Background:

There has been increasing interest in leveraging cells’ natural ability to migrate towards areas of disease, respond to local environmental cues, and carry out useful behaviors to diagnose and treat disease. Macrophages, and the circulating monocytes they differentiate from, are an interesting chassis for cellular therapy and diagnostics in cancer as they are actively recruited to tumors and can form up to 50% of a tumor’s mass. Depending on their environment and the immune signaling, macrophages play a variety of roles in solid tumors including phagocytosis of cancer cells, extracellular matrix remodeling, and cytokine secretion. Because of their recruitment and potential to mediate tumor destruction, macrophages have become increasingly studied as potential targets for tumor therapies. Their active recruitment also supports their use for cancer detection.

Methods:

THP-1 cells were cultured in RPMI 1640 media and transduced with lentivirus containing genetic elements required for gas vesicle production. Transduced THP-1 cells were induced with doxycycline to initiate gene expression and imaged via ultrasound in 1% agarose phantoms using a L22-14vX transducer. Chemotaxis and migration assays were conducted in gas vesicle expressing monocytes and gene circuits conferring hypoxia and antigen specificity were cloned and tested as above.

Results:

THP-1 cells are capable of producing non-linear ultrasound contrast and performing key functions like migration and phagocytosis while expressing gas vescile. Furthermore, gas vesicle expression and ultrasound contrast production can be transcriptionally controlled using genetic elements that confine gene expression to specfic tumor-associated signals like hypoxia and tumor antigen binding. 

Conclusions:

Our results use the THP-1 monocyte cell line to establish proof of concept studies demonstrating that both monocytes and macrophages are capable of expressing GVs as cellular ultrasound contrast agents. Furthermore, monocytes can migrate in a 3um transwell assay towards a chemokine while carrying intracellular GV cargo, and macrophages are capable of phagocytosis following expression of GV. These results suggest that GV expression does not abrogate critical cellular functions that may be useful in downstream applications leveraging GV expressing macrophages as ultrasound contrast agents.