Embryonic macrophages function during early life to determine invariant natural killer T cell levels at barrier surfaces.

It is increasingly recognized that immune development within mucosal tissues is under the control of environmental factors during early life. However, the cellular mechanisms that underlie such temporally and regionally restrictive governance of these processes are unclear. Here, we uncover an extrathymic pathway of immune development within the colon that is controlled by embryonic but not bone marrow-derived macrophages, which determines the ability of these organs to receive invariant natural killer T (iNKT) cells and allow them to establish local residency. Consequently, early-life perturbations of fetal-derived macrophages result in persistent decreases of mucosal iNKT cells and is associated with later-life susceptibility or resistance to iNKT cell-associated mucosal disorders. These studies uncover a host developmental program orchestrated by ontogenically distinct macrophages that is regulated by microbiota, and they reveal an important postnatal function of macrophages that emerge in fetal life.

Phase II study of E7777 in Japanese patients with relapsed/refractory peripheral and cutaneous T-cell lymphoma.

E7777 is a recombinant cytotoxic fusion protein composed of the diphtheria toxin fragments A and B and human interleukin-2. It shares an amino acid sequence with denileukin diftitox, but has improved purity and an increased percentage of active monomer. We undertook a multicenter, single-arm phase II study of E7777 in patients with relapsed or refractory peripheral T-cell lymphoma (PTCL) and cutaneous T-cell lymphoma (CTCL) to evaluate its efficacy, safety, pharmacokinetics, and immunogenicity. A total of 37 patients were enrolled, of which 17 and 19 patients had PTCL and CTCL, respectively, and one patient with another type of lymphoma (extranodal natural killer/T-cell lymphoma, nasal type), diagnosed by the Central Pathological Diagnosis Committee. Among the 36 patients with PTCL and CTCL, objective response rate based on the independent review was 36% (41% and 31%, respectively). The median progression-free survival was 3.1 months (2.1 months in PTCL and 4.2 months in CTCL). The common adverse events (AEs) observed were increased aspartate aminotransferase (AST) / alanine aminotransferase (ALT), hypoalbuminemia, lymphopenia, and pyrexia. Our results indicated that a 9 µg/kg/d dose of E7777 shows efficacy and a manageable safety profile in Japanese patients with relapsed or refractory PTCL and CTCL, with clinical activity observed across the range of CD25 expression. The common AEs were manageable, but increase in ALT / AST, hypoalbuminemia, and capillary leak syndrome should be carefully managed during the treatment.

Inhibition of neovascularisation in human endothelial cells using anti NRP-1 nanobody fused to truncated form of diphtheria toxin as a novel immunotoxin.

Neuropilin-1 (NRP-1) regulates a range of physiological and pathological processes, including angiogenesis. Targeting of NRP1 is considered a significant approach in cancer therapy. In the present study, a novel antiNRP1 immunotoxin (αNRP1 IT) was developed by genetic fusion of a single domain (VHH) anti-NRP-1 antibody fragment to a truncated diphtheria toxin. The αNRP1 IT was expressed into bacterial cells as an inclusion body (IB). Expression of αNRP1 IT was confirmed by SDS-PAGE and western blotting. Recombinant αNRP1 IT was purified using nickel affinity chromatography. Toxicity and antiangiogenesis effect of αNRP1 IT was investigated both in vitro and in vivo. Results showed that αNRP1 IT significantly reduced the viability of human umbilical vein endothelial cell line (HUVEC) (p < .05). The αNRP1 IT significantly inhibited tube formation of HUVEC cells (p < .001). Furthermore, αNRP1 IT inhibited angiogenesis in Chick Chorioallantoic Membrane (CAM) Assay. These data suggest the potential of αNRP1 IT as a novel therapeutic in targeted cancer therapy.

Half depletion of Foxp3+ regulatory T cells by diphtheria toxin for long-term study in vivo.

Depletion of regulatory T cells (Tregs) is an appropriate approach to study the function of Tregs in vivo, and most previous studies have focused on complete depletion. The purpose of the current study was to determine an appropriate dose and timing for half depletion of Tregs in vivo. DETREG (DEpletion of REGulatory T cells) mice were produced and injected with different doses of diphtheria toxin (DT) for 7 days and 14 days. The mice were then sacrificed to collect the spleen and mesenteric lymph nodes (MLN) for analysis using flow cytometry. Foxp3+eGFP+ cells were significantly reduced by DT injection. A dose of 5 ug/kg DT led to half depletion and no deaths. A DT dose of 25, 50, or 100 ug/kg led to a progressively higher depletion rate but also a higher mortality rate. In conclusion, a low dose of DT is effective for half depletion of Tregs and long-term study. Half depletion of Tregs may become a new method for the future study of Tregs in vivo.

