Nur77-eGFP BAC Tg reporter mouse

Nr4a1 (encoding the orphan nuclear hormone receptor Nur77) is an immediate-early gene that is rapidly upregulated by antigen receptor signaling. We described a Nur77-eGFP reporter mouse in 2012, in which T and B cells activated by antigen upregulate GFP expression. In doing so, we observed that reporter expression was sensitive to endogenous antigen recognition, and therefore marked self-reactivity within the naive mature B cell repertoire. We have since taken advantage of this tool in a growing set of inter-related projects to understand how endogenous antigen encounter shapes the transcriptome and functional capacity of self-reactive B cells, and to track antigen-dependent signaling in vitro and in vivo during development and immune responses.

Figure 1. Nur77-eGFP reporter is responsive to TCR and BCR stimulation in vitro. Zikherman et al. Nature 2012.

Differential responsiveness of IgM and IgD to self antigens

Since IgM and IgD BCRs differ in their Fc region, but not in their antigen-binding domain, it has been unclear why two different BCRs are co-expressed by naive mature B cells. Perhaps the most notable phenotypic and functional characteristic of self-reactive naive B cells that express high levels of Nur77-eGFP reporter is progressive downregulation of surface IgM BCR. By contrast, surface IgD BCR expression is unaltered. One focus of current research in the lab is trying to understand how downregulation of IgM in the face of persistent high IgD expression might impose tolerance on auto-reactive B cells. To address this question, we have introduced the Nur77-eGFP reporter of endogenous antigen-dependent signaling onto genetic backgrounds lacking either the IgM or the IgD BCRs. 

Figure 2. Nur77-eGFP reporter marks functional responsiveness of mature naive B cells. Zikherman et al. Nature 2012.

The role of Nur77 in B cell tolerance and in the context of humoral immune responses

Nur77-eGFP reporter expression reveals that endogenous Nur77 protein is upregulated in response to chronic endogenous antigen stimulation in B cells. Our lab is taking advantage of mice with conditional and germline deficiency in Nur77 to understand its function in the context of mouse models of B cell autoreactivity, and in the context of humoral immune responses.

Figure 3. Nur77-eGFP reporter and endogenous Nr4a1 transcript upregulation during B cell development requires endogenous antigen. Zikherman et al. Nature 2012; Sabouri et al. Nature Communications 2016.