No abstract

SunllTlaryStimulation of B and T cells via the antigen receptor, by phorbol ester or by phorbol ester and ionomycin, leads to nuclear translocation of the inducible transcription factor NF-KB, comprising the p50 and p65 re/-related polypeptides. In this report we show that c-tel is a component of the antigen receptor-induced xB binding proteins in both B and T cells. Whereas NF-~cB can be induced by phorbol ester alone, optimal induction of c-tel requires stimulation by both phorbol ester and ionomycin, the dual signal that is necessary for proliferation of untransformed lymphocytes. Furthermore, c-tel induction is blocked by the immunosuppressive drug FK506 that is known to inhibit B and T cell activation, c-rel-dependent transactivation of the interleukin-2 receptor ot chain (Ib2Rot) promoter is augmented by coexpression of calcineurin, suggesting the involvement of a calcineurin-dependent intracellular pathway. Our results identify c-tel as a target of immunosuppressive agents and illustrate the similarity of activation pathways in both B and T ceils. Stim. ulation of B and T cells via the antigen receptor triggers a signaling cascade that results in clonal cell proliferation characteristic of the immune response. Alteration of the resting program of gene expression is brought about by inducible nuclear factors that transmit the signal generated at the plasma membrane to the cell nucleus. Several transcription factors have been identified that are induced in response to antigen receptor cross-linking, or its pharmacologic equivalent, the combined action of phorbol esters and calcium ionophore. We, and others, have shown that antigenreceptor cross-linking on both B and T cells induces the factors NF-KB, AP-1, and cAMP response element-binding protein (CREB) (1)(2)(3)(4). However, these proteins can be induced by the action of phorbol ester alone on these cells, a signal that is not sufficient to induce proliferation. A notable exception is NF-AT, a nuclear factor present in activated T cells, whose induction requires the dual signal of phorbol ester and ionomycin (5). We have recently shown that both signals are also required to induce an indistinguishable DNA-binding protein in B cells, indicating that NF-AT should be regarded more generally as a factor of activated lymphoid ceils (6). Because it is likely that proteins that are induced in response to the complete proliferative signal will play a critical role in normal and aberrant immune responses, it is essential to identify factors that satisfy this criterion.The immunosuppressive drugs cyclosporin A and FKS06 block B and T cell proliferation (7-9). The intracellular pathway of drug action in T cells has been recently elucidated. Both drugs bind to distinct cytosolic receptors and the drug receptor complexes inhibit the enzymatic activity of the calcium and cahnodulin-dependent serine/threonine phosphatase, calcineurin (10-13). NF-ATp/c, the preexisting cytoplasmic component of NF-AT (14-16), has been implicated as a direct target of calcineurin (17,18). ...

Maul et al. show that proteins associated with variable gene hypermutation, such as Spt5 and AID, are recruited to the initiating form of RNA polymerase II specifically in cells activated in germinal centers but not in culture.

c-Rel induction in activated lymphocytes is suppressed by the immunosuppressive drug, FK506. Here we show that FK506-suppressible, delayed c-Rel induction is similar in B and T cells and is regulated by mRNA production. In contrast, rapid nuclear translocation of pre-existing cytoplasmic c-Rel occurs only in B cells, but not in T cells. Analysis of I-kappaBalpha and -beta in these cells showed that both I-kappaBalpha and I-kappaBbeta were rapidly degraded in response to stimulation in B cells, but only I-kappaBalpha was affected in T cells. These observations suggest that 1) different Rel proteins in the same cell may be sequestered in the cytoplasm differently, 2) the sequestration mechanism is cell-type specific, and 3) differential sensitivities of I-kappaBalpha and beta in B and T cells may regulate, in part, the rapid response of family members. We propose that subunit-specific and cell-specific regulation of nuclear translocation may help determine the varied cellular responses to different stimuli.

Thymocytes mature in response to the cues from the thymic micro-environment, which regulate stage-specific gene expression during development. We find that several proteins that bind the kappa B sequence in vitro are constitutively activated in freshly isolated thymocytes. These include the rel-related p50 homodimers, p50/p65 heterodimers, low levels of c-rel, and two other factors that may be thymus specific. Disruption of the thymic micro-environment resulted in loss of DNA-binding, suggesting that lymphocyte-stromal cell interactions induce and maintain these proteins in a DNA-binding form. Phorbol ester and ionomycin treatment induced p50, p65, and p68 c-rel kappa B DNA-binding activity. Expression of p68 c-rel protein, but not p50 or p65, was suppressed by the immunosuppressive drug FK506. Because FK506 specifically inhibits the appearance of mature single-positive thymocytes, gene expression regulated by p68 c-rel may play a role in selection and maturational signals involved in the double-positive to single-positive transition.