mTORC1 activation in B cells confers impairment of marginal zone microarchitecture by exaggerating cathepsin activity

Abstract

Mammalian target of rapamycin complex 1 (mTORC1) is a key regulator of cell metabolism and lymphocyte proliferation. It is inhibited by the tuberous sclerosis complex (TSC), a heterodimer of TSC1 and TSC2. Deletion of either gene results in robust activation of mTORC1. Mature B cells reside in the spleen at two major anatomical locations, the marginal zone (MZ) and follicles. The MZ constitutes the first line of humoral response against blood-borne pathogens and undergoes atrophy in chronic inflammation. In previous work, we showed that mice deleted for TSC1 in their B cells (TSC1 ) have almost no MZ B cells, whereas follicular B cells are minimally affected. To explore potential underlying mechanisms for MZ B-cell loss, we have analysed the spleen MZ architecture of TSC1 mice and found it to be severely impaired. Examination of lymphotoxins (LTα and LTβ) and lymphotoxin receptor (LTβR) expression indicated that LTβR levels in spleen stroma were reduced by TSC1 deletion in the B cells. Furthermore, LTα transcripts in B cells were reduced. Because LTβR is sensitive to proteolysis, we analysed cathepsin activity in TSC1 . A higher cathepsin activity, particularly of cathepsin B, was observed, which was reduced by mTORC1 inhibition with rapamycin in vivo. Remarkably, in vivo administration of a pan-cathepsin inhibitor restored LTβR expression, LTα mRNA levels and the MZ architecture. Our data identify a novel connection, although not elucidated at the molecular level, between mTORC1 and cathepsin activity in a manner relevant to MZ dynamics.

Resting innate-like B cells leverage sustained Notch2/mTORC1 signaling to achieve rapid and mitosis-independent plasma cell differentiation

“…Consistent with past work, at the 28-day timepoint frequencies of MZ B cells were decreased to roughly 50% of control mice (Supplemental Fig. 8A) (48). Notably, when harvested from mice given tamoxifen for 28 days, Tsc1-deficient CpG-stimulated follicular B cells were able to generate CD138 + cells within 48 hours and without cytokine supplementation (Fig.…”