Intein-mediated cytoplasmic reconstitution of a split toxin enables selective cell ablation in mixed populations and tumor xenografts.

The application of proteinaceous toxins for cell ablation is limited by their high on- and off-target toxicity, severe side effects, and a narrow therapeutic window. The selectivity of targeting can be improved by intein-based toxin reconstitution from two dysfunctional fragments provided their cytoplasmic delivery via independent, selective pathways. While the reconstitution of proteins from genetically encoded elements has been explored, exploiting cell-surface receptors for boosting selectivity has not been attained. We designed a robust splitting algorithm and achieved reliable cytoplasmic reconstitution of functional diphtheria toxin from engineered intein-flanked fragments upon receptor-mediated delivery of one of them to the cells expressing the counterpart. Retargeting the delivery machinery toward different receptors overexpressed in cancer cells enables selective ablation of specific subpopulations in mixed cell cultures. In a mouse model, the transmembrane delivery of a split-toxin construct potently inhibits the growth of xenograft tumors expressing the split counterpart. Receptor-mediated delivery of engineered split proteins provides a platform for precise therapeutic and experimental ablation of tumors or desired cell populations while also greatly expanding the applicability of the intein-based protein transsplicing.

Interleukin 1 beta and Matrix Metallopeptidase 3 Contribute to Development of Epidermal Growth Factor Receptor-Dependent Serrated Polyps in Mouse Cecum.

BACKGROUND & AIMS: Transgenic mice (HBUS) that express the epidermal growth factor receptor (EGFR) ligand HBEGF (heparin-binding epidermal growth factor-like growth factor) and a constitutively active G protein-coupled receptor (US28) in intestinal epithelial cells develop serrated polyps in the cecum. Development of serrated polyps depends on the composition of the gut microbiota and is associated with bacterial invasion of the lamina propria, accompanied by induction of inflammation and up-regulation of interleukin 1 beta (IL1B) and matrix metalloproteinase (MMP) 3 in the cecum. We investigated the mechanisms by which these changes contribute to development of serrated polyps. METHODS: We performed studies with C57BL/6 (control) and HBUS mice. To accelerate polyp development, we increased the exposure of the bacteria to the lamina propria by injecting HBUS mice with diphtheria toxin, which binds transgenic HBEGF expressed by the epithelial cells and causes apoptosis. Mice were given injections of IL1B-neutralizing antibody and the MMP inhibitor N-isobutyl-N-(4-methoxyphenylsulfonyl)glycyl hydroxamic acid. Intestinal tissues were collected from mice and analyzed by histology, reverse-transcription polymerase chain reaction, enzyme-linked immunosorbent assay, immunofluorescence, and flow cytometry. We examined fibroblast subsets in polyps using single-cell RNA sequencing. RESULTS: Administration of diphtheria toxin to HBUS mice accelerated development of serrated polyps (95% of treated mice developed polyps before 100 days of age, compared with 53% given vehicle). IL1B stimulated subsets of platelet-derived growth factor receptor alpha+ (PDGRFA+) fibroblasts isolated from cecum, resulting in increased expression of MMP3. Neutralizing antibodies against IL1B or administration of the MMP inhibitor reduced the number of serrated polyps that formed in the HBUS mice. Single-cell RNA sequencing analysis showed subsets of fibroblasts in serrated polyps that express genes that regulate matrix fibroblasts and inflammation. CONCLUSIONS: In studies of mice, we found that barrier breakdown and expression of inflammatory factors contribute to development of serrated polyps. Subsets of cecal PDGFRA+ fibroblasts are activated by release of IL1B from myeloid cells during the early stages of serrated polyp development. MMP3 produced by PDGFRA+ fibroblasts is important for serrated polyp development. Our findings confirm the functions of previously identified serrated polyp-associated molecules and indicate roles for immune and stromal cells in serrated polyp development.

Tagraxofusp: First Global Approval.

