Synthetic vaccine particles for durable cytolytic T lymphocyte responses and anti-tumor immunotherapy.

We previously reported that synthetic vaccine particles (SVP) encapsulating antigens and TLR agonists resulted in augmentation of immune responses with minimal production of systemic inflammatory cytokines. Here we evaluated two different polymer formulations of SVP-encapsulated antigens and tested their ability to induce cytolytic T lymphocytes (CTL) in combination with SVP-encapsulated adjuvants. One formulation led to efficient antigen processing and cross-presentation, rapid and sustained CTL activity, and expansion of CD8+ T cell effector memory cells locally and centrally, which persisted for at least 1-2 years after a single immunization. SVP therapeutic dosing resulted in suppression of tumor growth and a substantial delay in mortality in several syngeneic mouse cancer models. Treatment with checkpoint inhibitors and/or cytotoxic drugs, while suboptimal on their own, showed considerable synergy with SVP immunization. SVP encapsulation of endosomal TLR agonists provided superior CTL induction, therapeutic benefit and/or improved safety profile compared to free adjuvants. SVP vaccines encapsulating mutated HPV-16 E7 and E6/E7 recombinant proteins led to induction of broad CTL activity and strong inhibition of TC-1 tumor growth, even when administered therapeutically 13-14 days after tumor inoculation in animals bearing palpable tumors. A pilot study in non-human primates showed that SVP-encapsulated E7/E6 adjuvanted with SVP-encapsulated poly(I:C) led to robust induction of antigen-specific T and B cell responses.

TLR-adjuvanted nanoparticle vaccines differentially influence the quality and longevity of responses to malaria antigen Pfs25.

Transmission-blocking vaccines (TBVs) are considered an integral element of malaria eradication efforts. Despite promising evaluations of Plasmodium falciparum Pfs25-based TBVs in mice, clinical trials have failed to induce robust and long-lived Ab titers, in part due to the poorly immunogenic nature of Pfs25. Using nonhuman primates, we demonstrate that multiple aspects of Pfs25 immunity were enhanced by antigen encapsulation in poly(lactic-co-glycolic acid)-based [(PLGA)-based] synthetic vaccine particles (SVP[Pfs25]) and potent TLR-based adjuvants. SVP[Pfs25] increased Ab titers, Pfs25-specific plasmablasts, circulating memory B cells, and plasma cells in the bone marrow when benchmarked against the clinically tested multimeric form Pfs25-EPA given with GLA-LSQ. SVP[Pfs25] also induced the first reported Pfs25-specific circulating Th1 and Tfh cells to our knowledge. Multivariate correlative analysis indicated several mechanisms for the improved Ab responses. While Pfs25-specific B cells were responsible for increasing Ab titers, T cell responses stimulated increased Ab avidity. The innate immune activation differentially stimulated by the adjuvants revealed a strong correlation between type I IFN polarization, induced by R848 and CpG, and increased Ab half-life and longevity. Collectively, the data identify ways to improve vaccine-induced immunity to poorly immunogenic proteins, both by the choice of antigen and adjuvant formulation, and highlight underlying immunological mechanisms.

Improving the efficacy and safety of biologic drugs with tolerogenic nanoparticles.

The development of antidrug antibodies (ADAs) is a common cause for the failure of biotherapeutic treatments and adverse hypersensitivity reactions. Here we demonstrate that poly(lactic-co-glycolic acid) (PLGA) nanoparticles carrying rapamycin, but not free rapamycin, are capable of inducing durable immunological tolerance to co-administered proteins that is characterized by the induction of tolerogenic dendritic cells, an increase in regulatory T cells, a reduction in B cell activation and germinal centre formation, and the inhibition of antigen-specific hypersensitivity reactions. Intravenous co-administration of tolerogenic nanoparticles with pegylated uricase inhibited the formation of ADAs in mice and non-human primates and normalized serum uric acid levels in uricase-deficient mice. Similarly, the subcutaneous co-administration of nanoparticles with adalimumab resulted in the durable inhibition of ADAs, leading to normalized pharmacokinetics of the anti-TNFα antibody and protection against arthritis in TNFα transgenic mice. Adjunct therapy with tolerogenic nanoparticles represents a novel and broadly applicable approach to prevent the formation of ADAs against biologic therapies.

Polymeric synthetic nanoparticles for the induction of antigen-specific immunological tolerance.

