Platelet-derived microparticles differentially regulate macrophage polarization
Platelet-derived microparticles (PMPs) shed from platelets upon activation and constitute almost 90% of the circulating microparticles. Due to their versatile cargo, PMPs were associated with the generation of immunosuppressive microenvironment and and thought to promote tumor growth. They are also potential candidates for prevention and treatment of autoimmune diseases. Macrophages are one of the enigmatic cells of the immune system. They are either categorized as ‘M1-type’, mediating an inflammatory environment or ‘M2-type’, mediating an immune suppressive environment. Cardinal signals resulting M1-tropic or M2-tropic macrophage differentiation is not fully understood. However, it is crucial to understand the inducers of macrophage polarization for therapeutic approaches. We aimed to understand the interaction between PMPs with macrophages and wished to understand mechanistic alterations upon macrophages engage with PMPs. In this thesis, we showed that activated human platelets released microparticles and they were internalized by macrophages differentiated from THP1 monocytic cell line. Internalized PMPs co-localized with late endosomes. The phagocytic capacity of M2- polarized THP1 macrophages were greater than M1-polarized macrophages. Strikingly, when THP1 derived macrophages were treated with standalone PMPs our results revealed that these macrophage were unable to mount any detectable cytokine secretion related to M1 or M2 type identity. This prompted us to encapsulate TLR agonists within PMPs and harness them as a carrier system. Different TLR ligands including TLR7 (sensing ssRNA) and TLR9 (sensing ss/ds DNA expressing CpG motifs) ligands were incorporated within PMPs via dehydration-rehydration method that was developed in our laboratory. Upon screening of several TLR agonist candidates on healthy donor PBMCs as well as on purified monocytes, we found that M1-like macrophage differentiation was TLR9 agonist D-type CpG oligodeoxynucleotide loaded PMP dependent whereas M2-like macrophage differentiation was dependent on TLR7 agonist R848 loaded PMPs. In conclusion, this work implicated that PMP treatment of macrophages loaded with suitable ligand combinations might regulate M1/M2 type macrophage differentiation and could be used efficiently either to control tumor development (M1) or to alleviate symptoms of auto-immune/auto-inflammatory diseases (M2).