Tagraxofusp (tagraxofusp-erzs) [Elzonris™] is an intravenously administered CD123-directed cytotoxin (composed of human interleukin-3 and a truncated diphtheria toxin payload) that was developed by Stemline Therapeutics, Inc. for the treatment of blastic plasmacytoid dendritic cell neoplasm (BPDCN). In December 2018, tagraxofusp received its first global approval in the USA for the treatment of BPDCN in adults and in paediatric patients aged 2 years and older. A centralized registration application for the use of tagraxofusp in patients with BPDCN is under review in the EU. This article summarizes the milestones in the development of tagraxofusp leading to its first global approval for the treatment of BPDCN.

Second-generation IL-2 receptor-targeted diphtheria fusion toxin exhibits antitumor activity and synergy with anti-PD-1 in melanoma.

Denileukin diftitox (DAB-IL-2, Ontak) is a diphtheria-toxin-based fusion protein that depletes CD25-positive cells including regulatory T cells and has been approved for the treatment of persistent or recurrent cutaneous T cell lymphoma. However, the clinical use of denileukin diftitox was limited by vascular leak toxicity and production issues related to drug aggregation and purity. We found that a single amino acid substitution (V6A) in a motif associated with vascular leak induction yields a fully active, second-generation biologic, s-DAB-IL-2(V6A), which elicits 50-fold less human umbilical vein endothelial cell monolayer permeation and is 3.7-fold less lethal to mice by LD50 analysis than s-DAB-IL-2. Additionally, to overcome aggregation problems, we developed a production method for the fusion toxin using Corynebacterium diphtheriae that secretes fully folded, biologically active, monomeric s-DAB-IL-2 into the culture medium. Using the poorly immunogenic mouse B16F10 melanoma model, we initiated treatment 7 days after tumor challenge and observed that, while both s-DAB-IL-2(V6A) and s-DAB-IL-2 are inhibitors of tumor growth, the capacity to treat with higher doses of s-DAB-IL-2(V6A) could provide a superior activity window. In a sequential dual-therapy study in tumors that have progressed for 10 days, both s-DAB-IL-2(V6A) and s-DAB-IL-2 given before checkpoint inhibition with anti-programmed cell death-1 (anti-PD-1) antibodies inhibited tumor growth, while either drug given as monotherapy had less effect. s-DAB-IL-2(V6A), a fully monomeric protein with reduced vascular leak, is a second-generation diphtheria-toxin-based fusion protein with promise as a cancer immunotherapeutic both alone and in conjunction with PD-1 blockade.

Myofibroblasts acquire retinoic acid-producing ability during fibroblast-to-myofibroblast transition following kidney injury.

Tubular injury and interstitial fibrosis are the hallmarks of chronic kidney disease. While recent studies have verified that proximal tubular injury triggers interstitial fibrosis, the impact of fibrosis on tubular injury and regeneration remains poorly understood. We generated a novel mouse model expressing diphtheria toxin receptor on renal fibroblasts to allow for the selective disruption of renal fibroblast function. Administration of diphtheria toxin induced upregulation of the tubular injury marker Ngal and caused tubular proliferation in healthy kidneys, whereas administration of diphtheria toxin attenuated tubular regeneration in fibrotic kidneys. Microarray analysis revealed down-regulation of the retinol biosynthesis pathway in diphtheria toxin-treated kidneys. Healthy proximal tubules expressed retinaldehyde dehydrogenase 2 (RALDH2), a rate-limiting enzyme in retinoic acid biosynthesis. After injury, proximal tubules lost RALDH2 expression, whereas renal fibroblasts acquired strong expression of RALDH2 during the transition to myofibroblasts in several models of kidney injury. The retinoic acid receptor (RAR) RARγ was expressed in proximal tubules both with and without injury, and αB-crystallin, the product of an RAR target gene, was strongly expressed in proximal tubules after injury. Furthermore, BMS493, an inverse agonist of RARs, significantly attenuated tubular proliferation in vitro. In human biopsy tissue from patients with IgA nephropathy, detection of RALDH2 in the interstitium correlated with older age and lower kidney function. These results suggest a role of retinoic acid signaling and cross-talk between fibroblasts and tubular epithelial cells during tubular injury and regeneration, and may suggest a beneficial effect of fibrosis in the early response to injury.

BReDi mouse: A novel transgenic mouse strain to track and deplete B cells.

BReDi mice express a red fluorescent protein together with the diphtheria toxin receptor selectively in B cells. B cells can be effectively visualized by red fluorescence and can be efficiently depleted in a highly controlled fashion to study their functional capacity in vivo.

Depletion of regulatory T cells in ongoing paracoccidioidomycosis rescues protective Th1/Th17 immunity and prevents fatal disease outcome.

