Abstracts (complete list) - Wissenschaft Online

Monika Braun, Christine Sers, Ruprecht Kuner, Barbara Simm, Barbara Mosetter,
Dolores Schendel, Christine Falk
Oncogenic KRAS-signalling and DNMT activity are involved in
immune escape of tumor cells
The transformation process of cancer cells is often accompanied by an accumulation of
genetic and epigenetic abnormalities that lead to indefinite proliferation and evasion of
immune recognition by immune effector cells such as CTL and NK cells. The impact of
these malignant alterations on immune evasion strategies by tumor cells is still poorly
understood. HCT116 colon carcinoma cell lines were selected with respect to mutated Ki-
Ras that is associated with oncogenic constitutive MEK/ERK signalling. In addition,
variants of this cell line with aberrant DNA-methylation due to deletion of DNAmethyltransferase
DNMT1 and/or 3b were available.
These cells were treated with a MEK/ERK pathway inhibitor (U0126) and analysed for
mRNA expression profiles by microarray-analysis and RT-PCR, for promoter-methylation
patterns by bisulphite sequencing and for surface expression by FACS analysis. The
functional relevance for immune recognition was tested using HLA class I-restricted CTL
and allogeneic NK cells. In addition, the effect of IFN-α versus IFN-γ treatment in
combination with Ras-signalling inhibition and DNMT-deficiency was analysed with
respect to sensitivity to lysis by CTL and NK cells.
The analysis revealed that some immune genes are co-regulated via DNMT and MEK/
ERK signalling. HLA-A and NKG2D-ligand ULBP2 surface expression was up-regulated by
inhibition of MEK/ERK signalling and/or deletion of DNMT, with highest expression levels
in those cells where both systems were blocked. Using an HLA-A2-restricted CTL clone
as effector cell, these changes in the density of HLA-A could be sensed and led to
enhanced tumor lysis. In addition, treatment with IFN-γ could further enhance CTL lysis,
but diminished NK-mediated killing. In contrast, IFN-α treatment could increase CTL
lysis without decreasing the NK cell response. Taken together, our data indicate that
both, oncogenic Ras-signalling and DNMT activity contribute to the immune escape
against CTL recognition. Importantly, this escape could be reversed by IFN-α but not
IFN-γ with respect to both, CTL and NK recognition.