Immunoglobulin signal transduction guides the specificity of B cell-T cell interactions and is blocked in tolerant self-reactive B cells.

TLDR

Antibody responses rely on directing helper T cell signals to antigen‑binding B cells while avoiding self‑reactive B cells. The study compared activation requirements of hen egg lysozyme‑specific B cells from Ig‑transgenic mice with those of tolerant B cells expressing soluble HEL to uncover mechanisms preventing self‑reactive B cell activation. The authors used allogeneic bm12 T cells to provide in vivo help to C57BL/6 Ig‑transgenic B cells, thereby isolating the effects of surface Ig signaling. Non‑tolerant B cells required both allogeneic Th cells and soluble HEL for activation, whereas tolerant self‑reactive B cells failed to respond to this combination but remained responsive to IL‑4 and anti‑CD40; a proximal block in surface Ig signaling prevented tyrosine‑kinase activation, yet membrane‑bound HEL restored collaboration and antibody production, demonstrating that dual signals and surface Ig regulation govern self‑reactive B cell tolerance.

Abstract

The specificity of antibody (Ab) responses depends on focusing helper T (Th) lymphocyte signals to suitable B lymphocytes capable of binding foreign antigens (Ags), and away from nonspecific or self-reactive B cells. To investigate the molecular mechanisms that prevent the activation of self-reactive B lymphocytes, the activation requirements of B cells specific for the Ag hen egg lysozyme (HEL) obtained from immunoglobulin (Ig)-transgenic mice were compared with those of functionally tolerant B cells isolated from Ig-transgenic mice which also express soluble HEL. To eliminate the need for surface (s)Ig-mediated Ag uptake and presentation and allow the effects of sIg signaling to be studied in isolation, we assessed the ability of allogeneic T cells from bm12 strain mice to provide in vivo help to C57BL/6 strain-transgenic B cells. Interestingly, non-tolerant Ig-transgenic B cells required both allogeneic Th cells and binding of soluble HEL for efficient activation and Ab production. By contrast, tolerant self-reactive B cells from Ig/HEL double transgenic mice responded poorly to the same combination of allogeneic T cells and soluble HEL. The tolerant B cells were nevertheless normally responsive to stimulation with interleukin 4 and anti-CD40 Abs in vitro, suggesting that they retained the capacity to respond to mediators of T cell help. However, the tolerant B cells exhibited a proximal block in the sIg signaling pathway which prevented activation of receptor-associated tyrosine kinases in response to the binding of soluble HEL. The functional significance of this sIg signaling defect was confirmed by using a more potent membrane-bound form of HEL capable of triggering sIg signaling in tolerant B cells, which markedly restored their ability to collaborate with allogeneic Th cells and produce Ab. These findings indicate that Ag-specific B cells require two signals for mounting a T cell-dependent Ab response and identify regulation of sIg signaling as a mechanism for controlling self-reactive B cells.