Rap1 Is Essential for B-Cell Locomotion, Germinal Center Formation and Normal B-1a Cell Population.

Sayaka Ishihara,Akihiko Nakajima,Satoshi Sawai,Koko Katagiri,Ryota Miwa,Ai Kotani,Hidehiro Fukuyama,Tsuyoshi Sato,Risa Sugioka,Haruka Saito,Ryota Sato

doi:10.3389/fimmu.2021.624419

Abstract

Integrin regulation by Rap1 is indispensable for lymphocyte recirculation. In mice having B-cell-specific Rap1a/b double knockouts (DKO), the number of B cells in lymph nodes decreased to approximately 4% of that of control mice, and B cells were present in the spleen and blood. Upon the immunization with NP-CGG, DKO mice demonstrated the defective GC formation in the spleen, and the reduced NP-specific antibody production. In vitro, Rap1 deficiency impaired the movement of activated B cells along the gradients of chemoattractants known to be critical for their localization in the follicles. Furthermore, B-1a cells were almost completely absent in the peritoneal cavity, spleen and blood of adult DKO mice, and the number of B-cell progenitor/precursor (B-p) were reduced in neonatal and fetal livers. However, DKO B-ps normally proliferated, and differentiated into IgM+ cells in the presence of IL-7. CXCL12-dependent migration of B-ps on the VCAM-1 was severely impaired by Rap1 deficiency. Immunostaining study of fetal livers revealed defects in the co-localization of DKO B-ps and IL-7-producing stromal cells. This study proposes that the profound effects of Rap1-deficiency on humoral responses and B-1a cell generation may be due to or in part caused by impairments of the chemoattractant-dependent positioning and the contact with stromal cells.

Highlights

The small GTPase Ras-related protein1(Rap1) regulates multiple functions such as cell proliferation, differentiation, and adhesion [1]
As previously reported [33], L-selectin-dependent rolling was increased in double knockouts (DKO) B cells, but CXCL13 and lymphocyte function-associated antigen (LFA)-1dependent stable arrest was completely abrogated by Rap1deficiency (Figure S1B in Supplementary Material)
We propose from the data presented that the absence of Rap1deficient B cells in lymph nodes (LNs) occurs as a result of defective homing or retention, leading to localization of B cells in the blood and spleen, which were consistent with the previous papers [11, 12]

Summary

Introduction

The small GTPase Ras-related protein1(Rap1) regulates multiple functions such as cell proliferation, differentiation, and adhesion [1]. Integrin-mediated regulation of lymphocyte adhesion and migration is an integral process at each step of immunosurveillance [2, 3]. We demonstrated that Rap1-RAPL-Mst pathway is essential for LFA-1-dependent arrest of T and B cells on the high endothelial cells (HEV), which is critical step to home into peripheral lymph nodes [4, 9, 10]. Previous papers [11, 12] demonstrated the critical role of Rap1b in B-cell trafficking and differentiation using Rap1b null mice, because Rap1b is the dominant isoform of Rap in lymphoid cells. It is necessary to explore the exact roles of Rap in B-cell development using the mice having B-cell specific double knockout of Rap1a and Rap1b

Methods

Results

Conclusion