Strategies for Targeting Immunosuppressive Myeloid Cells in a Murine Breast Cancer Model

The immune system is the body’s most exacting defense, combating everything from pathogens to cancerous cells, with incredible precision and accuracy. However, the function of the immune system can be disrupted by its own regulatory components. Overexpression of immature immune cells, known as myeloid derived suppressor cells (MDSCs), can downregulate the body’s T-cell anti-tumor response in cancer patients through multiple inhibitory mechanisms. MDSCs also diminish the effectiveness of current immunotherapies, such as Chimeric Antigen Receptor (CAR) T-cell therapy, by creating a very immunosuppressive environment within solid tumors. Therefore, depleting or inhibiting the activity of MDSCs is an active area of research. One possible therapeutic target is the beta form of the folate receptor (FR), which is expressed on some populations of myeloid cells. Our study used an induced 4T1 murine breast cancer tumor model. The spleen, bone marrow and tumors were harvested at 4 weeks and analyzed via immunofluorescence microscopy and flow cytometry to visualize localization and numbers of MDSCs in the tissues. In some studies, the thymidine analog Bromodeoxyuridine (BrdU) was injected into the animals 1 hour prior to tumor harvest to identify proliferating cells. Our results indicate that some proliferation of MDSCs may occur outside the bone marrow; therefore, current treatments that only target MDSCs within bone marrow may be ineffective. We were also able to quantify the expression of FR+ subsets of MDSCs in the tissues. Interestingly, while splenic MDSCs had uniform FR expression, distinct high and low FRb expressing populations of MDSCs were found within the tumor tissue. Future studies will explore the possibility of using novel folic acid (FA)-conjugated therapeutics, such as FA-conjugated histone deacetylase inhibitors, to target and destroy MDSCs in tissue thereby increasing the body’s anti-tumor immune response and maximizing the efficiency of current immunotherapy technology