Activation of thymic B cells by signals of CD40 molecules plus interleukin‐10

Abstract

We have previously found that thymic B cells, particularly thymic CD5⁺ B cells, show low responsiveness to the usual B cell stimulants such as lipopolysaccharide or anti‐IgM plus interleukin (IL)‐4, although they proliferate and produce antibodies after direct interaction with major histocompatibility complex class II‐restricted T blasts. These findings raise the possibility that a CD40‐CD40 ligand (L) interaction is involved in the activation of thymic B cells. In the present study, we therefore examine this possibility using CD40L‐transfected Chinese hamster ovary (CHO) cells or anti‐CD40 monoclonal antibody (mAb). When B cells in the spleen and peritoneal cavity were stimulated, they proliferated and produced immunoglobulin (Ig) in the presence of CD40L‐CHO cells or anti‐CD40 mAb alone. However, another signal delivered by IL‐10 in addition to CD40L‐CHO cells or anti‐CD40 mAb was found to be necessary for thymic B cells to proliferate and secrete Ig. Other interleukins acting on B cells, such as IL‐4, IL‐5, and IL‐6, had no effect on the activation of thymic B cells, which thus have unique characteristics not found in peripheral B cells. This report discusses the physiological significance of IL‐10‐ and CD40‐driven signals in the activation of thymic B cells.