Naturally occurring regulatory T cells show reduced sensitivity toward oxidative stress–induced cell death

CD4⁺CD25^brightFOXP3⁺CD127^dim/low regulatory T cells (Tregs) represent 4% to 5% of the CD4⁺ T cells and have a potent immunosuppressive capacity. They are critical for the prevention of autoimmunity and play an emerging role in cancer immunology.^1,2  The authors of several studies have demonstrated the presence of elevated levels of Tregs in cancer patients, a phenomenon that may promote tumor progression and influence the course of disease.^3-6  Increasing evidence indicates that tumor cells can induce Tregs directly or indirectly through their microenvironment by, for example, nitric oxide (NO) production, COX-2/PGE₂ expression, or interleukin (IL)-10.^7,8  Malignant cells and, more importantly, cells recruited or induced by tumor cells within the microenvironment, such as tumor-associated macrophages, activated granulocytes, and myeloid-derived suppressor cells, produce significant amounts of reactive oxygen species (ROS).^9,10  The detrimental effect of ROS such as hydrogen peroxide (H₂O₂) and reactive nitrogen species such as NO on natural killer (NK) and T cells is well established and described in malignant and chronic inflammatory diseases.^11-14  Paradoxically, Treg levels can be increased in this hostile (for lymphocytes) milieu

We studied the sensitivity of freshly isolated Tregs from peripheral blood to oxidative stress–induced cell death and resulting functional alterations. Furthermore, we evaluated the antioxidative capacity and components of the antioxidative machinery to gain a better understanding of our observations.

We demonstrate for the first time that Tregs show reduced sensitivity to oxidative stress–induced cell death and maintained their suppressive activity even at H₂O₂ levels that are lethal for half of the conventional effector CD4⁺CD25^−/low T-cell population. Greater expression levels of surface thiols and a stronger intracellular antioxidative capacity observed in Tregs may contribute to their reduced sensitivity to oxidative stress.

Peripheral blood mononuclear cells from healthy donors were isolated by Ficoll-Paque separation (Amersham Biosciences, Sunnyvale, CA). Tregs and conventional CD4⁺ T cells were purified with the use of a CD4⁺CD25^bright Treg isolation kit (purity >90%) and frequencies of memory (63.20% ± 13.81%) and naive (36.40% ± 13.42%) Tregs were in the expected range (Figure S1, available on the Blood website; see the Supplemental Materials link at the top of the online article). CD45RA⁺ and CD45RA⁻ subpopulations (purity > 95%) were purified by the use of anti-CD45RA/RO microbeads (Miltenyi Biotec, Auburn, CA). Granulocytes were isolated from the pellet obtained after Ficoll-Paque gradient centrifugation, followed by erythrocyte lysis.

This study received institutional review board approval from the Karolinska Institutet, and informed consent was obtained from donors in accordance with the Declaration of Helsinki.

Cells were stained with the use of mouse monoclonal antibody (mAb) against human CD3-FITC (UCHT1), CD4-FITC/-PE/-PerCP (RPA-T4), CD25-PE/-APC (M-A251), CD45RA-FITC/-APC (HI100), CD127-PE/-APC (HIL-7R-M21), Bcl-2-FITC (3F11), and FOXP3-FITC/PE (PCH101, eBioscience, San Diego, CA). Antibodies were purchased from BD Biosciences (San Jose, CA) if not stated otherwise. Surface thiols were detected with Alexa-Fluor-488–coupled maleimides (Invitrogen, Carlsbad, CA). Cells were acquired and analyzed on a FACSCalibur with the use of Cellquest Pro (BD Biosciences) and FlowJo software (TreeStar, Ashland, OR).

Tregs were cultured (18 hours) in AIM-V serum-free medium (Invitrogen, Carlsbad, CA) in the absence or presence of 10 μmol/L H₂O₂, then washed thoroughly before coculture with carboxyfluorescein succinimidyl ester (CFSE)–labeled CD25^−/low T cells. A total of 5 × 10⁴ CD4⁺CD25^−/low T cells was plated alone or together with 2.5 × 10⁴ Tregs and stimulated with the use of anti-CD2, anti-CD3, and anti-CD28 beads (Miltenyi Biotec, Auburn, CA) for 3 or 5 days. Proliferation was assessed by flow cytometry by calculating the mean fluorescence index (MFI) of the CFSE staining (FL-1) of each cell-cycle division.

Intracellular oxidation levels were determined by 5-(and-6)-chloromethyl-2′,7′-dichlorodihydrofluorescein diacetate acetyl ester (Invitrogen) staining that is metabolized to fluorescent (FL-1) H_2-2′,7′-dichlorofluorescein (DCF) upon oxidation.

Short-term culturing conditions were compared (AIM-V vs X-VIVO with/without human serum) to minimize spontaneous cell death, and serum-free AIM-V medium was chosen. Oxidative stress was induced by either addition of H₂O₂ or coculture with autologous granulocytes. Granulocyte-generated ROS effects were antagonized by the addition of human catalase (100 U/mL; Sigma-Aldrich, St Louis, MO). Cell death was assessed by 7-amino-actinomycin D (7-AAD) staining and forward-to-side-scatter profile (see Figure S2).

