A novel technique for NO.253 lymph node dissection and left colic artery preservation to avoid potential postoperative internal hernia in laparoscopic radical resection for rectal cancer.

BACKGROUND: The preservation of the left colic artery (LCA) has emerged as a preferred approach in laparoscopic radical resection for rectal cancer. However, preserving the LCA while simultaneously dissecting the NO.253 lymph node can create a mesenteric defect between the inferior mesenteric artery (IMA), the LCA, and the inferior mesenteric vein (IMV). This defect could act as a potential "hernia ring," increasing the risk of developing an internal hernia after surgery. The objective of this study was to introduce a novel technique designed to mitigate the risk of internal hernia by filling mesenteric defects with autologous tissue. METHODS: This new technique was performed on eighteen patients with rectal cancer between January 2022 and June 2022. First of all, dissected the lymphatic fatty tissue on the main trunk of IMA from its origin until the LCA and sigmoid artery (SA) or superior rectal artery (SRA) were exposed and then NO.253 lymph node was dissected between the IMA, LCA and IMV. Next, the SRA or SRA and IMV were sequentially ligated and cut off at an appropriate location away from the "hernia ring" to preserve the connective tissue between the "hernia ring" and retroperitoneum. Finally, after mobilization of distal sigmoid, on the lateral side of IMV, the descending colon was mobilized cephalad. Patients'preoperative baseline characteristics and intraoperative, postoperative complications were examined. RESULTS: All patients' potential "hernia rings" were closed successfully with our new technique. The median operative time was 195 min, and the median intraoperative blood loss was 55 ml (interquartile range 30-90). The total harvested lymph nodes was 13.0(range12-19). The median times to first flatus and liquid diet intake were both 3.0 days. The median number of postoperative hospital days was 8.0 days. One patient had an injury to marginal arterial arch, and after mobolization of splenic region, tension-free anastomosis was achieved. No other severe postoperative complications such as abdominal infection, anastomotic leakage, or bleeding were observed. CONCLUSIONS: This technique is both safe and effective for filling the mesenteric defect, potentially reducing the risk of internal hernia following laparoscopic NO.253 lymph node dissection and preservation of the left colic artery in rectal cancer surgeries.

CD4+ T cells produce IFN-I to license cDC1s for induction of cytotoxic T-cell activity in human tumors.

CD4+ T cells can "help" or "license" conventional type 1 dendritic cells (cDC1s) to induce CD8+ cytotoxic T lymphocyte (CTL) anticancer responses, as proven in mouse models. We recently identified cDC1s with a transcriptomic imprint of CD4+ T-cell help, specifically in T-cell-infiltrated human cancers, and these cells were associated with a good prognosis and response to PD-1-targeting immunotherapy. Here, we delineate the mechanism of cDC1 licensing by CD4+ T cells in humans. Activated CD4+ T cells produce IFNß via the STING pathway, which promotes MHC-I antigen (cross-)presentation by cDC1s and thereby improves their ability to induce CTL anticancer responses. In cooperation with CD40 ligand (L), IFNß also optimizes the costimulatory and other functions of cDC1s required for CTL response induction. IFN-I-producing CD4+ T cells are present in diverse T-cell-infiltrated cancers and likely deliver "help" signals to CTLs locally, according to their transcriptomic profile and colocalization with "helped/licensed" cDCs and tumor-reactive CD8+ T cells. In agreement with this scenario, the presence of IFN-I-producing CD4+ T cells in the TME is associated with overall survival and the response to PD-1 checkpoint blockade in cancer patients.

Immune checkpoints targeting dendritic cells for antibody-based modulation in cancer.

Dendritic cells (DC) are professional antigen-presenting cells which link innate to adaptive immunity. DC play a central role in regulating antitumor T-cell responses in both tumor-draining lymph nodes (TDLN) and the tumor microenvironment (TME). They modulate effector T-cell responses via immune checkpoint proteins (ICPs) that can be either stimulatory or inhibitory. Functions of DC are often impaired by the suppressive TME leading to tumor immune escape. Therefore, better understanding of the mechanisms of action of ICPs expressed by (tumor-infiltrating) DC will lead to potential new treatment strategies. Genetic manipulation and high-dimensional analyses have provided insight in the interactions between DC and T-cells in TDLN and the TME upon ICP targeting. In this review, we discuss (tumor-infiltrating) DC lineage cells and tumor tissue specific "mature" DC states and their gene signatures in relation to anti-tumor immunity. We also review a number of ICPs expressed by DC regarding their functions in phagocytosis, DC activation, or inhibition and outline position in, or promise for clinical trials in cancer immunotherapy. Collectively, we highlight the critical role of DC and their exact status in the TME for the induction and propagation of T-cell immunity to cancer.

