Garp as a therapeutic target for modulation of T regulatory cell function

New insight into GARP striking role in cancer progression: application for cancer therapy

T regulatory cells play a crucial role in antitumor immunity suppression. Glycoprotein-A repetitions predominant (GARP), transmembrane cell surface marker, is mostly expressed on Tregs and mediates intracellular organization of transforming growth factor-beta (TGF-β). The physiological role of GARP is immune system homeostasis, while it may cause tumor development by upregulating TGF-β secretion. Despite the vast application of anti- programmed cell death protein-1 (PD-1)/programmed death-ligand 1 (PD-L1) and anti-cytotoxic T-lymphocyte Antigen-4 (CTLA-4) antibodies in immunotherapy, anti-GARP antibodies have the advantage of better response in patients who has resistance to anti-PD-1/PD-L1. Furthermore, simultaneous administration of anti-GARP antibody and anti-PD-1/PD-L1 antibody is much more effective than anti-PD-1/PD-L1 alone. It is worth mentioning that the GARP-mTGF-β complex is more potent than secretory TGF-β to induce T helper 17 cells differentiation in HIV + patients. On the other hand, TGF-β is an effective cytokine in cancer development, and some microRNAs could control its secretion by regulating GARP. In the present review, some information is provided about the undeniable role of GARP in cancer progression and its probable importance as a novel prognostic biomarker. Anti-GARP antibodies are also suggested for cancer immunotherapy.

GARP Polymorphisms Associated with Susceptibility to House Dust Mite-Sensitized Persistent Allergic Rhinitis in a Chinese Population

Background

Genetic variants in GARP (also known as LRRC32) have been reported to have significant associations with asthma and eczema in special populations, but little is known about allergic rhinitis. This study purposes to evaluate the association of single nucleotide polymorphisms (SNPs) in GARP with house dust mite (HDM)-sensitized persistent allergic rhinitis (PER) in a population of Han Chinese.

Methods

In this hospital-based case–control study, 534 HDM-sensitized PER patients and 451 healthy controls were recruited from East China. In this population, six SNPs in GARP were identified. Serum total and specific IgE levels were measured with ImmunoCAP. Secondary structure and minimum free energy were predicted by RNAfold.

Results

rs79525962 was associated with the risk of HDM-sensitized PER (P < 0.05). The individuals with CT+TT genotype demonstrated a higher risk of HDM-sensitized PER than those with CC genotype (adjusted OR = 1.393, 95% CI = 1.019–1.904). The homozygous genotype CC of rs3781699 rendered a lower risk of HDM-sensitized PER than the wild-type genotype AA (adjusted OR = 0.646, 95% CI = 0.427–0.976); however, the genotype and allele frequencies of rs3781699 demonstrated no associations with HDM-sensitized PER (P > 0.05). rs79525962 increased the risk of HDM-sensitized PER in the subgroup aged ≥16 years (adjusted OR = 1.745, 95% CI = 1.103–2.760), and this high risk was also found in the females (adjusted OR = 1.708, 95% CI = 1.021–2.856). The G-C haplotype of rs1320646-rs3781699 rendered a lower risk of HDM-sensitized PER than the common haplotype G-A (adjusted OR = 0.819, 95% CI = 0.676–0.993). The secondary structure of GARP altered in response to different genotypes of rs79525962 and rs3781699.

Conclusion

SNP rs79525962 in the GARP gene marks a risk locus of HDM-sensitized PER in Chinese Hans.

