Hiroyuki Kishi

B Cell Growth and Differentiation Factors and Mechanism of B Cell Activation

B cells, one of the best understood eukariotic cells, originate from pluripotent hematopoietic stem cells and differentiate into immunoglobulin {Ig)-secreting cells through multistep developmental stages, such as pre-pre-B cells, pre-B cells, immature B cells, mature B cells, activated B cells and Ig-secreting cells. An understanding of the stimuli which cause the activation, proliferation and differentiation of B cells is critical to the delineation of the normal regulation of the immune responses as well as proliferation and differentiation of eukariotic cells. Since the discovery of T-B collaboration in the antibody response, extensive studies on the regulatory molecules involved in B cell activation have been done. More than a decade ago, Dutton and his colleagues (1971) suggested the involvement of soluble helper factors in the T cell-dependent activation of B cells into Ig-secreting cells. Since then, hundreds of factors, antigen specific or nonspecific, have been reported and those results have supported the notion that helper or suppressor function of T cells in B cell activation can be replaced by the soluble products released from T cells. Kishimoto and Ishizaka (1975) and Kishimoto and his colleagues (1975) have shown Ig-induction in rabbit B cells by anti-Ig and T cell-derived helper factors, suggesting that 2 signals, crosslinkage of Ig-receptors and helper factors, could induce activation of B cells into Igproducing cells. This finding was confirmed by several investigators in murine or human B cells (Parker etal. 1979, Isakson etal. 1981, Yoshizaki et al. 1982)and also supported the notion that helper function of T cells was mediated by T cellderived helper factors.

ACTIVATION OF HUMAN MONOCLONAL B CELLS WITH ANTI-Ig AND T CELL-DERIVED HELPER FACTOR(S) AND BIOCHEMICAL ANALYSIS OF THE TRANSMEMBRANE SIGNALING IN B CELLS

ABSTRACT Ig production was induced in human leukemic B cells or a transformed B cell line (CESS) by anti-Ig or T cell-derived helper factor(s). It was shown that three signals, i.e., crosslinkage of Ig receptors by anti-Ig, B cell growth factor (BCGF) and T cell replacing factor (TRF), were required for the activation of leukemic B cells into Ig-producing cells. Establishment of human T hybridomas secreting monoclonal immunoregulatory molecules showed that BCGF or TRF was different from the factor required for the proliferation of T cells (IL-2). Injection of the cytoplasm from TRF-stimulated CESS cells into non-stimulated cells induced IgG production, suggesting TRF-induced generation of the cytoplasmic sub-stnace responsible for the signal transmission. The binding of TRF with TRF acceptors induced methylation of membrane phospholipids followed by the activation of serine esterase. TRF-activated serine esterase was shown to be involved in the generation of the cytoplasmic factor(s) which were responsible for the signal transmission from membrane to nuclei.

T Cell Replacing Factor (TRF)-Induced IgG Production in a Human B Cell Line and the Mechanism of Transmembrane Signaling through TRF-Acceptors

Binding of a given antigen with surface immunoglobulins (sIg) initiates a complex series of activation processes of B lymphocytes into immunoglobulin (Ig)-producing cells under the influence of helper T cells. In spite of intensive investigations of the triggering and regulation of immunocompetent cells, the subcellular biochemical mechanisms operative in those phenomena remain poorly understood. One of the central problems in analyzing such events is an extensive diversity of the cell types in the immune system as well as a complexity of interactions between them. The diversity is reflected not only in the large repertoire of antigenic specificities expressed by lymphocytes, but also in the many subsets of lymphocytes with distinct functions. In such situations, tumors derived from cells of the immune system of established cell lines have been used as models of clones of immune cells which can be subjected to biochemical and molecular analysisl1–5. If those neoplastic cells or cell lines can be affected by external signals and activated into different states of differentiation, they especially prove useful for the molecular analysis of signaling and activation of lymphocytes.

Regulation of human B lymphocytes differentiation: Characterization of B cell stimulatory factors

B lymphocytes originate from pluripotent hematopoietic stem cells and differentiate into antibody producing cells through several distinct differentiation stages. Commitment of the antigen specificity in each B cell clone occurs at the stage of pre B cells by two step DNA rearrangements including V, D and J segments, i.e. from DJ joining to the functional VDJ formation (1). Following the rearrangement of heavy chain genes, light chain gene rearrangements occur (2) and 7S IgM molecules are expressed as antigen receptors on the surface of mature B cells. A given antigen selects a B cell clone with a matched receptor and activates this clone into antibody producing cells under the influence of helper T cells.

Regulation of growth and differentiation of human B cells

Activation process of B lymphocytes into immunoglobulin (Ig)-secreting cells can be dissected into three steps, i.e., activation, proliferation and differentiation (1–3). It has been demonstrated by several investigators that B cell specific growth and differentiation factors are involved in the process of proliferation and differentiation of activated B cells (4–10). The presence of two different kinds of B cell growth factors, BCGF-I or BSF-pI and BCGF-II, was reported in human (11) as well as in murine systems (12). BCGF-I induces proliferation of anti-IgM-stimulated human or murine B cells as well as SAC (Staphylococcus aureus Cowan I)-stimulated human B cells. Activated but not resting B cells are able to adsorb the activity of BCGF-I, suggesting the induction of the expression of receptors for BCGF-I on activated B cells. However, a recent study showed an increased expression of Ia antigen on resting B cells by stimulation with BCGF-I, suggesting the presence of BCGF-I receptors even on resting B cells, although BCGF-I induces proliferation only in activated B cells (13).