Immunomodulatory effects of anti-angiogenic drugs - Nature

Leukemia (2011) 25, 899–905 & 2011 Macmillan Publishers Limited All rights reserved 0887-6924/11 www.nature.com/leu

REVIEW Immunomodulatory effects of anti-angiogenic drugs A Heine, SAE Held, A Bringmann, TAW Holderried and P Brossart University Hospital Bonn, Department of Hematology/Oncology, Bonn, Germany

Much progress and significant therapeutic changes have been made in the field of tumor therapy in the past decades. Besides chemotherapy and radiotherapy, a special focus was laid on targeted therapies such as small molecule tyrosine kinase inhibitors (TKIs) and other immunomodulatory drugs, which have become standard therapies and important combination partners in a variety of malignancies. In contrast to the widely established use of these often anti-angiogenic drugs, many functional molecular mechanisms are yet not completely understood. Recent analyses focused not only on their direct anti-tumor responses, but also on their influence on tumor microenvironment, as well as on their effects on malignant and healthy cells. Different anti-angiogenic compounds targeting the vascular endothelial growth factor (VEGF) or plateletderived growth factor pathways seem to be capable of modulating immune responses, in a positive, as well as apparently harmful manner. For an optimal clinical anti-cancer treatment, a better understanding of these immunomodulatory effects is necessary. Here we summarize recent reports on the immunomodulatory function of lately introduced clinically applied antiangiogenic compounds, such as the humanized monoclonal antibody against VEGF bevacizumab, the small molecule TKIs sunitinib, sorafenib, imatinib, dasatinib, nilotinib and the proteasome inhibitor bortezomib. Leukemia (2011) 25, 899–905; doi:10.1038/leu.2011.24; published online 25 February 2011 Keywords: angiogenesis; tyrosine kinase inhibitors; immunomodulation; VEGF

Introduction Tumor therapy has significantly changed during the past decades. We have not only learned about the complex process of tumor evolution and development, but also about compounds and processes inhibiting an effective anti-tumor immunity. The evolution from benign to malignant cells is a multiple step process, including a history of genetic alterations that result in proliferation and expansion of the malignant clone.1 Tumors bigger than 1–2 mm of size need to establish their own blood vessel infrastructure and initiate neoangiogenesis.2 Hence, angiogenesis is fundamental in tumor growth and metastatic spread, and the ‘switch’ to an angiogenic phenotype is considered to be the most important step in the course of a malignant process. An inefficient blood supply induces an increased expression of hypoxia-inducible factors (HIFs) in tumor cells. These proteins control the transcription of various genes that are important for angiogenesis and lead to an enhanced expression of Correspondence: Professor Dr Peter Brossart, Department of Hematology/ Oncology, University Hospital Bonn, Wilhelmstr. 35-37, Bonn 53111, Germany. E-mail: [email protected] Received 1 November 2010; revised 23 December 2010; accepted 11 January 2011; published online 25 February 2011

