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

• Published in: Immunology Vol. 155; no. 4; pp. 505 - 518
• Main Authors: Meena, Naresh Kumar, Pattanayak, Shakti Prasad, Ben‐Nun, Yael, Benhamron, Sandrine, Kumar, Saran, Merquiol, Emmanuelle, Hövelmeyer, Nadine, Blum, Galia, Tirosh, Boaz
• Format: Journal Article
• Language: English
• Published: England Wiley Subscription Services, Inc 01-12-2018; John Wiley and Sons Inc
• Subjects: activity based probes; Animals; Atrophy; B-Lymphocytes - immunology; Cathepsin B; cathepsins; Cathepsins - antagonists & inhibitors; Cathepsins - metabolism; Cell activation; Cell Line; CHO Cells; Clonal deletion; Computer architecture; Cricetulus; Follicles; Gene deletion; Gene expression; Immune response (humoral); Lymphocytes B; Lymphotoxin; Lymphotoxin beta Receptor - biosynthesis; Lymphotoxin-alpha - biosynthesis; Lymphotoxin-beta - biosynthesis; lymphotoxins; marginal zone; Mechanistic Target of Rapamycin Complex 1 - metabolism; Metabolism; Mice; Mice, Transgenic; mTOR; Original; Proteolysis; Rapamycin; Rodents; Sirolimus - pharmacology; Spleen; Spleen - cytology; Spleen - immunology; Stroma; TOR protein; Tuberous sclerosis; Tuberous Sclerosis Complex 1; Tuberous Sclerosis Complex 1 Protein - genetics; Tuberous Sclerosis Complex 2; Tuberous Sclerosis Complex 2 Protein - genetics; mTOR; activity based probes; lymphotoxins; cathepsins; marginal zone
• ISSN: 0019-2805; 1365-2567
• DOI: 10.1111/imm.12996

Summary
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 (TSC1BKO) 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 TSC1BKO 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 TSC1BKO. 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.
Marginal zone (MZ) B cells are absent upon activation of mammalian target of rapamycin complex 1 in a B‐cell‐specific TSC1 deletion mouse model. We observed that this is associated with an impairment in MZ architecture. Analysis of protease activity in the TSC1‐deleted B cells revealed an increase in cathepsin activity. Accordingly, in vivo administration of a pan‐cathepsin inhibitor restored MZ architecture.