Changes in circulating TCF1- and GARP-associated regulatory T cell subsets reflect the clinical status of patients with chronic HBV infection

This study aimed to clarify the expression changes and clinical significance of regulatory T (Treg) cells and follicular regulatory T (TFR) cell subsets divided by glycoprotein A repetitions predominant protein (GARP) and T cell factor 1(TCF1) in peripheral blood of patients with chronic HBV infection. The peripheral blood of 26 chronic hepatitis B (CHB) patients, 27 inactive HBsAg carriers and 32 healthy controls were collected and GARP + percentages in Treg and TFR cells were analyzed by flow cytometry. In addition, Treg and TFR cell subsets sorted by CD62L and TCF1 were analyzed and compared. Correlation analyses were performed between Treg and TFR cell subpopulations and clinical parameters as well as cytokine concentrations, including IL-21, IL-10 and TGF-β1 in plasma. Circulating Treg and TFR levels were elevated in CHB patients. Moreover, GARP and TCF1 were up-regulated in circulating Treg and TFR cells of CHB patients. TCF1 + CD62L- Treg cells were increased while TCF1-CD62L + Treg cells were decreased in CHB patients. TCF1 + CD62L- and TCF1-CD62L- TFR cells were increased while TCF1 + CD62L + TFR cells were decreased in CHB patients. TCF1 + CD62L- Treg cells were positively correlated with HBV DNA, ALT and plasma IL-10, while TCF1 + CD62L + TFR cells were negatively correlated with HBV DNA, HBeAg, HBsAg, ALT, AST, T-BIL and positively correlated with plasma IL-21. Treg and TFR subsets sorted by TCF1, CD62L and GARP were changed in CHB patients. Changes in Treg and TFR functional subsets are associated with antiviral immunity in CHB patients.

Mechanisms of chemotherapeutic resistance and the application of targeted nanoparticles for enhanced chemotherapy in colorectal cancer

Colorectal cancer is considered one of the major malignancies that threaten the lives and health of people around the world. Patients with CRC are prone to post-operative local recurrence or metastasis, and some patients are advanced at the time of diagnosis and have no chance for complete surgical resection. These factors make chemotherapy an indispensable and important tool in treating CRC. However, the complex composition of the tumor microenvironment and the interaction of cellular and interstitial components constitute a tumor tissue with high cell density, dense extracellular matrix, and high osmotic pressure, inevitably preventing chemotherapeutic drugs from entering and acting on tumor cells. As a result, a novel drug carrier system with targeted nanoparticles has been applied to tumor therapy. It can change the physicochemical properties of drugs, facilitate the crossing of drug molecules through physiological and pathological tissue barriers, and increase the local concentration of nanomedicines at lesion sites. In addition to improving drug efficacy, targeted nanoparticles also reduce side effects, enabling safer and more effective disease diagnosis and treatment and improving bioavailability. In this review, we discuss the mechanisms by which infiltrating cells and other stromal components of the tumor microenvironment comprise barriers to chemotherapy in colorectal cancer. The research and application of targeted nanoparticles in CRC treatment are also classified.

Boosting regulatory T cell function for the treatment of autoimmune diseases – That’s only half the battle!

Regulatory T cells (Treg) represent a subset of specialized T cells that are essential for the regulation of immune responses and maintenance of peripheral tolerance. Once activated, Treg exert powerful immunosuppressive properties, for example by inhibiting T cell-mediated immune responses against self-antigens, thereby protecting our body from autoimmunity. Autoimmune diseases such as multiple sclerosis, rheumatoid arthritis or systemic lupus erythematosus, exhibit an immunological imbalance mainly characterized by a reduced frequency and impaired function of Treg. In addition, there has been increasing evidence that – besides Treg dysfunction – immunoregulatory mechanisms fail to control autoreactive T cells due to a reduced responsiveness of T effector cells (Teff) for the suppressive properties of Treg, a process termed Treg resistance. In order to efficiently treat autoimmune diseases and thus fully induce immunological tolerance, a combined therapy aimed at both enhancing Treg function and restoring Teff responsiveness could most likely be beneficial. This review provides an overview of immunomodulating drugs that are currently used to treat various autoimmune diseases in the clinic and have been shown to increase Treg frequency as well as Teff sensitivity to Treg-mediated suppression. Furthermore, we discuss strategies on how to boost Treg activity and function, and their potential use in the treatment of autoimmunity. Finally, we present a humanized mouse model for the preclinical testing of Treg-activating substances in vivo.

