Application of paclitaxel in low non-cytotoxic doses supports vaccination with melanoma antigens in normal mice

Prodrugs of paclitaxel improve in situ photo-vaccination

Photodynamic therapy (PDT) effectively kills cancer cells and initiates immune responses that promote anticancer effects locally and systemically. Primarily developed for local and regional cancers, the potential of PDT for systemic antitumor effects [in situ photo-vaccination (ISPV)] remains underexplored. This study investigates: (1) the comparative effectiveness of paclitaxel (PTX) prodrug [Pc-(L-PTX)2] for PDT and site-specific PTX effects versus its pseudo-prodrug [Pc-(NCL-PTX)2] for PDT combined with checkpoint inhibitors; (2) mechanisms driving systemic antitumor effects; and (3) the prophylactic impact on preventing cancer recurrence. A bilateral tumor model was established in BALB/c mice through subcutaneous injection of CT26 cells. Mice received the PTX prodrug (0.5 μmole kg-1, i.v.), and tumors were treated with a 690-nm laser (75 mW cm-2 for 30 min, drug-light interval 0.5 h, light does 135 J cm-1), followed by anti-CTLA-4 (100 μg dose-1, i.p.) on days 1, 4, and 7. Notable enhancement in both local and systemic antitumor effectiveness was observed with [Pc-(L-PTX)2] compared to [Pc-(NCL-PTX)2] with checkpoint inhibitor. Immune cell depletion and immunohistochemistry confirmed neutrophils and CD8+ T cells are effectors for systemic antitumor effects. Treatment-induced immune memory resisted newly rechallenged CT26, showcasing prophylactic benefits. ISPV with a PTX prodrug and anti-CTLA-4 is a promising approach for treating metastatic cancers and preventing recurrence.

It's high-time to re-evaluate the value of induced-chemotherapy for reinforcing immunotherapy in colorectal cancer

Immunotherapy has made significant advances in the treatment of colorectal cancer (CRC), revolutionizing the therapeutic landscape and highlighting the indispensable role of the tumor immune microenvironment. However, some CRCs have shown poor response to immunotherapy, prompting investigation into the underlying reasons. It has been discovered that certain chemotherapeutic agents possess immune-stimulatory properties, including the induction of immunogenic cell death (ICD), the generation and processing of non-mutated neoantigens (NM-neoAgs), and the B cell follicle-driven T cell response. Based on these findings, the concept of inducing chemotherapy has been introduced, and the combination of inducing chemotherapy and immunotherapy has become a standard treatment option for certain cancers. Clinical trials have confirmed the feasibility and safety of this approach in CRC, offering a promising method for improving the efficacy of immunotherapy. Nevertheless, there are still many challenges and difficulties ahead, and further research is required to optimize its use.

The future of cancer immunotherapy: DNA vaccines leading the way

Immuno-oncology has revolutionized cancer treatment and has opened up new opportunities for developing vaccination methods. DNA-based cancer vaccines have emerged as a promising approach to activating the bodily immune system against cancer. Plasmid DNA immunizations have shown a favorable safety profile and there occurs induction of generalized as well as tailored immune responses in preclinical and early-phase clinical experiments. However, these vaccines have notable limitations in immunogenicity and heterogeneity and these require refinements. DNA vaccine technology has been focusing on improving vaccine efficacy and delivery, with parallel developments in nanoparticle-based delivery systems and gene-editing technologies such as CRISPR/Cas9. This approach has showcased great promise in enhancing and tailoring the immune response to vaccination. Strategies to enhance the efficacy of DNA vaccines include the selection of appropriate antigens, optimizing insertion in a plasmid, and studying combinations of vaccines with conventional strategies and targeted therapies. Combination therapies have attenuated immunosuppressive activities in the tumor microenvironment and enhanced the capability of immune cells. This review provides an overview of the current framework of DNA vaccines in oncology and focuses on novel strategies, including established combination therapies and those still under development.The challenges that oncologists, scientists, and researchers need to overcome to establish DNA vaccines as an avant-garde approach to defeating cancer, are also emphasized. The clinical implications of the immunotherapeutic approaches and the need for predictive biomarkers have also been reviewed upon. We have also tried to extend the role of Neutrophil extracellular traps (NETs) to the DNA vaccines. The clinical implications of the immunotherapeutic approaches have also been reviewed upon. Ultimately, refining and optimizing DNA vaccines will enable harnessing the immune system's natural ability to recognize and eliminate cancer cells, leading the world towards a revolution in cancer cure.

The mechanobiology of NK cells- 'Forcing NK to Sense' target cells

Natural killer (NK) cells are innate immune lymphocytes that recognize and kill cancer and infected cells, which makes them unique 'off-the-shelf' candidates for a new generation of immunotherapies. Biomechanical forces in homeostasis and pathophysiology accrue additional immune regulation for NK immune responses. Indeed, cellular and tissue biomechanics impact NK receptor clustering, cytoskeleton remodeling, NK transmigration through endothelial cells, nuclear mechanics, and even NK-dendritic cell interaction, offering a plethora of unexplored yet important dynamic regulation for NK immunotherapy. Such events are made more complex by the heterogeneity of human NK cells. A significant question remains on whether and how biochemical and biomechanical cues collaborate for NK cell mechanotransduction, a process whereby mechanical force is sensed, transduced, and translated to downstream mechanical and biochemical signalling. Herein, we review recent advances in understanding how NK cells perceive and mechanotransduce biophysical cues. We focus on how the cellular cytoskeleton crosstalk regulates NK cell function while bearing in mind the heterogeneity of NK cells, the direct and indirect mechanical cues for NK anti-tumor activity, and finally, engineering advances that are of translational relevance to NK cell biology at the systems level.

Recent Developments and Anticancer Therapeutics of Paclitaxel: An Update

Plants are a source of diverse classes of secondary metabolites with anticancer properties. Paclitaxel (Taxol) is an anticancer drug isolated from various Taxus species and is used as a chemotherapeutic agent against various cancers. The biosynthesis of paclitaxel is a complex pathway, making its total chemical synthesis commercially non-viable; hence, alternative novel sources - like plant cell culture and heterologous expression systems, are being investigated to overcome this issue. Advancements in the field of genetic engineering, microbial fermentation engineering, and recombinant techniques have significantly increased the achievable yields of paclitaxel. Indeed, paclitaxel selectively targets microtubules and causes cell cycle arrest in the G2/M phase, inducing a cytotoxic effect in a concentration and time-dependent manner. Innovative drug delivery formulations, like the development of albumin-bound nanoparticles, nano-emulsions, nano-suspensions, liposomes, and polymeric micelles, have been applied to enhance the delivery of paclitaxel to tumor cells. This review focuses on the production, biosynthesis, mechanism of action, and anticancer effects of paclitaxel.