CMTM6 shapes antitumor T cell response through modulating protein expression of CD58 and PD-L1.

The dysregulated expression of immune checkpoint molecules enables cancer cells to evade immune destruction. While blockade of inhibitory immune checkpoints like PD-L1 forms the basis of current cancer immunotherapies, a deficiency in costimulatory signals can render these therapies futile. CD58, a costimulatory ligand, plays a crucial role in antitumor immune responses, but the mechanisms controlling its expression remain unclear. Using two systematic approaches, we reveal that CMTM6 positively regulates CD58 expression. Notably, CMTM6 interacts with both CD58 and PD-L1, maintaining the expression of these two immune checkpoint ligands with opposing functions. Functionally, the presence of CMTM6 and CD58 on tumor cells significantly affects T cell-tumor interactions and response to PD-L1-PD-1 blockade. Collectively, these findings provide fundamental insights into CD58 regulation, uncover a shared regulator of stimulatory and inhibitory immune checkpoints, and highlight the importance of tumor-intrinsic CMTM6 and CD58 expression in antitumor immune responses.

CD4+ helper T cells endow cDC1 with cancer-impeding functions in the human tumor micro-environment.

Despite their low abundance in the tumor microenvironment (TME), classical type 1 dendritic cells (cDC1) play a pivotal role in anti-cancer immunity, and their abundance positively correlates with patient survival. However, their interaction with CD4+ T-cells to potentially enable the cytotoxic T lymphocyte (CTL) response has not been elucidated. Here we show that contact with activated CD4+ T-cells enables human ex vivo cDC1, but no other DC types, to induce a CTL response to cell-associated tumor antigens. Single cell transcriptomics reveals that CD4+ T-cell help uniquely optimizes cDC1 in many functions that support antigen cross-presentation and T-cell priming, while these changes don't apply to other DC types. We robustly identify "helped" cDC1 in the TME of a multitude of human cancer types by the overlap in their transcriptomic signature with that of recently defined, tumor-infiltrating DC states that prove to be positively prognostic. As predicted from the functional effects of CD4+ T-cell help, the transcriptomic signature of "helped" cDC1 correlates with tumor infiltration by CTLs and Thelper(h)-1 cells, overall survival and response to PD-1-targeting immunotherapy. These findings reveal a critical role for CD4+ T-cell help in enabling cDC1 function in the TME and may establish the helped cDC1 transcriptomic signature as diagnostic marker in cancer.

Radiomics-based signature of breast cancer on preoperative contrast-enhanced MRI to predict axillary metastasis.

Aim: This study aimed to predict axillary metastasis using radiology features in dynamic contrast-enhanced MRI. Methods: This study included 243 breast lesions confirmed as malignant based on axillary status. Most outcome-predictive features were selected using four machine-learning algorithms. Receiver operating characteristic analysis was used to reflect diagnostic performance. Results: Least absolute shrinkage and selection operator was used to dimensionally reduce 1137 radiomics features to three features. Three optimal radiomics features were used to model construction. The logistic regression model achieved an accuracy of 97% and 85% in the training and test groups. Clinical utility was evaluated using decision curve analysis. Conclusion: The novel combination of radiomics analysis and machine-learning algorithm could predict axillary metastasis and prevent invasive manipulation.

Trial watch: Dendritic cell (DC)-based immunotherapy for cancer.

Dendritic cell (DC)-based vaccination for cancer treatment has seen considerable development over recent decades. However, this field is currently in a state of flux toward niche-applications, owing to recent paradigm-shifts in immuno-oncology mobilized by T cell-targeting immunotherapies. DC vaccines are typically generated using autologous (patient-derived) DCs exposed to tumor-associated or -specific antigens (TAAs or TSAs), in the presence of immunostimulatory molecules to induce DC maturation, followed by reinfusion into patients. Accordingly, DC vaccines can induce TAA/TSA-specific CD8+/CD4+ T cell responses. Yet, DC vaccination still shows suboptimal anti-tumor efficacy in the clinic. Extensive efforts are ongoing to improve the immunogenicity and efficacy of DC vaccines, often by employing combinatorial chemo-immunotherapy regimens. In this Trial Watch, we summarize the recent preclinical and clinical developments in this field and discuss the ongoing trends and future perspectives of DC-based immunotherapy for oncological indications.

