Circulating monoclonal B cells may be detected in healthy adults, a condition called monoclonal B-cell lymphocytosis (MBL). MBL has also been identified in donated blood, but no systematic study of blood donors has been reported. Using sensitive and specific laboratory methods, we detected MBL in 149 (7.1%; 95% confidence interval, 6.0% to 8.3%) of 2098 unique donors ages 45 years or older in a Midwestern US regional blood center between 2010 and 2011. Most of the 149 donors had low-count MBL, including 99 chronic lymphocytic leukemia-like (66.4%), 22 atypical (14.8%), and 19 CD5(-) (12.8%) immunophenotypes. However, 5 donors (3.4%) had B-cell clonal counts above 500 cells per µL, including 3 with 1693 to 2887 cells per µL; the clone accounted for nearly all their circulating B cells. Four donors (2.7%) had 2 distinct MBL clones. Of 51 MBL samples in which immunoglobulin heavy chain (IGH)V-D-J genotypes could be determined, 71% and 29% used IGHV3- and IGHV4-family genes, respectively. Sequencing revealed 82% with somatic hypermutation, whereas 18% had >98% germ-line identity, including 5 with entirely germ-line sequences. In conclusion, MBL prevalence is much higher in blood donors than previously reported, and although uncommon, the presence of high-count MBL warrants further investigations to define the biological fate of the transfused cells in recipients.
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