When monocytes and platelets compete: The effect of platelet count on the flow cytometric measurement of monocyte CD36.

BACKGROUND

Flow cytometric measurement of monocyte surface CD36 is relevant to several conditions including diabetes, cardiovascular disease, lipid disorders, platelet isoimmunization, and susceptibility to P falciparum malaria. CD36 is also strongly expressed on platelets where it is also known as platelet glycoprotein IV.

METHODS

Whole blood samples, containing identical monocyte concentrations, were adjusted to contain platelets ranging from 20,000/uL to 600,000/uL, were stained with fluorescent-labeled anti-CD36, and analyzed by flow cytometry.

RESULTS

CD36 median fluorescent intensity (MFI) observed on monocytes decreased as the platelet concentration in the sample increased with more than a 50% decline in monocyte MFI over the normal range of platelet values. The effect was not abolished by using larger volumes of monoclonal antibody and was observed with different clones of reagent anti-CD36. The findings were most consistent with competition by platelets for the CD36 reagent. Similar findings were observed with antibody to class I HLA. Under defined assay conditions, monocyte CD36 MFI declined with rising platelet concentration in a predictable fashion following an inverse linear relationship.

CONCLUSIONS

Measurement of CD36 expression on monocytes by flow cytometry in whole blood samples is affected by the sample platelet count. When comparing the monocyte CD36 expression among different individuals, our approach can be used to adjust measured monocyte CD36 expression for the effect of the platelet concentration in the sample. Competition by platelets for monoclonal reagents may occur in other settings when whole blood assays are used and when the target antigen is strongly expressed on both platelets and leukocytes.