B-cell receptor (BCR) mediated early signaling events are critical for normal B cell development and B lymphoma growth

Egr-1 is a nuclear zinc finger containing transcription factor. Upon cross-linking of B-cell receptor, mature B cells undergo proliferation with an increase in Egr-1 whereas immature B cells become unresponsive with only a modest increase in Egr-1 levels. This differential Egr-1 expression pattern during different stages of B cell development could be important for induction of tolerance versus clonal expansion in B cells. To test this hypothesis, we developed transgenic mice expressing a dominant negative form of Egr-1, which lacks the transactivation domain but retains the DNA binding domain, in a B cell specific manner. There is a decrease in B lymphopoeisis in the bone marrow accompanied by reduction in splenic immature and mature B cells as well as marginal zone B cells in the transgenic mice. B cells from these transgenic mice showed reduced proliferative response to anti-IgM and respond poorly to T-dependent and T-independent type 2 antigens in vivo compared to wildtype littermate. Unlike normal B cells where Egr-1 expression is induced by BCR cross-linking, B lymphoma cells express Egr-1 constitutively. Egr-1 is regulated by Mitogen activated protein kinases (MAPKs) including ERK and JNK in many cell types. Consistent with the constitutive levels of Egr-1, several primary murine and human B lymphomas and cell lines were found to constitutively express high levels of activated form of JNK. Proliferation of several B lymphoma cell lines was strongly inhibited by JNK inhibitor in a dose dependent manner. Thus in the B-lymphoma model JNK appears to have a unique prosurvival role. MAPKs are regulated by proximal kinases including Syk. Accordingly, Syk is constitutively active in a variety of B lymphoma cells and blocking, Syk activity by selective inhibitors suppresses B lymphoma growth. In B cells, Syk activation is BCR dependent. Based on this and the fact that BCR is required for mature B cell survival we tested the requirement for continued expression of BCR for the growth and survival of B lymphoma cells. Using Ig-alpha or Ig-beta specific RNAi to inhibit BCR expression, we demonstrate that constitutive signaling by BCR is critical for survival and proliferation of B lymphoma cells.