Current treatments to control pathological or unwanted immune responses often use broadly immunosuppressive drugs. New approaches to induce antigen-specific immunological tolerance that control both cellular and humoral immune responses are desirable. Here we describe the use of synthetic, biodegradable nanoparticles carrying either protein or peptide antigens and a tolerogenic immunomodulator, rapamycin, to induce durable and antigen-specific immune tolerance, even in the presence of potent Toll-like receptor agonists. Treatment with tolerogenic nanoparticles results in the inhibition of CD4+ and CD8+ T-cell activation, an increase in regulatory cells, durable B-cell tolerance resistant to multiple immunogenic challenges, and the inhibition of antigen-specific hypersensitivity reactions, relapsing experimental autoimmune encephalomyelitis, and antibody responses against coagulation factor VIII in hemophilia A mice, even in animals previously sensitized to antigen. Only encapsulated rapamycin, not the free form, could induce immunological tolerance. Tolerogenic nanoparticle therapy represents a potential novel approach for the treatment of allergies, autoimmune diseases, and prevention of antidrug antibodies against biologic therapies.

Adjuvant-carrying synthetic vaccine particles augment the immune response to encapsulated antigen and exhibit strong local immune activation without inducing systemic cytokine release.

Augmentation of immunogenicity can be achieved by particulate delivery of an antigen and by its co-administration with an adjuvant. However, many adjuvants initiate strong systemic inflammatory reactions in vivo, leading to potential adverse events and safety concerns. We have developed a synthetic vaccine particle (SVP) technology that enables co-encapsulation of antigen with potent adjuvants. We demonstrate that co-delivery of an antigen with a TLR7/8 or TLR9 agonist in synthetic polymer nanoparticles results in a strong augmentation of humoral and cellular immune responses with minimal systemic production of inflammatory cytokines. In contrast, antigen encapsulated into nanoparticles and admixed with free TLR7/8 agonist leads to lower immunogenicity and rapid induction of high levels of inflammatory cytokines in the serum (e.g., TNF-a and IL-6 levels are 50- to 200-fold higher upon injection of free resiquimod (R848) than of nanoparticle-encapsulated R848). Conversely, local immune stimulation as evidenced by cellular infiltration of draining lymph nodes and by intranodal cytokine production was more pronounced and persisted longer when SVP-encapsulated TLR agonists were used. The strong local immune activation achieved using a modular self-assembling nanoparticle platform markedly enhanced immunogenicity and was equally effective whether antigen and adjuvant were co-encapsulated in a single nanoparticle formulation or co-delivered in two separate nanoparticles. Moreover, particle encapsulation enabled the utilization of CpG oligonucleotides with the natural phosphodiester backbone, which are otherwise rapidly hydrolyzed by nucleases in vivo. The use of SVP may enable clinical use of potent TLR agonists as vaccine adjuvants for indications where cellular immunity or robust humoral responses are required.

Precision intracellular delivery based on optofluidic polymersome rupture.

We present an optical approach for intracellular delivery of molecules contained within oxidation-sensitive polymersomes. The photosensitizer ethyl eosin is associated with the polymersome membrane to oxidatively increase the hydrophilicity of the hydrophobic block under optical excitation. This optofluidic interaction induces rapid polymersome rupture and payload release via the reorganization of the aggregate structure into smaller diameter vesicles and micelles. When the particles are endocytosed by phagocytes, such as RAW macrophages and dendritic cells, the polymersomes' payload escapes the endosome and is released in the cell cytosol within a few seconds of illumination. The released payload is rapidly distributed throughout the cytosol within milliseconds. The presented optofluidic method enables fast delivery and distribution throughout the cytosol of individual cells, comparable to photochemical internalization, but a factor of 100 faster than similar carrier mediated delivery methods (e.g., liposomes, polymersomes, or nanoparticles). Due to the ability to simultaneously induce payload delivery and endosomal escape, this approach can find applications in detailed characterizations of intra- and intercellular processes. As an example in quantitative cell biology, a peptide antigen was delivered in dendritic cells and MHC I presentation kinetics were measured at the single cell and single complex level.

Polymer micelles with pyridyl disulfide-coupled antigen travel through lymphatics and show enhanced cellular responses following immunization.