The immune modifying effects of chemotherapy and advances in chemo-immunotherapy

Immune checkpoint inhibitors (ICIs) have transformed the treatment paradigm for several malignancies. While the use of single-agent or combined ICIs has achieved acceptable disease control rates in a variety of solid tumors, such approaches have yet to show substantial therapeutic efficacy in select difficult-to-treat cancer types. Recently, select chemotherapy regimens are emerging as extensive modifiers of the tumor microenvironment, leading to the reprogramming of local immune responses. Accordingly, data is now emerging to suggest that certain anti-neoplastic agents modulate various immune cell processes, most notably the cross-presentation of tumor antigens, leukocyte trafficking, and cytokine biosynthesis. As such, the combination of ICIs and cytotoxic chemotherapy are beginning to show promise in many cancers that have long been considered poorly responsive to ICI-based immunotherapy. Here, we discuss past and present attempts to advance chemo-immunotherapy in these difficult-to-treat cancer histologies, mechanisms through which select chemotherapies modify tumor immunogenicity, as well as important considerations when designing such approaches to maximize efficacy and improve therapeutic response rates.

The effect of low-dose chemotherapy on the tumor microenvironment and its antitumor activity combined with anti-PD-1 antibody

Aim: This study aimed to explore the effects of low-dose chemotherapy in the tumor microenvironment (TME) on a gastric cancer xenograft and its antitumor activity combined with the anti-PD-1 antibody. Materials & methods: Mice with gastric cancer were divided into four groups. The body weight and tumor volume of the mice were recorded. The TME was analyzed using flow cytometry. Results: Low-dose paclitaxel increased the PD-L1 expression level and the number of CD8+ T cells, but not the CD4+ T and myeloid-derived suppressor cells or PD-1+ CD8+ T cells in the TME. Low-dose 5-fluorouracil reduced the number of myeloid-derived suppressor cells and PD-1+ CD8+ T cells, but the PD-L1 expression level and the number of CD4+ T and CD8+ T cells did not change in the TME. The anti-PD-1 antibody inhibited tumor growth, but the combination therapy did not show superior antitumor activity. Conclusion: Low-dose chemotherapy altered the TME but failed to improve the responses to the anti-PD-1 antibody.

The interactions of paclitaxel with tumour microenvironment

Today, it is well-known that the interactions and secretion within the tumour are crucial to consider for cancer therapy. Some novel cancer therapy modalities such as immunotherapy or tumour vaccination therapy work based on the control of interactions within the tumour microenvironment (TME). It has been revealed that anti-cancer drugs or radiotherapy can modulate some interactions in favour of cancer therapy. However, they may induce some mechanisms to increase the resistance of cancer cells to therapy. Paclitaxel is known as the first approved herbal derived chemotherapy drug. Although the main known anti-cancer effect of paclitaxel is the inhibition of the cell cycle, today, it has been well known that paclitaxel may suppress the tumour via modulating several interactions in TME. Furthermore, paclitaxel may increase the expression of some tumour resistance drivers. This review aims to discuss the interactions within TME following treatment with paclitaxel. The effects of paclitaxel on the anti-tumour immunity, immunosuppressive cells, hypoxia, and also angiogenesis will be discussed. The targeting of these interactions may be interesting to increase therapy efficiency using the combination modalities.

Myeloid-Derived Suppressor Cells in Prostate Cancer: Present Knowledge and Future Perspectives

Several lines of research are being investigated to better understand mechanisms implicated in response or resistance to immune checkpoint blockade in prostate cancer (PCa). Myeloid-derived suppressor cells (MDSCs) have emerged as a major mediator of immunosuppression in the tumor microenvironment that promotes progression of various tumor types. The main mechanisms underlying MDSC-induced immunosuppression are currently being explored and strategies to enhance anti-tumor immune response via MDSC targeting are being tested. However, the role of MDSCs in PCa remains elusive. In this review, we aim to summarize and present the state-of-the-art knowledge on current methodologies to phenotypically and metabolically characterize MDSCs in PCa. We describe how these characteristics may be linked with MDSC function and may influence the clinical outcomes of patients with PCa. Finally, we briefly discuss emerging strategies being employed to therapeutically target MDSCs and potentiate the long-overdue improvement in the efficacy of immunotherapy in patients with PCa.

NK Cells in the Tumor Microenvironment as New Potential Players Mediating Chemotherapy Effects in Metastatic Melanoma

Until the last decade, chemotherapy was the standard treatment for metastatic cutaneous melanoma, even with poor results. The introduction of immune checkpoints inhibitors (ICIs) radically changed the outcome, increasing 5-year survival from 5% to 60%. However, there is still a large portion of unresponsive patients that would need further therapies. NK cells are skin-resident innate cytotoxic lymphocytes that recognize and kill virus-infected as well as cancer cells thanks to a balance between inhibitory and activating signals delivered by surface molecules expressed by the target. Since NK cells are equipped with cytotoxic machinery but lack of antigen restriction and needing to be primed, they are nowadays gaining attention as an alternative to T cells to be exploited in immunotherapy. However, their usage suffers of the same limitations reported for T cells, that is the loss of immunogenicity by target cells and the difficulty to penetrate and be activated in the suppressive tumor microenvironment (TME). Several evidence showed that chemotherapy used in metastatic melanoma therapy possess immunomodulatory properties that may restore NK cells functions within TME. Here, we will discuss the capability of such chemotherapeutics to: i) up-regulate melanoma cells susceptibility to NK cell-mediated killing, ii) promote NK cells infiltration within TME, iii) target other immune cell subsets that affect NK cells activities. Alongside traditional systemic melanoma chemotherapy, a new pharmacological strategy based on nanocarriers loaded with chemotherapeutics is developing. The use of nanotechnologies represents a very promising approach to improve drug tolerability and effectiveness thanks to the targeted delivery of the therapeutic molecules. Here, we will also discuss the recent developments in using nanocarriers to deliver anti-cancer drugs within the melanoma microenvironment in order to improve chemotherapeutics effects. Overall, we highlight the possibility to use standard chemotherapeutics, possibly delivered by nanosystems, to enhance NK cells anti-tumor cytotoxicity. Combined with immunotherapies targeting NK cells, this may represent a valuable alternative approach to treat those patients that do not respond to current ICIs.