pro-angiogenic factors, such as vascular endothelial growth factor (VEGF), platelet-derived growth factor (PDGF) receptor or interleukin-8. Elevated VEGF levels result in the production of endothelial cells that are capable to persist, to migrate and to differentiate into cells, which offer a higher vascular permeability. Various malignancies are caused by an abnormal expression of the involved tyrosine kinases and signaling pathways that represent useful targets in tumor and leukemia therapy, such as targeting the VEGF receptor signaling pathway may result in destruction of endothelial cells and newly built tumor blood vessels.3 Numerous anti-angiogenic drugs have lately been introduced and have become a standard of care in a variety of malignancies. Most of them are well–tolerated, and with less of the typical accompanying adverse side effects seen in patients treated with radiotherapy or chemotherapy. However, intolerance and resistance have developed in a limited number of patients and many functional molecular mechanisms, as well as immunomodulatory effects are yet not completely discovered. There is evidence that some of the applied anti-angiogenic drugs can interfere with the induction of specific immune responses, leading to a reduced anti-tumor immunity. Anti-tumor directed immune responses and tumor-dependent angiogenesis are, among others, significantly influenced by an immunosuppressive microenvironment or a deregulation in endothelial cells, as well as tumor-associated stroma cells. Gr-1 þ CD11b þ myeloid-derived suppressor cells (MDSC) and CD4 þ CD25 þ Foxp3 þ regulatory T cells (Tregs) are two important cell populations that have a central role in the balance of this microenvironment and promotion of immune suppression. Among others, a special focus of this review was laid on the effects of anti-angiogenic compounds on MDSC, dendritic cells (DCs) and T cells. Further, after primarily discussing the more ‘typical’ anti-angiogenic compounds sunitinib, sorafenib and bevacizumab, we decided to include imatinib, dasatinib, nilotinib and the proteasome inhibitor bortezomib, even though not targeting VEGF or being a typical anti-angiogenic treatment in the common view. However, these compounds were shown to affect angiogenesis by interfering with the PDGF signaling or the downstream molecules.

Sunitinib and sorafenib Sunitinib (sutent) and sorafenib (nexavar) are two broadspectrum oral TKIs that inhibit a variety of tyrosine kinases (Table 1) and are mainly applied in patients with solid cancers such as renal cell carcinomas (RCC, Table 2). Several pre-clinical and clinical studies in mice and men investigated the immunomodulatory effects of sunitinib and sorafenib, which seem to perform differently in this context. Diverse approaches focused on their influence on the mentioned immunosuppressive microenvironment.

Immunomodulation by anti-angiogenic drugs A Heine et al

900 Table 1

Tyrosine kinase inhibitors and their targets

TKI

Target

Sunitinib

c-KIT VEGFR PDGFR FLT-3 c-KIT VEGFR PDGFR FLT-3 Raf-MEK-ERK abl-kinase PDGFR c-KIT M-CSF ARG (ABL related gene) abl-kinase c-kit PDGFR Scr-kinases Off-target kinases abl-kinases c-kit PDGFR

Sorafenib

Imatinib

Dasatinib

Nilotinib

Malignancy (exemplarily) RCC, GIST

Table 2

Tyrosine kinase expression in tumors

Receptor

Localisation

Function

PDGFR

Pericytes Tumor cells

VEGFR

Endothelial cells

KIT

Tumor cells

RET

Tumor cells

Tumor growth Solid tumors: angiogenesis RCC vessel maturation Colorectal carcinoma NSCLC HCC GIST Ovarian cancer Breast cancer Prostate cancer Osteosarcoma Melanoma Meningeoma Hematological malignancies: CML MDS Tumor growth Solid tumors: angiogenesis RCC vessel maturation Colorectal carcinoma NSCLC Breast cancer Prostate cancer Pancreatic cancer Melanoma Neuroendocrine tumors Hematological malignancies: CML MDS Multiple myeloma Tumor growth Solid tumors: GIST SCLC Breast cancer Ovarian cancer Cervix cancer Melanoma Mastocytoma Testis cancer Hematological malignancies: AML Tumor growth Thyroid cancer

RCC, HCC

CML, ALL, GIST

CML

CML

Abbreviations: ALL, acute lymphatic leukemia; CML, chronic myelogenous leukemia; ERK, extracellular signal regulated kinase; GIST, gastrointestinal stroma tumor; HCC, hepatocellular carcinoma; M-CSF, macrophage colony stimulating factor; PDGFR, platelet-derived growth factor receptor; RCC, Renal cell cancer; TKI, tyrosine kinase inhibitor; VEGFR, vascular endothelial growth factor receptor.

