Abstract
Progenitor and precursor B lymphocytes with the capacity of long-term proliferation on stromal cells in the presence of interleukin-7 (IL-7) can be cloned ex vivo from fetal liver, neonatal blood, and spleen, and from adult bone marrow (BM) in frequencies that are similar in different strains of mice and that change with age. A wave of clonable cells appears before birth and disappears after birth in liver. Up to 2 weeks after birth, high frequencies of clonable cells are present in spleen but become undetectable at 6 to 8 weeks of age. In BM, high frequencies (1 in 50) of clonable cells are present early after birth, and then decrease continuously to 10- to 20-fold lower levels at 6 to 8 months of age. The earliest clonable cells have at least part of their IgH genes in germline configuration. Clones of pro/pre B cells apparently continue to rearrange DH to JH segments on both chromosomes. Rearrangements without insertion of N-sequences at the DHJH joints are found in fetal liver, while DHJH joints in pre B cells of spleen and BM throughout life have N-regions inserted. At least half of all primary pre B-cell clones develop mitogen-reactive B cells after differentiation to sIg+ B cells. Clonable pro and pre B cells are enriched in B220- c-kit(low) as well as in B220+ c-kit+ and B220+ CD43+ cell populations of BM. The frequencies of clonable cells in the B220- c-kit(low) BM cell population decrease 10- to 20-fold during 8 months of life, while those in the B220+ c-kit+ population remain constant, although their absolute numbers drop 5- to 10-fold during that time. All long-term proliferating clones express the surrogate L chain VpreB/lambda 5 as well as c-kit and CD43 on all cells. The number of total clonable pro and pre B cells is at best 10% of the number of cells required to produce the estimated daily output of 5 x 10(7) B- lineage cells in a mouse. This suggests that the production of a relatively constant number of B cells during adulthood may be effected by precursors, which are not clonable on stromal cells and IL-7 with long-term proliferative capacity. On the other hand, BM transplantation experiments indicate that a mouse retains B220- progenitors throughout life, from which pre B and B cells can be generated in old mice in frequencies characteristic of young mice.
