ABSTRACT

ResponsesNanoparticle-mediated immune responses is a poorly understood set of complex interfacial and molecular processes that involve many elements of both innate and adaptive immunity, depending on nanoparticle physicochemical characteristics, dose, and the route of exposure [9-11]. For example, nanoparticle-mediated inadvertent activation of the complement system through any of the three complement initiation pathways may trigger consequential secondary responses with hemodynamic, respiratory, cutaneous, and subjective manifestations [12, 18, 19]. This is due to liberation of potent complement bioactive products (e.g., C3a, C5a, and C5b-9) with the ability to modulate the function of a variety of immune cells and vascular endothelial cells. Indeed, excessive production of C5a may down-regulate immune responses in some leukocytes, while overactivating other cell types [39, 40]. Triggering of mast cells and basophils by anaphylatoxins may lead to secretion of a cocktail of vasoactive meditors (e.g., histamine, thromboxanes, leukotrienes) and induce anaphylaxis and other undesirable effects [19]. On the other hand, C5b-9 complexes may elicit nonlytic stimulatory responses from vascular endothelial cells and modulate endothelial regulation of hemostasis and inflammatory cell recruitment [41]. Other complement activation products such as iC3b could induce up-regulation of certain adhesion molecules on neutrophils and endothelial cells [42]. Complement activation by nanoparticles-and its consequence-is discussed further in Chapter 9. Direct interaction of nanoparticles with a variety of immune cells such as M1, M2, and regulatory macrophages, mast cells, basophils, eosinophils, neutrophils, natural killer cells, and different subsets of lymphocytes and dendritic cells may initiate many immunostimulatory or immunosuppressive reactions [5, 9-11]. Nanoparticle (e.g., various metallic and polymeric particles,

carbon nanotubes) interaction and uptake by some immune cells may generate reactive oxygen species and trigger production of inflammatory cytokines, such as interleukin 1β, causing severe tissue damage [9-11]. Cytokine release by nanoparticles and potential immunotoxic effects can sometimes be ascribed to the type of surfactant used, presence of impurities, and instability (aggregation) of the formulation. Nanoparticles can also induce immunosuppression, either inadvertently through cytokine production (as in transforming growth factor β), T-cell impairment, and complement activation or intentionally through delivery of immunosuppressive agents [9-11]. Here we limit our discussion to the application of nanoparticles in therapeutics and medicine. 9.3.1 ImmunostimulationTwo interesting immunostimulatory properties of nanoparticles in medicine are antigenicity and adjuvanticity. 9.3.1.1 Antigenicity

Antigenicity is usually referred to a specific antibody response. The immune response to a typical nanoparticle may result in generation of antibodies (IgG and IgM classes) against some components of a nanoparticle. The presence of these antibodies may affect nanoparticle stability as well as pharmacokinetics [5, 6, 43-45]. For example, there are many naturally occurring antibodies to phospholipid headgroups and cholesterol with different titers, specificity, and interindividual variations [43, 44]. The binding of such antibodies to liposomes may trigger complement activation, which in turn will generate the lytic complex C5b-9 for insertion into the liposomal bilayer [44, 45]. This could result in substantial leakage of vesicular encapsulated aqueous cargo before it reaches its cellular targets. Others have shown that repeated injection of long-circulating polyethylene glycol (PEG)ylated liposomes can generate anti-PEG antibodies, since PEG acts as a type 2 T-cell-independent antigen and elicits a potent IgM response by direct stimulation of B-cells [46]. Consequently, in the presence of circulating anti-PEG antibodies PEGylated liposomes will no longer exhibit prolonged circulation profiles and become prone to sequestration by hepatic and splenic

macrophages as a result of antibody (IgM)-mediated complement fixation. Induction of anti-PEG antibodies can be prevented if PEGylated liposomes carry a cytotoxic drug such as doxorubicin [47]. This is due to doxorubicin-induced macrophage death and the inhibition of B-cell proliferation and/or killing of proliferated B-cells. In some cases, liposomal cargo, such as therapeutic nucleic acids, may act as a potent enhancer of immunogenicity. Antibody induction by therapeutic liposomes may also generate new complications such as infusion-related reactions or anaphylaxis [19]. Apart from liposomes, there are limited studies that have assessed antibody generation against nanoparticles. For example, there are inconsistent reports on antibody generation against fullerene nanoparticles [48-50]. While some reports have shown antibody generation against C60 and C70 fullerenes [48, 49], others did not detect fullerene antigenicity, even in the presence of a complete Freund’s adjuvant [50]. These discrepancies may be accounted for by differences in fullerene surface properties and animal choice. 9.3.1.2 AdjuvanticityNanoparticles may act as immune potentiators or adjuvants triggering early innate immune responses that subsequently assist the generation of potent and persistent adaptive immune responses. Accordingly, a wide variety of nanoparticles have been used as adjuvants for vaccine formulations, particularly to enhance the immunogenicity of subunit vaccines, not only through antigen protection and targeting to antigen-presenting cells, but also through direct immunostimualtion [16, 24]. For instance, cationic liposomes are potent activators of the innate immunity when combined with nucleic acid agonists of endosomally located Toll-like receptors 3, 7, 8, and 9 [51]. Nanoparticle surface decoration with appropriate ligands against a plethora of plasma membrane and internal dendritic cell receptors can augment antigen delivery, signalling events and intracellular processing (reviewed in Ref. [24]). These include ligands that target DEC-205, mannose receptor MRC1/CD206, DC-SIGN (CD209), LOX-1, langerin (CD207), CLEC4A, DECTIN-2, DORA, and Fcg receptor type I. However, these receptors are differently expressed by distinct subsets of dendritic cells, and their expression may vary with the state of dendritic cell maturation. Adjuvanticity may also depend on the ability of particles to activate

the complement system as in the case of liposomes, carbon nanotubes, and a variety of polymeric nanospheres and microspheres [12, 52-55]. Indeed, the complement split-product C3d can induce B-cell activation, whereas anaphylatoxins C3a and C5a may serve as “danger signals” [56]. C5a is also a chemoattractant to many immune cells [12, 42]. Furthermore, other complement components such as C1q may bind to anionic nanoparticles and induce dendritic cell maturation, activation (interleukin 12 and tumor necrosis factor alpha secretion), and elevation of their T-cell-stimulating capacity [12, 57]. Other potential adjuvants, such as chitosan and silica particles, as well as ISCOMS, may directly activate the NALP3 inflammasome complex (apoptosis-associated speck-like protein and caspase 1 protease), which in turn cleave and activate the immunostimulatory cytokine interleukin 1β [17, 24].