Poly(ethylene glycol)-stabilized poly(propylene sulfide) core (PEG-PPS) nanoparticles (NPs) smaller than 50 nm efficiently travel to draining lymph nodes and interact with antigen-presenting cells (APCs) to induce potent immune responses following intradermal immunization. To determine if a similar system could be developed that could be more easily and reproducibly prepared and eliminated faster in vivo, we created block copolymers of PEG-bl-PPS capable of self-assembling into 25-35 nm micelles (MCs). Biodistribution studies showed that these MCs were able to travel to draining lymph nodes, where they preferentially interacted with APCs. To couple cysteine-containing antigens to the surface of the MCs, a new polymer was synthesized with a terminal pyridyl disulfide (PDS), forming PDS-PEG-bl-PPS-benzyl. When mice were immunized in conjunction with free CpG as an adjuvant, ovalbumin-conjugated MCs (MC-Ova) generated more (2.4-fold) Ova-specific CD8(+) T cells in the blood and higher (1.7-fold) interferon-gamma levels from splenocytes upon restimulation than in mice immunized with free Ova and CpG. When comparing this MC platform to our PEG-PPS NPs with disulfide-linked Ova, no significant differences were found in the measured responses. These results indicate that PDS-functionalized MCs are efficient antigen delivery vehicles that enhance immune responses compared to immunization with free protein.

Nano-sized drug-loaded micelles deliver payload to lymph node immune cells and prolong allograft survival.

By delivering immunomodulatory drugs in vivo directly to lymph nodes draining an injection site, an opportunity exists to increase drug bioavailability to local immune cells. Importantly, particles smaller than 100 nm are efficiently transported through lymphatic vessels to draining lymph nodes. To investigate whether this approach could be used for local delivery of immunomodulatory drugs, amphiphilic poly(ethylene glycol)-bl-poly(propylene sulfide) (PEG-bl-PPS) block copolymers forming 50 nm micelles were used to encapsulate hydrophobic drugs. Micelle drainage was determined using fluorescent micelles and showed effective targeting of multiple immune cell subsets in lymph nodes. For functional studies of our formulations, two approaches were considered. To evaluate the efficacy of anti-inflammatory drug delivery, dendritic cell activation was shown to be prevented when mice were pretreated with micelles loaded with the glucocorticoid mometasone and then challenged with the TLR9 ligand, CpG. To evaluate whether immunosuppressive drug-loaded micelles were effective in prolonging MHC-mismatched allograft survival, BALB/c mice were treated for 14 consecutive days with drug-loaded micelles following transplantation of allogenic C57BL/6 tail skin. Micelles loaded with a mixture of rapamycin and tacrolimus prolonged allograft survival by 2-fold. Our results indicate that the drug-loaded micelle approach effectively targets the draining lymph nodes and exhibits proper immune regulation.

Enzymatic- and temperature-sensitive controlled release of ultrasmall superparamagnetic iron oxides (USPIOs).

BACKGROUND: Drug and contrast agent delivery systems that achieve controlled release in the presence of enzymatic activity are becoming increasingly important, as enzymatic activity is a hallmark of a wide array of diseases, including cancer and atherosclerosis. Here, we have synthesized clusters of ultrasmall superparamagnetic iron oxides (USPIOs) that sense enzymatic activity for applications in magnetic resonance imaging (MRI). To achieve this goal, we utilize amphiphilic poly(propylene sulfide)-bl-poly(ethylene glycol) (PPS-b-PEG) copolymers, which are known to have excellent properties for smart delivery of drug and siRNA. RESULTS: Monodisperse PPS polymers were synthesized by anionic ring opening polymerization of propylene sulfide, and were sequentially reacted with commercially available heterobifunctional PEG reagents and then ssDNA sequences to fashion biofunctional PPS-bl-PEG copolymers. They were then combined with hydrophobic 12 nm USPIO cores in the thin-film hydration method to produce ssDNA-displaying USPIO micelles. Micelle populations displaying complementary ssDNA sequences were mixed to induce crosslinking of the USPIO micelles. By design, these crosslinking sequences contained an EcoRV cleavage site. Treatment of the clusters with EcoRV results in a loss of R2 negative contrast in the system. Further, the USPIO clusters demonstrate temperature sensitivity as evidenced by their reversible dispersion at ~75°C and re-clustering following return to room temperature. CONCLUSIONS: This work demonstrates proof of concept of an enzymatically-actuatable and thermoresponsive system for dynamic biosensing applications. The platform exhibits controlled release of nanoparticles leading to changes in magnetic relaxation, enabling detection of enzymatic activity. Further, the presented functionalization scheme extends the scope of potential applications for PPS-b-PEG. Combined with previous findings using this polymer platform that demonstrate controlled drug release in oxidative environments, smart theranostic applications combining drug delivery with imaging of platform localization are within reach. The modular design of these USPIO nanoclusters enables future development of platforms for imaging and drug delivery targeted towards proteolytic activity in tumors and in advanced atherosclerotic plaques.

