DEPDC5 is a metabolic checkpoint regulator in the B cell AA-mTORC1 pathway

Abstract

Immunometabolism is a rapidly developing field that is central to understanding the homeostatic maintenance and induction of immune responses. Naïve B cells rest in quiescent states until antigen (Ag) is encountered, which simultaneously triggers a metabolic switch enabling enhanced growth, proliferation and differentiation in activated B cells. Central to this metabolic activation is the mechanistic target of rapamycin (mTOR) complex 1 (mTORC1) pathway, which is required for multiple inputs to determine if sufficient nutrients are available to drive these metabolic programs. In recent years amino acid (AA) sensing and availability have been shown to be important for the control of mTORC1 in various cell types, however their importance in B cell metabolism was largely unknown. DEPDC5 is a metabolic checkpoint protein that mediates crosstalk between AA availability and the mTORC1 pathway, but it’s role in B cells has not been explored. The results in this thesis showed that Depdc5 is expressed in B cells and most abundantly in germinal center (GC) B cells (GCB). A novel line of conditional knock-out mice was generated that lack DEPDC5 in mature B cells (Depdc5flox/flox x Cd23Cre). This deletion led to constitutive mTORC1 activation in mature B cells. Analysis of Depdc5flox/flox x Cd23Cre mice showed no impairment of B cell development and at homeostasis, mature B cell frequencies were unchanged from controls. Despite this, mature B cells in resting mice that lacked B cell expression of DEPDC5 exhibited a hyperactive phenotype with increased mTOR signalling, increased biomass and higher levels of expression of B cell activation markers. The effects of DEPDC5 deletion in B cells became more apparent in challenged animals. Fewer GC B cells were present in draining LNs following viral infection than in littermate controls. Furthermore, the absence of Depdc5 in B cells led to reduced generation of anti-viral IgM Ab, but not antiviral IgG, and impaired affinity maturation in a protein immunisation model. Mechanistically it was shown that DEPDC5 was required to protect GC B cells from late-stage apoptosis and restrain B cell proliferation following viral infection. Thus, Depdc5 was required to ensure the sustained expansion of GC B cells in humoral immune responses, likely by restraining their metabolic activity in response to the limited nutrient microenvironment in the GC. However, in contrast, an increase GC B cells frequency and number was also observed in the lung of Depdc5-deficient mice following IAV infection. Furthermore, DEPDC5 was observed to be dispensable in B cell activation within GALT-associated chronic GCs. These results broaden understanding of AA responsive metabolic checkpoint regulators and have implications for nutritional control of optimal humoral responses. Thus, this study contributed to broadening the understanding of how metabolic pathways shape immunity in health and disease.