It is known that elevated frequencies of MDSCFwhich interfere with effector T-cell functions and antigen presenting cells (APC)Fcan be found in various cancer patients,4–7 so that it might be clinically desirable to deplete these cell populations, especially before the initiation of immunotherapeutic treatment modalities. Sunitinib has shown the potential to modulate antitumor immunity by reversing the MDSC accumulation.8–10 The reduction in MDSC correlated with a reversal of type 1 T-cell suppression, an effect that could be reproduced by the depletion of the MDSC in vitro.8 Interestingly, MDSC reduction in response to sunitinib went in line with a reversal of Treg elevation. Elevated numbers of TregsFthat regulate self tolerance and can suppress T-cell responsesFwere also found in patients with advanced cancer4 and were shown to be associated with reduced survival.5 In contrast, depletion of Tregs can lead to elevated anti-tumor immune responses.6,7 Such a reduction of immunosuppressive cells and a decreased potential to inhibit T-cell functions under sunitinib treatment might therefore be of benefit in anti-cancer treatments striving for strong anti-tumor directed immune responses. However, although sunitinib treatment seems to provoke beneficial immunomodulatory changes in the context of anti-cancer treatment, no correlation could be observed between a change in tumor burden and a change in MDSC, Tregs or interferon gamma (IFN-g) production by T cells.8,9 Sunitinib application further might be able to prevent tumorspecific T-cell anergy.10 A significantly higher percentage and infiltration of CD8 þ and CD4 þ T cells was detected in tumors of sunitinib-treated animals, when compared with controltreated mice. The expression of the negative costimulatory receptors cytotoxic T-lymphocyte antigen-4 and programmed death-1 on both CD4 þ and CD8 þ T cells, as well as programmed cell death ligand 1 expression on MDSC and plasmacytoid DCs, was significantly decreased by sunitinib Leukemia

Tumor entitiy

Abbreviations: AML, acute myeloid leukemia; ALL, acute lymphatic leukemia; CML, chronic myelogenous leukemia; GIST, gastrointestinal stroma tumor; HCC, hepatocellular carcinoma; MDS, myelodysplastic syndome; NSCLC, non-small cell lung cancer; PDGFR, platelet-derived growth factor receptor; RCC, Renal cell cancer; SCLC, small cell lung cancer; VEGFR, vascular endothelial growth factor receptor. Heine et al.

treatment.10 In addition, sunitinib treatment resulted in reduced expression of interleukin-10, tumor growth factor-b and Foxp3, but enhanced expression of Th1 cytokine interferon-g (IFN-g) and increased cytotoxic T lymphocyte responses in isolated tumor-infiltrating leukocytes.10 Concerning the second TKI, sorafenib, it was found to be capable of inhibiting primary human T-cell activation and proliferation in vitro in concentrations similar to those used in patients. Above a defined dose of sorafenib, the induced inhibition of T-cell proliferation was irreversible, even after drug withdrawal. In higher doses, T-cell apoptosis was induced. In line with imatinib and sunitinib, also sorafenib led to a G0/G1


901 phase arrest. Above, sorafenib treatment induced an inhibition of CD25 and CD69 expression, interleukin-2 production and Lck phosphorylation in T cells.11 There are reports of a dramatic impairment of T cells after exposure to sorafenib and a reduction of antigen-specific tumor responses in melanoma patients after vaccination against the tumor-associated antigen surviving.12 In contrast to sunitinib, sorafenib treatment provoked a decreased growth of CD4 þ and CD8 þ T cells and even had a cytotoxic effect on Tregs.13 Moreover, sorafenib, but not sunitinib, altered the phenotype and function of DCs, characterized by reduced secretion of cytokines and expression of CD1a, major histocompatibility complex and costimulatory molecules in response to toll-like receptor ligands.14 In presence of sorafenib, a decreased induction of T-cell responses, as well as a reduced migratory capacity of T cells was found. The observed effects are mediated by the inhibition of PI3 kinases and mitogen-activated protein kinases, as well as nuclear factor-k B signaling.14 On the basis of these results, sunitinib seems to improve the function of APC and T cells in animal models and cancer patients by reducing the number of Tregs and MDSC.

