Ligation of CD40 with soluble CD40 ligand reverses anti-immunoglobulin-mediated negative signalling in murine B lymphoma cell lines but not in immature B cells from neonatal mice.

Ligation of surface immunoglobulin (sIg) on certain murine B-lymphoma lines has been shown to initiate a programme leading to growth arrest and death of the cells by apoptosis. The cell lines WEHI 231 and CH33 which respond in this way to receptor cross-linking have phenotypic characteristics resembling those of immature normal B cells, and their responses have been taken to model those responsible for clonal deletion or anergy. Cross-linking of sIg on normal neonatal B cells has also been shown to inhibit their responsiveness to polyclonal activators. We have examined the ability of various co-stimuli to modify the response of growth-inhibitable B lymphoma lines to sIg cross-linking. Our findings indicate that cell–cell contact between cells of the WEHI 231 or CH33 lines and activated T cells rescues these cells from growth arrest and apoptosis. Cell-free supernatants from some T-cell lines were also protective although recombinant IL-4 had no effect. Analysis of the most effective signals and timing for inducing this protection suggested that it might, in part, be mediated by CD40 ligand (CD40L) expressed on or secreted by activated T cells. Using a soluble recombinant CD40L-CD8 fusion protein we have now shown that co-ligation of CD40 is sufficient to rescue WEHI 231 and CH33 cells from anti-Ig-induced apoptosis. In contrast, the inhibitory effect of anti-Ig antibodies on the lipopolysaccharide (LPS)-driven proliferation of neonatal B cells was not relieved by co-ligation of CD40 with CD40L. These findings bring into question the usefulness of ‘immature’ B-cell lines as models for tolerance induction.