A Novel Technique With Ileal Mesentery to Reconstruct the Pelvic Peritoneum After Pelvic Dissection With End Colostomy for Rectal Cancer.

BACKGROUND: After abdominoperineal resection, low anterior resection, and end colostomy for lower rectal cancer, it is necessary to reconstruct the pelvic peritoneum to avoid small bowel obstruction, perineal hernia, and radiation enteritis in patients for whom postoperative radiotherapy is planned. However, pelvic peritoneal closure is technically difficult in patients who lack enough peritoneum to cover the defect or have received neoadjuvant radiation and have a rigid pelvis. IMPACT OF INNOVATION: The impact of this innovation is to reconstruct the pelvic peritoneum with the distal ileal mesentery laparoscopically. TECHNOLOGY, MATERIALS AND METHODS: After removal of the tumor, the distal ileal mesentery was selected to completely cover the defect. Subsequently, suturing of the ileal mesentery to the posterior wall of the urinary bladder and all sides of the pelvic cavity was performed. Finally, the patients were returned to the headfirst supine position to ensure that there was no small bowel falling into the pelvic dead space. PRELIMINARY RESULTS: All surgical procedures were successfully performed laparoscopically from January 2019 to April 2021. No perineal complications or intestinal obstructions occurred during the follow-up period. CONCLUSIONS AND FUTURE DIRECTIONS: This novel technique was found to be safe and effective. Moreover, it provided an economical method for the reconstruction of the pelvic peritoneum using autologous material, which could preserve the small intestine in the abdomen to avoid related complications. Additional larger series of patients with longer follow-up are needed to validate the safety and feasibility of this method.

Laparoscopic right hemicolectomy with complete mesocolic excision plus D3 lymphadenectomy (CME + D3): a new medial approach for central vascular ligation.

Due to the high risk of vascular injuries, it remains a technical challenge and time-consuming procedure for surgeons to perform CME and D3 lymph node dissection in laparoscopic right hemicolectomy. To overcome this difficulty, we developed a novel method of the vessel's management for central vascular ligation (CVL). The key feature of this new approach focused on vascular dissection in two aspects. The first one was to expose the superior mesentery vein (SMV) and the branches of the superior mesentery artery (SMA) at their roots from left to right after dividing the peritoneum near the left border of SMV, which has the advantage of exposing SMV and controlling bleeding. The second was to selectively ligate the colic tributaries of gastrocolic trunk of Henle (GTH) after expanding its surrounding spaces. We named this technique the "new approach (NA)". Thirty-eight patients who underwent laparoscopic right hemicolectomy with the new approach (NA) were retrospectively analyzed and compared with data from 35 patients, who underwent the conventional medial approach (TA) performed by the same surgical team from April 2017 to March 2021. There was no significant difference between the two groups in baseline data (all p > 0.05). All 38 operations were completed with this procedure successfully. The NA approach was associated with a shorter operation time (190.5 min vs.215.5 min; P < 0.05) and a smaller blood loss (50 ml vs. 95 ml; P < 0.05) compared with the conventional approach. Two cases of vascular injuries occurred in the TA group and had been managed laparoscopically. The lymph nodes count (15 vs. 16; P > 0.05) was not significantly different; additionally, no difference was observed regarding anastomotic leakage (both n = 0) and postoperative complications (3/31 vs. 3/30; P > 0.05). No mortality was observed. NA is feasible and can be an optional method of vessel's management in laparoscopic CME and D3 lymphadenectomy for right-sided colon cancer.

Clinically applicable CD34+-derived blood dendritic cell subsets exhibit key subset-specific features and potently boost anti-tumor T and NK cell responses.

