Table 2.
Immune cells infiltrating the tumor microenvironment and their role in tumor progression.
| Cell type | Representative infiltrating population | Outcome | Main features of infiltrating cells inside the tumor microenvironment |
|---|---|---|---|
| Macrophages | M1 | TR | Activation of immune responses by MyDD88/TLR pathways |
| M2 | TP | Promotion of angiogenesis; suppression of CTL function; recruitment of CCR6+TREG; positive modulation of the tumorigenic and angiogenic potential of CSC | |
| T lymphocytes | CTL | TR | Specific tumor cell killing activity |
| TREG | TP/TR | TP: functional suppression of CTL, DC, NK cells, and macrophages. | |
| TR: correlation with good prognosis in some solid tumors, hypothetically due to lacking of suppressor activity and other unidentified activities | |||
| γδ T cells | TP/TR | TP: inhibition of CTL and NK cell activity; promotion of angiogenesis | |
| TR: cytotoxic activity, IFN-γ production | |||
| Dendritic cells | CD8α+ DC | TR | Processing and presentation of soluble tumor-associated antigens; Type I IFN-dependent CD8+ T cell cross-priming against antigens released from dying tumor cells |
| Plasmacytoid DC | TR | Processing and presentation of soluble tumor associated antigens | |
| TIM-3+ DC | TP | Suppression of HMGB1-dependent innate immune responses | |
| NK cells | NKp46+ | TR | Specific tumor cell killing activity by secretion of perforin and granzyme B-containing granules as well as release of calcium ions; DC editing; killing activity against CSC |
| NKp30+ | |||
| DNAM-1+ | |||
| CD69+ | |||
| CD155+ | |||
| NKT cells | NK receptors | TP/TR | TP: CD1d-restricted cytotoxic activity; IFN-γ production; APC stimulation |
| TCR-α chain variants | |||
| CD1d-restricted | TR: Th2 cytokine production | ||
| CD57+ | |||
| Myeloid-derived suppressor cells | CD11b+Gr-1+ | TP | Repression of the effector function of T lymphocytes and NK cells; highly present in late stages of tumor progression; promotion of TREG functions; promotion and sustainment of angiogenesis; present in elevated number in highly aggressive microenvironments |
| Cancer stem cells | CD34+ CD133+ | TP | Self-renewal function; tumor initiating activity; promotion and sustainment of angiogenesis; tumor resistance; sustainment of the tumor mass |
TP, tumor progression; TR, tumor regression.