Reduced frequencies of Foxp3+GARP+ regulatory T cells in COPD patients are associated with multi-organ loss of tissue phenotype

Background

Expression of glycoprotein A dominant repeat (GARP) has been reported to occur only in activated human naturally occurring regulatory T cells (Tregs) and their clones, and not in activated effector T cells, indicating that GARP is a marker for bona fide Tregs. A different phenotype of chronic obstructive pulmonary disease (COPD) may have a different immunologic mechanism.

Objective

To investigate whether the distribution of Tregs defined by GARP is related to the multi-organ loss of tissue phenotype in COPD.

Methods

GARP expression on T cells from peripheral blood and bronchoalveolar lavage (BAL) collected from patients with COPD was examined by flow cytometry. The correlation of GARP expression to clinical outcomes and clinical phenotype, including the body mass index, lung function and quantitative computed tomography (CT) scoring of emphysema, was analyzed.

Results

Patients with more baseline emphysema had lower forced expiratory volume, body mass index (BMI), worse functional capacity, and more osteoporosis, thus, resembling the multiple organ loss of tissue (MOLT) phenotype. Peripheral Foxp3⁺GARP⁺ Tregs are reduced in COPD patients, and this reduction reversely correlates with quartiles of CT emphysema severity in COPD. Meanwhile, the frequencies of Foxp3⁺GARP⁻ Tregs, which are characteristic of pro-inflammatory cytokine production, are significantly increased in COPD patients, and correlated with increasing quartiles of CT emphysema severity in COPD. Tregs in BAL show a similar pattern of variation in peripheral blood.

Conclusion

Decreased GARP expression reflects more advanced disease in MOLT phenotype of COPD. Our results have potential implications for better understanding of the immunological nature of COPD and the pathogenic events leading to lung damage.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12931-022-02099-2.

FVB/N mouse strain regulatory T cells differ in phenotype and function from the C57BL/6 and BALB/C strains

Regulatory T cells (Treg) are vital to the maintenance of immune homeostasis. The genetic background of an inbred mouse strain can have a profound effect on the immune response in the animal, including Treg responses. Most Treg studies focus on animals created on the C57BL/6 or BALB/c background. Recent studies have demonstrated a difference in the phenotype and behavior of C57BL/6 and BALB/c Tregs. In this study, we have investigated the function of FVB/N Tregs compared to C57BL/6 and BALB/c. We observed that while FVB/N Tregs appear to suppress normally in a cell contact‐dependent system, FVB/N Tregs are less capable of suppressing when regulation depends on the secretion of a soluble factor. FVB/N Tregs produce IL‐10; however, TGF‐β was not detected in any culture from C57BL/6 or FVB/N. C57BL/6 Foxp3⁺ Tregs expressed more of the TGF‐β‐related proteins glycoprotein‐A repetitions predominant (GARP) and latency‐associated peptide (LAP) on the cell surface than both FVB/N and BALB/c, but C57BL/6 Tregs expressed significantly less Ctse (Cathepsin E) mRNA. Each strain displayed different abilities of thymic Tregs (tTreg) to maintain Foxp3 expression and had a varying generation of induced Tregs (iTregs). In vitro generated FVB/N iTregs expressed significantly less GARP and LAP. These results suggest Tregs of different strains have varying phenotypes and dominant mechanisms of action for the suppression of an immune response. This information should be taken into consideration when Tregs are examined in future studies, particularly for therapeutic purposes in a genetically diverse population.

Current Strategies to Modulate Regulatory T Cell Activity in Allergic Inflammation

Over the past decades, atopic diseases, including allergic rhinitis, asthma, atopic dermatitis, and food allergy, increased strongly worldwide, reaching up to 50% in industrialized countries. These diseases are characterized by a dominating type 2 immune response and reduced numbers of allergen-specific regulatory T (Treg) cells. Conventional allergen-specific immunotherapy is able to tip the balance towards immunoregulation. However, in mouse models of allergy adaptive transfer of Treg cells did not always lead to convincing beneficial results, partially because of limited stability of their regulatory phenotype activity. Besides genetic predisposition, it has become evident that environmental factors like a westernized lifestyle linked to modern sanitized living, the early use of antibiotics, and the consumption of unhealthy foods leads to epithelial barrier defects and dysbiotic microbiota, thereby preventing immune tolerance and favoring the development of allergic diseases. Epigenetic modification of Treg cells has been described as one important mechanism in this context. In this review, we summarize how environmental factors affect the number and function of Treg cells in allergic inflammation and how this knowledge can be exploited in future allergy prevention strategies as well as novel therapeutic approaches.

