UCB transplantation: miRNA involvement

In this issue of Blood, Weitzel and colleagues show the regulation of NFAT1 expression, an important transcription factor in T cells by microRNA-184, advancing our understanding of autoimmunity and GVHD.

Allogeneic hematopoietic stem cell transplantation is routinely used in clinical protocols for the treatment of high-risk hematologic malignancies following intensive chemotherapy and/or radiation. However, the limited number of available compatible unrelated donors is one of the major limitations to wider application, especially for ethnic minorities. Umbilical cord blood (UCB) transplantation is an alternative strategy and has been successfully implemented for both pediatric and adults recipients. Two of its advantages are rapid access and availability of cord blood units, and the less frequent and severe acute graft-versus-host disease (aGVHD) in a partially unmatched transplantation setting. Some major drawbacks, however, are limited numbers of nucleated cells per UCB unit, prolonged time to engraftment, and slow recovery of immunity, resulting in higher risk of severe infections 3 to 4 months after transplantation.¹ 

Donor T-cell activation and secretion of proinflammatory cytokines play an important role in the development of aGVHD. The nuclear factor of activated T cells (NFAT) family of proteins are important regulators of T-cell activation, differentiation, and self-tolerance.²  They also are indirectly inhibited by cyclosporin A and FK506. These immunosuppressive agents are widely used for preventing and counteracting severe aGVHD. NFAT1 protein-deficient mice show decreased IFN-γ production in response to T-cell receptor ligation. Since UCB T cells have a lower capacity for cytokine production than T cells derived from adult blood, it was striking that Weitzel et al identified a lower expression of NFAT1 protein in UCB CD4⁺ T cells as compared with adult CD4⁺ T cells in their previous work.^3,4  However, it remained mysterious as to why NFAT1-mRNA was not expressed at a markedly lower level in these cells. In this issue of Blood, Weitzel et al now report the translational repression of the NFAT1 protein by a microRNA, miR-184, in cord blood T cells, explaining this “contradiction.”⁵ 

MicroRNAs (miRNAs) constitute a large class of endogenous noncoding RNAs and are involved in a wide variety of processes including cell differentiation, apoptosis, and metabolic pathways.⁶  They are 19 to 24 nucleotides in length, are part of a ribonucleoprotein complex and direct this complex mostly to the 3′ UTR of the targeted mRNA. This results in translational repression of the mRNA by a wide variety of molecular mechanisms including destabilization of the mRNA and shuffling of the mRNA to structures without access to the ribosomal translation machinery.⁷ 

Weitzel et al nicely demonstrate that the translational repression of NFAT1 is most prominent in umbilical cord blood–derived CD4⁺ T cells with low NFAT1 levels and high miR-184 expression compared with adult blood-derived CD4⁺ T cells. However, this effect might also be due to the lower fraction of naive T cells in the adult CD4⁺ compartment, since the authors show that the miR-184 expression is higher in this naive fraction. Upon stimulation, inhibition in UCB CD4⁺ T cells is relieved by 2 different mechanisms. At the earlier time points, miRNA expression is reduced, resulting in higher NFAT1 protein levels, while at later time points, the NFAT1c2 mRNA is transferred from silenced cell structures to the polyribosomal fraction where the message is translated. The factors involved in the latter remain to be elucidated, but competing regulators binding to mRNA have been identified to counteract miRNA action in other cell types including human hepatoma cells, rat neurons, and fly olfactory neurons.⁷  The authors go on to define the binding site of the miRNA to the 3′ UTR on a molecular level, using a dual luciferase assay system and selectively inhibiting or overexpressing the miR-184 as well as mutating the binding site.

Since NFAT1 is a major therapeutic target for counteracting GVHD after transplantation, the findings of this study may have important clinical implications. For example, one might envision using miR-184 overexpression itself to target NFAT1 in GVHD, especially since this miRNA is not ubiquitously present in all cell types, and overexpression might have a direct effect on donor T cells. This approach could be more specific than pharmacologic agents, such as corticosteroids or cyclosporine A, that broadly suppress the immune system and are now widely used in the treatment of GVHD. Future animal studies will be required to test this hypothesis and to carefully address the potential side effects of miR-184 overexpression. Further studies addressing the role of miRNAs in T-cell activation and function are warranted.