Paclitaxel: Application in Modern Oncology and Nanomedicine-Based Cancer Therapy

Paclitaxel is a broad-spectrum anticancer compound, which was derived mainly from a medicinal plant, in particular, from the bark of the yew tree Taxus brevifolia Nutt. It is a representative of a class of diterpene taxanes, which are nowadays used as the most common chemotherapeutic agent against many forms of cancer. It possesses scientifically proven anticancer activity against, e.g., ovarian, lung, and breast cancers. The application of this compound is difficult because of limited solubility, recrystalization upon dilution, and cosolvent-induced toxicity. In these cases, nanotechnology and nanoparticles provide certain advantages such as increased drug half-life, lowered toxicity, and specific and selective delivery over free drugs. Nanodrugs possess the capability to buildup in the tissue which might be linked to enhanced permeability and retention as well as enhanced antitumour influence possessing minimal toxicity in normal tissues. This article presents information about paclitaxel, its chemical structure, formulations, mechanism of action, and toxicity. Attention is drawn on nanotechnology, the usefulness of nanoparticles containing paclitaxel, its opportunities, and also future perspective. This review article is aimed at summarizing the current state of continuous pharmaceutical development and employment of nanotechnology in the enhancement of the pharmacokinetic and pharmacodynamic features of paclitaxel as a chemotherapeutic agent.

Chemotherapeutic drugs: Cell death- and resistance-related signaling pathways. Are they really as smart as the tumor cells?

Chemotherapeutic drugs kill cancer cells or control their progression all over the patient's body, while radiation- and surgery-based treatments perform in a particular site. Based on their mechanisms of action, they are classified into different groups, including alkylating substrates, antimetabolite agents, anti-tumor antibiotics, inhibitors of topoisomerase I and II, mitotic inhibitors, and finally, corticosteroids. Although chemotherapeutic drugs have brought about more life expectancy, two major and severe complications during chemotherapy are chemoresistance and tumor relapse. Therefore, we aimed to review the underlying intracellular signaling pathways involved in cell death and resistance in different chemotherapeutic drug families to clarify the shortcomings in the conventional single chemotherapy applications. Moreover, we have summarized the current combination chemotherapy applications, including numerous combined-, and encapsulated-combined-chemotherapeutic drugs. We further discussed the possibilities and applications of precision medicine, machine learning, next-generation sequencing (NGS), and whole-exome sequencing (WES) in promoting cancer immunotherapies. Finally, some of the recent clinical trials concerning the application of immunotherapies and combination chemotherapies were included as well, in order to provide a practical perspective toward the future of therapies in cancer cases.

VLA-1 Binding to Collagen IV Controls Effector T Cell Suppression by Myeloid-Derived Suppressor Cells in the Splenic Red Pulp

Myeloid-derived suppressor cells (MDSCs) represent a major population controlling T cell immune responses. However, little is known about their molecular requirements for homing and T cell interaction to mediate suppression. Here, we investigated the functional role of the homing and collagen IV receptor VLA-1 (α1β1-integrin) on in vitro GM-CSF generated murine MDSCs from wild-type (WT) and CD49a/α1-integrin (Itga1^−/−) gene-deficient mice. Here, we found that effector (Teff) but not naive (Tn) CD4⁺ T cells express VLA-1 and monocytes further up-regulated their expression after culture in GM-CSF when they differentiated into the monocytic subset of resting MDSCs (R-MDSCs). Subsequent activation of R-MDSCs by LPS+IFN-γ (A-MDSCs) showed increased in vitro suppressor potential, which was independent of VLA-1. Surprisingly, VLA-1 deficiency did not influence A-MDSC motility or migration on collagen IV in vitro. However, interaction times of Itga1^−/− A-MDSCs with Teff were shorter than with WT A-MDSCs on collagen IV but not on fibronectin substrate in vitro. After injection, A-MDSCs homed to the splenic red pulp where they co-localized with Teff and showed immediate suppression already after 6 h as shown by inhibition of T cell proliferation and induction of apoptosis. Injection of A-MDSCs from Itga1^−/− mice showed equivalent homing into the spleen but a reduced suppressive effect. Interaction studies of A-MDSCs with Teff in the subcapsular red pulp with intravital two-photon microscopy revealed also here that MDSC motility and migration parameters were not altered by VLA-1 deficiency, but the interaction times with Teff were reduced. Together, our data point to a new role of VLA-1 adhesion to collagen IV as a prerequisite for extended contact times with Teff required for suppression.

Repurposing Food and Drug Administration-Approved Drugs to Promote Antitumor Immunity

There has been a major resurgence of interest in immune-based approaches to treat cancer, based largely on the success of checkpoint inhibitors (anti-cytotoxic T-lymphocyte-associated antigen 4, anti-programmed cell death 1, and anti-programmed cell death ligand 1 antibodies) in several malignancies. However, not all tumors respond to checkpoint therapy, and there is clearly a need for additional approaches for enhancing tumor immunity. We summarize the critical elements necessary for mounting an efficacious T-cell response to a tumor. We cite drugs approved by the Food and Drug Administration for no-cancer indications that could be repurposed and used as part of an antitumor immune cocktail. We also list cancer drugs not initially intended to impact tumor immunity (soft repurposing) but that have been found to modulate the immune system. We highlight those drugs that might be used in combination with checkpoint inhibitors to increase response rates and survival of cancer patients. Our focus will be on drugs for which there are limited but existing human data. We cite supporting mechanistic mouse data as well. Repurposing drugs to modulate antitumor immunity is an opportunity to rapidly bring new, effective, and affordable treatments to cancer patients.

Poly(cyclodextrin)-Polydrug Nanocomplexes as Synthetic Oncolytic Virus for Locoregional Melanoma Chemoimmunotherapy

Despite the approval of oncolytic virus therapy for advanced melanoma, its intrinsic limitations that include the risk of persistent viral infection and cost-intensive manufacturing motivate the development of analogous approaches that are free from the disadvantages of virus-based therapies. Herein, we report a nanoassembly comprised of multivalent host-guest interactions between polymerized paclitaxel (pPTX) and nitric oxide incorporated polymerized β-cyclodextrin (pCD-pSNO) that through its bioactive components and when used locoregionally recapitulates the therapeutic effects of oncolytic virus. The resultant pPTX/pCD-pSNO exhibits significantly enhanced cytotoxicity, immunogenic cell death, dendritic cell activation and T cell expansion in vitro compared to free agents alone or in combination. In vivo, intratumoral administration of pPTX/pCD-pSNO results in activation and expansion of dendritic cells systemically, but with a corresponding expansion of myeloid-derived suppressor cells and suppression of CD8+ T cell expansion. When combined with antibody targeting cytotoxic T lymphocyte antigen-4 that blunts this molecule's signaling effects on T cells, intratumoral pPTX/pCD-pSNO treatment elicits potent anticancer effects that significantly prolong animal survival. This formulation thus leverages the chemo- and immunotherapeutic synergies of paclitaxel and nitric oxide and suggests the potential for virus-free nanoformulations to mimic the therapeutic action and benefits of oncolytic viruses.

