The role of regulatory T cells in cancer immunology

Figures & data

Figure 1 The phenotypic profile and potential cellular origins of induced (i)Treg present in the tumor microenvironment.

Notes: In the presence of tumor-derived factors, nTreg (also known as thymic-derived or (t)Treg) or conventional CD4+CD25- T effector cells differentiate into iTreg (also known as peripheral (p)Treg) and up-regulate expression of a variety of surface-associated molecules. It has been suggested that a transcription factor, Kruppel-like factor 2 (KLF2), may be necessary for the development of iTreg.⁴⁹ In contrast to nTreg, iTreg may or may not be FOXP3+ and CD25+: they carry both CD39 and CD73 on the cell 71 surface and actively produce ADO. At the tumor sites, iTreg overexpress inhibitory receptors CTLA-4, PD-1, TIM-3 and LAG-3 and up-regulate expression of TGF-β-associated LAP and GARP molecules⁸⁵ and NRP-1.⁷⁶ HELIOS may be a marker of human iTreg, although this is still unresolved at present.¹⁰⁷ iTreg present in the peripheral blood of cancer patients tend to express CD122 and CD123 instead of CD25.³⁴
Abbreviations: Treg, T regulatory cells; nTreg, näive Treg; tTreg, thymus-derived Treg; iTreg, inducible Treg; pTreg, peripheral Treg; 5′AMP, adenosine-5′-monophosphate; ADO, adenosine; CTLA-4, cytotoxic lymphocyte antigen-4; PD-1, programmed death-1; TIM-3, T cell immunoglobulin mucin-3; LAG-3, lymphocyte activation gene-3; TGF-β, transforming growth factor-beta; LAP, latency-associated protein; GARP, glycoprotein A repetitions predominant; NRP-1, Neuropilin-1; ADP, adenosine diphosphate; KLF2, Kruppel-like factor 2; TCR, T cell receptor; ATP, adenosine-5′-triphosphate.

Figure 2 Treg accumulating in the TME (activated iTreg) utilize various suppressive mechanisms to inhibit functions of Teff.

Notes: These include consumption of the available IL-2,⁶² production of ADO and PGE₂,⁷² release of inhibitory cytokines including TGF-β and IL-10,² release of FasL⁴⁶ and/or GrB,⁶³ and upregulation of NRP-1 which interacts with its ligand semaphorin-4a on lymphocytes or dendritic cells, resulting in better survival of Treg and greater Treg-mediated suppression.⁷⁶ iTreg also produce scores of exosomes^71,106 which carry all of the above-listed iTreg products as well as nucleic acids and deliver them to Teff, thus contributing additional inhibitory signals. The suppressive factors released by iTreg interact with the cognate receptors present on the Teff surface, which process and direct the negative signals to the respective molecular pathways. The final result is a partial or complete loss of effector functions in responder immune cells.
Abbreviations: Treg, T regulatory cells; TME, tumor microenvironment; iTreg, inducible Treg; Teff, T effector cells; IL, interleukin; ADO, adenosine; TGF-β, transforming growth factor-beta; GrB, granzyme B; NRP-1, Neuropilin-1; ATP, adenosine-5′-triphosphate; ADP, adenosine diphosphate; TCR, T cell receptor; cAMP, cyclic adenosine monophosphate.

Table 1 Potential molecular targets for therapeutic depletion or re-programming of human Treg

