NFAT1 in Adult and Cord Blood-Derived Human CD4⁺ T-Cells: Regulatory Implications for Graft vs. Host Disease (GVHD).

Clinical benefits of umbilical cord blood (UCB) grafts include the lower incidence of acute graft-versus-host disease (aGVHD) following allogeneic transplantation. We have previously demonstrated dramatically lower expression of the Nuclear Factor of Activated T-Cells 1 (NFAT1) protein, its associated transcripts and known binding partners, but not NFAT1 transcripts in UCB CD4⁺ cells following primary stimulation when compared to T-cells obtained from adult blood (AB). As NFAT1 is a key transcriptional regulator of immune responses, we hypothesize that NFAT1 and NFAT1-dependent factors play a critical role in the increased proliferation and decreased cytokine production seen in UCB, which may contribute to the lower incidence of aGVHD following UCB transplantation observed clinically. We have now demonstrated in vitro via siRNA-mediated knockdown and subsequent analysis of adult-derived primary T-cells with RT-PCR and Cytometric Bead Assay (CBA) that expression of a GVHD-promoting, Th¹-type cytokine profile is dependent on expression of NFAT1, and that knockdown of NFAT1 results in a transcriptional and cytokine secretion profile similar to that seen in UCB T-cells with respect to key immunoregulatory cytokines and signal transducers following stimulation. Specifically, siRNA-mediated reduction of NFAT1 by at least 68% results in 93% and 28% reduction of IFN-γ, and TNF-αrespectively, as well as a 55% reduction in CTLA4 measured by RT-PCR. Additionally, short hairpin RNA (shRNA)-mediated stable knockdown of NFAT1 results in an enhanced and prolonged reduction of IFN-γ, IL-2, and TNF-αsecretion over transient siRNA transfection in both the Jurkat cell line and adult primary CD4⁺ T-cells. However, microarray and RT-PCR time point analyses indicate NFAT1 transcripts are not changed in UCB versus AB CD4⁺ cells. Under siRNA knockdown conditions, we observed at least a 24-hour delayed decrease in NFAT1 protein levels following mRNA reduction and an eventual rebound of both NFAT1 and its associated cytokines following transient transfection at 48 hours. This suggests a crucial regulatory role for post-translational events which may preferentially occur in UCB-derived T-cells in vivo.

Indeed, we have observed ubiquitination of NFAT1 and rapid rescue of NFAT1 expression following treatment with the calpain-proteasome inhibitor Acetyl-L-Leucyl-L-Leucyl-L-Norleucinal (ALLN) in UCB-derived T-cells in vitro by co-immunoprecipitation (co-IP) in a dose-dependent manner. NFAT1 in unstimulated UCB CD4⁺ cells is shown by co-IP to be significantly more ubiquitinated than in AB-derived cells. Overall, our data indicates that regulation of NFAT1 through ubiquitination and proteasomal degradation, and its subsequent downstream effects may constitute a key difference between T-cells of adult and neonatal origin; specifically that this post-translational modification may serve as a method by which UCB-derived T-cells may prevent NFAT1 activity, impacting their allogeneic response.