Regulation of cytoplasmic, surface and soluble forms of CD40 ligand in mouse B cells

Abstract

CD40 and CD40 ligand (CD40L) form one of most important receptor‐ligand pairs that dock during T‐B cell interactions as part of T‐dependent antibody responses. It has been reported that among other cell types, B cells can express CD40L. Here we show that a large proportion of mouse B cells express CD40L in their cytoplasm, but not on the surface and that this is readily released as a soluble molecule. Thus, in their resting state up to 50 % of mouse B cells express CD40L within their cytoplasm and both the proportion of cells expressing and the amount of CD40L is increased by signaling through immunoglobulin (Ig) or CD38. In contrast, T cell‐derived signals such as CD40L (anti‐CD40) or Th2‐type cytokines cause a decrease in CD40L expression that is related to a release of a soluble form of the molecule from the cell. Supernatants from B cells activated with anti‐Ig and anti‐CD40 contain CD40L in a variety of forms (18 kDa, 33 kDa and 66 kDa) that are readily detectable by immunoprecipitation with CD40‐Fcγ fusion protein (CD40‐Ig) followed by Western blotting with anti‐CD40L antibody (MR1). The 33‐kDa species is distinct from the 39‐kDa membrane‐bound molecule found in activated T cells or in resting B cells and appears to be a novel soluble form of CD40L. Inhibition of T cell‐independent in vitro stimulation of B cells with CD40‐Ig or anti‐CD40L suggests that the B cell‐derived soluble CD40L or CD40L expressed on the B cell surface can play a positive role in B cell proliferation.