Differences in mouse and human nonmemory B cell pools.

Identifying cross-species similarities and differences in immune development and function is critical for maximizing the translational potential of animal models. Coexpression of CD21 and CD24 distinguishes transitional and mature B cell subsets in mice. In this study, we validate these markers for identifying analogous subsets in humans and use them to compare the nonmemory B cell pools in mice and humans, across tissues, and during fetal/neonatal and adult life. Among human CD19(+)IgM(+) B cells, the CD21/CD24 schema identifies distinct populations that correspond to transitional 1 (T1), transitional 2 (T2), follicular mature, and marginal zone subsets identified in mice. Markers specific to human B cell development validate the identity of marginal zone cells and the maturation status of human CD21/CD24 nonmemory B cell subsets. A comparison of the nonmemory B cell pools in bone marrow, blood, and spleen in mice and humans shows that transitional B cells comprise a much smaller fraction in adult humans than mice. T1 cells are a major contributor to the nonmemory B cell pool in mouse bone marrow, in which their frequency is more than twice that in humans. Conversely, in spleen, the T1:T2 ratio shows that T2 cells are proportionally ∼ 8-fold higher in humans than in mice. Despite the relatively small contribution of transitional B cells to the human nonmemory pool, the number of naive follicular mature cells produced per transitional B cell is 3- to 6-fold higher across tissues than in mice. These data suggest differing dynamics or mechanisms produce the nonmemory B cell compartments in mice and humans.