CD8+ T Cell Responses to the Polycomb Protein EZH2 in Patients with Haemopoietic Malignancies.

Polycomb Group (PcG) genes encode transcriptional repressors that have been implicated in stem cell survival and cell fate determination (Lessard J et al. Blood. 1998). They are the human homologues of the Drosophila Polycomb and Posterior sex comb genes where they maintain segment-specific expression of the homeotic (Hox) genes. Hox gene products are transcription factors involved in cell fate determination and have been implicated in normal haematopoiesis and leukaemic transformation. In mammals, the PcG genes are highly conserved and essential for normal embryonal development and cell cycle control, as well as the regulation of haematopoiesis. Two distinct PcG gene subsets have been identified which demonstrate consistent and often mutually exclusive expression in haemopoietic cells, the patterns correlating with phenotypically distinct populations in the lymph nodes and thymus. BMI-1 and EZH2 are representative examples of these two subsets of PcG genes. BMI-1 expression is essential for stem cell self-renewal whereas EZH2 is required for cell proliferation. Disrupted expression of both of these genes has been demonstrated in a range of haemopoietic and non-haemopoietic malignancies and appears to deliver a proliferative advantage to the cell.

Humoral and T cell immunity to BMI-1 and EZH2 has been observed in patients with hepatocellular and prostate carcinoma, suggesting that these proteins may act as tumour antigens. However, CD8+ T cell responses to peptides derived from the proteins have also been demonstrated in healthy controls indicating that T cell tolerance might not be complete.

Using the cytokine secretion assay and HLA-tetramer staining, HLA A*0201-restricted CD8+ T cell responses to an EZH2-derived peptide have been identified in 7 out of 19 (37%) healthy controls. The frequency of IFNg-secreting cells ranged from 0.001–1.6% of the total CD8+ T cell pool. In the patient cohort, 4 out of 11 donors responded to the peptide (frequency 36%) with a frequency representing between 0.002 and 0.03% of CD8+ T cells. The frequency of the response is therefore comparable between the two groups. The potential role of regulatory CD4+CD25+ T cells in controlling auto-reactive immune responses to PcG proteins is being assessed. Depletion of this population can result in an increased expansion of the tumour-reactive T cell response in vitro.

Thus, although ectopic expression of polycomb proteins is often seen in haemopoietic malignancy there is currently little evidence for a tumour-selective immune response in such patients.