Comparative analysis of signal transduction by CD40 and the Epstein-Barr virus oncoprotein LMP1 in vivo

There is much evidence, based primarily on in vitro studies, indicating that the Epstein-Barr virus oncoprotein latent membrane protein 1 (LMP1) mimics an activated CD40 receptor. In order to investigate the extent of similarity between LMP1 and CD40 functions in vivo, we analyzed the cytoplasmic signaling properties of LMP1 and CD40 in B cells in a directly comparable manner. For this purpose, we generated transgenic mice expressing either LMP1 or a chimeric LMP1CD40 molecule, which constitutively activates the CD40 pathway, under the control of the CD19 promoter. LMP1 and LMP1CD40 were expressed at similar levels in a B-lymphocyte-specific manner. Similar to LMP1, LMP1CD40 suppressed germinal center (GC) formation and antibody production in response to thymus-dependent antigens, albeit to a greater extent than LMP1. Furthermore, the avidity of the antibodies produced against thymus-dependent antigens was lower for LMP1CD40 transgenic mice than for wild-type and LMP1 transgenic mice. GC suppression was linked to the ability of LMP1CD40 and LMP1 to downregulate mRNA and protein levels of BCL6 and to suppress the activity of the BCL6 promoter. In contrast to LMP1, LMP1CD40 caused an upregulation of CD69, CD80, and CD86 in B cells and a dramatic increase in serum immunoglobulin A In addition, LMP1CD40 but not LMP1 transgenic mice had elevated numbers of marginal-zone B cells and increased populations of polymorphonuclear cells and/or neutrophills. Consistent with these findings, LMP1CD40 but not LMP1 transgenic mice showed signs of spontaneous inflammatory reactions and the potential for autoimmunity.