In human paracoccidioidomycosis (PCM), a primary fungal infection typically diagnosed when the disease is already established, regulatory T cells (Treg) cells are associated with disease severity. Experimental studies in pulmonary PCM confirmed the detrimental role of these cells, but in most studies, Tregs were depleted prior to or early during infection. These facts led us to study the effects of Treg cell depletion using a model of ongoing PCM. Therefore, Treg cell depletion was achieved by treatment of transgenic C57BL/6DTR/eGFP (DEREG) mice with diphtheria toxin (DT) after 3 weeks of intratracheal infection with 1 × 106 Paracoccidioides brasiliensis yeasts. At weeks 6 and 10 post-infection, DT-treated DEREG mice showed a reduced number of Treg cells associated with decreased fungal burdens in the lungs, liver and spleen, reduced tissue pathology and mortality. Additionally, an increased influx of activated CD4+ and CD8+ T cells into the lungs and elevated production of Th1/Th17 cytokines was observed in DT-treated mice. Altogether, our data demonstrate for the first time that Treg cell depletion in ongoing PCM rescues infected hosts from progressive and potentially fatal PCM; furthermore, our data indicate that controlling Treg cells could be explored as a novel immunotherapeutic procedure.

Efferocytosis of apoptotic alveolar epithelial cells is sufficient to initiate lung fibrosis.

Type II alveolar epithelial cell (AEC) apoptosis is a prominent feature of fibrotic lung diseases and animal models of pulmonary fibrosis. While there is growing recognition of the importance of AEC injury and apoptosis as a causal factor in fibrosis, the underlying mechanisms that link these processes remain unknown. We have previously shown that targeting the type II alveolar epithelium for injury by repetitively administering diphtheria toxin to transgenic mice expressing the diphtheria toxin receptor off of the surfactant protein C promoter (SPC-DTR) develop lung fibrosis, confirming that AEC injury is sufficient to cause fibrosis. In the present study, we find that SPC-DTR mice develop increased activation of caspase 3/7 after initiation of diphtheria toxin treatment consistent with apoptosis within AECs. We also find evidence of efferocytosis, the uptake of apoptotic cells, by alveolar macrophages in this model. To determine the importance of efferocytosis in lung fibrosis, we treated cultured alveolar macrophages with apoptotic type II AECs and found that the uptake induced pro-fibrotic gene expression. We also found that the repetitive intrapulmonary administration of apoptotic type II AEC or MLE-12 cells induces lung fibrosis. Finally, mice lacking a key efferocytosis receptor, CD36, developed attenuated fibrosis in response to apoptotic MLE-12 cells. Collectively, these studies support a novel mechanism linking AEC apoptosis with macrophage pro-fibrotic activation via efferocytosis and reveal previously unrecognized therapeutic targets.

Targeting macrophages for cancer therapy disrupts bone homeostasis and impairs bone marrow erythropoiesis in mice bearing Lewis lung carcinoma tumors.

Macrophages are represented in all tissues by phenotypically distinct resident populations that show great functional diversity. Macrophages generally play a protumoral role, and they are attractive targets for cancer therapy. In this study, we found that CD169+ macrophages depletion inhibited the growth of established Lewis lung carcinoma tumors in mice. Benefits must be weighed against potential adverse effects in cancer therapy. Here, we investigated the adverse effects of CD169+ macrophages depletion on bone and bone marrow in mice bearing Lewis lung carcinoma tumors. Our studies showed that depletion of CD169+ macrophages in LLC tumor-bearing mice disrupted bone homeostasis, including bone weight loss and bone mineral density decrease. Further studies revealed that bone marrow erythropoiesis was severely impaired after depletion of CD169+ macrophages in LLC tumor-bearing mice. Our findings suggest that depletion of macrophages for cancer therapy may be associated with potential adverse effects that need to be recognized, prevented, and optimally managed.

Dosing optimization of CCR4 immunotoxin for improved depletion of CCR4+ Treg in nonhuman primates.

Recently, we have developed a diphtheria toxin-based recombinant anti-human CCR4 immunotoxin for targeting CCR4+ tumors and Tregs. In this study, we further optimized the dosing schedule for improved CCR4+ Treg depletion. We have demonstrated that up to a 90% depletion was achieved and the depletion extended to approximately 2 weeks in the peripheral blood and more than 48 days in the lymph node at 25 µg·kg-1 , BID for 8 consecutive days in cynomolgus monkeys. Expansion was observed including monocytes and NK cells. Antibody against the CCR4 immunotoxin was detected after approximately 2 weeks, affecting further depletion efficacy for multiple course treatment.

