Fundamentally Different Roles of HDAC1 and HDAC2 in Control of Foxp3+ T-Regulatory (Treg) Cell Function.

Abstract

Understanding the molecular mechanisms by which Foxp3+ Tregs function is key to their pharmacologic regulation and/or use in cell therapy of transplant (Tx) recipients. The histone deacetylases (HDAC), HDAC1 and HDAC2, are highly homologous (>80%) and essential epigenetic enzymes that function in multiprotein complexes (Sin3, NuRD, CoREST) to regulate gene transcription. We undertook the first analysis of the effects of their targeting on alloresponses by conditionally deleting HDAC1 or 2 in Tregs, through mating Foxp3-cre and floxed HDAC1 or HDAC2 mice. In each case, HDAC deletion had no effect on thymic Treg development or on the numbers of Foxp3+ Tregs in peripheral lymphoid tissues. Expression profiling of peripheral Tregs showed that whereas HDAC2 deletion induced significant increases in CTLA4, IL-10 and TGF-b transcription, HDAC1 deletion resulted in significant increases in IL-2 and IFN-g transcription. When activated in vitro with CD3/CD28, and cultured in the presence of IL-2 (50 U/ml) and IL-6 (50 ng/ml), Foxp3 levels were unaffected in both populations, but HDAC1-/- Treg produced more IL-17 and IFN-g. Consistent with these data, the in vitro suppressive function of HDAC2 KO increased, but that of HDAC1 KO was decreased, compared to WT Tregs. To test in vivo Treg function, we transplanted BALB/c cardiac allografts into Rag1-/- C57BL/6 recipients and adoptively transferred 1 million WT conventional T cells and a half million WT or HDAC1-/- Treg cells. WT Treg maintained allograft survival for >100 days post-Tx, whereas use of HDAC1-/- Treg led to acute rejection within 2 weeks of engraftment. Likewise, allograft tolerance was readily attained by costimulation blockade therapy in WT recipients, but was ineffectual in HDAC1-/- recipients. In contrast, whereas even WT Treg could not promote allograft survival when transferred at a 4:1 ratio of T cells to Tregs, this ratio of HDAC2-/- Treg to WT conventional T cells significantly extended allograft survival. In summary, despite evidence that HDAC1 and 2 have complimentary functions in other cell types, they showed differential functions in Tregs. HDAC1 enhances Treg function by inhibiting IL-2 and IFN-g production, whereas HDAC2 inhibits Treg function by down-regulating CTLA4, IL-10 and TGF-b expression. These data provide a powerful rationale for the use of selective HDAC inhibitors to regulate the phenotype and functions of Tregs post-Tx.