Recent studies show that besides having a direct cytotoxic effect on tumor cells, chemotherapeutic drugs can also potentiate the immune system [
20,
21]. Here, we show that the chemotherapeutic platinum compound oxaliplatin modulated TLR9-induced signaling in human pDCs. This modulation led to increased IFNα production, decreased inflammatory cytokine and chemokine secretion, and decreased STAT1 and STAT3 expression by human pDCs. Furthermore, we showed that oxaliplatin enhanced PD-L1 expression levels on pDCs, resulting in impaired T-cell proliferation depending on the maturation stimuli. Generally, oxaliplatin is used in combination with other cytotoxic drugs as a treatment regimen for gastrointestinal, colorectal, esophago-gastric and/or pancreatic cancer. However, it remains to be elucidated whether the other active compounds in these treatment regimens may also affect the immunogenicity of human DC subsets.
We studied STATs and protein tyrosine phosphatases, which are key regulators of intracellular signaling as an obvious mechanism exploited by oxaliplatin to modulate TLR-induced signaling. The protein tyrosine phosphatases SHP-1 was reported to block TLR-induced production of proinflammatory cytokines by inhibiting the activation of NF-κB pathway and MAPKs by direct dephosphorylation [
26,
31,
32]. Moreover, An et al. recently described that in mice SHP-1 increased TLR- and RIG-I-induced type I IFN production by directly binding to the kinase domain of IRAK1 and thus inhibiting IRAK1 activity in bone marrow DCs, peritoneal macrophages and splenic CD11c
+ DCs [
26]. We obtained the same observations in the TLR9-induced cytokine secretion profile after oxaliplatin treatment; however, a role for SHP-1 was excluded based on experiments which showed that sodium stibogluconate had no effect. Interestingly, suramin, a broad phosphatase inhibitor, enhanced the oxaliplatin-induced effects. These data indicate that oxaliplatin impairs the activity of a certain phosphatase rather than increasing the expression of a phosphatase. Besides phosphatases, also kinases are involved in signaling upon ligand binding by TLRs [
33]. Larangé et al. demonstrated that using specific inhibitors for JNK or JAK upon TLR7 or TLR8 stimulation, significant changes were observed in the expression of certain surface molecules and the production cytokines [
33]. These findings underscore that inhibition of mediators involved in downstream signaling after TLR stimulation has subtle but significant influences on cell functionality. The observed increased expression of PD-L1 related to oxaliplatin treatment implicates undesired effects on the immunological elimination of tumor cells outcome in cancer patients. It was reported that tumor-infiltrating dendritic cells which express PD-L1 help to maintain the immunosuppressive environment [
10]. PD-L1 expression has also been detected on the surface of a wide variety of human cancers, including lung, colon, breast, ovary, renal cell, glioma and melanoma [
13,
34,
35], and has been correlated with poor prognosis [
34,
36‐
39]. Tumors escape from the host immune system by attenuation of tumor-specific T-cell responses via PD-L1/PD-1 interactions. Normally, PD-L1 expression is controlled by STAT3 and becomes upregulated by tumor cells and other immune cells upon IFN or IL-6 stimulation [
34,
40‐
43]. Recently, a study showed that chemotherapeutic agents also induced PD-L1 surface expression in breast cancer cells and promoted PD-L1-mediated T-cell apoptosis [
44]. Additionally, we found that other platinum-based chemotherapeutic also induced PD-L1 and decreased CD80 and CD86 expression on pDCs, and that oxaliplatin also induced PD-L1 expression on tumor cells (Suppl. Figure 4). In summary, the expression of PD-L1 on tumors has a negative effect on the immunological outcome and is considered as a key feature of tolerogenic DCs [
43].
In the present study, we demonstrated that human pDCs, treated with oxaliplatin in the presence of TLR9-induced type I IFN, markedly increased expression levels of PD-L1. Aspord et al. demonstrated that in breast cancer tumors CD4
+ T cells are present secreting IFNγ and IL-13 [
45]. These T cells provide an additional source of IFNs which in combination with chemotherapeutic agents can lead to enhanced PD-L1 expression on either tumor cells or on tumor-infiltrating pDCs. Moreover, infiltration of pDCs and expression of PD-L1 in severable tumors correlated with poor prognosis. Recently, Lesterhuis et al. described that platinum compounds positively affect the immunostimulatory potential of myeloid DCs by impairing the upregulation of PD-L2 [
24]. Together, this demonstrates that platinum compounds have dissimilar effects on different DC subsets dependent on the presence of environmental cues. In cancer patients, the therapeutic off-target effect of platinum compounds will depend on the presence and the amount of the different tumor-infiltrating DC subsets, the TLR expression on those DC subsets, and the micro-environment in which they reside. Based on the present study, when tumors are infiltrated with high numbers of pDCs, it would be beneficial to provide additional TLR7-agonists rather than TLR9-agonists in combination with platinum-based chemotherapeutics. Combining chemo- with immunotherapy is already reported, showing that combining chemotherapy with immunotherapy improves patient survival compared to patients treated with only chemotherapy [
46]. However, in order to become even more effective, therapeutic strategies should have direct cytotoxic effects but also be tailored based on the presence of tumor-infiltrating DCs and the TLR expression by those tumor-infiltrating DCs. This tailor-made combination of chemotherapy with additional immunotherapy has great potential in inducing superior anti-tumor responses and ultimately will lead to enhanced tumor clearance and patient survival.