Red blood cell alloimmunization is associated with lower expression of FcγR1 on monocyte subsets in patients with sickle cell disease

BACKGROUND Despite the clinical significance of red blood cell (RBC) alloantibodies, there are currently no laboratory tests available to predict which patients may be at risk of antibody formation after transfusion exposure. Given their phagocytic and inflammatory functions, we hypothesized that differences in circulating monocytes may play a role in alloimmunization. STUDY DESIGN AND METHODS Forty‐two adults with sickle cell disease (SCD) were recruited, with data extracted from the electronic medical record and peripheral blood analyzed by flow cytometry for total monocytes, monocyte subsets (CD14 high/CD16 low+ classical monocytes, CD14 high/CD16 high+ intermediate monocytes, and CD14 intermediate/CD16 high+ non‐classical/inflammatory monocytes), and FcγR1 (CD64) expression. Thirteen “non‐responder” patients (non‐alloimmunized patients with documented RBC transfusion at the study institution) were compared to 20 alloimmunized “responder” patients, who had a total of 44 RBC alloantibodies identified. RESULTS There were no significant differences in the percentages of total monocytes, monocyte subsets, or measured cytokines between non‐responders and responders. However, non‐responders had higher CD64 expression on classical monocytes (MFI mean 3424 ± standard deviation 1141) compared to responders (MFI mean 2285 ± 1501), p = 0.029, and on intermediate monocytes (MFI mean 3720 ± 1191) compared to responders (MFI mean 2497 ± 1640), p = 0.033. CONCLUSIONS Monocytes and the inflammatory milieu increasingly are being appreciated to play a role in some complications of SCD. The differences in FcγR1 expression on monocyte subsets noted between responders and non‐responders, which cannot be directly explained by the serum cytokines evaluated, warrant further investigation.