The cytokine transcription profiles of developing T helper 1 and T helper 2 cells are imprinted and induced appropriately following stimulation of differentiated cells. Epigenetic regulation combines several mechanisms to ensure the inheritance of transcriptional programs. We found that the expression of the polycomb group proteins, whose role in maintaining gene silencing is well documented, was induced during development in both T helper lineages. Nevertheless, the polycomb proteins, YY1, Mel-18, Ring1A, Ezh2, and Eed, bound to the Il4 and Ifng loci in a differential pattern. In contrast to the prevailing dogma, the binding activity of the polycomb proteins in differentiated T helper cells was associated with cytokine transcription. The polycomb proteins bound to the cytokine genes under resting conditions, and their binding was induced dynamically following stimulation. The recruitment of the polycomb proteins Mel-18 and Ezh2 to the cytokine promoters was inhibited in the presence of cyclosporine A, suggesting the involvement of NFAT. Considering their binding pattern at the cytokine genes and their known function in higher order folding of regulatory elements, we propose a model whereby the polycomb proteins, in some contexts, positively regulate gene expression by mediating long-distance chromosomal interactions.

Following antigen recognition, naive T helper (Th; CD4+) cells can differentiate toward one of several effector lineages such as Th1 and Th2; each expressing distinctive transcriptional profiles of cytokine genes. These cytokines eventually instruct the strategy of the immune response. In our search for factors that propagate the transcriptional programs of differentiated Th cells, we previously found that Polycomb group (PcG) proteins, which are known as epigenetic regulators that maintain repressive chromatin states, bind differentially the signature cytokine genes. Unexpectedly, their binding to the Ifng (Interferon-g) in Th1 cells and Il4 (Interleukin-4) in Th2 cells, was correlated with transcriptional activation. Therefore, in this study we aimed to determine the functional role of PcG proteins in the regulation of the expression of the signature cytokine genes.

Molecular mismatch load analysis was recently introduced as a means for risk stratification following organ transplantation. However, while good correlation was demonstrated between molecular mismatch load and generation of de‐novo donor‐specific HLA antibody (DSA), quite a few exceptions exist, and the underlying factors that define HLA immunogenicity remain unclear. Herein we present a new paradigm to interrogate differences between molecular mismatches that lead to the generation of de‐novo DSA and those that do not (the 2 MM 1 DSA cohort). Specifically, patients transplanted across 2 HLA‐DQ mismatches, who formed de‐novo DSA only to one mismatch (foe) but not the other (friend), provide a unique environment in which patient‐specific factors that affect the immune response other than immunogenicity, such as infection and immunosuppression, can be controlled for. It further permits focusing on mismatches uniquely exhibited by the de‐novo DSA allele, rather than mismatches shared by both DSA and non‐DSA alleles. This concept paper illustrates several examples, highlights the need for center‐specific or population‐specific cutoff values for post‐transplant risk stratification, and mostly argues that if there is no direct correlation between molecular mismatch load and immunogenicity, then molecular mismatch load must not be adopted as an approach for equitable organ allocation.

We have previously shown that in differentiated T‐helper (Th)1 and Th2 cells, polycomb group (PcG) proteins are associated differentially with the promoters of the signature cytokine genes. The correlation of the binding activity of PcG proteins with gene expression is unusual, since they are well known as epigenetic regulators that maintain transcriptional silencing. Here we show that in Th17 cells, the more phenotypically flexible Th lineage, the PcG proteins Mel‐18 and less strikingly Ezh2 are associated differentially with the Il17a promoter. Using the RNAi approach, we found that Mel‐18 and Ezh2 positively regulate the expression of Il17a and Il17f. The inducible binding of Mel‐18 and Ezh2 at the Il17a promoter was dependent on signaling pathways downstream of the TCR. However, a continuous presence of TGF‐β, the cytokine that is necessary to maintain Il17a expression, was required to preserve the binding activity of Mel‐18, but not of Ezh2, following restimulation. The binding of Mel‐18 at the Il17a promoter was correlated with the recruitment of the lineage‐specifying transcription factor RORγt. Altogether, our results suggest that in Th17 cells the TCR and polarizing cytokines synergize to modulate the binding activity of Mel‐18 at the Il17a promoter, and consequently to facilitate Il17a expression.