Regulated Expansion and Survival of Chimeric Antigen Receptor-Modified T Cells Using Small Molecule-Dependent Inducible MyD88/CD40

Anti-tumor efficacy of T cells engineered to express chimeric antigen receptors (CARs) is dependent on their specificity, survival, and in vivo expansion following adoptive transfer. Toll-like receptor (TLR) and CD40 signaling in T cells can improve persistence and drive proliferation of antigen-specific CD4⁺ and CD8⁺ T cells following pathogen challenge or in graft-versus-host disease (GvHD) settings, suggesting that these costimulatory pathways may be co-opted to improve CAR-T cell persistence and function. Here, we present a novel strategy to activate TLR and CD40 signaling in human T cells using inducible MyD88/CD40 (iMC), which can be triggered in vivo via the synthetic dimerizing ligand, rimiducid, to provide potent costimulation to CAR-modified T cells. Importantly, the concurrent activation of iMC (with rimiducid) and CAR (by antigen recognition) is required for interleukin (IL)-2 production and robust CAR-T cell expansion and may provide a user-controlled mechanism to amplify CAR-T cell levels in vivo and augment anti-tumor efficacy.

B cell receptor-mediated sustained c-Rel activation facilitates late transitional B cell survival through control of B cell activating factor receptor and NF-kappaB2

Signaling from the BCR and B cell activating factor receptor (BAFF-R or BR3) differentially regulates apoptosis within early transitional (T1) and late transitional (T2; CD21(int)-T2) B cells during selection processes to generate mature B lymphocytes. However, molecular mechanisms underlying the differential sensitivity of transitional B cells to apoptosis remain unclear. In this study, we demonstrate that BCR signaling induced more long-term c-Rel activation in T2 and mature than in T1 B cells leading to increased expression of anti-apoptotic genes as well as prosurvival BAFF-R and its downstream substrate p100 (NF-kappaB2). Sustained c-Rel activation required de novo c-Rel gene transcription and translation via Btk-dependent mechanisms. Like T1 cells, mature B cells from Btk- and c-Rel-deficient mice also failed to activate these genes. These findings suggest that the gain of survival potential within transitional B cells is dependent on the ability to produce a long-term c-Rel response, which plays a critical role in T2 B cell survival and differentiation in vivo by inducing anti-apoptotic genes, BAFF-R and NF-kappaB2, an essential component for BAFF-R survival signaling. Thus, acquisition of resistance to apoptosis during transitional B cell maturation is achieved by integration of BCR and BAFF-R signals.

Bruton’s Tyrosine Kinase Mediates NF-κB Activation and B Cell Survival by B Cell-Activating Factor Receptor of the TNF-R Family

Loss of Bruton's tyrosine kinase (Btk) function results in mouse Xid disease characterized by a reduction in mature B cells and impaired humoral immune responses. These defects have been mainly attributed to impaired BCR signaling including reduced activation of the classical NF-kappaB pathway. In this study we show that Btk also couples the receptor for B cell-activating factor (BAFF) of the TNF family (BAFF-R) to the NF-kappaB pathway. Loss of Btk results in defective BAFF-mediated activation of both classical and alternative NF-kappaB pathways. Btk appears to regulate directly the classical pathway in response to BAFF such that Btk-deficient B cells exhibit reduced kinase activity of IkappaB kinase gamma-containing complexes and defective IkappaBalpha degradation. In addition, Btk-deficient B cells produce reduced levels of NF-kappaB2 (p100) basally and in response to stimulation via the BCR or BAFF-R, resulting in impaired activation of the alternative NF-kappaB pathway by BAFF. These results suggest that Btk regulates B cell survival by directly regulating the classical NF-kappaB pathway under both BCR and BAFF-R, as well as by inducing the expression of the components of alternative pathway for sustained NF-kappaB activation in response BAFF. Thus, impaired BCR- and BAFF-induced signaling to NF-kappaB may contribute to the observed defects in B cell survival and humoral immune responses in Btk-deficient mice.

Differential regulation of NF-κB survival signaling in transitional and mature Fo B cells (86.13)

The earliest transitional (T1) B cells are significantly more sensitive to cell death than the late transitional (T2) and mature follicular (Fo) B cells and aberrant survival of T1 B cells can lead to autoimmune disease. While it is known that survival of splenic B cells is predominantly regulated by B cell antigen receptor (BCR) and B cell activation factor receptor (BAFF-R), the mechanisms by which these receptors mediate survival is less clear. We have investigated the developmental stage-specific regulation of anti-apoptotic genes and the transcription factor NF-κB, which is a major modulator of these genes. We found that in vivo, T1 B cells express the highest levels of Mcl-1 and Bcl-xL, whereas mature Fo B cells predominantly express Bcl-2. Consistent with T1 cell sensitivity to cell death following BCR crosslinking, T1 B cells only weakly induced the expression of anti-apoptotic genes (Mcl-1 and Bcl-xL) relative to robust induction in T2 and mature Fo B cells following BCR stimulation. BCR-induced expression of NF-κB (NF-κB1, RelA, cRel, NF-κB2 and RelB) coincides with that of anti-apoptotic genes; lowest in T1 and highest in mature Fo B cells. These findings suggest that T2 and mature Fo B cell survival may be linked to sustained expression of NF-κB and anti-apoptotic genes.

This study is supported by NIH RO1 A1060729

B-cell antigen receptor activates transcription factors NFAT (nuclear factor of activated T-cells) and NF-κB (nuclear factor κB) via a mechanism that involves diacylglycerol

Engagement of the B-cell antigen receptor (BCR) induces the activation of various transcription factors, including NFAT (nuclear factor of activated T-cells) and NF-κB (nuclear factor κB), which participate in long-term biological responses such as proliferation, survival and differentiation of B-lymphocytes. We addressed the biochemical basis of this process using the DT40 chicken B-cell lymphoma. We discovered that Bruton's tyrosine kinase (BTK) and phospholipase C-γ2 (PLC-γ2) are required to activate NFAT and NF-κB, and to produce the lipid second messenger diacylglycerol in response to BCR cross-linking. Therefore the functional integrity of the BTK/PLC-γ2/diacylglycerol signalling axis is crucial for BCR-directed activation of both transcription factors NFAT and NF-κB.

B Cell receptor directs the activation of NFAT and NF-κB via distinct molecular mechanisms

BCR engagement initiates intracellular calcium ([Ca2+]i) mobilization which is critical for the activation of multiple transcription factors including NF-kappaB and NFAT. Previously, we showed that Bruton's tyrosine kinase (BTK)-deficient (btk-/-) B cells, which display a modestly reduced calcium response to BCR crosslinking, do not activate NF-kappaB. Here we show that BTK is also essential for the activation of NFAT following BCR engagement. Pharmacological mobilization of [Ca2+]i in BTK-deficient DT40 B cells (DT40.BTK) does not rescue BCR directed activation of NF-kappaB and only partially that of NFAT, suggesting existence of additional BTK-signaling pathways in this process. Therefore, we investigated a requirement for BTK in the production of diacylglycerol (DAG). We found that DT40.BTK B cells do not produce DAG in response to BCR engagement. Pharmacological inhibition of PKC isozymes and Ras revealed that the BCR-induced activation of NF-kappaB requires conventional PKCbeta, whereas that of NFAT may involve non-conventional PKCdelta and Ras pathways. Consistent with an essential role for BTK in the regulation of NFAT, B cells from btk-/- mice display defective expression of CD5, a gene under the control of NFAT. Together, these results suggest that BCR employs distinct BTK-dependent molecular mechanisms to regulate the activation of NF-kappaB versus NFAT.