Minimal residual disease in advanced or metastatic solid cancers: The G0-G1 state and immunotherapy are key to unwinding cancer complexity

In the last decade, a large amount of research has focused on elucidating the mechanisms that account for homing disseminated cancer cells (DCCs) from solid tumours to distant organs, which successively progress to overt metastatic disease; this is currently incurable. A better understanding of DCC behaviour is expected to allow detectable metastasis prevention by more effectively targeting 'metastatic seeds before they sprout'. As DCC biology co-evolved with that of the primary tumour, and due to the many similarities between them, the term 'niche' has been borrowed from normal adult stem cells (ASCs) to define the site of DCC metastatic colonisation. Moreover, heterogeneity, survival, protection, stemness and plasticity as well as the prolonged G0-G1 dormant state in the metastatic niche have been the main aspects of intense investigation. Consistent with these findings, in solid cancers with minimal residual disease (MRD), it has been proposed to prolong adjuvant therapy by targeting specific molecular pathway(s) involving DCC dormancy. However, so far, few disappointing clinical data have been reported. As an alternative strategy, because immune-surveillance contributes to the steady state of the DCC population and likely to the G0-G1 state of cancer cells, we have used prolonged immune-modulatory cytostatic chemotherapy, active immune stimulation with an INF-β/IL-2 sequence or drugs inhibiting myeloid-derived suppressor cell (MDSC)/Treg-mediated immune suppression. This strategy, mainly aimed at boosting the immune response, is based on recent findings suggesting the downregulation of immune escape mechanisms as well as other principal hallmarks during the G0-G1 state and/or in MRD. Preliminary clinical and/or laboratory data suggest the efficacy of this strategy in gastrointestinal and some endocrine-dependent cancers. Following this, we propose therapeutic schedules to prevent DCC activation and proliferation in solid cancers at a high risk of relapse or as maintenance therapy in metastatic patients after complete response (CR) to conventional treatment.

Low-dose docetaxel enhances the anti-tumour efficacy of a human umbilical vein endothelial cell vaccine

Our previous studies have indicated that human umbilical vein endothelial cell (HUVEC) vaccination appears to be a potentially promising anti-angiogenesis therapy, but the modest therapeutic anti-tumour efficiency limits its clinical use. This highlights the importance of identifying more potent therapeutic HUVEC vaccine strategies for clinical testing. In the present study, the immune-modulating doses of docetaxel (DOC) was combined with 1 × 10⁶ viable HUVECs as a means to enhance the therapeutic anti-tumour efficiency of the HUVEC vaccine. Our results demonstrated that 5 mg/kg DOC administrated prior to HUVEC vaccine could most effectively assist HUVEC vaccine to display a remarkable suppression of tumour growth and metastasis as wells as a prolongation of survival time in a therapeutic procedure. CD31 immunohistochemical analysis of the excised tumours confirmed a significant reduction in vessel density after treatment with the HUVEC vaccine with 5 mg/kg DOC. Additionally, an increased HUVEC-specific antibody level, activated CTLs and an elevated IFN-γ level in cultured splenocytes were revealed after treatment with HUVEC vaccine with 5 mg/kg DOC. Finally, 5 mg/kg DOC coupled with the HUVEC vaccine led to induction of significant increases in CD8⁺T cells and decrease in Tregs in the tumour microenvironment. Taken together, all the results verified that 5 mg/kg DOC could assist HUVEC vaccine to elicit strong HUVEC specific humoral and cellular responses, which could facilitate the HUVEC vaccine-mediated inhibition of cancer growth and metastasis. These findings provide the immunological rationale for the combined use of immune-modulating doses of DOC and HUVEC vaccines in patients with cancer.

Cancer DNA vaccines: current preclinical and clinical developments and future perspectives

The recent developments in immuno-oncology have opened an unprecedented avenue for the emergence of vaccine strategies. Therapeutic DNA cancer vaccines are now considered a very promising strategy to activate the immune system against cancer. In the past, several clinical trials using plasmid DNA vaccines demonstrated a good safety profile and the activation of a broad and specific immune response. However, these vaccines often demonstrated only modest therapeutic effects in clinical trials due to the immunosuppressive mechanisms developed by the tumor. To enhance the vaccine-induced immune response and the treatment efficacy, DNA vaccines could be improved by using two different strategies. The first is to increase their immunogenicity by selecting and optimizing the best antigen(s) to be inserted into the plasmid DNA. The second strategy is to combine DNA vaccines with other complementary therapies that could improve their activity by attenuating immunosuppression in the tumor microenvironment or by increasing the activity/number of immune cells. A growing number of preclinical and clinical studies are adopting these two strategies to better exploit the potential of DNA vaccination. In this review, we analyze the last 5-year preclinical studies and 10-year clinical trials using plasmid DNA vaccines for cancer therapy. We also investigate the strategies that are being developed to overcome the limitations in cancer DNA vaccination, revisiting the rationale for different combinations of therapy and the different possibilities in antigen choice. Finally, we highlight the most promising developments and critical points that need to be addressed to move towards the approval of therapeutic cancer DNA vaccines as part of the standard of cancer care in the future.

Augmenting the synergies of chemotherapy and immunotherapy through drug delivery

Despite the recent approvals of multiple cancer immunotherapies, low tumor immunogenicity and immunosuppressive tumor microenvironments prevent a large portion of patients from responding to these treatment modalities. Given the immunomodulatory and adjuvant effects of conventional chemotherapy as well as its widespread clinical use, the use of chemotherapy in combination with immunotherapy (so-called chemoimmunotherapy) is an attractive approach to potentiate the effects of immunotherapy in more patient populations. However, due to the limited extent of tumor accumulation, poorly controlled interactions with the immune system, and effects on systemic healthy tissues by chemotherapeutic drugs, the incorporation of anti-cancer agents into biomaterial-based structures, such as nanocarriers, is highly attractive to improve the safety and efficacy of chemoimmunotherapy. Herein, we review the recent progress in drug delivery systems (DDSs) for potentiating the immunomodulatory effects of chemotherapeutics in chemoimmunotherapy, which represent among the most promising next generation strategies for cancer treatment in the immunotherapy era. STATEMENT OF SIGNIFICANCE: Given the benefits of cancer immunotherapy in inducing durable, albeit low rates, of patient response, interest in the immunomodulatory and adjuvant effects of conventional chemotherapy has been re-invigorated. This review article discusses the recent progress towards understanding the synergies between these two treatment types, how they can be used in combination (so-called chemoimmunotherapy), and the potential for drug delivery systems to optimize their effects in translational settings.

