Absence of lipopolysaccharide high‐dose paralysis in B‐cell responses: Implications for the one‐signal theory

Abstract

IntroductionLipolysaccharide (LPS), a component of the cell wall of gram-negative bacteria, is able to induce B cell activation resulting in proliferation and Ig production.1,2 Recently, Tolllike receptor 4 (Tlr4) has been implicated in the signalling pathway that leads to LPS-induced activation.3 However, how Tlr4 or other cellular components interact with LPS and induce cellular changes associated with differentiation is still poorly understood. Despite this lack of mechanistic information, a comprehensive picture of the dynamics and doseresponse features of LPS stimulation was reported by Coutinho and Möller in a series of classic studies (reviewed by Coutinho and Möller 4 ). A feature of these experiments was the observation of two bell-shaped dose-response curves. In the first, the generation of antibody secreting cells (ASC) increased with LPS concentration to reach an optimum and then reduced as the concentration was increased further. The second bell curve occurred when haptenated LPS was used as the stimulus and the production of antihapten specific cells was measured. Here, the bell curve appeared at substantially lower overall LPS concentrations than for the first example. These authors argued that the two bell curves were analagous, with high LPS concentrations inhibiting the development of ASC by a form of cellular paralysis. The reason for the shift in sensitivity for the antigen-specific cells, they reasoned, was that surface immunoglobulin (sIg) played a passive (non-signalling) role in B cell activation by focusing LPS to the surface of the hapten-specific B cells, thereby promoting antigen-specific responses at much lower total LPS concentrations. 2,5,6 Although Coutinho and Möller's experiments were based on the mode of action of a highly potent B cell mitogen, they generalized their concept to include T-dependent B cell activation in their 'one-signal theory' of B cell activation. By this view, the antigen receptor on the B cell, once bound to antigen, serves to attract an appropriate T cell to deliver a potent activating signal, which does not require a signal from the B cell receptor itself. This latter prediction 4 has been borne out by experiments that revealed that, in the absence of an antigen (sIg-mediated) signal, activated T cell membranes or the interaction between CD40 and its ligand are sufficient to stimulate B cell proliferation and, in combination with Th2 cytokines, antibody secretion. Summary 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 ...

Cross-Linking Surface Ig Delays CD40 Ligand- and IL-4-Induced B Cell Ig Class Switching and Reveals Evidence for Independent Regulation of B Cell Proliferation and Differentiation

“…The varied effects of the different anti-Ig mAbs on cell division suggested that these reagents might differ qualitatively in their ability to signal via sIg. Different mitogenic properties of various anti-Ig reagents have been noted previously and attributed to qualitatively different signals delivered to the B cell as a result of different sIg cross-linking capacities (18,(51)(52)(53), their binding of different sIg epitopes (53,54), or coligation of Fc␥RIIb (55,56). This latter possibility was eliminated in this study by the inclusion of an antiFc␥RIIb mAb in selected experiments.…”

Regulation of lipopolysaccharide‐induced B‐cell activation: Evidence that surface immunoglobulin mediates two independently regulated signals

“…B cells were prepared from mouse spleen as described previously 10 , 19 . Briefly, a single cell suspension of murine spleen cells was prepared and consecutively depleted of red blood cells, with a lysis buffer (10 mmol/L potassium bicarbonate, 0.15 mol/L ammonium chloride, 0.1 mmol/L EDTA and 5% HI FCS, pH 7.3); adherent cells, by allowing cells to adhere to plastic Petri dishes; and T cells, using a cocktail of anti‐T cell antibodies (antimouse CD4, RL172 anti‐CD8, 31M and anti‐Thy1, 30H‐12 20 ) followed by rabbit complement (Cederlane Laboratories, Hornby, Ontario, Canada).…”

“…When high concentrations of LPS are focused to the B cell surface by surface (s)Ig, however, the specific response is inhibited. Coutinho and Möller's one‐signal model assumed that this inhibition was due to high‐dose paralysis, 9 a phenomenon that our accompanying paper disputes 10 . As an alternative, we have proposed the hypothesis that the inhibition of ASC formation at high concentrations is due to B cells receiving a sIg‐mediated signal at the same time as the TI‐1 activation signal 11 .…”

High-Affinity B Cell Receptor Ligation by Cognate Antigen Induces Cytokine-Independent Isotype Switching

“…Low-dose, low-affinity stimulation does not inhibit ASC differentiation but also has little effect on isotype switching. Thus, most Ab produced following these activation conditions will be low-affinity IgM Ab, as previously postulated (47,48). Although we remain unsure of the physiological relevance of these observations, it is possible that isotype-switched, high-affinity B cells are preferentially directed to a memory pool rather than to plasma cell differentiation.…”