Phosphoinositide 3-kinase δ inhibition promotes antitumor responses but antagonizes checkpoint inhibitors.

Multiple modes of immunosuppression restrain immune function within tumors. We previously reported that phosphoinositide 3-kinase δ (PI3Kδ) inactivation in mice confers resistance to a range of tumor models by disrupting immunosuppression mediated by regulatory T cells (Tregs). The PI3Kδ inhibitor idelalisib has proven highly effective in the clinical treatment of chronic lymphocytic leukemia and the potential to extend the use of PI3Kδ inhibitors to nonhematological cancers is being evaluated. In this work, we demonstrate that the antitumor effect of PI3Kδ inactivation is primarily mediated through the disruption of Treg function, and correlates with tumor dependence on Treg immunosuppression. Compared with Treg-specific PI3Kδ deletion, systemic PI3Kδ inactivation is less effective at conferring resistance to tumors. We show that PI3Kδ deficiency impairs the maturation and reduces the capacity of CD8+ cytotoxic T lymphocytes (CTLs) to kill tumor cells in vitro, and to respond to tumor antigen-specific immunization in vivo. PI3Kδ inactivation antagonized the antitumor effects of tumor vaccines and checkpoint blockade therapies intended to boost the CD8+ T cell response. These findings provide insights into mechanisms by which PI3Kδ inhibition promotes antitumor immunity and demonstrate that the mechanism is distinct from that mediated by immune checkpoint blockade.

Compensation between CSF1R+ macrophages and Foxp3+ Treg cells drives resistance to tumor immunotherapy.

Redundancy and compensation provide robustness to biological systems but may contribute to therapy resistance. Both tumor-associated macrophages (TAMs) and Foxp3+ regulatory T (Treg) cells promote tumor progression by limiting antitumor immunity. Here we show that genetic ablation of CSF1 in colorectal cancer cells reduces the influx of immunosuppressive CSF1R+ TAMs within tumors. This reduction in CSF1-dependent TAMs resulted in increased CD8+ T cell attack on tumors, but its effect on tumor growth was limited by a compensatory increase in Foxp3+ Treg cells. Similarly, disruption of Treg cell activity through their experimental ablation produced moderate effects on tumor growth and was associated with elevated numbers of CSF1R+ TAMs. Importantly, codepletion of CSF1R+ TAMs and Foxp3+ Treg cells resulted in an increased influx of CD8+ T cells, augmentation of their function, and a synergistic reduction in tumor growth. Further, inhibition of Treg cell activity either through systemic pharmacological blockade of PI3Kδ, or its genetic inactivation within Foxp3+ Treg cells, sensitized previously unresponsive solid tumors to CSF1R+ TAM depletion and enhanced the effect of CSF1R blockade. These findings identify CSF1R+ TAMs and PI3Kδ-driven Foxp3+ Treg cells as the dominant compensatory cellular components of the immunosuppressive tumor microenvironment, with implications for the design of combinatorial immunotherapies.

Modulation of subsets of cardiac B lymphocytes improves cardiac function after acute injury.

Despite the long-standing recognition that the immune response to acute myocardial injury contributes to adverse left ventricular (LV) remodeling, it has not been possible to effectively target this clinically. Using 2 different in vivo models of acute myocardial injury, we show that pirfenidone confers beneficial effects in the murine heart through an unexpected mechanism that depends on cardiac B lymphocytes. Naive hearts contained a large population of CD19+CD11b-CD23-CD21-IgD+IgMlo lymphocytes, and 2 smaller populations of CD19+CD11b+ B1a and B1b cells. In response to tissue injury, there was an increase in neutrophils, monocytes, macrophages, as well as an increase in CD19+ CD11b- B lymphocytes. Treatment with pirfenidone had no effect on the number of neutrophils, monocytes, or macrophages, but decreased CD19+CD11b- lymphocytes. B cell depletion abrogated the beneficial effects of pirfenidone. In vitro studies demonstrated that stimulation with lipopolysaccharide and extracts from necrotic cells activated CD19+ lymphocytes through a TIRAP-dependent pathway. Treatment with pirfenidone attenuated this activation of B cells. These findings reveal a previously unappreciated complexity of myocardial B lymphocytes within the inflammatory infiltrate triggered by cardiac injury and suggest that pirfenidone exerts beneficial effects in the heart through a unique mechanism that involves modulation of cardiac B lymphocytes.