Tumour growth and immune evasion as targets for a new strategy in advanced cancer

It has become clearer that advanced cancer, especially advanced breast cancer, is an entirely displayed pathological system that is much more complex than previously considered. However, the direct relationship between tumour growth and immune evasion can represent a general rule governing the pathological cancer system from the initial cancer cells to when the system is entirely displayed. Accordingly, a refined pathobiological model and a novel therapeutic strategy are proposed. The novel therapeutic strategy is based on therapeutically induced conditions (undetectable tumour burden and/or a prolonged tumour ‘resting state’), which enable an efficacious immune response in advanced breast and other types of solid cancers.

Optimized dendritic cell vaccination induces potent CD8 T cell responses and anti-tumor effects in transgenic mouse melanoma models

Despite melanoma immunogenicity and remarkable therapeutic effects of negative immune checkpoint inhibitors, a significant fraction of patients does not respond to current treatments. This could be due to limitations in tumor immunogenicity and profound immunosuppression in the melanoma microenvironment. Moreover, insufficient tumor antigen processing and presentation by dendritic cells (DC) may hamper the development of tumor-specific T cells. Using two genetically engineered mouse melanoma models (RET and BRAF^V600E transgenic mice), in which checkpoint inhibitor therapy alone is not efficacious, we performed proof-of-concept studies with an improved, multivalent DC vaccination strategy based on our recently developed genetic mRNA cancer vaccines. The in vivo expression of multiple chimeric MHC class I receptors allows a simultaneous presentation of several melanoma-associated shared antigens tyrosinase related protein (TRP)-1, tyrosinase, human glycoprotein 100 and TRP-2. The DC vaccine induced a significantly improved survival in both transgenic mouse models. Vaccinated melanoma-bearing mice displayed an increased CD8 T cell reactivity indicated by a higher IFN-γ production and an upregulation of activation marker expression along with an attenuated immunosuppressive pattern of myeloid-derived suppressor cells (MDSC) and regulatory T cells (Treg). The combination of DC vaccination with ultra-low doses of paclitaxel or anti-PD-1 antibodies resulted in further prolongation of mouse survival associated with a stronger reduction of MDSC and Treg immunosuppressive phenotype. Our data suggest that an improved multivalent DC vaccine based on shared tumor antigens induces potent anti-tumor effects and could be combined with checkpoint inhibitors or targeting immunosuppressive cells to further improve their therapeutic efficiency.

Evaluation of the cytotoxicity of the Bithionol-paclitaxel combination in a panel of human ovarian cancer cell lines

Previously, Bithionol (BT) was shown to enhance the chemosensitivity of ovarian cancer cell lines to cisplatin treatment. In the present study, we focused on the anti-tumor potential of the BT-paclitaxel combination when added to a panel of ovarian cancer cell lines. This in vitro study aimed to 1) determine the optimum schedule for combination of BT and paclitaxel and 2) assess the nature and mechanism(s) underlying BT-paclitaxel interactions. The cytotoxic effects of both drugs either alone or in combination were assessed by presto-blue cell viability assay using six human ovarian cancer cell lines. Inhibitory concentrations to achieve 50% cell death (IC₅₀) were determined for BT and paclitaxel in each cell line. Changes in levels of cleaved PARP, XIAP, bcl-2, bcl-xL, p21 and p27 were determined via immunoblot. Luminescent and colorimetric assays were used to determine caspases 3/7 and autotaxin (ATX) activity. Cellular reactive oxygen species (ROS) were measured by flow cytometry. Our results show that the efficacy of the BT-paclitaxel combination depends upon the concentrations and sequence of addition of paclitaxel and BT. Pretreatment with BT followed by paclitaxel resulted in antagonistic interactions whereas synergistic interactions were observed when both drugs were added simultaneously or when cells were pretreated with paclitaxel followed by BT. Synergistic interactions between BT and paclitaxel were attributed to increased ROS generation and enhanced apoptosis. Decreased expression of pro-survival factors (XIAP, bcl-2, bcl-xL) and increased expression of pro-apoptotic factors (caspases 3/7, PARP cleavage) was observed. Additionally, increased expression of key cell cycle regulators p21 and p27 was observed. These results show that BT and paclitaxel interacted synergistically at most drug ratios which, however, was highly dependent on the sequence of the addition of drugs. Our results suggest that BT-paclitaxel combination therapy may be effective in sensitizing ovarian cancer cells to paclitaxel treatment, thus mitigating some of the toxic effects associated with high doses of paclitaxel.

In vivo amelioration of endogenous antitumor autoantibodies via low-dose P4N through the LTA4H/activin A/BAFF pathway

Cancer progression is associated with the development of antitumor autoantibodies in patients' sera. Although passive treatment with antitumor antibodies has exhibited remarkable therapeutic efficacy, inhibitory effects on tumor progression by endogenous antitumor autoantibodies (EAAs) have been limited. In this study, we show that P₄N, a derivative of the plant lignan nordihydroguaiaretic acid (NDGA), enhanced the production of EAAs and inhibited tumor growth at low noncytotoxic concentrations via its immunoregulatory activity. Intratumoral injection of P₄N improved the quantity and quality of EAAs, and passive transfer of P₄N-induced EAAs dramatically suppressed lung metastasis formation and prolonged the survival of mice inoculated with metastatic CT26 tumor cells. P₄N-induced EAAs specifically recognized two surface antigens, 78-kDa glucose-regulated protein (GRP78) and F1F0 ATP synthase, on the plasma membrane of cancer cells. Additionally, P₄N treatment led to B-cell proliferation, differentiation to plasma cells, and high titers of autoantibody production. By serial induction of autocrine and paracrine signals in monocytes, P₄N increased B-cell proliferation and antibody production via the leukotriene A4 hydrolase (LTA4H)/activin A/B-cell activating factor (BAFF) pathway. This mechanism provides a useful platform for studying and seeking a novel immunomodulator that can be applied in targeting therapy by improving the quantity and quality of the EAAs.