Allogeneic stem cell transplantation (alloSCT), following induction chemotherapy, can be curative for hemato-oncology patients due to powerful graft-versus-tumor immunity. However, disease recurrence remains the major cause of treatment failure, emphasizing the need for potent adjuvant immunotherapy. In this regard, dendritic cell (DC) vaccination is highly attractive, as DCs are the key orchestrators of innate and adaptive immunity. Natural DC subsets are postulated to be more powerful compared with monocyte-derived DCs, due to their unique functional properties and cross-talk capacity. Yet, obtaining sufficient numbers of natural DCs, particularly type 1 conventional DCs (cDC1s), is challenging due to low frequencies in human blood. We developed a clinically applicable culture protocol using donor-derived G-CSF mobilized CD34+ hematopoietic progenitor cells (HPCs) for simultaneous generation of high numbers of cDC1s, cDC2s and plasmacytoid DCs (pDCs). Transcriptomic analyses demonstrated that these ex vivo-generated DCs highly resemble their in vivo blood counterparts. In more detail, we demonstrated that the CD141+CLEG9A+ cDC1 subset exhibited key features of in vivo cDC1s, reflected by high expression of co-stimulatory molecules and release of IL-12p70 and TNF-α. Furthermore, cDC1s efficiently primed alloreactive T cells, potently cross-presented long-peptides and boosted expansion of minor histocompatibility antigen-experienced T cells. Moreover, they strongly enhanced NK cell activation, degranulation and anti-leukemic reactivity. Together, we developed a robust culture protocol to generate highly functional blood DC subsets for in vivo application as tailored adjuvant immunotherapy to boost innate and adaptive anti-tumor immunity in alloSCT patients.

Flagellin/TLR5 Stimulate Myeloid Progenitors to Enter Lung Tissue and to Locally Differentiate Into Macrophages.

Toll-like receptor 5 (TLR5) is the receptor of bacterial Flagellin. Reportedly, TLR5 engagement helps to combat infections, especially at mucosal sites, by evoking responses from epithelial cells and immune cells. Here we report that TLR5 is expressed on a previously defined bipotent progenitor of macrophages (MΦs) and osteoclasts (OCs) that resides in the mouse bone marrow (BM) and circulates at low frequency in the blood. In vitro, Flagellin promoted the generation of MΦs, but not OCs from this progenitor. In vivo, MΦ/OC progenitors were recruited from the blood into the lung upon intranasal inoculation of Flagellin, where they rapidly differentiated into MΦs. Recruitment of the MΦ/OC progenitors into the lung was likely promoted by the CCL2/CCR2 axis, since the progenitors expressed CCR2 and type 2 alveolar epithelial cells (AECs) produced CCL2 upon stimulation by Flagellin. Moreover, CCR2 blockade reduced migration of the MΦ/OC progenitors toward lung lavage fluid (LLF) from Flagellin-inoculated mice. Our study points to a novel role of the Flagellin/TLR5 axis in recruiting circulating MΦ/OC progenitors into infected tissue and stimulating these progenitors to locally differentiate into MΦs. The progenitor pathway to produce MΦs may act, next to monocyte recruitment, to fortify host protection against bacterial infection at mucosal sites.

A New Medial-to-Lateral Approach for Laparoscopic D3 Lymphadenectomy plus Complete Mesocolic Excision (D3 + CME) for Right-Sided Colon Cancer.

BACKGROUND: D3 lymphadenectomy is important for accurate staging and provides long-term benefits, especially for T3-4/N + tumors.1,2 Both D3 lymphadenectomy as well as complete mesocolic excision (CME) require central ligation of vessels at their origins to ensure radical resection.3 Currently, superior mesentery vein (SMV) is navigated by ileocolic vessels while its sheath is dissected stepwise from caudal to cranial.4-6 This report describes a new medial-to-lateral approach for laparoscopic right hemicolectomy with D3 + CME. METHODS: The patient was a 47-year-old man with diagnosis of hepatic flexure cancer (cT4N1M0). First, the pedicle of the middle colic vessels and ileocolic vessels were both grasped, then the sheath of SMV was dissected at its left side as there are fewer blood vessels entering here compared with its right side. Second, after identification of middle colic artery (MCA), SMV was skeletonized from medial to lateral and no. 213 and no. 203 lymph nodes were dissected. Third, MCA and ileocolic vein and artery (ICV and ICA) were ligated at their roots. After separating the transverse mesocolon from the duodenum, the branches of the Henle trunk were exposed and no. 223 lymph nodes were dissected. Accessory right colic vein, right colic vein, and middle colic vein were ligated respectively. Fourth, the ascending mesocolon was separated from the retroperitoneal tissues through the front side of Toldt's fascia, the mesocolon was mobilized completely, and the tumor was removed en bloc. RESULTS: The operation time was 175 min, with estimated blood loss of 50 ml. The patient recovered well without bleeding complications and was discharged on postoperative day 7. Histology revealed moderately differentiated adenocarcinoma with 5 of 24 lymph nodes involved (pT3N2M0). CONCLUSIONS: The medial-to-lateral approach presented in the video might be helpful for standardization of laparoscopic D3 + CME for right-sided colon cancer.

Guizhi Fuling wan for chronic pelvic inflammatory disease protocol: A protocol for systematic review and meta analysis.

