p16(Ink4a) and senescence-associated β-galactosidase can be induced in macrophages as part of a reversible response to physiological stimuli

Constitutive p16^Ink4a expression, along with senescence-associated β-galactosidase (SAβG), are commonly accepted biomarkers of senescent cells (SCs). Recent reports attributed improvement of the healthspan of aged mice following p16^Ink4a-positive cell killing to the eradication of accumulated SCs. However, detection of p16^Ink4a/SAβG-positive macrophages in the adipose tissue of old mice and in the peritoneal cavity of young animals following injection of alginate-encapsulated SCs has raised concerns about the exclusivity of these markers for SCs. Here we report that expression of p16^Ink4a and SAβG in macrophages is acquired as part of a physiological response to immune stimuli rather than through senescence, consistent with reports that p16^Ink4a plays a role in macrophage polarization and response. Unlike SCs, p16^Ink4a/SAβG-positive macrophages can be induced in p53-null mice. Macrophages, but not mesenchymal SCs, lose both markers in response to M1- [LPS, IFN-α, Poly(I:C)] and increase their expression in response to M2-inducing stimuli (IL-4, IL-13). Moreover, interferon-inducing agent Poly(I:C) dramatically reduced p16^Ink4a expression in vivo in our alginate bead model and in the adipose tissue of aged mice. These observations suggest that the antiaging effects following eradication of p16^Ink4a-positive cells may not be solely attributed to SCs but also to non-senescent p16^Ink4a/SAβG-positive macrophages.

Aging of mice is associated with p16(Ink4a)- and β-galactosidase-positive macrophage accumulation that can be induced in young mice by senescent cells

Senescent cells (SCs) have been considered a source of age-related chronic sterile systemic inflammation and a target for anti-aging therapies. To understand mechanisms controlling the amount of SCs, we analyzed the phenomenon of rapid clearance of human senescent fibroblasts implanted into SCID mice, which can be overcome when SCs were embedded into alginate beads preventing them from immunocyte attack. To identify putative SC killers, we analyzed the content of cell populations in lavage and capsules formed around the SC-containing beads. One of the major cell types attracted by secretory factors of SCs was a subpopulation of macrophages characterized by p16(Ink4a) gene expression and β-galactosidase activity at pH6.0 (β-gal(pH6)), thus resembling SCs. Consistently, mice with p16(Ink4a) promoter-driven luciferase, developed bright luminescence of their peritoneal cavity within two weeks following implantation of SCs embedded in alginate beads. p16(Ink4a)/β-gal(pH6)-expressing cells had surface biomarkers of macrophages F4/80 and were sensitive to liposomal clodronate used for the selective killing of cells capable of phagocytosis. At the same time, clodronate failed to kill bona fide SCs generated in vitro by genotoxic stress. Old mice with elevated proportion of p16(Ink4a)/β-gal(pH6)-positive cells in their tissues demonstrated reduction of both following systemic clodronate treatment, indicating that a significant proportion of cells previously considered to be SCs are actually a subclass of macrophages. These observations point at a significant role of p16(Ink4a)/β-gal(pH6)-positive macrophages in aging, which previously was attributed solely to SCs. They require re-interpretation of the mechanisms underlying rejuvenating effects following eradication of p16(Ink4a)/β-gal(pH6)-positive cells and reconsideration of potential cellular target for anti-aging treatment.

Regulatory Rebound in IL-12-Treated Tumors Is Driven by Uncommitted Peripheral Regulatory T Cells

IL-12 promotes a rapid reversal of immune suppression in the tumor microenvironment. However, the adjuvant activity of IL-12 is short-lived due to regulatory T cell (Treg) reinfiltration. Quantitative analysis of Treg kinetics in IL-12-treated tumors and tumor-draining lymph nodes revealed a transient loss followed by a rapid 4-fold expansion of tumor Treg between days 3 and 10. Subset-specific analysis demonstrated that the posttreatment rebound was driven by the CD4(+)CD25(+)Foxp3(+) neuropilin-1(low) peripheral Treg (pTreg), resulting in a 3-5-fold increase in the pTreg to CD4(+)CD25(+)Foxp3(+) neuropilin-1(high) thymic Treg ratio by day 10. The expanding pTreg displayed hypermethylation of the CpG islands in Treg-specific demethylated region, CTLA-4 exon 2, and glucocorticoid-induced TNFR exon 5, were phenotypically unstable, and exhibited diminished suppressive function consistent with an uncommitted in vitro-induced Treg-like phenotype. In vitro culture of posttherapy Treg populations under Th1-promoting conditions resulted in higher levels of IFN-γ production by pTreg compared with thymic Treg, confirming their transitional state. Blockade of selected molecular mechanisms that are known to promote Treg expansion identified IDO-positive dendritic cells as the primary mediator of post-IL-12 pTreg expansion. Clinical implications of these findings are discussed.