An immunostimulatory dual-functional nanocarrier that improves cancer immunochemotherapy

Immunochemotherapy combines a chemotherapeutic agent with an immune-modulating agent and represents an attractive approach to improve cancer therapy. However, the success of immunochemotherapy is hampered by the lack of a strategy to effectively co-deliver the two therapeutics to the tumours. Here we report the development of a dual-functional, immunostimulatory nanomicellar carrier that is based on a prodrug conjugate of PEG with NLG919, an indoleamine 2,3-dioxygenase (IDO) inhibitor currently used for reversing tumour immune suppression. An Fmoc group, an effective drug-interactive motif, is also introduced into the carrier to improve the drug loading capacity and formulation stability. We show that PEG_2k-Fmoc-NLG alone is effective in enhancing T-cell immune responses and exhibits significant antitumour activity in vivo. More importantly, systemic delivery of paclitaxel (PTX) using the PEG_2k-Fmoc-NLG nanocarrier leads to a significantly improved antitumour response in both breast cancer and melanoma mouse models.

The use of immunostimulatory agents to enhance the efficacy of chemotherapy is a promising strategy in cancer therapy. Here, the authors report on a micellar nanoparticle that can effectively co-deliver chemo- and immunotherapeutics, resulting in an improved in vivo antitumour response.

The Role of Myeloid-Derived Suppressor Cells (MDSC) in Cancer Progression

The immunosuppressive tumor microenvironment represents not only one of the key factors stimulating tumor progression but also a strong obstacle for efficient tumor immunotherapy. Immunosuppression was found to be associated with chronic inflammatory mediators including cytokines, chemokines and growth factors produced by cancer and stroma cells. Long-term intensive production of these factors induces the formation of myeloid-derived suppressor cells (MDSCs) representing one of the most important players mediating immunosuppression. Moreover, MDSCs could not only inhibit anti-tumor immune reactions but also directly stimulate tumor growth and metastasis. Therefore, understanding the mechanisms of their generation, expansion, recruitment and activation is required for the development of novel strategies for tumor immunotherapy.

Paclitaxel and Its Evolving Role in the Management of Ovarian Cancer

Paclitaxel, a class of taxane with microtubule stabilising ability, has remained with platinum based therapy, the standard care for primary ovarian cancer management. A deeper understanding of the immunological basis and other potential mechanisms of action together with new dosing schedules and/or routes of administration may potentiate its clinical benefit. Newer forms of taxanes, with better safety profiles and higher intratumoural cytotoxicity, have yet to demonstrate clinical superiority over the parent compound.

Paclitaxel and Its Evolving Role in the Management of Ovarian Cancer

Paclitaxel, a class of taxane with microtubule stabilising ability, has remained with platinum based therapy, the standard care for primary ovarian cancer management. A deeper understanding of the immunological basis and other potential mechanisms of action together with new dosing schedules and/or routes of administration may potentiate its clinical benefit. Newer forms of taxanes, with better safety profiles and higher intratumoural cytotoxicity, have yet to demonstrate clinical superiority over the parent compound.

Paclitaxel and Its Evolving Role in the Management of Ovarian Cancer

Paclitaxel, a class of taxane with microtubule stabilising ability, has remained with platinum based therapy, the standard care for primary ovarian cancer management. A deeper understanding of the immunological basis and other potential mechanisms of action together with new dosing schedules and/or routes of administration may potentiate its clinical benefit. Newer forms of taxanes, with better safety profiles and higher intratumoural cytotoxicity, have yet to demonstrate clinical superiority over the parent compound.

Myeloid-derived suppressor cells in malignant melanoma

Melanoma is known for its rapid progression, metastasis to distant organs and therapeutic resistance. Despite high melanoma immunogenicity, the results of immunotherapeutic clinical studies are mostly unsatisfactory. One explanation is the development of strong immunosuppression mediated by highly immunosuppressive regulatory leukocytes, in particular, myeloid-derived suppressor cells (MDSCs). These cells were found to be enriched and activated in the melanoma microenvironment, inducing a profound impairment of anti-tumor immune responses and leading to the tumor progression. Therefore, understanding the mechanisms of MDSC generation, migration to the tumor site and activation as well as their targeting is important for the development of novel strategies for effective melanoma immunotherapy. We suggest that such therapeutic approaches should involve the inhibition of MDSC-mediated immunosuppressive melanoma microenvironment combined with other immunologic treatments.

Hampering immune suppressors: therapeutic targeting of myeloid-derived suppressor cells in cancer

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of immature myeloid cells with suppressive properties that preferentially expand in cancer. Myeloid-derived suppressor cells mainly suppress T-cell proliferation and cytotoxicity, inhibit natural killer cell activation, and induce the differentiation and expansion of regulatory T cells. The wide spectrum of MDSC suppressive activity in cancer and its role in tumor progression have rendered these cells a promising target for effective cancer immunotherapy. In this review we briefly discuss the origin of MDSCs and their main mechanisms of suppression and focus more on the approaches developed up to date targeting of MDSCs in tumor-bearing animals and cancer patients.

Repurposing Drugs in Oncology (ReDO)-mebendazole as an anti-cancer agent

Mebendazole, a well-known anti-helminthic drug in wide clinical use, has anti-cancer properties that have been elucidated in a broad range of pre-clinical studies across a number of different cancer types. Significantly, there are also two case reports of anti-cancer activity in humans. The data are summarised and discussed in relation to suggested mechanisms of action. Based on the evidence presented, it is proposed that mebendazole would synergise with a range of other drugs, including existing chemotherapeutics, and that further exploration of the potential of mebendazole as an anti-cancer therapeutic is warranted. A number of possible combinations with other drugs are discussed in the Appendix.

Paclitaxel: new uses for an old drug

Paclitaxel (Taxol), one of the most important anticancer drugs, has been used for therapy of different types of cancers. Mechanistically, paclitaxel arrests cell cycle and induces cell death by stabilizing microtubules and interfering with microtubule disassembly in cell division. Recently, it has been found that low-dose paclitaxel seems promising in treating non-cancer diseases, such as skin disorders, renal and hepatic fibrosis, inflammation, axon regeneration, limb salvage, and coronary artery restenosis. Future studies need to understand the mechanisms underlying these effects in order to design therapies with specificity.