GARP: A Key Target to Evaluate Tumor Immunosuppressive Microenvironment

Simple Summary

Tumors are not only composed of cancer cells but also of various infiltrating cells constituting the tumor microenvironment (TME); all these cells produce growth factors which contribute to tumor progression and invasiveness. Among them, transforming growth factor-β1 (TGF-β1) has been shown to be a potent immunosuppressive cytokine favoring cell proliferation and invasion and to be associated with resistance to anticancer treatments. Glycoprotein-A repetition predominant (GARP) plays a critical role in the activation of TGF-β1 and has been shown to be expressed at the membrane of cancer cells and also of regulatory T cells and platelets in the TME. An increased GARP expression has been shown in a variety of cancers. The objective of this review is to highlight GARP’s expression and function in cancer and to evaluate its potential as a predictive and therapeutic follow-up biomarker that could be assessed, in real time, by molecular imaging.

Abstract

Glycoprotein-A repetitions predominant (GARP) is the docking receptor for latent transforming growth factor (LTGF-β) and promotes its activation. In cancer, increased GARP expression has been found in many types of cancer. GARP is expressed by regulatory T cells and platelets in the tumor microenvironment (TME) and can be also expressed by tumor cells themselves. Thus, GARP can be widely present in tumors in which it plays a major role in the production of active TGF-β, contributing to immune evasion and cancer progression via the GARP-TGF-β pathway. The objective of this review is to highlight GARP expression and function in cancer and to evaluate the potential of membrane GARP as a predictive and therapeutic follow-up biomarker that could be assessed, in real time, by molecular imaging. Moreover, as GARP can be secreted, a focus will also be made on soluble GARP as a circulating biomarker.

GARP and GARP-Treated tDC Prevented the Formation of Atherosclerotic Plaques in ApoE-/- Mice

Purpose

This study aims to clarify the specific mechanism by which GARP affects the atherosclerotic plaques in ApoE^−/- mice and the effect of GARP-tDC on atherosclerosis.

Methods

The mice were randomly divided into three groups: the control group, the GARP-overexpressed group and the GARP-inhibited group. After 12 weeks, all the mice were euthanized, and the specimens were collected. In vitro, experiments were conducted to observe the effect of GARP on DC phenotype and the changes of the proportion of CD4⁺CD25⁺Foxp3⁺ Treg cells when GARP-tDCs were co-cultured with CD4⁺ T cells. Furthermore, adoptive transmission of GARP-tDCs was used to observe the effect on atherosclerotic plaque in mice.

Results

The GARP-overexpressed group enhanced the biological activity of Foxp3⁺ CD4⁺CD25⁺ Tregs and resulted in increased expression of LAP in T cells. In addition, the GARP-overexpressed group significantly suppressed the function of Th1 and Th17, and decreased the secretion of INF-γ and IL-17A. Thus, GARP had a protective effect on atherosclerosis. In vitro, we found that GARP-tDC had a tolerance-inducing phenotype, and GARP-tDC also had the ability to induce tolerance when co-cultured with CD4⁺ T cells. More importantly, adoptive transmission of GARP-tDCs reduced the size of atherosclerotic plaques.

Conclusion

GARP and the GARP-tDC play protective roles in atherosclerosis. The protective effect of GARP on atherosclerosis is achieved by increasing CD4⁺CD25⁺Foxp3⁺ Treg cells and inhibiting the production of IFN-γ and IL-17A.