Analytical ultracentrifugation to support the development of biomaterials and biomedical devices.

Analytical ultracentrifugation (AUC) primarily serves to investigate hydrodynamic and thermodynamic properties of natural and synthetic macromolecules and colloids in solution, dispersion or suspension. Beside such more conventional use, AUC can support materials development particularly by combining different optical systems, if the AUC is equipped with such, or using complementary data evaluation approaches. In this context, an Optima XL-I equipped with absorbance (AO) and interference optics (IO) was used alone or complementary to study the success of conjugation of biopolymers, to evaluate the completeness of the incorporation of macromolecules into micelles and vesicles, and to analyze the composition and homogeneity of macromolecular assemblies. The combination of AO and IO proved covalent binding of concanavalin A to dextran without macromolecular degradation as well as the formation of mixed micelles composed of two types of block copolymers. Further, AUC contributed to analyze the homogeneity, purity, size and size distribution of carbon monoxide-releasing macromolecular assemblies. These case studies revealed that the application possibilities of AUC are by far not completely discovered but can still be extended.

Engineering complement activation on polypropylene sulfide vaccine nanoparticles.

The complement system is an important regulator of both adaptive and innate immunity, implicating complement as a potential target for immunotherapeutics. We have recently presented lymph node-targeting, complement-activating nanoparticles (NPs) as a vaccine platform. Here we explore modulation of surface chemistry as a means to control complement deposition, in active or inactive forms, on polypropylene sulfide core, block copolymer Pluronic corona NPs. We found that nucleophile-containing NP surfaces activated complement and became functionalized in situ with C3 upon serum exposure via the alternative pathway. Carboxylated NPs displayed a higher degree of C3b deposition and retention relative to hydroxylated NPs, upon which deposited C3b was more substantially inactivated to iC3b. This in situ functionalization correlated with in vivo antigen-specific immune responses, including antibody production as well as T cell proliferation and IFN-γ cytokine production upon antigen restimulation. Interestingly, inactivation of C3b to iC3b on the NP surface did not correlate with NP affinity to factor H, a cofactor for protease factor I that degrades C3b into iC3b, indicating that control of complement protein C3 stability depends on architectural details in addition to factor H affinity. These data show that design of NP surface chemistry can be used to control biomaterials-associated complement activation for immunotherapeutic materials.

Synthesis of pyridyl disulfide-functionalized nanoparticles for conjugating thiol-containing small molecules, peptides, and proteins.

Previously we reported emulsion polymerization of propylene sulfide with Pluronic F127 as an emulsifier, yielding nanoparticles (NPs) in the 25 nm size range. Immunologically functional NPs were prepared by adding an antigen-Pluronic conjugate to the polymerization mixture ( Reddy , S. T. , et al. ( 2007 ) Nat. Biotechnol. 25, 1159 ). We sought a more flexible scheme for conjugation of antigens and other biomolecules to the NP surfaces that would allow for milder reaction conditions than achievable during the polymerization step. Here, we present the synthesis of such functionalizable NPs in the form of NPs that carry thiol-reactive groups, to which thiol-containing antigens (peptide or protein) or other biomolecules can be conjugated under mild conditions to yield immunofunctional NPs. The Pluronic-stabilized poly(propylene sulfide) (PPS) NPs with thiol-reactive pyridyl disulfide groups are prepared in two steps by (1) emulsion polymerization of propylene sulfide in the presence of a carboxylate-Pluronic and (2) reaction of the carboxylic acid groups on the NP surface with cysteamine pyridyl disulfide and a water-soluble carbodiimide reagent. We choose pyridyl disulfide groups to have a reduction-sensitive disulfide bond linking the antigen to the NP surface, allowing efficient release of antigen inside the cell in response to the reductive conditions within the endosome. The functionalizable NPs are characterized by proton NMR, dynamic light scattering (DLS), UV/vis spectroscopy, and transmission electron microscopy (TEM). Conjugation of small molecules and protein to the NP surface is presented.

