ROLES OF IGH-CHAIN AND L-CHAIN AND OF SURROGATE H-CHAIN AND L-CHAIN IN THE DEVELOPMENT OF CELLS OF THE B-LYMPHOCYTE LINEAGE

Proteins expressed from productively rearranged H and L chain gene loci have been implied in the regulation of Ig gene rearrangements during B lymphopoiesis. However, recent findings suggest that early B cell development can occur without expression of surrogate L chain, without deposition of mu H chains into membranes, without productive H chain gene rearrangements, and even without any rearrangements of Ig gene loci. In bone marrow, 2-5% of all B220(-), sIgM(-), c-kit(+) cells are pro B cells that undergo differentiation from B220(+) via B220(+), c-kit(+), CD43(+), clonable long-term proliferating pre B-I cells to B220(+), c-kit(+), CD43(+), IL-2 receptor(+) pre B-II cells and immature B cells, only to die by apoptosis in situ within less than 4 days. A membrane-bound complex of surrogate H chain (gp130/gp35-65) and surrogate L chain expressed on pro B and pre B-I cells has apparently no influence on this early development. Pre B-I cells carrying D(H)J(H)-rearrangements in reading frame (rf) II are counterselected, probably because they can express an Ig-like complex of truncated D(H)J(H)C mu-protein and surrogate L chain, while pre B-I cells D(H)J(H)-rearranged in rf I or III are not suppressed. Immature sIg(+) B cells, also from bcl-2 transgenic mice, can continue to rearrange L chain gene loci. Thus, mere membrane deposition of Ig, even with concomitant expression of bcl-2, terminates neither expression of RAG-1 and 2, nor secondary L chain gene rearrangements, nor does it allow the development of mature B cells, Membrane-bound expression of an Ig-like complex of mu H chains and surrogate L chains appears to be needed to generate the 50-70 million pre B-II cells in bone marrow. However, the membrane-bound expression of Ig is mandatory for negative and positive selection of immature B cells. Autoantigens delete or anergize self-reactive B cells. We speculate that all mature, resting, primary antigen-reactive B cells in the periphery have been selected from immature sIg(+) B cells by unknown antigens and have, thereby, changed their lifestyle from rapid death by apoptosis to longevity.