Oncogenic Rag GTPase signalling enhances B cell activation and drives follicular lymphoma sensitive to pharmacological inhibition of mTOR

The humoral immune response requires that B cells undergo a sudden anabolic shift and high cellular nutrient levels, which are required to sustain the subsequent proliferative burst. Follicular lymphoma (FL) originates from B cells that have participated in the humoral response, and 15% of FL samples harbour point-activating mutations in RRAGC, an essential activator of mTORC1 downstream of the sensing of cellular nutrients. The impact of recurrent RRAGC mutations in B cell function and lymphoma is unexplored. RRAGC mutations, targeted to the endogenous locus in mice, confer a partial insensitivity to nutrient deprivation, but strongly exacerbate B cell responses and accelerate lymphomagenesis, while creating a selective vulnerability to pharmacological inhibition of mTORC1. This moderate increase in nutrient signalling synergizes with paracrine cues from the supportive T cell microenvironment that activate B cells via the PI3K–Akt–mTORC1 axis. Hence, Rragc mutations sustain induced germinal centres and murine and human FL in the presence of decreased T cell help. Our results support a model in which activating mutations in the nutrient signalling pathway foster lymphomagenesis by corrupting a nutrient-dependent control over paracrine signals from the T cell microenvironment.