CD40-CD40L cognate interaction mediates maintenance of thymic B cells

Objectives: Progression of periodontitis depends on the balance between pathogen and host immunity. Recent study discovered a certain number of thymic B cells are required to shape optimal immune system. Thus, we postulate disturbance of thymic B cell homeostasis leads to deteriorate periodontal disease. Therefore, we investigated the mechanism(s) of homeostasis of thymic B cells.
Methods: Thymic B cell frequency was analyzed in the mice with genetically altered thymic environment using flow cytometer. Cell cycle and apoptosis were analyzed by DAPI and Caspase3 staining.
Results: Thymic B cell frequency is lower in T cell receptor a (TCRa) KO mice, which lack mature T cells, indicating necessity of T cells for optimal thymic B cells. To elucidate molecular interactions with thymic T cells and B cells, we analyzed thymic B cell frequency in Major Histocompatibility complex II (MHCII) KO and co-stimulatory KO mice. MHCII KO mice have similar frequency of thymic B cells compared with WT, indicating MHCII-TCR interaction is not required for thymic B cell maintenance. In co-stimulatory KO mice, CD40L and CD40KO mice, thymic B cell frequency is extremely decreased compared with those in WT, indicating CD40L-CD40 interaction between thymic T cells and B cells mediates maintenance of thymic B cells. We further analyzed mechanisms by which CD40 signaling mediates thymic B cell maintenance. Cell cycle analysis showed CD40 signaling is required for proliferation of thymic B cells, while measurement of Caspase3 expression as a marker of apoptosis showed that disturbance of CD40 signaling does not affect thymic B cell apoptosis.
Conclusions: Thymic B cell population is maintained by proliferation induced by CD40L-CD40 interaction between thymic T cells and B cells. The disturbance of CD40-CD40L interaction may mediate the failure of thymic B cell maintenance, resulting in the deterioration of various diseases including periodontitis.