Oral interleukin-10 alleviates polyposis via neutralization of pathogenic T-regulatory cells

Immune dysregulation drives the pathogenesis of chronic inflammatory, autoimmune, and dysplastic disorders. While often intended to address localized pathology, most immune modulatory therapies are administered systemically and carry inherent risk of multiorgan toxicities. Here, we demonstrate, in a murine model of spontaneous gastrointestinal polyposis, that site-specific uptake of orally administered IL10 microparticles ameliorates local and systemic disease to enhance survival. Mechanistic investigations showed that the therapeutic benefit of this treatment derived from neutralization of disease-promoting FoxP3(+)RoRγt(+)IL17(+) pathogenic T-regulatory cells (pgTreg), with a concomitant restoration of FoxP3(+)RoRγt(-)IL17(-) conventional T-regulatory cells (Treg). These findings provide a proof-of-principle for the ability of an oral biologic to restore immune homeostasis at the intestinal surface. Furthermore, they implicate local manipulation of IL10 as a tractable therapeutic strategy to address the inflammatory sequelae associated with mucosal premalignancy.

Characterization of iNOS(+) Neutrophil-like ring cell in tumor-bearing mice

Background

Myeloid-derived Suppressor Cells (MDSC) have been identified as tumor-induced immature myeloid cells (IMC) with potent immune suppressive activity in cancer. Whereas strict phenotypic classification of MDSC has been challenging due to the highly heterogeneous nature of cell surface marker expression, use of functional markers such as Arginase and inducible nitric oxide synthase (iNOS) may represent a better categorization strategy. In this study we investigated whether iNOS could be utilized as a specific marker for the identification of a more informative homogenous MDSC subset.

Methods

Single-cell suspensions from tumors and other organs were prepared essentially by enzymatic digestion. Flow cytometric analysis was performed on a four-color flow cytometer. Morphology, intracellular structure and localization of iNOS⁺ ring cells in the tumor were determined by cytospin analysis, immunofluorescence microscopy and immunohistochemistry, respectively. For functional analysis, iNOS⁺ ring subset were sorted and tested in vitro cell culture experiments. Pharmacologic inhibition of iNOS was performed both in vivo and in vitro.

Results

The results showed that intracellular iNOS staining distinguished a granular iNOS⁺ SSC^hi CD11b⁺ Gr-1^dim F4/80⁺ subset with ring-shaped nuclei (ring cells) among the CD11b⁺ Gr-1⁺ cell populations found in tumors. The intensity of the ring cell infiltrate correlated with tumor size and these cells constituted the second major tumor-infiltrating leukocyte subset found in established tumors. Although phenotypic analysis demonstrated that ring cells shared characteristics with tumor-associated macrophages (TAM), morphological analysis revealed a neutrophil-like appearance as detected by cytospin and immunofluorescence microscopy analysis. The presence of distinct iNOS filled granule-like structures located next to the cell membrane suggested that iNOS was stored in pre-formed vesicles and available for rapid release upon activation. Tumor biopsies showed large areas with infiltrating ring cells primarily surrounding necrotic areas. Importantly, these cells significantly impaired CD8⁺ T-cell proliferation and induced apoptotic death. The intratumoral accumulation and suppressive activity of ring cells could be blocked through pharmacologic inhibition of iNOS, demonstrating the critical role of this enzyme in mediating both the differentiation and the activity of these cells.

Conclusions

In this study, iNOS expression was linked to a homogeneous subset; ring cells with a particular phenotype and immune suppressive function, in a common and well-established murine tumor model; 4T-1. Since the absence of a Gr-1 homolog in humans has made the identification of MDSC much more challenging, use of iNOS as a functional marker of MDSC may also have clinical importance.

Dichotomous effects of IFN-γ on dendritic cell function determine the extent of IL-12-driven antitumor T cell immunity

Sustained intratumoral delivery of IL-12 and GM-CSF can overcome tumor immune suppression and promote T cell-dependent eradication of established disease in murine tumor models. However, the antitumor effector response is transient and rapidly followed by a T suppressor cell rebound. The mechanisms that control the switch from an effector to a regulatory response in this model have not been defined. Because dendritic cells (DC) can mediate both effector and suppressor T cell priming, DC activity was monitored in the tumors and the tumor-draining lymph nodes (TDLN) of IL-12/GM-CSF-treated mice. The studies demonstrated that therapy promoted the recruitment of immunogenic DC (iDC) to tumors with subsequent migration to the TDLN within 24-48 h of treatment. Longer-term monitoring revealed that iDC converted to an IDO-positive tolerogenic phenotype in the TDLN between days 2 and 7. Specifically, day 7 DC lost the ability to prime CD8(+) T cells but preferentially induced CD4(+)Foxp3(+) T cells. The functional switch was reversible, as inhibition of IDO with 1-methyl tryptophan restored immunogenic function to tolerogenic DC. All posttherapy immunological activity was strictly associated with conventional myeloid DC, and no functional changes were observed in the plasmacytoid DC subset throughout treatment. Importantly, the initial recruitment and activation of iDC as well as the subsequent switch to tolerogenic activity were both driven by IFN-γ, revealing the dichotomous role of this cytokine in regulating IL-12-mediated antitumor T cell immunity.