Immunological Mechanisms of Low and Ultra-Low Dose Cancer Chemotherapy

Traditionally, anticancer chemotherapy has been generally considered to be strongly immunosuppressive. However, increasing evidence suggests that certain chemotherapeutic agents rely on the induction of antitumor immune responses, in both experimental animal models and patients with cancer. Many of these chemotherapeutic agents exert immunogenic effects via the induction and release of immunostimulatory "danger" signals from dying cancerous cells when used in low doses. New data suggests that several common chemotherapeutic agents may also display direct stimulating effects on immune cells even when applied in ultra-low concentrations (chemoimmunomodulation). Importantly, it is becoming clear that both immune effector cells and immune regulatory cells can be targeted by various chemotherapeutic agents to produce favorable antitumor immune responses. Therefore, utilizing cancer drugs to enhance host antitumor immunity should be considered a feasible therapeutic approach; and recent characterization of the immunomodulatory mechanisms of anticancer chemotherapy using both new and traditional cytotoxic agents suggests that combinations of these approaches with "classical" immunomodulatory agents could lead to a viable new therapeutic paradigm for the treatment of cancer.

The significant increase and dynamic changes of the myeloid-derived suppressor cells percentage with chemotherapy in advanced NSCLC patients

PURPOSE

To investigate the correlations between myeloid-derived suppressor cells (MDSCs) in the peripheral blood and cancer stage, immune function, and chemotherapy.

METHODS

Percentages of MDSCs (CD11b(+)CD14(-)CD33(+) cells) and lymphocyte subsets in peripheral blood mononuclear cells (PBMCs) of 94 patients with Non-small cell lung cancer (NSCLC) who were treated naïve and 30 healthy individuals were measured. Changes of the MDSCs percentage were further detected in patients with advanced NSCLC treated with systemic chemotherapy. Finally, coculture with CD8(+) cells was developed to determine effect of MDSCs on IFN-γ secretion of T lymphocytes.

RESULTS

MDSCs percentage of 94 patients with NSCLC was significantly higher than that of 30 healthy subjects (P < 0.05), the percentages were increased with tumor progression, in patients with stage III and IV percentages were significantly higher than those in stage I and II patients (P = 0.013). The MDSCs percentage was negatively related to percentage of CD8(+) cells in the peripheral blood (r = -0.354, n = 38, P = 0.029), and when they were cocultured, IFN-γ secretion of CD8(+) cells was significantly decreased (P < 0.05). In 20 patients with advanced NSCLC who received systemic chemotherapy, nine partial remission (PR) cases got MDSCs percentage significantly decreased (P < 0.001), three stable disease (SD) cases remained invariable (P = 0.307) and eight progressive disease (PD) cases got significantly increased (P = 0.024).

CONCLUSION

The percentage of MDSCs in the patients was significantly higher than that of the healthy control subjects and it increased with tumor progression partially by inhibiting the CD8(+) cell function. The dynamic changes of MDSCs percentage reflected the efficacy of systemic chemotherapy.

Prospects of combinatorial synthetic peptide vaccine-based immunotherapy against cancer

The insight that the immune system is involved in tumor resistance is gaining momentum and this has led to the development of immunotherapeutic strategies aiming at enhancement of immune-mediated tumor destruction. Although some of these strategies have moderate clinical benefit, most stand-alone therapies fail to significantly affect progressive disease and survival or do so only in a minority of patients. Research on the mechanisms underlying the generation of immune responses against tumors and the immune evasion by tumors has emphasized that various mechanisms simultaneously prevent effective immunity against cancer including inefficient presentation of tumor antigens by dendritic cells and induction of negative immune regulation by regulatory T-cells (Tregs) and myeloid derived suppressor cells (MDSCs). Thus the design of therapies that simultaneously improve effective tumor immunity and counteract immune evasion by tumors seems most desirable for clinical efficacy. As it is unlikely that a single immunotherapeutic strategy addresses all necessary requirements, combinatorial strategies that act synergistically need to be developed. Here we discuss the current knowledge and prospects of treatment with synthetic peptide vaccines that stimulate tumor-specific T-cell responses combined with adjuvants, immune modulating antibodies, cytokines and chemotherapy. We conclude that combinatorial approaches have the best potency to accomplish the most significant tumor destruction but further research is required to optimize such approaches.

The role of TLR4 in the paclitaxel effects on neuronal growth in vitro

Paclitaxel (Pac) is an antitumor agent that is widely used for treatment of solid cancers. While being effective as a chemotherapeutic agent, Pac in high doses is neurotoxic, specifically targeting sensory innervations. In view of these toxic effects associated with conventional chemotherapy, decreasing the dose of Pac has been recently suggested as an alternative approach, which might limit neurotoxicity and immunosuppression. However, it remains unclear if low doses of Pac retain its neurotoxic properties or might exhibit unusual effects on neuronal cells. The goal of this study was to analyze the concentration-dependent effect of Pac on isolated and cultured DRG neuronal cells from wild-type and TLR4 knockout mice. Three different morphological parameters were analyzed: the number of neurons which developed neurites, the number of neurites per cell and the total length of neurites per cell. Our data demonstrate that low concentrations of Pac (0.1 nM and 0.5 nM) do not influence the neuronal growth in cultures in both wild type and TLR4 knockout mice. Higher concentrations of Pac (1–100 nM) had a significant effect on DRG neurons from wild type mice, affecting the number of neurons which developed neurites, number of neurites per cell, and the length of neurites. In DRG from TLR4 knockout mice high concentrations of Pac showed a similar effect on the number of neurons which developed neurites and the length of neurites. At the same time, the number of neurites per cell, indicating the process of growth cone initiation, was not affected by high concentrations of Pac. Thus, our data showed that Pac in high concentrations has a significant damaging effect on axonal growth and that this effect is partially mediated through TLR4 pathways. Low doses of Pac are devoid of neuronal toxicity and thus can be safely used in a chemomodulation mode.

