UC Irvine

As humans, we have evolved to build a complex humoral immune response against infection. The process of differentiating B cells into antibody secreting cells is highly regulated and when dysfunctional can lead to various human malignancies. Central to both an effective host response and disease progression is the PI3K/AKT/mTOR network and targeting this pathway has already led to some success in treating certain B-cell malignancies and autoimmune diseases. However, there are still many questions about how this process is regulated by this signaling network. This thesis investigates the immunomodulatory effects of pharmacological and genetic perturbations on B cell function.

Chapter 2 of this thesis characterizes the effects of novel PI3Kδ and PI3Kδ/γ inhibitors on B cell function and differentiation. I show in vitro that B cell survival, proliferation, and plasmablast differentiation are reduced at nanomolar concentrations of these inhibitors. PI3Kδ was found to be the dominant isoform involved and the inhibitors also potently increased antibody class switching.

Chapter 3 provides mechanistic investigation of mTORC1 and mTORC2 function in antibody class switching in vitro. I tested both rapamycin and mTOR kinase inhibitors (TORKi) and found that they had opposite effects on antibody class switching. Genetic deletion of Raptor and Rictor for mTORC1 and mTORC2 inhibition in B cells phenocopied the effects of the inhibitors.

In Chapter 4, I further investigate the mechanism of mTORC1 in B cell differentiation and characterized the two downstream substrates S6Ks and 4E-BPs, and their effects on antibody class switching. The main finding of this chapter was that mTORC1 promotes switching in part by inhibiting 4E-BPs, thereby elevating eIF4E activity and cap-dependent translation to increase expression of AID protein.

Chapter 5 characterizes the effects of reduced eIF4E protein on antibody class switching in normal B cells and tumorigenesis in a mouse model of leukemia. These experiments demonstrate that primary B cells can regulate their ratio of 4E-BPs to eIF4E but reduced eIF4E is rate-limiting for tumorigenesis.

The work presented here establishes the contributions of the PI3K/mTOR/eIF4E signaling network to B cell differentiation and provides insight in how the humoral immune response is regulated.