Nitric oxide short-circuits interleukin-12-mediated tumor regression

Interleukin-12 (IL-12) can promote tumor regression via activation of multiple lymphocytic and myelocytic effectors. Whereas the cytotoxic mechanisms employed by T/NK/NKT cells in IL-12-mediated tumor kill are well defined, the antitumor role of macrophage-produced cytotoxic metabolites has been more controversial. To this end, we investigated the specific role of nitric oxide (NO), a major macrophage effector molecule, in post-IL-12 tumor regression. Analysis of tumors following a single intratumoral injection of slow-release IL-12 microspheres showed an IFNγ-dependent sevenfold increase in inducible nitric oxide synthase (iNOS) expression within 48 h. Flow cytometric analysis of tumor-resident leukocytes and in vivo depletion studies identified CD11b(+) F4/80(+) Gr1(lo) macrophages as the primary source of iNOS. Blocking of post-therapy iNOS activity with N-nitro-L: -arginine methyl ester (L-NAME) dramatically enhanced tumor suppression revealing the inhibitory effect of NO on IL-12-driven antitumor immunity. Superior tumor regression in mice receiving combination treatment was associated with enhanced survival and proliferation of activated tumor-resident CD8+ T-effector/memory cells (Tem). These findings demonstrate that macrophage-produced NO negatively regulates the antitumor activity of IL-12 via its detrimental effects on CD8+ T cells and identify L-NAME as a potent adjuvant in IL-12 therapy of cancer.

Activated CD8+ T-effector/memory cells eliminate CD4+ CD25+ Foxp3+ T-suppressor cells from tumors via FasL mediated apoptosis

Tumor-resident CD8(+) T cells display a quiescent effector/memory phenotype that is maintained in part by infiltrating CD4(+) CD25(+) Foxp3(+) T-suppressor cells. Intratumoral delivery of IL-12, in contrast, can restore cytotoxic function to tumor-associated CD8(+) T cells and induce the apoptotic death of T-suppressor cells. Depletion of CD8(+) T cells from tumors before IL-12 treatment resulted in the abrogation of treatment-mediated T-suppressor cell apoptosis revealing a link between CD8(+) T cell activation and T-suppressor elimination. Furthermore, IL-12 failed to induce T-suppressor cell loss in IFN-gamma- or FasL-deficient mice demonstrating a requirement for IFN-gamma and FasL in this process. Adoptive transfer of wild-type CD8(+) T cells to FasL-knockout mice restored posttherapy T-suppressor cell elimination from tumors establishing that expression of FasL on CD8(+) T cells was sufficient to promote T-suppressor cell death. IL-12 failed to induce FasL on T-effectors in IFN-gamma-knockout mice demonstrating a requirement for IFN-gamma in FasL up-regulation. Adoptive transfer of wild-type CD8(+) T cells induced T-suppressor cell death in IFN-gamma-knockout mice confirming that autocrine IFN-gamma was sufficient for CD8(+) T cell FasL expression. These findings reveal a mechanism by which cytotoxic T cells can abrogate regulatory cell activity.

Central role of tumor-associated CD8+ T effector/memory cells in restoring systemic antitumor immunity

Sustained delivery of IL-12 and GM-CSF to tumors induces the activation of tumor-resident CD8(+) T effector/memory cells (Tem) followed by cytotoxic CD8(+) T effector cell expansion. To determine whether the secondary effectors expanded from tumor-associated Tem or were primed de novo, activation kinetics of tumor-draining lymph node (TDLN) CD8(+) T cells were analyzed. Treatment promoted a 4-fold increase in the numbers of TDLN CD8(+) T cells displaying a CD69(+)CCR5(+)CD62L(-) periphery-homing effector phenotype by day 4 posttherapy. Pulse labeling of tumor and TDLN T cells with BrdU confirmed that proliferation occurred exclusively within the draining lymph nodes between days 1 and 4 with subsequent migration of primed CD8(+) T effectors to tumors on day 7. Day 4 CD8(+) T effector cells preferentially homed to and lysed experimental, but not control, tumors, establishing tumor specificity. To determine whether the secondary CD8(+) T effector cell response was dependent on activation of tumor-resident CD8(+) Tem, mice that were selectively depleted of tumor-infiltrating CD8(+) T cells were treated and monitored for T effector priming. In the absence of tumor-resident CD8(+) Tem, T effector cell expansion was completely abrogated in the TDLN, revealing that restoration of CD8(+) Tem function was critical to the induction of secondary T effectors. T cell priming failed to occur in IFN-gamma or perforin knockout mice, demonstrating that the requirement for Tem activation was associated with induction of Tem cytotoxicity. These data confirm that intratumoral IL-12 plus GM-CSF induces de novo priming of tumor-specific CD8(+) T effector cells in the TDLN and establish the critical role of preexisting intratumoral CD8(+) Tem in driving this process.