The surprising role of Indoleamine 2,3-dioxygenase (IDO) in supporting the survival of bone marrow resident long lived plasma cells (LLPC)

Abstract

Abstract Long lived plasma cells (LLPC) are a key component of protective humoral immunity. The ability of these cells to maintain longevity and a durable antibody response is largely dependent on the complex nature of the bone marrow (BM) microenvironment in which these cells reside. Our lab has demonstrated that the co-stimulatory receptor, CD28, is essential for the survival and function of LLPC. Furthermore, when CD28 on LLPC ligates with CD80/CD86 expressed on DC, it induces upregulation of the enzyme indoleamine 2,3-dioxygenase (IDO), which catabolizes tryptophan (Trp) into kynurenine (kyn), a well-known ligand of the aryl hydrocarbon receptor (AhR). AhR is a nuclear transcription factor that can modulate immune responses and is highly expressed in plasma cells (PC). Our preliminary data reveals that IDO knock-out (KO) mice in comparison to wild type (Wt) mice have a decrease in bone marrow resident PCs and a decrease in overall Ig levels. Additionally, 15 weeks post vaccination IDO KO mice have significantly less antigen-specific long lived antibody titers compared to Wt. It was also observed that interaction with DC both in vitro and in vivo is needed for LLPC survival and function. Moreover, AhR activation by kyn causes an increase in CD28 mRNA in LLPC. This leads us to propose a model where CD28 interaction with CD80/CD86 induces IDO production, which through its metabolite kyn activates AhR which can then regulate CD28 expression- thus creating a mechanism that promotes LLPC survival and function. These findings challenge the traditional idea of IDO as an inhibitory molecule and provide the rationale for investigating its essential role in maintaining LLPC survival and sustained antibody responses.