Bevacizumab Bevacizumab (avastin) is a recombinant monoclonal antibody that binds and neutralizes VEGF.15 It has been approved for the treatment of several types of tumors, such as advanced RCC and colorectal carcinoma. It has been suggested that DC differentiation might be negatively associated with VEGF levels. Interestingly, the administration of bevacizumab in cancer patientsFwhich were reported to show an impaired DC function and maturationFwas associated with a decrease in the accumulation of immature DC progenitors.16 It was found that VEGF and supernatants of RCC cell lines cultured under hypoxia alter the differentiation of human monocytes to DCs. These DCs showed impaired activity, as assessed by the alloreactive mixed T-lymphocyte reaction. Bevacizumab and sorafenib, but not sunitinib, reversed the inhibitory effects of VEGF, but not of those mediated by tumor supernatants. DCs that matured under the influence of VEGF expressed less human leukocyte antigenDR and CD86, an effect that could be restored by bevacizumab and sorafenib. Finally, tumor cell supernatants decreased the interleukin-12 production by mature DCs, and this inhibition was not recovered by any of the tested anti-angiogenic drugs, delivered either as single agents or in combination.17 Additionally, there is some evidence that bevacizumab improves peripheral blood mononuclear cell-induced, but not IFN-g-induced, neointimal formation of the vessels.18 VEGF further seems to contribute to vascular remodeling in human arteries through a direct effect on human T cells, leading to an increased recruitment of T cells to the vessel.18 Little is known about the effects of bevacizumab on T-cell populations. In a small study with few patients, a significant decrease of Tregs was found after bevacizumab exposure.19 On the basis of these results, blocking of VEGF by specific antibodies seems to improve the function of APC such as DCs.

Imatinib Imatinib (glivec) is the first multi-target TKI, which was approved for the treatment of patients with Philadelphia -chromosome positive chronic myelogenous leukemia (CML). It inhibits the

BCR/ABL-kinase, but also targets other receptor tyrosine kinases (Table 1). Although not targeting the VEGF pathway, it interferes with the signaling through the PDGF receptor pathway important for the growth and function of pericytes and stabilization of newly developed vessels. Imatinib treatment was shown to result in significant immunomodulatory changes in APC, T cells and leukemia cells. Besides a reduced expression of CD1a, costimulatory molecules and human leukocyte antigen-class I and II molecules of DCs cultured with imatinib, DCs were also unable to elicit primary immune responses, as well as T-cell responses against a recall antigen. These effects were thought to be mainly caused by imatinib-induced suppressive effects on T cells.20,21 Similar results were reported by others, confirming the inhibitory effects of imatinib on T-cell activation, proliferation and expansion.22,23 Interestingly, imatinib-treated mice also showed an activation of natural killer (NK) cells, and an increased NK celldependent anti-tumor activity was found when imatinib was applied in humans.24 DCs of leukemia patients are usually defective and differ from normal DCs. It was shown by Mohty et al.,25 that plasmacytoid DC function is restored in CML patients under imatinib therapy. Other groups reported that in vitro treatment with imatinib leads to an enhanced APC function and was able to prevent the in vivo induction of tolerance of tumor-specific CD4 þ T cells. By preserving the responsiveness of tumor antigen-specific CD4 þ T cells, imatinib treatment also resulted in an enhanced efficacy of therapeutic vaccination.26 It was repeatedly demonstrated that imatinib acts immunosuppressive by inhibiting DC and T-cell effector functions. In addition, imatinib can affect the immunogenicity of CML cells by downregulation of TAAs in malignant cells due to inhibition of BCR/ABL.27

