Maria Grazia Petrillo

Pharmacological modulation of GITRL/GITR system: therapeutic perspectives

Glucocorticoid‐induced TNFR‐related (gitr) is a gene coding for a member of the TNF receptor superfamily. GITR activation by its ligand (GITRL) influences the activity of effector and regulatory T cells, thus participating in the development of immune response against tumours and infectious agents, as well as in autoimmune and inflammatory diseases. Notably, treating animals with GITR‐Fc fusion protein ameliorates autoimmune/inflammatory diseases while GITR triggering, by treatment with anti‐GITR mAb, is effective in treating viral, bacterial and parasitic infections, as well in boosting immune response against tumours. GITR modulation has been indicated as one of the top 25 most promising research areas by the American National Cancer Institute, and a clinical trial testing the efficacy of an anti‐GITR mAb in melanoma patients has been started. In this review, we summarize results regarding: (i) the mechanisms by which GITRL/GITR system modulates immune response; (ii) the structural and functional studies clearly demonstrating differences between GITRL/GITR systems of mice and humans; (iii) the molecules with pharmacological activities including anti‐GITR mAbs, GITR‐Fc and GITRL‐Fc fusion proteins, GITRL in monomer or multimer conformation; and (iv) the possible risks deriving from GITRL/GITR system pharmacological modulation. In conclusion, GITR triggering and inhibition could be useful in treating tumours, infectious diseases, as well as autoimmune and inflammatory diseases. However, differences between mouse and human GITRL/GITR systems suggest that further preclinical studies are needed to better understand how safe therapeutic results can be obtained and to design appropriate clinical trials.

CD4+CD25lowGITR+ cells: A novel human CD4+ T-cell population with regulatory activity

Treg subsets play a role in sustaining peripheral tolerance, are characterized by markers such as forkhead winged‐helix transcription factor (FOXP3) and CD25, and produce suppressive cytokines, such as IL‐10 and TGF‐β. Glucocorticoid‐induced TNF receptor family‐related (GITR) protein has been suggested to regulate Treg activity in mice. The aim of our study was to investigate GITR expression in human CD4+ T lymphocytes and its possible role in Treg function. Results indicate that a subset of CD4+ T cells in the peripheral blood expresses GITR and low levels of CD25 (CD4+CD25lowGITR+). These cells show Treg features as they express FOXP3, IL‐10, TGF‐β and are anergic but, as opposed to natural Tregs, express low levels of CTLA‐4 and are CD127high. CD4+CD25lowGITR+ cells represent a low percentage of the CD4+ T‐cell population (0.32–1.74%) and are mostly memory cells. Functional experiments demonstrated that CD4+CD25lowGITR+ cells have relevant suppressive activity that depends on TGF‐β. Moreover, an anti‐GITR Ab inhibited their suppressive activity, as observed in CD4+CD25+ murine Tregs. Taken together, these data indicate that human CD4+CD25lowGITR+ cells represent a distinct Treg subpopulation.

Glucocorticoid-Induced Tumour Necrosis Factor Receptor-Related Protein: A Key Marker of Functional Regulatory T Cells

Glucocorticoid-induced tumour necrosis factor receptor-related protein (GITR, TNFRSF18, and CD357) is expressed at high levels in activated T cells and regulatory T cells (Tregs). In this review, we present data from mouse and human studies suggesting that GITR is a crucial player in the differentiation of thymic Tregs (tTregs), and expansion of both tTregs and peripheral Tregs (pTregs). The role of GITR in Treg expansion is confirmed by the association of GITR expression with markers of memory T cells. In this context, it is not surprising that GITR appears to be a marker of active Tregs, as suggested by the association of GITR expression with other markers of Treg activation or cytokines with suppressive activity (e.g., IL-10 and TGF-β), the presence of GITR+ cells in tissues where Tregs are active (e.g., solid tumours), or functional studies on Tregs. Furthermore, some Treg subsets including Tr1 cells express either low or no classical Treg markers (e.g., FoxP3 and CD25) and do express GITR. Therefore, when evaluating changes in the number of Tregs in human diseases, GITR expression must be evaluated. Moreover, GITR should be considered as a marker for isolating Tregs.

