Killing magnetically-loaded cells by the application of alternating magnetic fields.

Applications of MNPs currently in preclinical stages include the  cell-targeted delivery of anticancer agents and molecular diagnosis. Recently, magnetic hyperthermia strategies passed preclinical trial stages and received regulatory approval as  a  clinical protocol for thermotherapy. Thus, with proper control of size and surface functionalisation, magnetic nanoparticles could become multifunctional intracellular agents for MRI, drug carriers and/or heat generators for hyperthermia. 

Loading MNPs with anticancer drugs and targeting to specific tumour sites is a promising strategy for the  detection and elimination of neoplastic tissue, even at the single-cell level.  However, a major obstacle for this approach is that the reticuloendothelial system  (RES) detects and  phagocytoses  MNPs, preventing  their 

targeting and therapeutic action. Attempts to overcome this difficulty (i.e., to evade the RES) have been made through the functionalisation of MNPs to mimic  biologic entities already present in the blood system, resulting in the generation of ‘stealth’ carriers.

Eukaryotic cells can easily be ‘targeted’ in vitro with MNPs of different sizes. Different mechanisms have been reported for the detection and/or uptake of inorganic nanoparticles by cells, depending on the type of core material and surface coating of the particles used. The general consensus is that the mechanisms involved in particle incorporation are strongly dependent on the cell type.

Dendritic cells (DCs) are the most important antigen-presenting cells, as they have a key role in the first steps of most immune responses. Therefore, DCs are good candidates as therapeutic tools for immune diseases  and malignant neoplasms. 

DCs are derived from bone marrow precursors and migrate to non-lymphoid tissues where they differentiate into dendritic cells. As antigen-presenting cells, DCs are active at the membrane level and are thus good candidates to  incorporate MNPs. Indeed, previously published  reports have demonstrated  that DCs can  incorporate peptides, viral RNA, bacterial DNA and other molecules. The uptake of antigens  by DCs  may occur by different processes, such as macropinocytosis, phagocytosis or receptor-mediated endocytosis; more recently, the ability of DCs to incorporate several types of solid particles within a broad size range has also been described.

The first results for a new strategy based on the use of dendritic cells as natural carriers of  magnetic nanoparticles have been developed at INA.  This investigation had two major goals: 

a) to study the inclusion and toxicity of magnetic nanoparticles in dendritic cells and 

b) to induce cell death by applying a time-varying magnetic field to the MNP-loaded DCs. 

The reason for using DCs as carriers of MNPs was  to mimic biological units and to elude the  immune response of the body. The advantage of using this strategy is that the cargo of the DCs could be designed to be either ‘nude’ magnetic MNPs for hyperthermia therapy or functionalised MNPs with specific drugs that could be released by the application of alternating magnetic fields (AMFs).

SEM images of DCs after the application of the AMF for 30 min: a) and b) cells not loaded with COOH(MNP)-; c) and d) DCs loaded with MNPs. Note the evident loss of membrane structure and ‘shrinking’ of the cells in c) and d). Note also in c) the large channels opened in the membrane.