Dasatinib Dasatinib (sprycel) is another extremely potent multi-kinase TKI, which was approved for the treatment of CML patients resistant or intolerant to imatinib. Besides targeting the bcr/abl and scr/ abl kinase domains (Table 1), it also seems to inhibit off-target kinases. Scr kinases are involved in the induction of innate and adaptive immunity and are essential in physiological T-cell activation. Dasatinib was further described to bind to several key kinase targets of the immune system, such as Lyn and Btk in B cells, T cells, mast cells and basophils. Recent results have shown that dasatinib not only exerts anti-neoplastic effects in CML patients, but also acts as an immunosuppressive agent. Schade et al.,28 demonstrated that dasatinib inhibited T-cell receptor (TCR)-mediated signal transduction, cellular proliferation, cytokine production and in vivo T-cell responses, which were not caused by apoptosis. Dasatinib potently inhibited Lck, which was supposed to be strongly involved in transmitting signals from the TCR. Hence, dasatinib seems to display specifity for TCR signaling. It was also suggested thatFamong other kinasesFthe inhibition of Lck might be responsible for the T-cell suppressive effects of dasatinib.29 Dasatinib was shown to block antigen-specific murine CD4 þ and CD8 þ transgenic T-cell proliferation in vitro and in vivo.30 Furthermore, there was no CD4 þ and CD8 þ T-cell induction at all, as well as sufficient major histocompatibility complex class I-deficient cell elimination by NK cells upon immunization with a recombinant virus. In line with these results, it was reported that dasatinib inhibited human T-cell activation, Leukemia


902 proliferation, cytokine production and degranulation in a dose-dependent manner in vitro.31 Interestingly, CD4 þ T cells seemed to be more sensitive to these effects than CD8 þ T cells, and naive T cells were more sensitive than memory subsets. It is notable that these effects occurred at therapeutically relevant concentrations. Fei et al.,32 went in line with these studies and were able to confirm the impairment of proliferation and function of CD8 þ T cells through TCR and nuclear factor-k B signaling, without induction of apoptosis. The same group demonstrated in a subsequent study that dasatinib also inhibited the proliferation of Tregs and CD4 þ CD25 þ T cells, again in a dose-dependent manner and in therapeutically relevant doses.33 Above, treatment of Tregs with dasatinib inhibited the suppressive capacity of Tregs, which could be mediated by a G0/G1 phase cell cycle arrest, and a down-regulation of the transcription factor forkhead box P3, glucocorticoid-induced tumor necrosis factor receptor and cytotoxic T-lymphocyte antigen-4, a key negative regulator for T cells. Interestingly, in a study with 22 patients on dasatinib therapy, a marked lymphoproliferation in the blood was detected. These T-cell expansions were found to be clonal.34 The authors mentioned that they had performed another study showing a superior survival in the patient group experiencing lymphocytosis and suggested that dasatinib might induce a state of aberrant immune reactivity associated with good clinical responses. The NK- or T cell-associated lymphocytosis under dasatinib exposure was confirmed by others, and the lymphocytosis was observed to go in line with large granular lymphocyte morphologies.35 Analyses of blood samples from CML patients revealed that in 83% (15/18) of patients, a sizeable clonal, BCR/ABL negative lymphocyte population at diagnosisFwhich was not common in healthy individuals (8%, 1/12)Fwas detected. This lymphocyte population persisted at low levels during imatinib treatment, whereas it expanded in a proportion of dasatinib-treated CML patients, resulting in absolute lymphocytosis in the blood. In contrast to patients without lymphocytosis, most patients with lymphocytosis showed a TCR-delta rearrangement. The authors suggest that a clonal expansion of these specific lymphocytes might be part of leukemia immunosurveillance.36 When investigating the effects of imatinib, dasatinib and nilotinib on NK-cell reactivity in vitro, all three BCR/ABL inhibitors were found to reduce NK-cell cytotoxicity and IFN-g production after exposure to CML cells in pharmacological concentrations. In regard to direct effects on NK-cell responses to the cell line K-562 and primary CML cells, as well as activating cytokines, dasatinib-decreased NK-cell cytotoxicity and cytokine production. In contrast, imatinib had no influence on NK-cell reactivity, whereas nilotinib did not modulate cytotoxicity, but at higher levels, impaired NK-cell cytokine production. Interestingly, nilotinib, but not imatinib and dasatinib, increased cell death within the preferentially cytokine-secreting CD56 (bright), CD16 () NK cell subset.37

