No abstract

In a primary immune response a signal from interleukin 2 (IL-2) induces B lymphocytes to express the gene for the IgM joining component, the J chain. The signaling mechanism was pursued in this study by examining the J-chain gene S' flanking region for regulatory sequences and interacting nuclear factors. The analyses identified a major control region located between -75 and -45 that encodes two adjacent elements: a T-rich sequence (JA) containing a single positive regulatory motif and an A+G-rich sequence (JB) containing overlapping positive and negative regulatory motifs. Dissection of the two elements indicated that the bifunctional JB sequence is the likely target of the IL-2 signal. The evidence was based on findings that (i) JB activity correlated with J-chain gene transcription-i.e., JB acts as a repressor in J-chain-silent B cells and as an activator in J-chain-expressing cells, and (ii) JB activator function is mediated by a B-cell-specific nuclear protein, NF-JB, that exhibits an IL-2-responsive binding pattern.The synthesis of J chain by B lymphocytes has several features that make it an attractive model for analyzing the mechanism of lymphokine signaling. First, the expression of J chain is developmentally regulated at the transcriptional level (1). The gene remains silent during the early antigenindependent stages of B-lymphocyte differentiation and becomes activated only during a primary immune response when the J-chain product is used for the assembly and secretion of pentamer IgM antibody (2). Second, the extracellular cues for J-chain gene activation are known; they are provided by either of two lymphokines-interleukin 2 (IL-2) or interleukin 5 (IL-5) (3, 4)-that are secreted by antigenactivated T-helper cells. Finally, and most importantly, the activation events can be reproduced under defined conditions in vitro; a cloned murine B-cell line, BCL1, can be stimulated to transcribe the J-chain gene by treatment with physiological doses of recombinant IL-2 (5) and/or recombinant .By taking advantage of these features, some progress has been made in defining the mechanism of IL-2 signaling. Analyses of signal input have established that the J-chain response is initiated by IL-2 binding to high-affinity IL-2 receptors expressed by antigen-activated B cells (6) and their counterpart BCL1 cells (7). The 75-kDa component of the bimolecular receptor then delivers the IL-2 message to a relay system within the cell (7,8). Although the relay system has yet to be completely defined, two candidates for early events in the pathway have recently been described. One is a tyrosine kinase that is associated with the IL-2 receptor (9-11) and the other is a glycosylated form of phosphatidylinositol that is rapidly hydrolyzed in response to IL-2 to generate two potential second messengers (12, 13).Analyses of IL-2 signal outcome have shown that transcription of the J-chain gene correlates with chromatin changes in the 5' region. A single nuclease hypersensitive site is coinduced with J-chain gene expression ...

In a primary mmune response, B cells reque signals from the T-cell lymphokines interleukins 2 and 5 (IL-2 and IL-5) to develop into IgM-secreting cells. One role of IL-2 and IL-5 is to activate transcription of the gene encoding the IgM joining component, the J chain. In this study the activation mechanism was investigated by using an inducible B-lymphoma cell line to examine J-chain RNA expression and factor binding to the J-chain promoter. The analyses revealed that both IL-2 and IL-5 trigger a decrease in the binding of two promoter-specific nuclear proteins that precedes the appearance of J-chain RNA. In combination the two lymphokines effected nearly additive changes in factor binding and J-chain RNA abundance. Both effects were reversed upon withdrawal of the lymphokine stimulus and both were inhibited in the presence of the T-cell lymphokine IL-4. These findings indicate that the IL-2 and IL-5 signal pathways converge to deliver a common signal that regulates the repressor activities of two lymphokine-responsive promoter elements T-cell lymphokines. play a critical role in the development of an immune response (1, 2). Their interaction with cell surface receptors generates the signals required for the clonal expansion of the participating B and T cells and for the subsequent differentiation of these cells to antibodysecreting or killer populations. Although considerable progress has been made in defining the properties of the lymphokines and their receptors, little is known about how the signals generated elicit a particular outcome in their target cell nuclei (1-3). The major difficulty has been the lack of appropriate experimental models in which the response to a single lymphokine signal can be assessed in a homogeneous target population. In the studies reported here, this difficulty has been surmounted by using a cloned lymphoma cell line to analyze a single lymphokine-induced event in a B-cell primary immune response: the activation ofthe immunoglobulin J-chain gene. Studies of the primary immune response have shown that either interleukin. 2 (IL-2) or IL-5 stimulates antigenactivated B cells to express the J-chain gene and thus provide the final protein component necessary for the assembly and secretion of pentameric (IgM)5J antibody (4-6). The J-chain response can be duplicated in two subclones of the BCL1 lymphoma, 3b3 (7, 8) and CL-3 (9), which are representative of antigen-activated B cells. In 3b3 cells treated with IL-2, J-chain RNA becomes detectable after 12-24 hr and reaches a plateau-after 2-3 days that is maintained as long as the cells are provided with the lymphokine (7). Coincident with the appearance ofJ-chain RNA is the development of a nucleasehypersensitive-site in the immediate 5' region of the J-chain gene (7, 10). Thus, the IL-2 signal appears to induce an opening of the chromatin in the promoter region of the gene, presumably allowing access ofthe transcriptional machinery.To pursue the activation mechanism, the interaction of nuclear proteins with the hypersensitive se...