not securely established. This methodical study provides a comparison of four different protocols from the literature and summarizes major pitfalls of measuring and interpreting leukocyte-platelet aggregates. The different techniques vary in the workup of the blood samples, applying variable washing and centrifugation steps or the use of erythrocyte lysis. All samples were finally analyzed by flow cytometry. Leukocyte subsets were stained with specific antibodies and platelet aggregates were identified by additional expression of CD41. The different procedures generated very heterogeneous data from the same blood sample which highlight the abundance of error measuring LPA. The most reproducible technique turned out to be a two-color whole blood flow cytometry assay with erythrocyte lysis and without washing or centrifugation steps avoiding platelet activation and artificial aggregate formation to achieve data mirroring the true situation in peripheral blood.