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Peixoto et al. Cancer Drug Resist 2018;1:219-29 I http://dx.doi.org/10.20517/cdr.2018.17 Page 221

in antigen presentation machinery or a decrease in major histocompatibility complex (MHC) expression;
(2) dysfunctions of signaling pathways such as the Wnt/βcatenin, phosphatase and tensin homolog,
phosphoinositide-3-kinase (PI3K) or Janus kinase pathways [9-12] ; and (3) prolonged T-cell stimulation, which
is frequently observed in chronic diseases such as chronic infection or cancer. This stimulation frequently
leads to the exhaustion of immune cells, phenotype which is characterized by a loss of both immune
and proliferative capacities. This state is usually accompanied by a loss of expression of cytokines such
as interleukin-2 (IL-2), tumor necrosis factor α (TNFα) and interferon (IFN) and induction of immune
checkpoint inhibitors such as programed cell death 1 (PD-1) (PDCD1), cytotoxic T-lymphocyte associated
[13]
protein 4 (CTLA-4) , lymphocyte activating 3 (LAG-3) and T-cell immunoglobulin mucin 3 (TIM-3) .
Immune checkpoint inhibitors are proteins controlling the immune system: PD-1 is expressed at the surface
of various immune cells, such as T-lymphocytes, myeloid cells or NKs, and is particularly expressed in
exhausted T-cells. PD-1 ligand (PD-L1) (also referred as CD274) or PD-L2 are expressed at the cell surface
of tumor cells and can bind to the PD-1 receptor [Figure 1]. The PD-1/PD-L1/2 interaction induces a
+
negative signaling cascade which leads to the inhibition of CD8 T-cell proliferation, cytokine secretion and
inflammation leading to a decrease in tumor cell elimination. Moreover, interaction of the other immune
check point inhibitors with their respective ligands (CTLA-4 with the ligands CD80/CD86 expressed on
regulator T-cells, LAG-3 with MHC class II, LAG-3 with galectin-9, high mobility group protein B1 or with
carcinoembryonic antigen cell adhesion molecule 1 and phosphatidylserine) also decrease the anti-tumour
immune response.

Although the identification of these mechanisms is recent and most of the factors involved still
remain largely unknown. Anti-PD-1 and anti-PD-L1 immunotherapy protocols (such as nivolumab or
pembrolizumad) have been developed and used to restore immune edition in cancers [Figure 2]. This
process is called immune checkpoint blockade (ICB) and, in the last few years, this new treatment has
been tested and shown promising results in many different types of cancers such as: non-small cell lung
carcinoma (NSCLC), melanoma, mesothelioma, renal cell carcinoma (RCC), bladder cancer, head and
neck squamous cell carcinoma (HNSCC) [14-19] . Unfortunately, although some very interesting results were
obtained in some patients, resistance to ICB are observed in a large percentage of cases. For example, only
20% of NSCLC or triple negative breast cancer (TNBC) patients treated with an anti-PD-1 therapy presented
a significant positive response [20,21] . These results suggest that immunotherapy resistance is present in a large
proportion of patients before treatment and may be due to a low immune checkpoint inhibitor expression or
an absence of T-cell infiltration in the solid tumors.

IMMUNE CHECKPOINT INHIBITORS ARE EPIGENETICALLY CONTROLLED
Epigenetics includes DNA methylation, histones post-translational modifications and non-coding RNA
and regulates gene expression in a transmissible but reversible manner. DNA methylation is processed by
DNA methyl transferases (DNMTs) which catalyze the addition of a 5methyl cytosine on the 5th position
of cytosines in CpGs. This mark is generally repressive and many genes are normally controlled by DNA
methylation during embryogenesis. Aberrant DNA methylation also frequently occurs in pathologies,
specifically in cancers, where it contributes to tumor suppressor gene silencing or inactivation of apoptosis
[22]
(for a review ). Histones are organized in nucleosomes whose local compaction is regulated by histones
post-translational modifications. These modifications are varied but the most studied are acetylation and
methylation. Acetylation on lysines in histone tails is promoted by histone acetyl transferases and provokes
a relaxed chromatin favorable to transcription factor recruitment, opposite to deacetylation which is
catalyzed by histones deacetylases (HDACs). The pro- or anti-transcriptional role of histone lysine and
arginine methylation is dependent of both the position of the amino acid and the level of methylation. For
example, the H3K4me2/3 mark is permissive whereas the H3K9me2/3 or H3K27me2/3 are repressive. These
modifications are catalyzed by histone methyl transferases (HMTs) and removed by histone demethylases. In