Macrophages with highly polarized functions coexist in tissues throughout the body to ensure the modulation of immune responses. Traditionally, macrophages can be subdivided into classically activated M1 and alternatively activated M2 phenotypes. M1 cells provide the first line of immune defense and activate both innate and adaptive immunity, while M2 macrophages are responsible for the regulation of tissue regeneration, are involved in the clearance of apoptotic bodies and contribute to the immune suppression [
1]. Controlling the balance of pro-inflammatory versus anti-inflammatory macrophages may have paramount therapeutic benefit in all the world’s leading causes of death, such as in cardiovascular diseases [
2,
3], infections [
4], cancers [
5,
6], chronic inflammation [
7], diabetes [
8] or autoimmune reactions [
9].
The success of immunotherapy highlights the effectiveness of the immune system in tumor eradication. Tumors still develop in spite of the immune attack, because tumors are surrounded by immunosuppressive cells and can escape from immune surveillance by hampering the onset of an effective anti-tumor immune response [
10]. The tumor microenvironment (TME) consists of various immune cells, where macrophages form one of the most abundant cell populations. Solid tumors manipulate macrophage recruitment and regulate macrophage differentiation. Reprogrammed tumor-associated macrophages (TAM) supports tumor formation through upregulation of angiogenesis, growth factor production or immunosuppression, and these cells also promote metastasis and increase drug resistance [
5,
6], Unlike monocytes, macrophages have a long life span of months to years [
11]. Accordingly, macrophages are relatively resistant to most apoptotic stimuli, but are highly sensitive to two newly described inflammatory forms of regulated cell death, necroptosis [
12,
13] and pyroptosis [
14]. Necroptosis is a regulated event rather than an accidental cell death process in which the most critical contributors are receptor-interacting protein 1 (RIPK1) [
15], RIPK3 [
16] and mixed lineage kinase domain like pseudokinase (MLKL) [
17]. Necroptosis is known to play an important role in the pathogenesis of many diseases, such as neurodegenerative or inflammatory disorders, gastrointestinal, cardiovascular and pulmonary diseases [
18]. The critical receptors of macrophages such as death, pattern recognition, DNA binding, cytokine and adhesion receptors all have been identified as potential inducers of necroptosis [
19]. Necroptosis can be activated when apoptosis is blocked and pro-necroptotic proteins are released from caspase-8-mediated inhibition [
15]. Active caspase-8 blocks the necroptotic mode [
15] of action preferentially through the cleavage of RIPK1 [
20], RIPK3 [
21] and the cylindromatosis (CYLD) protein, which mediates deubiqutination of RIPK1 [
22]. The ubiquitination of RIPK1 by inhibitors of apoptosis proteins (cIAPs) initiates cell survival [
23]. The created ubiquitin network allows the activation of TGF-activated kinase 1 (TAK1), which mediates survival signals by (1) activating the NFκB and MAPK signaling pathways and thereby increasing the expression of several prosurvival molecules [
24,
25], (2) preventing the interaction between RIPK1 and cell death-related molecules [
26], (3) regulating RIPK1 phosphorylation directly [
27] or indirectly by activating I kappa B
kinases (IIKKα/IKKβ) [
28] or mitogen-activated protein kinase-activated protein kinase 2 (p38MAPK/MK2) [
29]. In addition to TAK1- and cIAP-mediated downregulation, more than 70 molecules play a role in the regulation of necroptosis [
18], among them Aurora kinase A (AURKA), which interacts directly with RIPK1 and RIPK3 in nontreated cells to reduce unwanted necroptosis [
30]. Its downstream target glycogen synthase kinase 3β (GSK3β) regulates the formation of the necrosome and suppresses necroptosis [
30]. In the absence of ubiquitylation and/or phosphorylation, RIPK1 transduces cell death signals, and when apoptotic pathways are blocked, necroptosis is activated. Thus, necroptosis is most frequently induced in in vitro experimental systems by utilizing pan caspase inhibitors in combination either with IAP antagonists, termed SMAC mimetics (SM) to block RIPK1 ubiquitination [
12], or with TAK1 inhibitors to prevent the phosphorylation of RIPK1 [
13]. Necroptotic cell death of macrophages has already been shown following treatment with SM [
31] or TAK1 inhibitors [
14].
Many clinical trials aim to modify the M1/M2 ratio, but currently, the targeted depletion of a unique macrophage subtype by specific cell death signals is not a therapeutic option. We aimed to identify circumstances in which M2 cells or TAMs are susceptible to cell death signals, but M1 cells remain resistant. We found that M2 macrophages were highly sensitive, while M1 macrophages were unaffected by TAK1 inhibitor-generated necroptosis. The resistant M1 macrophages harness AURKA-mediated inhibition in the downregulation of cell death. In contrary to TAK1 inhibitor, SM treatment results in necroptosis in both macrophage populations, highlighting that at least two different necroptotic pathways operate in macrophages. TAK1 inhibitor-induced necroptosis pushes the ratio of M1/M2 macrophages toward an inflammatory phenotype, which rationalizes the activation of necroptosis for therapeutic intervention in any disease where M1 functions are preferred.