I reviewed this again after our last discussion on this topic and I came to the conclusion that the MOA is two fold with an FC gamma binding and by blocking the binding of phosphatidylserine-positive microvesicles to phosphatidylserine receptors on TAMs. Of course this is hypothesized MOA based on this study done by UTSW . This the discussion from that paper:
Recent evidence indicates that exposure of phosphatidylserine in the tumor microenvironment contributes to the immunosuppressed state of tumors (36, 43, 44). This suggests that the benefit of chemotherapy, radiotherapy, and other treatments that trigger tumor cell apoptosis is undermined by the increase in local tumor immunosuppression caused by phosphatidylserine exposed on dying tumor cells and their microvesicles (25, 43, 45). Here, we show that treatment of tumor-bearing mice with a phosphatidylserine-targeting antibody counteracts the tumor immunosuppression caused by chemotherapy and activates innate tumor immunity.
We show that 2aG4 functions at several levels to restore tumor immunity: (i) TAMs, which are predominantly in an immunosuppressive M2-like state in untreated or docetaxel-treated tumors, become tumoricidal M1-like TAMs; (ii) highly immunosuppressive monocytic MDSCs in the tumor become depleted, whereas their M1-like TAM and dendritic cell progeny increases; and (iii) immature dendritic cells in tumors become mature and express T-cell costimulatary molecules, indicating their potential to function as APCs. It should be noted that as this study was carried out in immunodeficient animals, the impact of Tregs and other memory T cells on the phenotype-switch of the myeloid infiltrate cannot be assessed. However, our previous studies have shown that 2aG4 treatment allows dendritic cells to present tumor antigens and generate glioma-specific cytotoxic T cells in an immunocompetent rat glioma model (36). These results suggest that 2aG4 treatment reactivates both innate and adaptive tumor immunity.
The M1-like TAMs generated by 2aG4 treatment caused the destruction of tumor endothelium, vascular shutdown, and tumor cell death. M1-like TAMs were the only cell type observed in contact with intact and disintegrating vascular endothelium. Most likely, M1-like TAMs bind via activating Fc? receptors to the antibody-coated endothelial cells and kill them by antibody-dependent cell-mediated cytotoxicity. Indeed, we have previously shown that macrophages lyse 2aG4-coated vascular endothelial cells in an Fc-dependent manner in vitro (36). 2aG4 does not mediate direct lysis of phosphatidylserine-expressing endothelial cells by complement (mouse or human). We attempted to deplete TAMs by systemic administration of liposomal clodronate but found, as have others (46), that TAMs were not depleted, most likely because of rapid liposome clearance by the liver and spleen. In addition to their vascular-damaging action, we show here that the M1-like TAMs synthesize NO and efficiently kill PC3 tumor cells in vitro, suggesting that they have direct tumoricidal activity in vivo.
One of the mechanisms by which 2aG4 induces TAM repolarization to an M1-like state is by binding to phosphatidylserine on the cell surface of TAM in an Fc-dependent manner. Our electron microscopy studies show that phosphatidylserine on the cell surface of TAM is due to the presence of phosphatidylserine-expressing microvesicles. It is likely that the phosphatidylserine-expressing microvesicles bind to phosphatidylserine receptors on the cell surface, sending signals that maintain TAMs in an anti-inflammatory M2-like state, similar to apoptotic cells (19–21). We hypothesize that 2aG4 binds to the microvesicles and ligates activating Fc? receptors on the same cell or adjacent cells, sending signals that override the anti-inflammatory phosphatidylserine receptor signal and activate M1 differentiation (Fig. 7). The identity of the phosphatidylserine receptors on TAMs responsible for the anti-inflammatory signals is unknown, but it could be Tim3 (30). Tim4-positive cells have been observed to bind microvesicles (47). Thus, 2aG4 could also block the binding of phosphatidylserine-positive microvesicles to phosphatidylserine receptors on TAMs in vivo. Although we cannot rule out the possibility that M1 progenitors are recruited from the blood, our data indicate that repolarization of resident M2-TAMs is the primary mechanism.
The reduction in MDSCs and the overall increase in the number of TAMs and mature dendritic cells in 2aG4-treated tumors suggest that differentiation of MDSCs is also inhibited by phosphatidylserine-expressing microvesicles in the tumor microenvironment. We speculate that 2aG4 binds to and stimulates MDSCs differentiation into TAMs and dendritic cells. We isolated monocytic MDSCs (CD11b+, Ly6Glo, and Ly6Chi) from the spleens of tumor-bearing mice (48), and cultured them with 2aG4 in the absence of additional growth factors. We found that 2aG4 treatment in vitro induces the differentiation of monocytic MDSCs into M1-like macrophages and dendritic cells (Fig. 6). These progeny secreted NO and their cytokine profile switched from immunosuppressive to immunostimulatory. Electron microscopy studies confirmed that MDSCs, like TAMs, are phosphatidylserine-positive due to the presence of phosphatidylserine-expressing microvesicles on their surface (Fig. 6). It is likely that dendritic cells are also prevented from maturing in tumors by the exposed phosphatidylserine, as dendritic cells have exposed phosphatidylserine and annexin 5A facilitates dendritic cell maturation in vitro (49). 2aG4 may stimulate dendritic cell maturation in tumors by a mechanism analogous to that shown in Fig. 7.
Bavituximab, the human chimeric version of 2aG4, in combination with chemotherapy, is being used to treat patients with cancer in randomized clinical trials (38–40). Impressive antitumor effects in patients with cancer have been obtained with other antibodies that enhance tumor immunity, including anti-PD1, anti-PD-1L, and ipilimumab (anti-CTLA-4; ref. 50). Unlike these antibodies, which inhibit negative feedback pathways in immune cell activation, bavituximab seems to act by reversing the immunosuppressive effects of exposed phosphatidylserine in the tumor microenvironment, resulting in activation of local tumor immunity and damage to tumor vasculature.
