2. How nanostructures can improve the treatment approaches

Nanomaterials can interact directly with the immune system triggering immune responses, consequently they have been considered as potential adjuvants.97 Their functionalisation with ligands produces a particular effect in a more controlled way. NPs have been considered as adequate platforms to conjugate ligands that can tune the physico-chemical properties of these nanostructures, but also to facilitate the interaction with target cells for a selective drug delivery. In addition, they can encapsulate allergens to be delivered selectively in cells of the immune system.131
CpG oligodeoxynucleotides (CpG-ODNs), a conserved sequence present in bacteria and viruses, are recognised by TLR9 receptors, inducing a Th1 response. CpG-ODNs have been used as potent and efficient adjuvants in allergic models. Mesoporous silica NPs, boron nitride particles, AuNPs, liposomes, CNTs, polymers, among other platforms have been extensively used to deliver CpG-ODNs into cells. The NPs not only prevent the degradation of the ODNs but also increase the cell uptake, facilitating the internalisation, and inducing the enhancement of the immunostimulation, and the release of pro-inflammatory cytokines.132
NPs have been also used for topical administration of drugs such as betamethasone, hydrocortisone, hydroxytyrosol, tacrolimus, anti-histamine drugs, etc. with the aim to treat atopic dermatitis. NPs improve the solubility of these drugs, their stability against degradation and facilitate the skin penetration capabilities, reducing side-effects.133
The use of nanomaterials has been also extensively applied to the treatment of asthma.134 Different types, as PAMAM and Polyethyleneimine dendrimers, polyethylenglycol (PEG) NPs, and liposomes have been used as carriers of CpG, DNA plasmids, beclomethasone dipropionate, dexamethasone, salbutamol, etc.
NPs based on chitosane, poly(D,L-lactic-co-glycolic) acid (PLGA), ceramic, plygammaglutamic acid, etc. have been used as mucosa adjuvants encapsulating antigens for oral vaccination. The properties of the polymers are fundamental for an adequate allergen release. PEG functionalisation can retain the NPs in the mucosa, reducing the treatment dose. Microbial adhesins functionalisation enhances NPs bioadhesive properties to colonise the gut.135Besides, a water soluble self-assembled micellar formulation, using PEG400, propylene glycerol and ethanol, has been used to encapsulate Cannflavin A, a flavone derivative with anti-inflammatory activity and poorly soluble in water. This composition was tested in an experimental asthma/chronic obstructive pulmonary disease in rats enhancing the anti-inflammatory effects in vivo .136
Protein-cage NPs using ferritin of 12 nm of diameter have been functionalised on their surface with 24 units of a small peptide (AP-1), a ligand for the receptor IL-R4 which abolishes the IgE response and has been tested in a murine model of allergic asthma. The treatment clearly decreased the severity of the symptoms.137
NPs combined with allergens have been applied for treating allergy. PLGA nanospheres particles loaded with Bet v 1, the major allergen of birch pollen, prevent the production of IgE and modify the Th2 response producing IFN-γ and IL-10.138 This can be considered the first example of that an allergen entrapped in PLGA polymer for allergy treatment reduces the predominance of the Th2 response.92 Poly(anhydride) NPs containing cashew nut proteins can induce a strong Th1 and Treg immune response after oral administration.92,139 This can be considered the first example of how an allergen entrapped in PLGA polymer for allergy treatment reduces the predominance of the Th2 response.
As result of the previous research 87,90, the first peanut oral immunotherapy clinical trial has started. It is a multicentre, double-blind, randomised, placebo-controlled phase I/II study (ClinicalTrials.gov Identifier: NCT04163562).
A copolymer based on poly(hydroxyethyl)aspartamide (PHEA) functionalised in the side chains with butyryl and succinyl moieties has been used to form nanoaggregates of 90 nm of diameter with the allergen lipid transfer protein Par j 1 and 2 (from Parietaria judaica pollen).140
After that, several examples demonstrate the use of NPs for allergy treatment in animal models. PLGA NPs have been used to load rChe a 3 (the most abundant allergen in Chenopodium album pollen) with the aim to modulate a Th2 immune responses by sublingual immunotherapy in a mouse model of allergic rhinitis.141 The Polyethyleneimine polymer loaded with Bet v 1 plasmid has demonstrated to induce a reduction of IgE and an increase of Th1 response in mice against birch pollen.142 Der f 1 plasmid complexed with a copolymer of propylene oxide and ethylene oxide has been used to reduce inflammation in asthmatic mice.143 Ara h 2 (peanut allergen) together with the adjuvant CpG was combined with protamine (arginine rich protein) NPs to induce a Th1 response.144 Natural polysaccharides have also been used as adjuvant together with allergens to decrease allergic responses. Alginate, extracted from algae, in combination with grass pollen extracts, induces a reduction of IgE in mice.145Studies using chitin as adjuvant produce a reduction of IgE and Th2 cytokines in allergic mice.146 Many other examples using biodegradable and non-biodegradable NPs have been revised recently,31 indicating that these platforms provide interesting approaches for the treatment of allergy.
Dendrosomes (hyperbranched dendritic spheroidal particles) have been used as adjuvants with plasmid for Bet v 1 in DNA vaccination in birch pollen allergy. In mice, footpath administration of these dendrosomes produces the inhibition of the allergic reaction, inhibiting the production of IgE, and maintaining the Th1/Th2 balance.142
Small CdSe/ZnS core/shell QDs NPs with a negative surface (using glutathione) show immunosuppressive effects when tested in skin allergy and used topically in combination with the allergen 1-fluoro-2,4-dinitrobenzene. The NPs penetrate the skin, facilitating their effect.147
Besides NPs, dendrimers have also been used to treat allergic diseases (Figure 3). Glycodendrimers containing mannoses (Table 1) have been conjugated with a peptide corresponding with a T-cell epitope of the major allergen of olive tree pollen (Ole e 1). This compound has demonstrated to induce Treg proliferation in vitro , in particular in cells from allergic donors.148 The same glycodendrimer conjugated with an epitope for the Pru p 3 lipid transfer protein induces long-term tolerance in a peach anaphylactic mice model when administered sublingually.149 These promising results indicate that glycodendrimers can be considered as promising adjuvants to be applied in allergy.