RNA was extracted (RNeasy mini kit; QIAGEN, Valencia, CA) and cDNA prepared (iScript cDNA synthesis kit; Bio-Rad, Hercules, CA). Messenger RNA levels were quantified by RT quantitative PCR (qRT-PCR; SYBR Green Supermix, iCycler; Bio-Rad). Relative gene expression was determined by normalizing the expression of each target gene to β-actin. For primer sequences (β-actin, catalase, Mn-/Zn-SOD, and FOXP3), see Table S1.

Previous reports have shown that T-cell subsets can differ in their susceptibility to oxidative stress–induced cell death.^13,15  In comparison with their conventional CD4⁺CD25^−/low counterparts, Tregs were significantly more resistant to cell death induced by exogenous H₂O₂ and granulocytes at all tested concentrations and cell-to-cell ratios (Figure 1A,B). In addition to the pure populations (majority of isolated Tregs were FOXP3⁺CD127^dim/low; see Figure S3), we performed identical experiments with negatively selected mixed CD4⁺ T-cell populations to rule out any triggering effects through the CD25 receptor during cell selection. The majority of the gated “untouched” CD4⁺CD25^bright Tregs were CD127^dim/low (87% ± 5%; see Figure S4), ruling out any false-positive gating on activated conventional CD4 T cells and displayed the same pattern of resistance to H₂O₂ as the bead-selected Tregs (Figure 1Aii).¹⁶  The addition of human catalase almost completely protected conventional T cells from granulocyte-mediated cell death, indicating that this effect was mediated by ROS (Figure 1Bii). Naive CD4⁺CD25^−/lowCD45RA⁺ T cells showed, as previously described, a significantly greater resistance to H₂O₂ than memory cells but were significantly less resistant to oxidative stress–induced cell death compared with Tregs when exposed to 20 μmol/L H₂O₂ (Figure 1C).¹⁵  This difference is even more prominent when comparing naive conventional cells with naive Tregs (Figure 1D), a recently described subpopulation that is less sensitive to death receptor (CD95) ligation-induced cell death.¹⁷  The proportion of naive Tregs among the viable Treg fraction increased upon induction of oxidative stress (Figure 1E) and correlated positively (R² = 0.94; P = .03) with the extent of H₂O₂ exposure, indicating a potential oxidative stress–driven modification of the Treg pool.¹⁸ 

The sensitivity to oxidative stress can be modified by expression of pro- and antiapoptotic molecules such as Bcl-2.¹⁹  Bcl-2 is differently expressed in various lymphoid subsets.²⁰  Tregs exhibited significantly lower Bcl-2 levels compared with conventional T cells, confirming previous results, and these levels remained stable upon exposure to oxidative stress (Figure 2A).²¹  Despite their lower Bcl-2 levels, Tregs show reduced activation-induced cell death and the underlying mechanism behind this action may be related to our findings.^21,22  Several mechanisms contribute to cellular protection against oxidative stress, one being proteins containing free thiol groups expressed at various levels on lymphoid subsets.^23,24  The amount of thiols per cell was significantly greater in the Treg population, which is similar to the observation made in oxidative stress-resistant CD56^bright NK cells (Figure 2B).²⁵  Tregs showed significantly lower basal intracellular oxidation levels when freshly isolated and upon H₂O₂ treatment, indicating a greater antioxidative capacity (Figure 2C). Well-described antioxidative enzymes protecting lymphocytes from ROS-induced cell death via up-regulation of Bcl-2 levels and direct metabolizing effects did not have a greater expression in the Tregs (Figure 2D).¹⁹ 

Lymphocytes exhibit aberrant function upon exposure to H₂O₂, a mechanism considered part of the immunosuppressive tumor microenvironment.¹³  Functional alteration of Tregs was evaluated by overnight exposure to 10 μmol/L H₂O₂, a concentration that is lethal for 50% of the conventional CD4⁺ T-cell population (LD₅₀; Figure 1A). These Tregs were then subjected to a suppression assay at a 1:2 Treg to T4 conv responder ratio. A significant loss of suppressive function could not be observed (Figure 2E,F).

Taken together, we demonstrate that ROS, besides impairing the immune system by functional alterations and apoptosis induction of NK- and T cells, may further exacerbate the immune dysfunctions by selective enrichment of Tregs. Several mechanisms leading to induction of peripheral Tregs in cancer have been reported but here, for the first time, a potential mechanism contributing to the selective enrichment of naturally occurring Tregs is described.^7,8 

We thank Dr Raja Choudhury for the critical review of the manuscript.

This work was supported by grants from the Swedish Cancer Society, the Cancer Society of Stockholm, the Karolinska Institutet, and an ALF-Project grant from the Stockholm City Council.

Contribution: D.M. and C.C.J. designed and performed research, analyzed data, and wrote the manuscript; and R.K. designed research and supervised data analysis and writing of the manuscript.

Conflict-of-interest disclosure: The authors declare no competing financial interests.

Correspondence: Rolf Kiessling, Professor, Department of Oncology and Pathology, Karolinska University Hospital, Solna, Cancer Center Karolinska R8:01, 171 76 Stockholm, Sweden; e-mail: Rolf.Kiessling@ki.se.