Preclinical high intensity focused ultrasound: mechanisms and applications

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Abstract

Pulsed FUS (pFUS) employs non-continuous exposures, that lower the rate of energy deposition and allow cooling to occur between pulses, thereby minimizing thermal effects and emphasizing the effects created by non-thermal mechanisms of FUS (i.e. acoustic radiation forces and acoustic cavitation). However, little is understood about the FUS effects these exposures have on tissue, especially with regard to cellular pro-homing factors (growth factors, cytokines, cell adhesion molecules). We examined changes in mice hamstring muscle following pFUS or cFUS and demonstrate that pFUS, unlike cFUS, has little effect on the histological integrity of muscle and does not induce cell death. Infiltration of macrophages was observed 3 and 8 days following pFUS or cFUS exposures. pFUS increased expression of several cytokines (e.g. IL-1alpha;, IL-1beta;, TNFalpha;, INFgamma;, MIP-1alpha;, MCP-1, GMCSF) creating a local cytokine gradient on days 0 and 1 following pFUS that returns to baseline levels by day 3 post-pFUS. pFUS exposures induced up-regulation of other signaling molecules (e.g. VEGF, FGF, PlGF, HGF, SDF-1alpha;) and cell adhesion molecules (e.g. ICAM-1 and VCAM-1) on muscle vasculature. The observed molecular changes in muscle following pFUS may be utilized to target cellular therapies, by increasing homing to areas of pathology. Bone marrow stromal cells (BMSCs) have shown significant promise in the treatment of disease, but their therapeutic efficacy is often limited by inefficient homing of systemically administered cells, which results in low number of cells accumulating at sites of pathology. BMSC home to areas of inflammation where local expression of integrins and chemokine gradients is present. We demonstrated that pFUS exposures that emphasize the mechanical effects of ultrasound-tissue interactions induced local and transient elevations of chemoattractants (i.e., cytokines, integrins, and growth factors) in the murine kidney. pFUS-induced upregulation of cytokines occurred through approximately 1 day post-treatment and returned to contralateral kidney levels by day 3. This window of significant increases in cytokine expression was accompanied by local increases of other trophic factors and integrins that have been shown to promote BMSC homing. When BMSCs were intravenously administered following pFUS treatment to a single kidney, enhanced homing, permeability, and retention of BMSC was observed in the treated kidney versus the contralateral kidney. Histological analysis revealed up to eight times more BMSC in the peritubular regions of the treated kidneys on days 1 and 3 post-treatment. Furthermore, cytokine levels in pFUS-treated kidneys following BMSC administration were found to be similar to controls, suggesting modulation of cytokine levels by BMSC. These results suggest that pFUS could potentially improve cell-based therapies as a noninvasive modality to target homing by establishing local chemoattractant gradients and increasing expression of cell adhesion molecules to enhance tropism of cells toward treated tissues.