GITR+ regulatory T cells in the treatment of autoimmune diseases

Autoimmune diseases decrease life expectancy and quality of life for millions of women and men. Although treatments can slow disease progression and improve quality of life, all currently available drugs have adverse effects and none of them are curative; therefore, requiring patients to take immunosuppressive drugs for the remainder of their lives. A curative therapy that is safe and effective is urgently needed. We believe that therapies promoting the in vivo expansion of regulatory T cells (Tregs) or injection of in vitro expanded autologous/heterologous Tregs (cellular therapy) can alter the natural history of autoimmune diseases. In this review, we present data from murine and human studies suggesting that 1) glucocorticoid-induced tumor necrosis factor receptor-related protein (GITR) plays a crucial role in thymic Treg (tTreg) differentiation and expansion; 2) GITR plays a crucial role in peripheral Treg (pTreg) expansion; 3) in patients with Sjögren syndrome and systemic lupus erythematosus, CD4(+)GITR(+) pTregs are expanded in patients with milder forms of the disease; and 4) GITR is superior to other cell surface markers to differentiate Tregs from other CD4(+) T cells. In this context, we consider two potential new approaches for treating autoimmune diseases consisting of the in vivo expansion of GITR(+) Tregs by GITR-triggering drugs and in vitro expansion of autologous or heterologous GITR(+) Tregs to be infused in patients. Advantages of such an approach, technical problems, and safety issues are discussed.

Characterization of a new regulatory CD4+ T cell subset in primary Sjögren’s syndrome

OBJECTIVE CD4(+)CD25(low)GITR(+) T lymphocytes expressing FoxP3 and showing regulatory function have been recently described in healthy donors (HD). The objective of the study was to investigate their presence and role in patients with primary SS (pSS). METHODS CD4(+)CD25(low)GITR(+) cells circulating in peripheral blood (PB) of patients with pSS were isolated by MACS technique, their phenotype was studied by flow cytometry and real-time PCR, and their function was studied by in vitro co-culture. CD4(+)CD25(low)GITR(+) cells infiltrating salivary glands (SGs) were revealed by immunohistochemistry. RESULTS Results indicated that conventional CD4(+)CD25(high) regulatory T cells (Tregs) are decreased, whereas CD4(+)CD25(low)GITR(+) cells are expanded in the PB of pSS as compared with HD. Phenotypic analysis demonstrated that CD4(+)CD25(low)GITR(+) cells display Treg markers, including FoxP3, TGF-β and IL-10, and functional experiments demonstrated that they exert a strong inhibitory activity against autologous effector cells. CD4(+)CD25(low)GITR(+) cells were detectable in great number in the SG inflammatory infiltrate. Interestingly, PB CD4(+)CD25(low)GITR(+) cell expansion was evident only in patients with inactive disease, while conventional CD4(+)CD25(high) Treg number was not associated with disease activity. CONCLUSION The present data demonstrate that circulating CD4(+) cells expressing GITR, but with low levels of CD25 (CD4(+)CD25(low)GITR(+)), are detectable in pSS patients. These cells, displaying Treg phenotype and function, are present in SG inflamed tissues and are expanded in the PB of subjects with inactive disease. Data suggest that the expansion of CD4(+)CD25(low)GITR(+) cells in pSS may represent a counter-regulatory attempt against autoimmune-driven inflammation and may provide a new target for future treatment strategies.