Novel anti-GARP antibody DS-1055a augments anti-tumor immunity by depleting highly suppressive GARP+ regulatory T cells

Regulatory T (Treg) cells, which are essential for maintaining self-tolerance, inhibit anti-tumor immunity, consequently hindering protective cancer immunosurveillance, and hampering effective anti-tumor immune responses in tumor-bearing hosts. Here, we show that depletion of Treg cells via targeting glycoprotein A repetitions predominant (GARP) induces effective anti-tumor immune responses. GARP was specifically expressed by highly suppressive Treg cells in the tumor microenvironment (TME) of multiple cancer types in humans. In the periphery, GARP was selectively induced in Treg cells, but not in effector T cells, by polyclonal stimulation. DS-1055a, a novel afucosylated anti-human GARP monoclonal antibody, efficiently depleted GARP+ Treg cells, leading to the activation of effector T cells. Moreover, DS-1055a decreased FoxP3+CD4+ T cells in the TME and exhibited remarkable anti-tumor activity in humanized mice bearing HT-29 tumors. We propose that DS-1055a is a new Treg-cell-targeted cancer immunotherapy agent with augmentation of anti-tumor immunity.

Control of regulatory T cell homeostasis

CD4⁺ Foxp3⁺ T Regulatory (Treg) cells play a critical role in the homeostasis and maintenance of the immune system. The understanding of different aspects of Treg cells biology remains an intensively investigated subject as altering their generation, stability, or function by drugs or biologics may have therapeutic value in the treatment of autoimmune and inflammatory diseases as well as cancers. This review will focus on recent studies on the role of cytokines, T Cell Receptor (TCR) and co-stimulatory/co-inhibitory molecules signaling, location and metabolism on the homeostasis and stability of Treg cells. The potential for therapeutic manipulation of each of these factors will be discussed.

CD19-targeted CAR regulatory T cells suppress B cell pathology without GvHD

Regulatory T cells (Tregs) play essential roles in maintaining immunological self-tolerance and preventing autoimmunity. The adoptive transfer of antigen-specific Tregs has been expected to be a potent therapeutic method for autoimmune diseases, severe allergy, and rejection in organ transplantation. However, effective Treg therapy has not yet been established because of the difficulty in preparing a limited number of antigen-specific Tregs. Chimeric antigen receptor (CAR) T cells have been shown to be a powerful therapeutic method for treating B cell lymphomas, but application of CAR to Treg-mediated therapy has not yet been established. Here, we generated CD19-targeted CAR (CD19-CAR) Tregs from human PBMCs (hPBMCs) and optimized the fraction of the Treg source as CD4+CD25+CD127loCD45RA+CD45RO-. CD19-CAR Tregs could be expanded in vitro while maintaining Treg properties, including high expression of the latent form of TGF-β. CD19-CAR Tregs suppressed IgG antibody production and differentiation of B cells via a TGF-β-dependent mechanism. Unlike conventional CD19-CAR CD8+ T cells, CD19-CAR Tregs suppressed antibody production in immunodeficient mice that were reconstituted with hPBMCs, reducing the risk of graft-versus-host disease. Therefore, the adoptive transfer of CD19-CAR Tregs may provide a novel therapeutic method for treating autoantibody-mediated autoimmune diseases.

CTRP1 Attenuates UUO-induced Renal Fibrosis via AMPK/NOX4 Pathway in Mice

C1q/TNF-related protein 1 (CTRP1), a conserved protein of the C1q family, plays a key role in cardiovascular and metabolic diseases. However, the role of CTRP1 in renal injury is unclear. The purpose of this study is to explore the role of CTRP1 in unilateral ureteral obstruction (UUO)-induced renal fibrosis and to elucidate the underlying mechanism. Using gene delivery system, CTRP1 was overexpressed in the kidney, then the mice were operated to induce UUO model after adenovirus transfection. It was found that the expression of CTRP1 in the renal tissue was decreased in mice after UUO. CTRP1 overexpression decreased the kidney function and kidney weight index. Moreover, CTRP1 reduced oxidative stress and renal collagen deposition in vivo. As expected, we found that CTRP1 activated AMP-activated kinase (AMPK) and decreased NOX4 expression, while silencing AMPKα1 abolished the protective effects of CTRP1 overexpression in mice after UUO. In conclusion, CTRP1 may protect against UUO-induced renal injury via AMPK/NOX4 signaling. Our results indicate that CTRP1 exhibits potential effects to treat renal fibrosis caused by UUO.