Microfluidic assays for DNA manipulation based on a block copolymer immobilization strategy.

Methods to manipulate and visualize isolated DNA and oligonucleotide strands are important for investigation of their biophysics as well as their interactions with proteins. Herein, we report such a method by combining a block copolymer surface functionalization strategy with microfluidics. The copolymer poly(l-lysine-graft-polyethylene glycol) (PLL-g-PEG) coated one surface of the microfluidic channels, rendering it passive to adsorption and thus minimizing any noise arising from nontargeted adsorbed molecules. Single lambda-phage DNA molecules were immobilized and were extended by molecular combing. Their extension did not exceed their contour length, which we attribute to the low surface tension of the coated surface. To demonstrate further the applicability of our method, the anchored DNA was extended by hydrodynamic flow. We propose this method for exploring DNA-protein interactions due to the copolymer's enhanced capacity for single-molecule detection, stability under wet or dry conditions, hydrophilicity, full compatibility with microfluidics and simplicity being a one-step process.

Aggregation behavior of poly(ethylene glycol-bl-propylene sulfide) di- and triblock copolymers in aqueous solution.

Block copolymers of poly(ethylene glycol)-bl-poly(propylene sulfide) (PEG-PPS) have recently emerged as a new macromolecular amphiphile capable of forming a wide range of morphologies when dispersed in water. To understand better the relationship between stability and morphology in terms of the relative and absolute block compositions, we have synthesized a collection of PEG-PPS block copolymers and quantified their critical aggregation concentration and observed their morphology using cryogenic transmission electron microscopy after thin film hydration with extrusion and after solvent dispersion from tetrahydrofuran, a solvent for both blocks. By understanding the relationship between aggregate character and block copolymer architecture, we have observed that whereas the relative block lengths control morphology, the stability of the aggregates upon dilution is determined by the absolute block length of the hydrophobic PPS block. We have compared results obtained with PEG-PPS to those obtained with poly(ethylene glycol)-bl-poly(propylene oxide)-bl-poly(ethylene glycol) block copolymers (Pluronics). The results reveal that the PEG-PPS aggregates are substantially more stable than Pluronic aggregates, by more than an order of magnitude. PEG-PPS can form a wide variety of stable or metastable morphologies in dilute solution within normal time and temperature ranges, whereas Pluronics can generally form only spherical micelles under the same conditions. On the basis of these results, block copolymers of PEG with poly(propylene sulfide) may present distinct advantages over those with poly(propylene glycol) for a number of applications.

A novel method for the encapsulation of biomolecules into polymersomes via direct hydration.

One of the major engineering challenges for the implementation of block copolymer vesicles, or polymersomes, as therapeutic drug carriers is obtaining high encapsulation efficiencies for biomolecules. Here we present a novel method for encapsulation of proteins with high encapsulation efficiency within polymersomes formed from block copolymers of poly(ethylene glycol)-bl-poly(propylene sulfide). By formulation of the neat block copolymer with a low molecular weight poly(ethylene glycol), direct hydration of the formulated mixture yielded polymersomes. We were able to achieve encapsulation efficiencies for ovalbumin at 37%, bovine serum albumin at 19%, and bovine gamma-globulin at 15% when the proteins were included in the hydration solution. The formulation process and the dispersion of polymersomes from the preparation in phosphate-buffered saline were characterized using confocal microscopy, cryogenic transmission electron microscopy, and fluorimetry. We were also successful in the encapsulation of proteinase K, a proteolytic enzyme, and demonstrated by SDS-PAGE that the enzyme was contained inside polymersomes when dispersed in a solution of ovalbumin.

Extracellular matrix binding mixed micelles for drug delivery applications.

We present the formation of collagen-binding mixed micelles and their potential suitability to deliver therapeutic drugs to the vessel wall. We modified poly(ethylene oxide)-bl-poly(propylene oxide)-bl-poly(ethylene oxide) (Pluronic F-127) to display sulfate groups on the terminus of the PEO block to act as a heparin mimics and bind to collagen in the extracellular matrix. This functionalized macroamphiphile was incorporated into a mixed micelle with poly(propylene sulfide)-bl-poly(ethylene oxide), a macroamphiphile that demonstrates improved micellar stability relative to Pluronic F-127 micelles. The mixed micelles were examined using analytical ultracentrifugation, dynamic light scattering, transmission electron microscopy, and measures of the critical micellar concentration using surface tensiometry. Encapsulation and in vitro release of Sirolimus, an immunosuppressant drug of interest in coronary artery treatment, was considered as an example. Mixed micelles with the sulfate functionality demonstrated enhanced binding to collagen I coated surfaces, suggestive of the potential for binding to the extracellular milieu.