Expansion of regulatory GITR+CD25low/-CD4+ T cells in systemic lupus erythematosus patients

IntroductionCD4+CD25low/-GITR+ T lymphocytes expressing forkhead box protein P3 (FoxP3) and showing regulatory activity have been recently described in healthy donors. The objective of the study was to evaluate the proportion of CD4+CD25low/-GITR+ T lymphocytes within CD4+ T cells and compare their phenotypic and functional profile with that of CD4+CD25highGITR- T lymphocytes in systemic lupus erythematosus (SLE) patients.MethodsThe percentage of CD4+CD25low/-GITR+ cells circulating in the peripheral blood (PB) of 32 patients with SLE and 25 healthy controls was evaluated with flow cytometry. CD4+CD25low/-GITR+ cells were isolated with magnetic separation, and their phenotype was compared with that of CD4+CD25highGITR- cells. Regulatory activity of both cell subsets was tested in autologous and heterologous co-cultures after purification through a negative sorting strategy.ResultsResults indicated that CD4+CD25low/-GITR+ cells are expanded in the PB of 50% of SLE patients. Expansion was observed only in patients with inactive disease. Phenotypic analysis demonstrated that CD4+CD25low/-GITR+ cells display regulatory T-cell (Treg) markers, including FoxP3, cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), transforming growth factor-beta (TGF-β), and interleukin (IL)-10. In contrast, CD4+CD25highGITR- cells appear to be activated and express low levels of Treg markers. Functional experiments demonstrated that CD4+CD25low/-GITR+ cells exert a higher inhibitory activity against both autologous and heterologous cells as compared with CD4+CD25highGITR- cells. Suppression is independent of cell contact and is mediated by IL-10 and TGF-β.ConclusionsPhenotypic and functional data demonstrate that in SLE patients, CD4+CD25low/-GITR+ cells are fully active Treg cells, possibly representing peripheral Treg (pTreg) that are expanded in patients with inactive disease. These data may suggest a key role of this T-cell subset in the modulation of the abnormal immune response in SLE. Strategies aimed at expanding this Treg subset for therapeutic purpose deserve to be investigated.

Expansion of regulatory GITR

IntroductionCD4+CD25low/-GITR+ T lymphocytes expressing forkhead box protein P3 (FoxP3) and showing regulatory activity have been recently described in healthy donors. The objective of the study was to evaluate the proportion of CD4+CD25low/-GITR+ T lymphocytes within CD4+ T cells and compare their phenotypic and functional profile with that of CD4+CD25highGITR- T lymphocytes in systemic lupus erythematosus (SLE) patients.MethodsThe percentage of CD4+CD25low/-GITR+ cells circulating in the peripheral blood (PB) of 32 patients with SLE and 25 healthy controls was evaluated with flow cytometry. CD4+CD25low/-GITR+ cells were isolated with magnetic separation, and their phenotype was compared with that of CD4+CD25highGITR- cells. Regulatory activity of both cell subsets was tested in autologous and heterologous co-cultures after purification through a negative sorting strategy.ResultsResults indicated that CD4+CD25low/-GITR+ cells are expanded in the PB of 50% of SLE patients. Expansion was observed only in patients with inactive disease. Phenotypic analysis demonstrated that CD4+CD25low/-GITR+ cells display regulatory T-cell (Treg) markers, including FoxP3, cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), transforming growth factor-beta (TGF-β), and interleukin (IL)-10. In contrast, CD4+CD25highGITR- cells appear to be activated and express low levels of Treg markers. Functional experiments demonstrated that CD4+CD25low/-GITR+ cells exert a higher inhibitory activity against both autologous and heterologous cells as compared with CD4+CD25highGITR- cells. Suppression is independent of cell contact and is mediated by IL-10 and TGF-β.ConclusionsPhenotypic and functional data demonstrate that in SLE patients, CD4+CD25low/-GITR+ cells are fully active Treg cells, possibly representing peripheral Treg (pTreg) that are expanded in patients with inactive disease. These data may suggest a key role of this T-cell subset in the modulation of the abnormal immune response in SLE. Strategies aimed at expanding this Treg subset for therapeutic purpose deserve to be investigated.