Nilotinib Nilotinib (tasigna) is an aminopyrimidine inhibitor that represents a TKI, which is highly potent in CML treatment and targets several TKs (Table 1). A reduced antigen-specific CD8 þ T-cell proliferation after nilotinib exposure was reported, which was supposed to be mediated through the inhibition of the phosphorylation of ZAP-20, Lck and extracellular signal-regulated kinase 1/2 as well as the nuclear factor-k B signal transduction Leukemia

pathway.38 It was further found that nilotinib inhibits the proliferation and suppressive capacity of Tregs in a dosedependent manner. Interestingly, these effects occurred only when high doses of nilotinib were applied and not in clinically relevant doses.39 As mentioned above, exposure of CML cells to nilotinib resulted in reduced NK-cell cytotoxicity and IFN-g production.37

Bortezomib Bortezomib (velcade) is a selective inhibitor of the 26S proteasome, which was shown to be highly effective in the treatment of multiple myeloma (MM). The proteasome is a multicatalytic protease, which is responsible for the generation of most major histocompatibility complex class-I ligands and might therefore interfere with antigen processing. Lately, proteasome inhibitors have been shown to exert immunomodulatory effects,40 such as inhibition of DC function41 and even apoptosis of human monocyte-derived DCs.42 Furthermore, inhibition of nuclear factor k-B and modulations of the tumor microenvironment, such as cytokine expression and stromal cell interactions, seem to be mediated by bortezomib.43,44 Interestingly, it was also described that bortezomib acts as an anti-angiogenic compound. HIF usually accumulates during hypoxia and has a central role in regulating tumor angiogenesis by modulating VEGF transcription. Proteasome inhibition seems to result in the accumulation of HIF-1a in tissue culture under aerobic conditions.45 A selective disruption of the hypoxia response might be of clinical benefit. Bortezomib was shown to be capable of dramatically decreasing plasma VEGF levels indicating a possibly very potent inhibition of hypoxia responses in tumors.46 Bone marrow angiogenesis is associated with progress of malignancies such as MM, and high HIF-1a-expression can be found in MM cells. Recent data demonstrated that proteasome inhibitors such as bortezomib can decrease HIF-1a in MM cells, resulting in reduced expansion and proliferation of MM cells.47 Another group went in line with these findings by demonstrating that bortezomib treatment results in a downregulated HIF-1a expression in cells that display activation of the unfolded protein response by stimulating phosphorylation of eukaryotic initiation factor-2a and inhibiting HIF-1a translation.48 When anti-angiogenic and anti-tumor effects of bortezomib were studied in vitro and in vivo in a murine neuroblastoma model, bortezomib exposure resulted in a dose- and timedependent reduction of cell proliferation because of, among others, cell cycle arrest and angiogenesis inhibition.49 Moschetta et al.,50 investigated the effects of bortezomib and zoledronic acid in MM patients. It is thought that bone marrow macrophages exert various angiogenic and vasculogenic activities and modulate, beside others, tumor progression. In this study, it was found that bortezomib and zoledronic acid display synergistic inhibitory effects on cell proliferation, adhesion, migration and expression of pro-angiogenic cytokines. In view of its immunological effects, bortezomib resulted in both decreased numbers of CD4 þ , as well as CD8 þ T-cell responses.51,52 It was further described that exposure to bortezomib led to an increased incidence of varicella-zoster infections and interferes with the priming of naive T cells.52 Additionally, proteasome inhibition triggered the mitochondrial pathway of apoptosis by activating mutually independent apoptotic pathways53 and suppressed essential immune functions of human CD4 þ T cells.54