TGF-β1 - A truly transforming growth factor in fibrosis and immunity

'Jack of all trades, master of everything' is a fair label for transforming growth factor β1 (TGF-β) - a cytokine that controls our life at many levels. In the adult organism, TGF-β1 is critical for the development and maturation of immune cells, maintains immune tolerance and homeostasis, and regulates various aspects of immune responses. Following acute tissue damages, TGF-β1 becomes a master regulator of the healing process with impacts on about every cell type involved. Divergence from the tight control of TGF-β1 actions, for instance caused by chronic injury, severe trauma, or infection can tip the balance from regulated physiological to excessive pathological repair. This condition of fibrosis is characterized by accumulation and stiffening of collagenous scar tissue which impairs organ functions to the point of failure. Fibrosis and dysregulated immune responses are also a feature of cancer, in which tumor cells escape immune control partly by manipulating TGF-β1 regulation and where immune cells are excluded from the tumor by fibrotic matrix created during the stroma 'healing' response. Despite the obvious potential of TGF-β-signalling therapies, globally targeting TGF-β1 receptor, downstream pathways, or the active growth factor have proven to be extremely difficult if not impossible in systemic treatment regimes. However, TGF-β1 binding to cell receptors requires prior activation from latent complexes that are extracellularly presented on the surface of immune cells or within the extracellular matrix. These different locations have led to some divergence in the field which is often either seen from the perspective of an immunologists or a fibrosis/matrix researcher. Despite these human boundaries, there is considerable overlap between immune and tissue repair cells with respect to latent TGF-β1 presentation and activation. Moreover, the mechanisms and proteins employed by different cells and spatiotemporal control of latent TGF-β1 activation provide specificity that is amenable to drug development. This review aims at synthesizing the knowledge on TGF-β1 extracellular activation in the immune system and in fibrosis to further stimulate cross talk between the two research communities in solving the TGF-β conundrum.

Treg-mediated acquired resistance to immune checkpoint inhibitors

T Regulatory cells (Tregs) act as a double-edged sword by regulating immune homeostasis (protective role) and inhibiting immune responses in different disease settings (pathological role). They contribute to cancer development and progression by suppressing T effector cell (Teff) functions. Decreased ratios of intratumoral CD8⁺ T cells to Tregs have been associated with poor prognosis in most cancer types. Targeting immune checkpoints (ICs), such as cytotoxic T lymphocyte-associated antigen-4 (CTLA-4) and programmed cell death-1 (PD-1), by immune checkpoint inhibitors (ICIs) in cancer patients has been beneficial in inducing anti-tumor immune responses and improving clinical outcomes. However, response rates remain relatively low, ranging from 15 to 40% depending on cancer type. Additionally, a significant proportion of patients who initially demonstrates a clinical response can acquire resistance overtime. This acquired resistance could occur due to the emergence of compensatory mechanisms within the tumor microenvironment (TME) to evade the anti-tumor effects of ICIs. In this review, we describe the immunosuppressive role of Tregs in the TME, the effects of currently approved ICIs on Treg phenotype and function, and the mechanisms of acquired resistance to ICIs mediated by Tregs within the TME, such as the over-expression of ICs, the up-regulation of immunosuppressive molecules, and apoptotic Treg-induced immunosuppression. We also describe potential therapeutic strategies to target Tregs in combination with ICIs aiming to overcome such resistance and improve clinical outcomes. Elucidating the Treg-mediated acquired resistance mechanisms should benefit the designing of well-targeted therapeutic strategies to overcome resistance and maximize the therapeutic efficacy in cancer patients.