CD8+ T cells: GITR matters.

As many members of the tumor necrosis factor receptor superfamily, glucocorticoid-induced TNFR-related gene (GITR) plays multiple roles mostly in the cells of immune system. CD8+ T cells are key players in the immunity against viruses and tumors, and GITR has been demonstrated to be an essential molecule for these cells to mount an immune response. The aim of this paper is to focus on GITR function in CD8+ cells, paying particular attention to numerous and recent studies that suggest its crucial role in mouse disease models.

Glucocorticoid-Induced Tumor Necrosis Factor Receptor Family-Related Ligand Triggering Upregulates Vascular Cell Adhesion Molecule-1 and Intercellular Adhesion Molecule-1 and Promotes Leukocyte Adhesion

The interaction of glucocorticoid-induced tumor necrosis factor receptor-family related (GITR) protein with its ligand (GITRL) modulates different functions, including immune/inflammatory response. These effects are consequent to intracellular signals activated by both GITR and GITRL. Previous results have suggested that lack of GITR expression in GITR−/− mice decreases the number of leukocytes within inflamed tissues. We performed experiments to analyze whether the GITRL/GITR system modulates leukocyte adhesion and extravasation. For that purpose, we first evaluated the capability of murine splenocytes to adhere to endothelial cells (EC). Our results indicated that adhesion of GITR−/− splenocytes to EC was reduced as compared with wild-type cells, suggesting that GITR plays a role in adhesion and that this effect may be due to GITRL-GITR interaction. Moreover, adhesion was increased when EC were pretreated with an agonist GITR-Fc fusion protein, thus indicating that triggering of GITRL plays a role in adhesion by EC regulation. In a human in vitro model, the adhesion to human EC of HL-60 cells differentiated toward the monocytic lineage was increased by EC pretreatment with agonist GITR-Fc. Conversely, antagonistic anti-GITR and anti-GITRL Ab decreased adhesion, thus further indicating that GITRL triggering increases the EC capability to support leukocyte adhesion. EC treatment with GITR-Fc favored extravasation, as demonstrated by a transmigration assay. Notably, GITRL triggering increased intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1) expression and anti–ICAM-1 and anti-VCAM-1 Abs reversed GITR-Fc effects. Our study demonstrates that GITRL triggering in EC increases leukocyte adhesion and transmigration, suggesting new anti-inflammatory therapeutic approaches based on inhibition of GITRL-GITR interaction.

Glucocorticoid-Induced TNFR family Related gene (GITR) enhances dendritic cell activity.

Glucocorticoid-Induced TNFR family Related gene (GITR), a Tumor Necrosis Factor Receptor Superfamily (TNFRSF) member involved in immune/inflammatory processes, has been previously shown to regulate T cell activation. To study GITR role in antigen presenting cells, we evaluated the capability of bone marrow derived dendritic cells (BMDC) from GITR(-/-) mice to stimulate the activation of CD4(+)CD25(-) T lymphocytes. We found that GITR(-/-) BMDC are weaker stimulators of T cell proliferation than GITR(+/+) BMDC, either in syngenic or allogenic BMDC/T cell co-cultures. Expression of GITR in GITR(-/-) BMDC restored their ability to activate T cells while GITR silencing in GITR(+/+) BMDC inhibited the capability to stimulate T cells. GITR(-/-) BMDC showed a reduced production of the pro-inflammatory cytokine IL-6 and an increased production of the anti-inflammatory cytokine IL-10. Notably, co-culture of CD4(+)CD25(-) cells with GITR(-/-) BMDC originated FoxP3(+) cells, secreting IL-10 and TGF-β. Finally, in vivo injection of GITR(-/-) OVA-loaded BMDC led to a lower cell number and a lower activated cell number in draining lymph nodes than in GITR(+/+) OVA-loaded BMDC injected mice. Together, these results indicate that GITR plays a role in regulating BMDC activity.