Antitumor effect of paclitaxel is mediated by inhibition of myeloid-derived suppressor cells and chronic inflammation in the spontaneous melanoma model

The antitumor effects of paclitaxel are generally attributed to the suppression of microtubule dynamics resulting in defects in cell division. New data demonstrated that in ultralow noncytotoxic concentrations, paclitaxel modulated in immune cells in vitro the activity of small Rho GTPases, the key regulators of intracellular actin dynamics. However, the immunomodulatory properties of paclitaxel in vivo have not been evaluated. In this study, using the ret transgenic murine melanoma model, which mimics human cutaneous melanoma, we tested effects of ultralow noncytotoxic dose paclitaxel on functions of myeloid-derived suppressor cells (MDSCs), chronic inflammatory mediators, and T cell activities in the tumor microenvironment in vivo. Administration of paclitaxel significantly decreased accumulation and immunosuppressive activities of tumor-infiltrating MDSCs without alterations of the bone marrow hematopoiesis. This was associated with the inhibition of p38 MAPK activity, TNF-α and production, and S100A9 expression in MDSCs. The production of mediators of chronic inflammation in the tumor milieu also was diminished. Importantly, reduced tumor burden and increased animal survival upon paclitaxel application was mediated by the restoration of CD8 T cell effector functions. We suggest that the ability of paclitaxel in a noncytotoxic dose to block the immunosuppressive potential of MDSCs in vivo represents a new therapeutic strategy to downregulate immunosuppression and chronic inflammation in the tumor microenvironment for enhancing the efficacy of concomitant anticancer therapies.

Tumor microenvironment and myeloid-derived suppressor cells

Tumor progression has been demonstrated to be supported by chronic inflammatory conditions developed in the tumor microenvironment and characterized by the long-term secretion of various inflammatory soluble factors (including cytokines, chemokines, growth factors, reactive oxygen and nitrogen species, prostaglandins etc.) and strong leukocyte infiltration. Among leukocytes infiltrating tumors, myeloid-derived suppressor cells (MDSCs) represent one of the most important players mediating immunosuppression. These cells may not only strongly inhibit an anti-tumor immune reactions mediated by T cells but also directly stimulate tumorigenesis, tumor growth and metastasis by enhancing neoangiogenesis and creating a suitable environment for the metastatic formation. This review provides an overview of interactions between MDSCs and tumor cells leading to MDSC generation, activation and migration to the tumor site, where they can strongly enhance tumor progression. Better understanding of the MDSC-tumor interplay is critical for the development of new strategies of tumor immunotherapy.

Effective cooperation of monoclonal antibody and peptide vaccine for the treatment of mouse melanoma

mAbs binding to tumor-associated surface Ags are therapeutically applied in a range of malignancies. Therapeutic vaccination only recently met with clinical success, and the first cancer vaccine received U.S. Food and Drug Administration approval last year. To improve current protocols, we combined peptide vaccines with mAb to the tyrosinase-related protein (TRP)-1 surface Ag for the treatment of B16F10 skin melanoma. Vaccine formulations with synthetic long peptides failed to elicit strong CD8 T cell responses to self-differentiation Ags gp100 and TRP-2, whereas altered peptide sequences recruited gp100-specific CD8 T cells from the endogenous repertoire with frequencies of 40%. However, these high frequencies were reached too late; large, progressively growing melanomas had already emerged. Addition of the TRP-1-directed mAb TA99 to the treatment protocol mediated eradication of s.c. lesions. The mode of action of the Ab did not depend on complement factor C3 and did not lead to improved Ag presentation and CD8 T cell immunity; rather, it recruited FcγR-bearing innate immune cells during early tumor control, thereby creating a window of time for the generation of protective cellular immunity. These data support the concept of combination therapy, in which passive transfer of mAbs is supplemented with cancer peptide vaccines. Moreover, we advocate that tumor Ag-specific T cell immunity directed against self-proteins can be exploited from the endogenous repertoire.

Myeloid-derived suppressor cells interact with tumors in terms of myelopoiesis, tumorigenesis and immunosuppression: thick as thieves

Tumor progression is often associated with chronic inflammation in the tumor microenvironment, which is mediated by numerous cytokines, chemokines and growth factors produced by cancer and stroma cells. All these mediators support tumor development and immunosuppression in autocrine and/or paracrine ways. Neutralization of chronic inflammatory conditions can lead to the restoration of anti-tumor immune responses. Among stroma cells infiltrating tumors, myeloid-derived suppressor cells (MDSCs) represent one of the most important players mediating immunosuppression. These cells may not only inhibit an anti-tumor immunity but also directly stimulate tumorigenesis as well as tumor growth and expansion. Therefore, understanding the mechanisms of generation, migration to the tumor site and activation of MDSC is necessary for the development of new strategies of tumor immunotherapy.

Immunomodulatory effects of low dose chemotherapy and perspectives of its combination with immunotherapy

Given that cancer is one of the main causes of death worldwide, many efforts have been directed toward discovering new treatments and approaches to cure or control this group of diseases. Chemotherapy is the main treatment for cancer; however, a conventional schedule based on maximum tolerated dose (MTD) shows several side effects and frequently allows the development of drug resistance. On the other side, low dose chemotherapy involves antiangiogenic and immunomodulatory processes that help host to fight against tumor cells, with lower grade of side effects. In this review, we present evidence that metronomic chemotherapy, based on the frequent administration of low or intermediate doses of chemotherapeutics, can be better than or as efficient as MTD. Finally, we present some data indicating that noncytotoxic concentrations of antineoplastic agents are able to both up-regulate the immune system and increase the susceptibility of tumor cells to cytotoxic T lymphocytes. Taken together, data from the literature provides us with sufficient evidence that low concentrations of selected chemotherapeutic agents, rather than conventional high doses, should be evaluated in combination with immunotherapy.

Paclitaxel promotes differentiation of myeloid-derived suppressor cells into dendritic cells in vitro in a TLR4-independent manner

Myeloid cells play a key role in the outcome of anti-tumor immunity and response to anti-cancer therapy, since in the tumor microenvironment they may exert both stimulatory and inhibitory pressures on the proliferative, angiogenic, metastatic, and immunomodulating potential of tumor cells. Therefore, understanding the mechanisms of myeloid regulatory cell differentiation is critical for developing strategies for the therapeutic reversal of myeloid derived suppressor cell (MDSC) accumulation in the tumor-bearing hosts. Here, using an in vitro model system, several potential mechanisms of the direct effect of paclitaxel on MDSC were tested, which might be responsible for the anti-tumor potential of low-dose paclitaxel therapy in mice. It was hypothesized that a decreased level of MDSC in vivo after paclitaxel administration might be due to (i) the blockage of MDSC generation, (ii) an induction of MDSC apoptosis, or (iii) the stimulation of MDSC differentiation. The results revealed that paclitaxel in ultra-low concentrations neither increased MDSC apoptosis nor blocked MDSC generation, but stimulated MDSC differentiation towards dendritic cells. This effect of paclitaxel was TLR4-independent since it was not diminished in cell cultures originated from TLR4-/- mice. These results support a new concept that certain chemotherapeutic agents in ultra-low non-cytotoxic doses may suppress tumor progression by targeting several cell populations in the tumor microenvironment, including MDSC.