IGHG, IGKC, and FCGR genes and endogenous antibody responses to GARP in patients with breast cancer and matched controls

Glycoprotein-A repetitions predominant (GARP) is a transmembrane protein that is highly expressed in breast cancer. Its overexpression correlates with worse survival, and antibodies to GARP appear to play a protective role in a mouse model. No large-scale studies of immunity to GARP in humans have yet been undertaken. In this investigation, using a large multiethnic cohort (1738 subjects), we aimed to determine whether the magnitude of anti-GARP antibody responsiveness was significantly different in patients with breast cancer from that in matched healthy controls. We also investigated whether the allelic variation at the immunoglobulin GM (γ marker), KM (κ marker), and Fcγ receptor (FcγR) loci contributed to the interindividual variability in anti-GARP IgG antibody levels. A combined analysis of all subjects showed that levels of anti-GARP antibodies were significantly higher in patients with breast cancer than in healthy controls (mean ± SD: 7.4 ± 3.5 vs. 6.9 ± 3.5 absorbance units per mL (AU/μL), p < 0.0001). In the two populations with the largest sample size, the probability of breast cancer generally increases as anti-GARP antibody levels increase. Several significant individual and epistatic effects of GM, KM, and FcγR genotypes on anti-GARP antibody responsiveness were noted in both patients and controls. These results, if confirmed by independent investigations, will aid in devising personalized GARP-based immunotherapeutic strategies against breast cancer and other GARP-overexpressing malignancies.

Mesenchymal stem cells inhibit T cell activation by releasing TGF-β1 from TGF-β1/GARP complex

Intervention with mesenchymal stem cells (MSCs) reveals a promising therapeutic tool to treat transplantation and autoimmune disease due to their immunoregulation capability. But the mechanisms of action are not fully investigated yet. Transforming growth factor-β1 (TGF-β1) exhibit multiple effects in migration, differentiation, and immunomodulation of MSCs. Glycoprotein A repetitions predominant (GARP) is an important marker of activated Treg (regulatory T cells). GARP binds latent TGF-β1 to regulate its activation, which is the indispensable step in Treg suppressing effector T cells. So far we don't know whether GARP present on MSCs and its association with MSCs function. Our study show that MSCs express GARP which binds latent TGF-β1 on their cell surface. We also found that TGF-β1+/- MSCs produce less TGF-β1 and exhibit reduced capacity in inhibiting T cells. When TGF-β1 signaling pathway was blocked, MSCs show decreased activity in inhibiting T cells. Importantly, silencing GARP expression distinctively damaged the capacity of MSCs to inhibit IFN-γ production. These findings indicated the expression of GARP on MSCs and its functionality in activating LAP, thus demonstrating GARP as a novel biomarker and new target to improve the therapeutic efficacy of MSCs.

Glycoprotein A repetitions predominant (GARP) positively regulates transforming growth factor (TGF) β3 and is essential for mouse palatogenesis

Glycoprotein A repetitions predominant (GARP) (encoded by the Lrrc32 gene) plays important roles in cell-surface docking and activation of TGFβ. However, GARP's role in organ development in mammalian systems is unclear. To determine the function of GARP in vivo, we generated a GARP KO mouse model. Unexpectedly, the GARP KO mice died within 24 h after birth and exhibited defective palatogenesis without apparent abnormalities in other major organs. Furthermore, we observed decreased apoptosis and SMAD2 phosphorylation in the medial edge epithelial cells of the palatal shelf of GARP KO embryos at embryonic day 14.5 (E14.5), indicating a defect in the TGFβ signaling pathway in the GARP-null developing palates. Of note, the failure to develop the secondary palate and concurrent reduction of SMAD phosphorylation without other defects in GARP KO mice phenocopied TGFβ3 KO mice, although GARP has not been suggested previously to interact with TGFβ3. We found that GARP and TGFβ3 co-localize in medial edge epithelial cells at E14.5. In vitro studies confirmed that GARP and TGFβ3 directly interact and that GARP is indispensable for the surface expression of membrane-associated latent TGFβ3. Our findings indicate that GARP is essential for normal morphogenesis of the palate and demonstrate that GARP plays a crucial role in regulating TGFβ3 signaling during embryogenesis. In conclusion, we have uncovered a novel function of GARP in positively regulating TGFβ3 activation and function.