Regulatory T-Cells Mediate IFN-α-Induced Resistance against Antigen-Induced Arthritis.

Objective: CD4+FoxP3+CD25+ regulatory T-cells (Tregs) are important for preventing tissue destruction. Here, we investigate the role of Tregs for protection against experimental arthritis by IFN-α. Methods: Arthritis was triggered by intra-articular injection of methylated bovine serum albumin (mBSA) in wild-type mice, Foxp3DTReGFP+/- mice [allowing selective depletion of Tregs by diphtheria toxin (DT)] and CD4-Cre+/- IFNA1R flox/flox mice (devoid of IFNAR signaling in T-cells) earlier immunized with mBSA, with or without treatment with IFN-α or the indoleamine 2,3-dioxygenase (IDO)-metabolite kynurenine. Tregs were depleted in DT-treated Foxp3DTReGFP+/- mice and enumerated by FoxP3 staining. Suppressive capacity of FACS-sorted CD25+highCD4+ Tregs was tested in vivo by adoptive transfer and ex vivo in cocultures with antigen-stimulated CFSE-stained T-responder (CD25-CD4+) cells. IDO was inhibited by 1-methyl tryptophan. Results: Both control mice and mice devoid of IFNAR-signaling in T helper cells were protected from arthritis by IFN-α. Depletion of Tregs in the arthritis phase, but not at immunization, abolished the protective effect of IFN-α and kynurenine against arthritis. IFN-α increased the number of Tregs in ex vivo cultures upon antigen recall stimulation but not in naïve cells. IFN-α also increased the suppressive capacity of Tregs against mBSA-induced T-responder cell proliferation ex vivo and against arthritis when adoptively transferred. The increased suppressive activity against proliferation conferred by IFN-α was clearly reduced by in vivo inhibition of IDO at immunization, which also abolished the protective effect of IFN-α against arthritis. Conclusion: By activating IDO during antigen sensitization, IFN-α activates Tregs, which prevent arthritis triggered by antigen rechallenge. This is one way by which IFN-α suppresses inflammation.

Optimal route of diphtheria toxin administration to eliminate native nephron progenitor cells in vivo for kidney regeneration.

To address the lack of organs for transplantation, we previously developed a method for organ regeneration in which nephron progenitor cell (NPC) replacement is performed via the diphtheria toxin receptor (DTR) system. In transgenic mice with NPC-specific expression of DTR, NPCs were eliminated by DT and replaced with NPCs lacking the DTR with the ability to differentiate into nephrons. However, this method has only been verified in vitro. For applications to natural models, such as animal fetuses, it is necessary to determine the optimal administration route and dose of DT. In this study, two DT administration routes (intra-peritoneal and intra-amniotic injection) were evaluated in fetal mice. The fetus was delivered by caesarean section at E18.5, and the fetal mouse kidney and RNA expression were evaluated. Additionally, the effect of the DT dose (25, 5, 0.5, and 0.05 ng/fetus-body) was studied. Intra-amniotic injection of DT led to a reduction in kidney volume, loss of glomeruli, and decreased differentiation marker expression. The intra-peritoneal route was not sufficient for NPC elimination. By establishing that intra-amniotic injection is the optimal administration route for DT, these results will facilitate studies of kidney regeneration in vivo. In addition, this method might be useful for analysis of kidney development at various time points by deleting NPCs during development.

Self-assembling toxin-based nanoparticles as self-delivered antitumoral drugs.

Loading capacity and drug leakage from vehicles during circulation in blood is a major concern when developing nanoparticle-based cell-targeted cytotoxics. To circumvent this potential issue it would be convenient the engineering of drugs as self-delivered nanoscale entities, devoid of any heterologous carriers. In this context, we have here engineered potent protein toxins, namely segments of the diphtheria toxin and the Pseudomonas aeruginosa exotoxin as self-assembling, self-delivered therapeutic materials targeted to CXCR4+ cancer stem cells. The systemic administration of both nanostructured drugs in a colorectal cancer xenograft mouse model promotes efficient and specific local destruction of target tumor tissues and a significant reduction of the tumor volume. This observation strongly supports the concept of intrinsically functional protein nanoparticles, which having a dual role as drug and carrier, are designed to be administered without the assistance of heterologous vehicles.