Targeted Treg marker	Therapeutic agent	Result	Reference
CD25 (IL-2R)	Anti-CD25 Ab* (daclizumab)	Reduced Treg	^Citation95
		Re-programming
	Denileukin diftitox* (Ontak)	Reduced Treg	^Citation109
		Reduced suppression
	Bivalent IL-2 fusion toxin (high affinity)	Depleted Treg function inhibited	^Citation110
TGF-β pathway	TGF-β antagonism
LAP, GARP, active TGF-β	Neutralizing anti-TGF-β Ab (GC1008; fresolimumab)*	Reduced Treg	^Citation111
		Inhibition of Treg generation
TGF-βR I/II	TGF-βR kinase inhibitors (LY2157299)*	Impairment of Treg function?	^Citation111
	Soluble TGF-βR I/II
Immune checkpoints	Blocking Abs	Treg reduced
CTLA-4	Ipilimumab*	Impaired Treg activity	^Citation112^–^Citation114
	Tremelimumab*	Treg re-programming?
PD-1	Pembrolizumab*
	Nivolumab
TIM-3, LAG-3	Abs in development
Adenosine pathway	Selective blockade	Selective Treg depletion
CD39	Neutralizing Abs	Treg reduced	^Citation100,Citation101
CD73
A2A R	Pharmacologic inhibition	Inhibition of ADO production	^Citation65,Citation68
		Impaired Treg activity	^Citation55,Citation94,Citation99
COX-2 pathway	Selective blockade	Inhibition of PGE2 production	^Citation45,Citation72,Citation94
	Celecoxib
	Indomethacin
	Diclofenac	Impaired Treg activity
	Ibuprofen
TNFR2	Blocking Ab	Impaired Treg activity and expansion	^Citation115,Citation116
GITR	Anti-GITR Ab	Attenuation of suppressor activity	^Citation117
Chemokine receptors (CCR 4, 5, 6, 10)	Receptor blockade migration	Impaired Treg	^Citation17
All Treg	High-dose cyclophosphamide*	Global Treg depletion (apoptosis)	^Citation88
	Low-dose cyclophosphamide*	Selective Treg depletion	^Citation88, ^Citation118^–^Citation120
		Decreased suppression
	Sunitinib*	Treg reduced	^Citation91
	Fludarabine*	Treg reduced	^Citation121
		Ameliorates Treg activity
PI(3)K p110(delta)	Pharmacologic inhibition	Inhibition of Treg suppression	^Citation122

Notes: Strategies that are listed have been used in preclinical studies.

* Those that have been translated to the clinic.

Abbreviations: Treg, T regulatory cells; IL, interleukin; Ab, antibody; TGF-β, transforming growth factor-beta; LAP, latency-associated protein; GARP, glycoprotein A repetitions predominant; CTLA-4, cytotoxic lymphocyte antigen-4; PD-1, programmed death-1; TIM-3, T cell immunoglobulin mucin-3; LAG-3, lymphocyte activation gene-3; ADO, adenosine; TNFR, tumor necrosis factor receptor 2.

Table 2 Immunotherapy clinical trials incorporating strategies for Treg depletion in patients with solid or hematologic malignancies

Cancer	Clinical trial gov number	Treg-targeting agent	Immunotherapy	Trial status
AML relapsed	NCT 01106950 Phase II	Cyclophosphamide Denileukin diftitox	Haploidentical NK cells Fludarabine	Terminated^Citation121
Colorectal metastatic to liver/lung	NCT 00986518 Phase I/II	Ex vivo sorting to deplete Treg Cyclophosphamide	Adoptive transfer of autologous lymphocytes Fludarabine	Completeda
Relapsed hematologic cancers	NCT 00675831 Phase I	Anti-CD25 Ab using CliniMACs	Depleted donor lymphocyte infusion	Completeda
Renal cell cancer metastatic	NCT 01462214 Phase I/II	Metronomic low-dose cyclophosphamide	Everolimus	Recruiting^Citation122
Melanoma stages IIIc–IV	NCT 022009384 Phase II	Denileukin diftitox	Ipilimumab	Recruiting
Melanoma	NCT 00847106 Phase I/II	Daclizumab	DC-based antitumor vaccine	Completeda
Melanoma recurrent stage IV	NCT 01307618 Phase II	Daclizumab	Peptide-based antitumor vaccine	Active, not recruiting
Metastatic breast, lung, colorectal, pancreatic cancers	NCT 00128622 Phase I	Denileukin diftitox	TRICOM DC-based vaccine	Completeda
Solid tumors (advanced, recurrent)	NCT 01929486 Phase I	Mogamulizumab	None	Recruiting
Melanoma stage IV	NCT 00056134 Phase I/II	Denileukin diftitox	Tumor peptide-loaded autologous DC	Completeda
Melanoma metastatic	NCT 00515528 Phase II	Denileukin diftitox	4-Peptide antitumor vaccine	Active, not recruiting
Mesothelioma	NCT 01241682 Phase I	Low-dose cyclophosphamide	Vaccine with tumor-lysate pulsed DC	Completeda

Notes: The above list of clinical trials includes 12/20 that can be found on the http://www.clinicaltrials.gov site. Clinical trials that were withdrawn or terminated are not listed.

a Results of these completed trials have not been posted.

Abbreviations: Treg, T regulatory cells; AML, acute myeloid lymphoma; NCT, national clinical trial; NK, natural killer; Ab, antibody; DC, dendritic cell.

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