[24, 70] Given that M1 macrophages do not express legumain, this legumain-based FDA-approved Drug Library DNA vaccine may be particularly useful for destroying M2-like TAMs. Another membrane protein involved in T-cell-mediated TAM depletion is CD1d, a strict target of Vα24-invariant natural killer T (NKT) cells. NKT cells are an independent factor

for favourable outcome in various human cancers.[71] The earlier explanation for the tumoricidal role of NKT cells emphasized the expression of CD1d on tumour cells, such as leukaemia and lymphoma cells.[71] However, this explanation faced a great challenge because the majority of human tumour cells are actually CD1d-negative. How do NKT cells reject CD1d-negative tumours? Song et al.[72, 73] provided an alternative answer to this question. They stated that TAMs were the major CD1d-positive cells co-localizing with NKT cells in primary human neuroblastomas and in AZD1208 mouse xenografts of neuroblastoma, and that TAMs were the major targets of NKT cells in CD1d-negative tumours. This discovery is important because it may guide the designs of NKT-mediated immunotherapy, alone or

in combination with other standard therapies. According to this notion, the agents that can promote the expression of CD1d in TAMs may improve the tumoricidal function of NKT cells. One such agent is retinoic acid, which can strongly up-regulate the CD1d expression in macrophages[74] and is now used as a standard therapeutic drug for high-risk neuroblastoma Teicoplanin in clinic.[71] However, the contribution of the NKT–TAM axis to the effects of retinoic acid on tumour suppression needs to be further explored. Although most TAMs exhibit immunosuppressive M2-like properties, they remain the plasticity for polarization,[75] which provides a potential for TAMs to re-polarize from tumour-promoting M2-type to tumoricidal M1-type. It is known that the polarization of macrophages largely depends on the local cytokine profiles. In detail, when high levels of Th1 cytokines, such as tumour necrosis factor (TNF), IL-12 and interferons (IFNs), are present, the pro-inflammatory M1 macrophages

will be established; whereas when exposed to Th2 cytokines, such as IL-4, IL-10, IL-13 and transforming growth factor-β (TGF-β), macrophages will polarize to M2 status.[4] Until now, several signalling pathways, especially the nuclear factor-κB (NF-κB) and the signalling transducer and activator of transcription (STAT) pathways are known to play pivotal roles in the transcriptional profile of macrophages.[6] Among those transcriptional factors, STAT1 and canonical NF-κB (p50p65 heterodimer) are essential for the M1 tumoricidal functions and trigger the expression of pro-inflammatory cytokines.[6] In contrast, TAMs harbouring activated STAT3 and STAT6 are not tumoricidal; instead, they exhibit M2 properties and facilitate cancer development.