Monoclonal B-cell lymphocytosis in healthy blood donors: an unexpectedly common finding

Abstract

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.

Figure 1

Gating strategy used for the detection of 3 distinct immunophenotypes of MBL. Representative immunophenotypes are shown for the 3 categories: CLL-like (CD19⁺CD5⁺CD20^dim), atypical (CD19⁺CD5⁺CD20^bright), and CD5ˉ (CD19⁺CD5ˉ) MBL.^, (A) Two parameter histograms of CD20 peridinin chlorophyll protein complex (PerCP) on x-axis and CD19 (allophycocyanin Cy7 or Pacific Blue) on y-axis were based on initial CD45 expression and SSC signal. B cells are gated for subsequent analysis. (B) κ (FITC) λ (PE) analysis on gated B cells. Cases 97 and 118 have bimodal κ populations showing dim clone and bright polyclonal κ cells. Case 3 shows 80.9% λ population. (C) Gated B cells are displayed on a CD20 vs CD5 plot. Case 97 is subgated on CD20^dim and CD5^+,bright B cells. Case 118 is subgated on CD5⁺ B cells. In case 3, all the B cells are CD5ˉ. (D) Subgated B cells from panel C are displayed on a κ vs λ plot. Case 97 shows 98.6% κ⁺ clone on κ-λ analysis on gated CD20^dim/CD5⁺ B cells. Case 118 shows κ-λ analysis on gated CD5⁺ B cells showing 97.8% κ⁺ clone. Case 3 shows a skewed κ-λ analysis on gated CD5ˉ B cells showing 81.9% λ⁺ cells. FITC, fluorescein isothiocyanate; PE, phycoerythrin.

Figure 2

Clonal B-cell counts among 101 CLL-like MBL cases.

Figure 3

Clonal B-cell counts among 23 atypical and 21 CD5^– MBL cases.

Figure 4

IGHV gene repertoires in 51 MBL cases.