Biofunctional polymer nanoparticles for intra-articular targeting and retention in cartilage.

The extracellular matrix of dense, avascular tissues presents a barrier to entry for polymer-based therapeutics, such as drugs encapsulated within polymeric particles. Here, we present an approach by which polymer nanoparticles, sufficiently small to enter the matrix of the targeted tissue, here articular cartilage, are further modified with a biomolecular ligand for matrix binding. This combination of ultrasmall size and biomolecular binding converts the matrix from a barrier into a reservoir, resisting rapid release of the nanoparticles and clearance from the tissue site. Phage display of a peptide library was used to discover appropriate targeting ligands by biopanning on denuded cartilage. The ligand WYRGRL was selected in 94 of 96 clones sequenced after five rounds of biopanning and was demonstrated to bind to collagen II alpha1. Peptide-functionalized nanoparticles targeted articular cartilage up to 72-fold more than nanoparticles displaying a scrambled peptide sequence following intra-articular injection in the mouse.

A benzene-core trinuclear GdIII complex: towards the optimization of relaxivity for MRI contrast agent applications at high magnetic field.

A novel ligand, H(12)L, based on a trimethylbenzene core bearing three methylenediethylenetriamine-N,N,N'',N''-tetraacetate moieties (-CH(2)DTTA(4-)) for Gd(3+) chelation has been synthesized, and its trinuclear Gd(3+) complex [Gd(3)L(H(2)O)(6)](3-) investigated with respect to MRI contrast agent applications. A multiple-field, variable-temperature (17)O NMR and proton relaxivity study on [Gd(3)L(H(2)O)(6)](3-) yielded the parameters characterizing water exchange and rotational dynamics. On the basis of the (17)O chemical shifts, bishydration of Gd(3+) could be evidenced. The water exchange rate, k(ex)(298)=9.0+/-3.0 s(-1) is around twice as high as k(ex)(298) of the commercial [Gd(DTPA)(H(2)O)](2-) and comparable to those on analogous Gd(3+)-DTTA chelates. Despite the relatively small size of the complex, the rotational dynamics had to be described with the Lipari-Szabo approach, by separating global and local motions. The difference between the local and global rotational correlation times, tau(lO)(298)=170+/-10 ps and tau(gO)(298)=540+/-100 ps respectively, shows that [Gd(3)L(H(2)O)(6)](3-) is not fully rigid; its flexibility originates from the CH(2) linker between the benzene core and the poly(amino carboxylate) moiety. As a consequence of the two inner-sphere water molecules per Gd(3+), their close to optimal exchange rate and the appropriate size and limited flexibility of the molecule, [Gd(3)L(H(2)O)(6)](3-) has remarkable proton relaxivities when compared with commercial contrast agents, particularly at high magnetic fields (r(1)=21.6, 17.0 and 10.7 mM(-1)s(-1) at 60, 200 and 400 MHz respectively, at 25 degrees C; r(1) is the paramagnetic enhancement of the longitudinal water proton relaxation rate, referred to 1 mM concentration of Gd(3+)).

Exploiting lymphatic transport and complement activation in nanoparticle vaccines.

Antigen targeting and adjuvancy schemes that respectively facilitate delivery of antigen to dendritic cells and elicit their activation have been explored in vaccine development. Here we investigate whether nanoparticles can be used as a vaccine platform by targeting lymph node-residing dendritic cells via interstitial flow and activating these cells by in situ complement activation. After intradermal injection, interstitial flow transported ultra-small nanoparticles (25 nm) highly efficiently into lymphatic capillaries and their draining lymph nodes, targeting half of the lymph node-residing dendritic cells, whereas 100-nm nanoparticles were only 10% as efficient. The surface chemistry of these nanoparticles activated the complement cascade, generating a danger signal in situ and potently activating dendritic cells. Using nanoparticles conjugated to the model antigen ovalbumin, we demonstrate generation of humoral and cellular immunity in mice in a size- and complement-dependent manner.