Transcription Factor BACH2 Inhibits AP1 Proteins JunB and FosL1 in Umbilical Cord Blood (UCB) CD4+ T-Cells.

Introduction: Umbilical cord blood (UCB) CD4⁺ T-cells have been shown to express significant levels of BACH2 transcription factor protein compared to adult blood (AB) CD4⁺ T-cells. Previously, NFAT1 siRNA knockdown of UCB T-cells exhibited a significantly higher BACH2 mRNA expression, and IFN-γ, TNF-α. and CTLA-4 mRNA levels were significantly suppressed. BACH2, a member of the b-Zip family, has been shown to act as a heterodimer with the bZip protein MafK, as a transcriptional inhibitor via recruitment of a histone deacetylase class II complex (HDAC II) in differentiating B-cells, and neurons. Due to observed inverse expression of BACH2 and NFAT1 in UCB CD4⁺ T-cells, we hypothesized that BACH2 may regulate transcription factors known to bind with NFAT1 including AP-1 proteins JunB and FosL1. We tested this by siRNA knockdown of BACH2 in primary UCB-derived CD4⁺ T-cells. Key developmental transcription factors JUNB, FosL1, NFAT1 and downstream IFN-γ, and TNF-α were mRNA analyzed.

Methods: UCB T-cells were purified using autoMACs system (Miltenyi). After overnight culture, T-cells were transfected with BACH2 siRNA (Dharmacon) using Amaxa Nucleofector system (Amaxa Inc). Both siRNA treated and control cells were incubated in media for 18 hours, and then stimulated using anti-CD3/anti-CD28 antibodies (BD BioScience). Aliquots of cells were collected at specified time points post-stimulation for protein and total RNA isolation. The relative change in mRNA levels for BACH2, JUNB, FosL1, IFN-γ, NFAT1, and TNF-α were determined by Lightcycler SybrGreen real time RT-PCR system (Roche). siRNA knockdown of BACH2 protein in transfected UCB T-cells was confirmed by western blot.

Results: Real-time RT-PCR of BACH2 siRNA treated UCB CD4⁺ T-cells stimulated with anti-CD3/CD28 antibodies and analyzed after 6 hrs of stimulation showed a 4 log increase in FosL1 and NFAT1 mRNA, a 3 log increase in JunB mRNA, a 5 log increase in IFN-γ as compared to stimulated control UCB T-cells. TNF-α mRNA was decreased by 5 logs in BACH2 siRNA treated UCB T-cells as compared to control. CD3/CD28 stimulated untransfected UCB T-cells were previously shown to have decrease expression of NFAT1, JunB, FosL1, IFN-γ, and TNF-α, and in UCB T-cells compared to stimulated AB T-cells.

Conclusions: BACH2 expression correlates with an inhibition of expression of AP1 transcription regulatory proteins in UCB T-cells during primary CD3/CD28 stimulation. The complete activation of the T-cell requires the activation of AP1 by CD28 pathway otherwise the antigen presenting cell signals the T-cell to enter anergy. In UCB CD4⁺ T-cells express BACH2, which acts as a transcriptional inhibitor of two critical AP1 genes, JUNB and FosL1, which mediate the CD28 co-stimulatory pathway. These results further suggests that expression of BACH2 in UCB T-cells may contribute to lower incidence of alloreactivity observed in leukemia patients receiving UCB stem cells compared to AB bone marrow stem cells and thus leads to low GVHD, and contribute to the weak Th1 response seen in stimulated UCB T-cells by reduced amounts of AP1 protein available for activating the T-cell.