*Nanomedical Applications: Immunotherapy*

Immunotherapy has become one of the effective treatment modalities for cancer: cytokine therapy, checkpoint-blockade therapy, adoptive T-cell transfer, and Chimeric Antigen Receptor T(CAR-T) cell therapy [120]. Immunotherapy not only treats primary tumors but also prevents metastasis and recurrence. Another opportunity for combinatorial immunotherapy is based on NP platforms because of their improved methods for tumor-cell detection, tumor imaging, and their ability to efficiently deliver drugs to target sites and protect drugs from endogenous enzymes [121]. Therefore, it is relevant to highlight how NPs may be engineered to overcome immunotherapy obstacles. In this mini review, we have discussed how NPs properties affect a biological mechanism and how they influence cellular internalization, biodistribution, and elimination. Therefore, we have enough information to understand how they alter immune responses.

NPs can release agents in response to biochemical changes in the target micro-environment (pH, redox potential, and enzymes) or to external stimuli (light, electrical, and magnetic fields) [122]. Due to that, targeted delivery of NPs and controlled drug release may allow the activation of immunotherapies in the action sites [123]. The use of NPs for delivery antigens, adjuvants, and other therapeutic agents resulted in more specific targeting and a better outcome in contrast to conventional immunotherapy. Advanced biomaterials and drug delivery systems, such as NPs and the use of T cells, have been designed to improve immunotherapy [124]. Moreover, NPs can deliver cytotoxic agents to tumor cells killing most of all the target cells with low concentrations of immune-stimulating drugs thanks to their potential to amplify T cell responses [120].

NP physicochemical properties can be tuned to stimulate the innate immune cells and to promote NP-immune cell interactions, which is a good therapeutic option [125]. Different strategies to enhance the efficacy of NP immunotherapy are the following [126,127]: Controlling the hydrophobicity surface (using hydrophilic polymers such as PEG) and a shape and rigidity optimization of NPs must reduce nonspecific uptake, which results in an efficient internalization. Enhancing tissue and cell penetration has been possible using peptide and chemical modifications to the NP surface such as cyclic iRGD peptide (CRGDK/RGDP/EC). Another important factor is targeting NPs and their bio-distribution to immune cells with ligands on NPs, such as T lymphocyte or B lymphocyte targeting. Nano-sized NPs have the advantage of accumulating within the tumor microenvironment with specific targeting, which minimizes o ff-target toxicity [125]. Then, once NPs reach the target cell, their biological activity occurs when they travel to the suitable intracellular compartment [127]. As a result, cationic polymers, pH-sensitive biomaterials, virus-derived cell-penetrating peptides, and direct cytosolic delivery must be used on NP in order to conduct appropriate intracellular delivery of NPs. Lastly, another approach to control immunotherapy is controlling the release kinetic.

The most common nanocarriers allowing specificity are liposomes, micelles, dendrimers, gold NPs, iron oxide NPs, carbon NPs, and quantum dots (NPs for tumor immunotherapy). Liposomes [128] are highly biocompatible and can be functionalized. However, they are widely studied for cancer immunotherapy. Micelles have a range application in cancer treatment because of their biodegradability and nontoxicity formulations, which makes them suitable for carrying therapeutic payloads. In addition, dendrimers [129] o ffer a highly specific NP physical properties thanks to their stepwise branching synthesis. Inorganic nanoparticles are well studied, such as gold NPs [123]. AuNPs are bio-inert and non-toxic nanocarriers, which, depending on their size, charge, shape, and functional group, may contribute to the e fficacy in accumulating di fferent immune cells [125]. The most studied functionalization for cancer immunotherapy is nanoparticles based on poly(lactic-co-glycolic acid) (PLGA NPs) because of their acceptance and biodegradability. Rosalia et al. [130] studied PLGA NPs functionalized with a αCD40-monoclonal antibody agonistic vaccine targeting dendritic cells (DCs). Two di fferent adjuvants targeting the toll like receptor (TLR) were encapsulated into PLGA NPs to induce potent CD8+ T cell responses. In vivo experiments in murine melanoma-OVA mouse model indicated that active targeting of DCs and vaccine delivery resulted in e fficient priming of CD8+ T cells, tumor control, and prolonged survival of the tumor-bearing mice.

Several programs work in integrated and interconnected research focused on therapeutically modifying the tumour micro-environment, (re)activation of anti-cancer immunity, and corresponding Drug Delivery System (DDS) [131]. Initially, they aim to develop new tumor-targeted drugs to selectively block key innate and adaptive immune checkpoints, such as PD-1, TIM-3, and CD47, in the tumor micro-environment. Furthermore, they aim to develop new tumor-targeted drugs to selectively activate key co-stimulatory receptors of the tumor necrosis factor receptor superfamily (TNFRSF) in the tumor micro-environment.

On the other hand, suitable drug delivery systems (DDS) could be developed using modern drug formulations based on nanotechnology and surface chemistry to achieve tumor-localized release and optimal localized co-stimulation of anti-cancer immunity. These developments will be attended by label-free detection of protein interactions by means of advanced bioanalysis methods [132]. That could ensure induction and execution of anti-cancer immune responses in the absence of systemic immune-related side-e ffects.

Lastly, immunotherapies help to amplify the knowledge and manipulation of the immune system and nanotechnology may be the cause of engineering remarkable mechanisms to produce an e ffective and long-lasting immune response against cancer.
