Garvin L. Warner

Role of T cell-derived lymphokines in two models of B-cell tolerance.

Engagement of surface immutioglobulin (slg) receptors by antigen initiates a series of biochemical signals that affect B-cell growth and differentiation. When this interaction occurs with immature (e.g. neonatal) B cells, a state of unresponsiveness or tolerance usually results. In contrast, mature B cells can be activated (i.e. positively signalled to proliferate/difTerentiate) by crosslinking of their slg receptors by antigen or anti-Ig antibodies. However, mature B cells also can be rendered unresponsive by concomitant interaction of anti-Ig/tolerogen with Fc receptors (Klaus et al. 1987, Waldschmidt & Vitetta 1985, Warner and Scott, unpublished). Several model systems employing polyclonal populations of normal B cells have been used to examine the molecular pathways of positive signalling via slg (Moller 1987). Over the past decade, our laboratory has been examining the biochemical events associated with both positive and negative signalling in relatively homogeneous populations of antigen-specific B cells (Scott et al. 1980, Pillai et al. 1984). These studies have allowed us to compare hapten-specific cells derived from normal, primed or tolerant individuals in terms of their ability to be stimulated by a variety of known B-cell activators (antigen, anti-Ig, mitogen, T-cell factors) as measured by increase in cell size, hyper-Ia expression, entry in the S phase of the cell cycle, and the appearance of antibody-secreting cells. The low frequency of antigen-specific B cells, and thus the difficulty in obtaining large numbers of cells, has severely limited our ability to examine early

Lymphoma models for B cell activation and tolerance: VIII. Cross-desensitization by slgM and slgD and its effects on growth regulation by anti-isotype antibodies

The product of the retinoblastoma gene, RB-1, is the prototype of a class of tumor suppressor genes that is expressed in most mammalian cells. The RB protein is phosphorylated in a cell cycle-dependent manner and is modulated during cellular differentiation. We have shown previously that anti-immunoglobulin M (anti-mu) treatment of WEHI-231 and CH31 B-lymphoma cells caused cell cycle blockade and apoptosis. In such arrested cells, pRB was predominantly in the underphosphorylated (active) form, in contrast to hyperphosphorylated pRB in control log phase cells. Herein we examine the modulation of pRB phosphorylation by anti-mu and its effect on a cyclin:kinase complex that can act on pRB in murine B-lymphoma cells. In unsynchronized B-lymphoma cells, anti-mu cross-linking of membrane immunoglobulin M leads to an accumulation of the hypophosphorylated form of pRB, a decrease in the abundance of one form of cyclin A, and inhibition of cyclin A and cdk2-associated kinase activity. Using centrifugal elutriation, we also show that anti-mu treatment prevents the phosphorylation of the retinoblastoma gene product only when added in early G1. In addition, there is a critical point after which membrane immunoglobulin M cross-linking is no longer effective at preventing this process. We suggest that anti-mu-mediated growth arrest is due to the direct or indirect inactivation of an active kinase complex capable of pRB phosphorylation.