The AKT isoforms 1 and 2 drive B cell fate decisions during the germinal center response.

Zilu Zhu,Parham Ramezani-Rad,Ashima Shukla,John R Apgar,Robert C Rickert

doi:10.26508/lsa.201900506

Abstract

The PI3K pathway is integral for the germinal center (GC) response. However, the contribution of protein kinase B (AKT) as a PI3K effector in GC B cells remains unknown. Here, we show that mice lacking the AKT1 and AKT2 isoforms in B cells failed to form GCs, which undermined affinity maturation and antibody production in response to immunization. Upon B-cell receptor stimulation, AKT1/2-deficient B cells showed poor survival, reduced proliferation, and impaired mitochondrial and metabolic fitness, which collectively halted GC development. By comparison, Foxo1 T24A mutant, which cannot be inactivated by AKT1/2 phosphorylation and is sequestered in the nucleus, significantly enhanced antibody class switch recombination via induction of activation-induced cytidine deaminase (AID) expression. By contrast, repression of FOXO1 activity by AKT1/2 promoted IRF4-driven plasma cell differentiation. Last, we show that T-cell help via CD40, but not enforced expression of Bcl2, rescued the defective GC response in AKT1/2-deficient animals by restoring proliferative expansion and energy production. Overall, our study provides mechanistic insights into the key role of AKT and downstream pathways on B cell fate decisions during the GC response.

Highlights

Germinal centers (GCs) are specialized microenvironments within the secondary lymphoid tissues where antigen-activated B cells undergo clonal expansion, immunoglobulin class switching, and affinity maturation
GC B cells iteratively migrate between the DZ and LZ, undergoing clonal expansion and somatic hypermutation (SHM) in the DZ followed by B-cell receptor (BCR) affinity-based selection in the LZ
Because AKT deficiency affected the development of peripheral B-cell subpopulations, we crossed Akt1f/f (Cho et al, 2001b) and Akt2−/− (Dummler et al, 2006) mice or Akt1f/f and Akt3−/− (Tschopp et al, 2005) mice to mice expressing Cre recombinase in B cells activated by T cell– dependent immunization (Cγ1Cre) (Casola et al, 2006) to generate Cγ1Cre × Akt1f/f × Akt2−/− or Cγ1Cre × Akt1f/f × Akt3−/− mice

Summary

Introduction

Germinal centers (GCs) are specialized microenvironments within the secondary lymphoid tissues where antigen-activated B cells undergo clonal expansion, immunoglobulin class switching, and affinity maturation. Recent studies found that BCR signaling to be critical for efficient selection in the LZ because it synergizes with T cell help to enhance the GC and plasmablast response (Turner et al, 2018) and induce expression of c-Myc, a critical driver of B cell proliferation (Luo et al, 2018). These findings indicate that GC B cells can integrate variable inputs from T cells and the BCR in vivo for an optimal process of selection and differentiation. Understanding the intracellular signaling pathways that effect these distinct differentiation pathways is an ongoing challenge

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