Cambier and his colleagues closely examined the B cells in mice less than 3 months of age and mice older than 22 months. That is roughly equivalent to studying B cells from a teenager and from a 75-year-old person. They used a number of cell characteristics, including cell-surface receptors and antibody expression, to distinguish antigen-experienced B cells from naive ones. One kind of naive B cell, called a follicular cell, decreased from an average of 70% of the B-cell population in young mice to 32% in older mice. In another analysis, results indicated that only about 5% of the B cells in young mice were antigen-experienced, but they accounted for 76% of the B cells in older mice.
These numbers are averages, said co-author Sarah Johnson. The B cell populations of individual mice varied, with some retaining large numbers of naive B cells and other carrying almost none. The mice with many naive B cells would presumably be better at fighting off infections.
In a follow up to these findings, Cambier and his colleagues have transplanted hematopoietic stem cells from young mice into old mice. Initial results suggest that the transplantation restores the B cell population to a younger, more nave and adaptable profile.