Inhibition of the MEK/ERK signaling pathway blocks a subset of B cell responses to antigen.

Signal transduction initiated by B cell Ag receptor (BCR) cross-linking plays an important role in the development and activation of B cells. Therefore, considerable effort has gone into determining the biochemical signaling events initiated by the BCR and delineating which events participate in specific biological responses to Ag. We used two inhibitors of mitogen-activated protein kinase/extracellular signal-regulated kinase kinase (MEK) 1 and MEK2, PD98059, and U0126, to assess the role the Ras-mitogen-activated protein kinase pathway plays in several BCR-induced responses. PD98059 or U0126 treatment substantially inhibited the BCR-induced activation of the extracellular signal-regulated kinase (ERK) forms of mitogen-activated protein kinase in the immature B cell line WEHI-231, in immature splenic B cells, and in mature splenic B cells. However, MEK-ERK inhibition did not block BCR-induced growth arrest or apoptosis of WEHI-231 cells or apoptosis of immature splenic B cells, indicating that the MEK-ERK pathway is not required for these events. In contrast, PD98059 and U0126 treatment did inhibit the up-regulation of specific BCR-induced proteins, including the transcription factor Egr-1 in WEHI-231 and mature splenic B cells, and the CD44 adhesion molecule and CD69 activation marker in mature splenic B cells. Moreover, both inhibitors suppressed BCR-induced proliferation of mature splenic B cells, in the absence and in the presence of IL-4. Therefore, activation of the MEK-ERK pathway is necessary for a subset of B cell responses to Ag.

Absence of lipopolysaccharide high-dose paralysis in B-cell responses: implications for the one-signal theory.

Over 20 years ago, Coutinho and Möller reported that high concentrations of LPS were paralytic for the development of antibody secreting cells (ASC). This data was used to explain bell-shaped dose-response curves observed for antihapten antibody formation in response to haptenated LPS. In turn, this bell curve was used to formulate the one-signal model of B cell activation, which argued that antigen signalling was generally unimportant to B cell responses. The present paper re-examines LPS dose-response curves and finds results that do not support the view that high doses of LPS inhibit B cell differentiation to ASC. If high-dose paralysis is not an attribute of LPS stimulation, then the bell-shaped dose curve for hapten-specific ASC originally observed by Coutinho and Möller required an alternative explanation. Through the use of haptenated Ficoll, it was possible to show that the generation of LPS-induced antitrinitrophenol ASC could be inhibited by antigen presented on an inert substrate. Thus, the transmission of surface Ig-mediated (antigen) signals at higher concentrations can explain the antihapten bell-shaped dose curves, in contradiction to the conclusions of the one-signal model.

Regulation of lipopolysaccharide-induced B-cell activation: evidence that surface immunoglobulin mediates two independently regulated signals.

An antigen-specific B cell response can be induced by low concentrations of haptenated lipopolysaccharide (LPS), whereas high concentrations are inhibitory. Two explanations have been proposed for the latter phenomenon. In the first, specific surface Ig focuses LPS to the B cell membrane, where high local concentrations of the mitogen become paralytic for B cell responses. In the alternative, transmission of an antigen signal at higher concentrations of hapten LPS actively inhibits the development of antibody secreting cells (ASC). In the present paper, the immunosuppressant cyclosporine A (CsA) was used to attempt to distinguish between these two models. Cyclosporine A did not block the inhibitory effects of goat anti-IgM (galphaIgM) on development of ASC induced by LPS and therefore was unsuitable for testing between the two models. However, surprisingly, in the presence but not the absence of CsA, even low concentrations of galphaIgM became inhibitory for LPS-induced B cell proliferation. Thus, a CsA-insensitive signal could inhibit both B cell proliferation and the development of ASC. In contrast, the CsA-sensitive signal induced by sIg required high concentrations of galphaIgM for triggering and enhanced the LPS proliferative response without affecting development of ASC. Evidence is presented that these two signals are regulated independently, suggesting that together they may transmit information about the physical form of an antigen to the B cell.

The one signal theory of B cell activation revisited: a role for surface immunoglobulin in regulating T-independent antibody responses?

Early theories of antibody production by B cells achieved considerable success in predicting B cell behaviour with simple deductive models. One such model, the one signal theory, postulated that the antigen receptor on B cells played only a passive non-signalling role in focusing non-specific activating signals to the B cell surface. This prediction is at least partially consistent with recent discoveries concerning the helper signals delivered to B cells by T cells. Here, we re-examine the foundation of this theoretical prediction with the benefit of recent information. The experimental basis for the theory was a study of B cell activation by LPS and, in particular, the interpretation of a bell-shaped dose-response curve. The logic applied is appropriate to explain some, but not all, forms of B cell behaviour because, as is now clear, the role played by the antigen signal varies depending upon the method of activation. This re-examination suggests an alternative interpretation of the LPS-induced bell-shaped curves that incorporates a role for an antigen signal. If correct, the mechanism would ensure that T-independent responses are drawn from low affinity precursors.