Early mortality in sickle cell disease (SCD) is attributed to increased infections due to loss of splenic function. Marginal zone B cells are important for initial opsonization of pathogens and can be absent in spleen histopathology in SCD. The frequency of unswitched memory B cells (UMBC), the circulating correlate of marginal zone B cells, reflects the immunologic function of the spleen. We hypothesized that asplenia in SCD is associated with alterations in the peripheral blood lymphocyte population and explored whether UMBC deficiency was associated with a clinical phenotype. We analyzed B cell subsets and clinical history for 238 children with SCD and 63 controls. The median proportion of UMBCs was lower in children with SCD compared with controls (4.7% vs. 6.6%, p < .001). Naïve B cells were higher in SCD compared with controls (80.6 vs. 76.3%, respectively, p = .02). UMBC frequency declined by 3.4% per year increase in age in SCD (95% CI: 2%, 4.7%, p < .001), but not in controls. A majority of children in all cohorts had an IgM concentration in the normal range for age and there were no differences between groups (p = .13). Subjects developed titers adequate for long-term protection to fewer serotypes in the polysaccharide vaccine than controls (14.7 vs. 19.4, p < .001). In this cohort, bacteremia was rare and specific clinical complications were not associated with UMBC proportion. In summary, UMBC deficiency occurs in SCD and is associated with age. Future studies should investigate B cell subsets prospectively and identify the mechanism of B cell loss in the spleen.
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