BACKGROUND: Chronic pelvic inflammatory disease (CPID) is one of common diseases of department of gynaecology, point to female inside genital and circumferential organization to suffer from infection of all sorts of pathogenic bacteria and cause chronic inflammation sex disease, also cause one of main factors of infertile of female of childbearing age period. Due to its insidious onset, it is not easy to find out in the early stage. Therefore, it is difficult to obtain satisfactory curative effect by taking routine treatment with antibiotics. In recent years, TCM has made great strides in the treatment of chronic pelvic inflammation, a number of clinical studies have shown that Guizhi Fuling wan combined with antibiotics can significantly improve the clinical symptoms and enhance the therapeutic effect. Therefore, we intend to conduct a system review and meta-analysis to further clarify the effectiveness and safety of GZFLW for CPID. METHODS: We will search each database from the built-in until September2020.The English literature mainly searches Cochrane Library, PubMed, EMBASE, and Web of Science, while the Chinese literature comes from CNKI, CBM, VIP, and Wangfang database. Simultaneously we will retrieval clinical registration tests and grey literatures. This study only screens the clinical randomized controlled trials (RCTs) about GZFLW for CPID to assess its efficacy and safety. The 2 researchers worked independently on literature selection, data extraction, and quality assessment. The dichotomous data is represented by relative risk (RR), and the continuous is expressed by mean difference (MD) or standard mean difference (SMD), eventually the data is synthesized using a fixed effect model (FEM) or a random effect model (REM) depending on whether or not heterogeneity exists. The clinical efficacy, pelvic effusion and mass were evaluated as the main outcomes. The serum interleukin-6 (IL-6), C-reactive protein (CRP), tumor necrosis factor (TNF)-α, erythrocyte sedimentation rate (ESR), erythrocyte specific volume was secondary outcomes. Finally, meta-analysis was conducted by RevMan software version 5.3. RESULTS: This study will provide high-quality evidence for treatment of CPID with GZFLW in terms of effectiveness and safety. CONCLUSION: This systematic review aims to provide new options for GZFLW treatment of CPID in terms of its efficacy and safety. ETHICS AND DISSEMINATION: This study does not require ethical approval. We will disseminate our findings by publishing results in a peer-reviewed journal. OSF REGISTRATION NUMBER: DOI 10.17605 / OSF.IO / R9NVT.

Bone marrow-derived myeloid progenitors as driver mutation carriers in high- and low-risk Langerhans cell histiocytosis.

Langerhans cell histiocytosis (LCH) is a myeloid neoplasia, driven by sporadic activating mutations in the MAPK pathway. The misguided myeloid dendritic cell (DC) model proposes that high-risk, multisystem, risk-organ-positive (MS-RO+) LCH results from driver mutation in a bone marrow (BM)-resident multipotent hematopoietic progenitor, while low-risk, MS-RO- and single-system LCH would result from driver mutation in a circulating or tissue-resident, DC-committed precursor. We have examined the CD34+c-Kit+Flt3+ myeloid progenitor population as potential mutation carrier in all LCH disease manifestations. This population contains oligopotent progenitors of monocytes (Mo's)/macrophages (MΦs), osteoclasts (OCs), and DCs. CD34+c-Kit+Flt3+ cells from BM of MS-RO+ LCH patients produced Langerhans cell (LC)-like cells in vitro. Both LC-like and DC offspring from this progenitor carried the BRAF mutation, confirming their common origin. In both high- and low-risk LCH patients, CD34+c-Kit+Flt3+ progenitor frequency in blood was higher than in healthy donors. In one MS-RO+ LCH patient, CD34+c-Kit+Flt3+ cell frequency in blood and its BRAF-mutated offspring reported response to chemotherapy. CD34+c-Kit+Flt3+ progenitors from blood of both high- and low-risk LCH patients gave rise to DCs and LC-like cells in vitro, but the driver mutation was not easily detectable, likely due to low frequency of mutated progenitors. Mutant BRAF alleles were found in Mo's /MΦs, DCs, LC-like cells, and/or OC-like cells in lesions and/or Mo and DCs in blood of multiple low-risk patients. We therefore hypothesize that in both high- and low-risk LCH, the driver mutation is present in a BM-resident myeloid progenitor that can be mobilized to the blood.

Hydrothermal synthesis of nitrogen and copper co-doped carbon dots with intrinsic peroxidase-like activity for colorimetric discrimination of phenylenediamine isomers.

