Dirk Haasner

The surrogate light chain in B-cell development

The proteins encoded by the VpreB and lambda 5 genes associate with each other to form a light (L) chain-like structure, the surrogate L chain. It can form Ig-like complexes with three partners-the classical heavy (H) chain, the DHJHC mu-protein, or the newly discovered p55 chain; these are expressed on the surface of pre-B cells at different stages of development. Here, Fritz Melchers and colleagues review the structures of the VpreB and lambda 5 genes in mouse and their relatives in humans, describe their pattern of expression, and speculate on their possible evolution and functions.

Two Pathways of B‐Lymphocyte Development in Mouse Bone Marrow and the Roles of Surrogate L Chain in this Development

The commitment of pluripotent stem cells and lymphoid progenitors to the Blineage pathway of differentiation in bone marrow of mice has been investigated in many laboratories. One aim of such studies is to analyze the molecular steps and cellular stages in which B-lineage progenitors and precursors rearrange immunoglobulin (Ig) heavy (H) and light (L) chain gene segments (V, D, J) to express first H, then L chains, and thereby generate a potential repertoire of B lymphocytes. The selection of these cells into a repertoire available in the peripheral pool of mature B cells has also been extensively examined (for reviews see Osmond 1991, Tsubata & Nishikawa 1991, Hardy 1992, Rajewsky 1992, Chen & Alt 1993, Rolink & Melchers 1993a, b). Fig. 1 summarizes some of our current knowledge of the size and cell cycle status of the major B-lineage precursor subpopulations in bone marrow, their expression of markers, and their potential for in vitro proliferation and for in vivo transplantability. It describes two pathways of differentiation, one of which (pathway 1) selects for productively H chain-gene-rearranged, fiH chain-expressing precursors, while the other pathway (2) appears to occur without selection for products of Ig gene rearrangements, and will proceed via a cellular program of surface marker and functional changes even without Ig-gene rearrangements. These two pathways will be explained in more detail. In this review article we summarize our recent work which is concerned with

B-Lymphocyte Lineage-Committed, IL-7 and Stroma Cell- Reactive Progenitors and Precursors, and Their Differentiation to B Cells

Long term lymphoid cell cultures from fetal liver and bone marrow1 have been used to study T- and B-lymphocyte development ‘in vitro’ and by repopulation experiments of suitable hosts ‘in vitro’2. Cell contacts between the progenitors of lymphocytes and stromal cells, as well as cytokines produced upon these contacts3 were found to be required for lymphopoiesis, specifically of the B-lymphocyte lineage4-6. Multiple contacts between stromal cells and progenitors have since been identified in this adhesion7-11. These early forms of progenitor and precursor cultures set the stage and raised the hope to dissect the hierarchy of hemopoietic-lymphopoietic differentiation to clone the differently committed cells, and to determine their capacity to proliferate and to differentiate.

Birth, Life and Death of a B Cell

The generation of B lymphocytes from progenitors and precursors, first during embryonic development and later continuously throughout life, is characterized by the expression of lineage-related markers, by successive rearrangements of the Ig loci, by development of cells with different proliferative and differentiating potentials, and by differential potentials of B lineage-cells to populate different B cell compartments in severe combined immunodeficient (SOD) hosts. Recent reviews have dealt with the molecular processes which accompany the Ig gene rearrangements (Alt et al., 1992), with selective expression of markers and their potential functions, and proliferative and differentiating capacities of progenitors and precursors of the B-lymphocytic lineages (Kincade et al., 1989; Dorshkind, 1990; Rolink and Melchers, 1991, 1993) of Ly1+ and Ly1- B cells (Hardy, 1992).

Progenitor and Precursor B Lymphocytes of Mice. Proliferation and Differentiation In Vitro and Population, Differentiation and Turnover in SCID Mice In Vivo of Normal and Abnormal Cells

Mouse progenitor B cells (pro B cells*), with H and L chain genes in germline configuration, and precursor B cells with L chain gene loci in germline configuration and H chain gene loci in DHJH-rearranged forms (pre B-I cells, ref. 1) can be grown from single cells of fetal liver, of neonatal liver, spleen, blood and bone marrow, and from adult bone marrow for several months in serum-substituted media on stromal cells in the presence of IL7. When IL7 is removed from the medium, more than 90% of the progenitors and precursors loose the capacity to proliferate on stromal cells in the presence of IL7 within 2 days. In these two days they rearrange their H and L chain gene loci in- and out-of-frame and thereby generate slg- and slg+ B cells which both die rapidly by apoptosis. The slg+ B cells can be activated by lipopolysaccharide (LPS), antigen and IL2 to a primary response of proliferation and maturation to IgM-secreting, plaque forming cells (PFC). Most of these findings have been published by Rolink et al. [7].

B Cell Development in the Mouse: Scenarios for Defects Expressed in the B Cell Lineage

The molecular steps and cellular stages which define the development of B lymphocytes in the mouse have been reviewed by us in several publications (Rolink and Melchers 1991, 1993a, b; Melchers 1993 and references therein). This presentation, given as an introduction to normal B cell development in relation to immunodeficiencies and autoimmune diseases, summarizes the major conclusions for normal B cell development reached in these reviews. It gives one example of a B lineage-expressed immunodeficiency induced by targeted disruption by a defective copy of the gene for λ5, one component of the surrogate L chain (Kitamura et al. 1992; Rolink et al. 1993), and describes the example of the New Zealand black (NZB) × New Zealand white (NZW) mouse strain, where recent experimental progress in the analysis of normal B cell development in vitro and in vivo has helped to define defects expressed in the B lineage which lead to lupus-like autoimmune disease (Reininger et al. 1992).