903 Naturally occurring Tregs were resistant to pro-apoptotic effects of bortezomib. Long-term culture of CD4 þ T cells in the presence of bortezomib promoted the emergence of a Treg population that significantly inhibited proliferation, IFN-g production and CD40L expression, among stimulated effector T cells.55 Bortezomib could therefore be considered in the prevention or treatment of graft-versus-host disease and in the generation of certain Treg populations. Above, bortezomib might be useful in the therapy of autoimmune diseases. It was demonstrated that the sensitivity of myeloma cells towards proteasome inhibition directly correlates with their extremely high immunoglobulin production rate56 and that it efficiently eliminated plasma cells in mice.57 In fact, it eliminated not only short, but also long-lived, and both autoimmune as well as normal plasma cells. Bortezomib impressively decreased auto-antibody-secreting cells in mice with lupus-like disease. Interestingly, resting B and T cells, monocytes, DCs and antigen presentation by DCs were not or only moderately affected. It is thought that bortezomib exposure impairs plasma-cell survival also by modulating the ‘plasmacell niches’ comprising different cytokines (interleukin-5, interleukin-6, tumor necrosis factor-a etc.), vascular cell adhesion molecule-1 and stroma cell-derived factor 1. Therefore, the authors of the study proposed proteasome inhibition as a new efficient treatment in antibody-mediated diseases. Anti-CD20 antibodies such as rituximab might not always be capable of targeting these plasma cells as they often lack CD20 expression,57 so that bortezomib could fill in the blank.

unknown abilities of TKIs has provided a framework for interesting combination therapies for a patient group with rather few other therapy options. In view of its immunological effects, the proteasome inhibitor bortezomib also decreases the proliferation of CD4 þ , as well as CD8 þ T cells. However, it seems to be capable of impressively eliminating auto-antibody-secreting cells while not or only moderately affecting resting B and T cells, monocytes, DCs and antigen presentation by DCs, making this drug an interesting compound to be used to treat rheumatologic and autoimmune diseases and in the field of graft-versus-host disease suppression. However, we are far from knowing the entire complex signaling pathways and interactions so that more studies are warranted. It is of interest to further investigate these immunomodulatory effectsFwhether in the context of a combination regime with tumor immunotherapy or with regard to autoimmune diseases and transplantation medicineFwith a special focus on graftversus-host disease and graft-versus-leukemia disease. In light of the different effects of the anti-angiogenic drugs, careful choice and consideration should be given to the individual patient and treatment decision.

Conflict of interest The authors declare no conflict of interest. Author contributions All authors were involved in writing the manuscript.

Conclusion References Much progress and significant therapeutic changes have been made in the field of tumor therapy in the past decades. VEGF, which once was thought to be solely involved in vascular growth and permeability, has emerged to have a major role as an immune modulator in tumor microenvironments. Intensive basic research and clinical practice have accentuated the important correlation between immunomodulation and angiogenesis. Intelligent anti-cancer treatment regimes need to consider these diverse immunomodulatory interactions. In regard to the widely applied TKIs, recent data showed an impaired maturation of DCs and a reduced T-cell proliferation when imatinib or sorafenib were applied. Also dasatinib, and at higher doses of nilotinib, were shown to inhibit the induction of CD4 þ and CD8 þ T cells and modulate cytokine production, signal transduction and NK-cell cytotoxicity. In contrast, sunitinib treatment did not alter DC maturation and differentiation and might therefore be a more suitable candidate for the induction of efficient immune responses. Additionally, in presence of sunitinib, a reduction in circulating Tregs and MDSC was foundFwhich again might help to reverse a tumor-induced immunosuppression. It is thought that these immunomodulatory effects of sunitinib are likely to be independent of its anti-cancer effect, so that the compound might be of benefit for a variety of tumor types. Hence, sunitinib might be a good combination partner in immunotherapeutic treatment regimes because of its synergistic manner. In contrast, sorafenib could be used, for example, in the setting of allogenic stem cell transplantation to treat graft-versushost disease. In summary, certain TKIs might represent a novel strategy for the treatment of certain malignancies, especially in the context of enhancing the therapeutic efficacy of existing immune-based therapies. The discovery of these previously

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