Carbon dots doped with nitrogen and copper have been synthesized via a hydrothermal method. They possess favorable peroxidase-like catalytic activity over a wide range of pH values and temperatures. Specifically, they were used to catalyze the oxidation of ortho- and para-phenylenediamine (OPD and PPD) by H2O2. The resulting products possess different colors (yellow for OPD and brown for PPD), which can be visually discriminated. The corresponding typical absorption peaks of oxidized products for OPD and PPD are at 413 nm and 500 nm, respectively. The method displays excellent discrimination ability and selectivity over potential interferents. The detection limits are 1.1 µM for OPD and 1.9 µM for PPD. The respective linear ranges are from 5 to 200 µM for OPD and from 2.5 to 700 µM for PPD. The method was applied to the quantification of OPD and PPD in spiked natural waters. Graphical abstract Schematic presentation of the synthesis of nitrogen and copper co-doped carbon dots (N,Cu-CDs) and their application as an enzyme mimic for colorimetric discrimination of ortho-, meta- and para-phenylenediamine (OPD, MPD and PPD).

Hydrothermal synthesis of carbon dots codoped with nitrogen and phosphorus as a turn-on fluorescent probe for cadmium(II).

Nitrogen and phosphorus co-doped carbon dots (N,P-CDs) have been synthesized via hydrothermal method starting from o-phosphorylethanolamine and citric acid. The blue-green fluorescence of the N,P-CDs (with excitation/emission peaks at 325/435 nm) is gradually enhanced on sequential addition of Cd(II) ions. This fluorometric assay works in the 0.5 µM to 12.5 µM Cd(II) concentration range and has a 0.16 µM detection limit. The phenomenon may be attributed to chelation enhanced fluorescence that is induced by the formation of Cd(II)-N,P-CDs complex with functional groups present on the surface. The method has applied to the detection of Cd(II) in spiked serum and urine samples and gave satisfying results. Graphical abstract Schematic presentation of the synthesis of nitrogen and phosphorus co-doped carbon dots (N,P-CDs) and their application as a turn-on fluorescence probe for Cd(II) detection.

CD4+ T Cell Help Confers a Cytotoxic T Cell Effector Program Including Coinhibitory Receptor Downregulation and Increased Tissue Invasiveness.

CD4+ T cells optimize the cytotoxic T cell (CTL) response in magnitude and quality, by unknown molecular mechanisms. We here present the transcriptomic changes in CTLs resulting from CD4+ T cell help after anti-cancer vaccination or virus infection. The gene expression signatures revealed that CD4+ T cell help during priming optimized CTLs in expression of cytotoxic effector molecules and many other functions that ensured efficacy of CTLs throughout their life cycle. Key features included downregulation of PD-1 and other coinhibitory receptors that impede CTL activity, and increased motility and migration capacities. "Helped" CTLs acquired chemokine receptors that helped them reach their tumor target tissue and metalloprotease activity that enabled them to invade into tumor tissue. A very large part of the "help" program was instilled in CD8+ T cells via CD27 costimulation. The help program thus enhances specific CTL effector functions in response to vaccination or a virus infection.

Identification of CMTM6 and CMTM4 as PD-L1 protein regulators.

The clinical benefit for patients with diverse types of metastatic cancers that has been observed upon blockade of the interaction between PD-1 and PD-L1 has highlighted the importance of this inhibitory axis in the suppression of tumour-specific T-cell responses. Notwithstanding the key role of PD-L1 expression by cells within the tumour micro-environment, our understanding of the regulation of the PD-L1 protein is limited. Here we identify, using a haploid genetic screen, CMTM6, a type-3 transmembrane protein of previously unknown function, as a regulator of the PD-L1 protein. Interference with CMTM6 expression results in impaired PD-L1 protein expression in all human tumour cell types tested and in primary human dendritic cells. Furthermore, through both a haploid genetic modifier screen in CMTM6-deficient cells and genetic complementation experiments, we demonstrate that this function is shared by its closest family member, CMTM4, but not by any of the other CMTM members tested. Notably, CMTM6 increases the PD-L1 protein pool without affecting PD-L1 (also known as CD274) transcription levels. Rather, we demonstrate that CMTM6 is present at the cell surface, associates with the PD-L1 protein, reduces its ubiquitination and increases PD-L1 protein half-life. Consistent with its role in PD-L1 protein regulation, CMTM6 enhances the ability of PD-L1-expressing tumour cells to inhibit T cells. Collectively, our data reveal that PD-L1 relies on CMTM6/4 to efficiently carry out its inhibitory function, and suggest potential new avenues to block this pathway.