Preparing Frozen Tissue Sections for Staining

Cell-staining studies on mammalian tissue are often performed on frozen sections. This is the gentlest method for the preparation of samples and gives good preservation of cell structure and antigens. Its principal disadvantages are that the specimens must be stored frozen and a special microtome, known as a cryostat, is required. In addition, many clinical specimens are not available in this form, and most classical histological descriptions of tissue structure and pathology are based on the use of formalin-fixed, paraffin-embedded (FFPE) material.

Embedding Cultured Cells in Matrigel for Staining

Adherent cells normally are prepared for cell staining by growing on a suitable support. Suspension cells can be fixed directly or can be attached to a solid support by centrifugation, chemical cross-linking, or simple dehydration. An alternative approach for cells grown in culture is to prepare a fixed cell pellet that is embedded in a scaffold, such as Matrigel, and then processed, embedded in paraffin, and stained in a manner identical to formalin-fixed tissue specimens.

Neoadjuvant Nivolumab or Nivolumab Plus Ipilimumab in Untreated Oral Cavity Squamous Cell Carcinoma: A Phase 2 Open-Label Randomized Clinical Trial

Importance

Novel approaches are needed to improve outcomes in patients with squamous cell carcinoma of the oral cavity. Neoadjuvant immunotherapy given prior to surgery and combining programmed cell death protein 1 (PD-1) and cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) immune checkpoint inhibitors are 2 strategies to enhance antitumor immune responses that could be of benefit.

Design, Setting, and Participants

In this randomized phase 2 clinical trial conducted at 1 academic center, 29 patients with untreated squamous cell carcinoma of the oral cavity (≥T2, or clinically node positive) were enrolled between 2016 to 2019.

Interventions

Treatment was administered with nivolumab, 3 mg/kg, weeks 1 and 3, or nivolumab and ipilimumab (ipilimumab, 1 mg/kg, given week 1 only). Patients had surgery 3 to 7 days following cycle 2.

Main Outcomes and Measures

Safety and volumetric response determined using bidirectional measurements. Secondary end points included pathologic and objective response, progression-free survival (PFS), and overall survival. Multiplex immunofluorescence was used to evaluate primary tumor immune markers.

Results

Fourteen patients were randomized to nivolumab (N) and 15 patients to nivolumab/ipilimumab (N+I) (mean [SD] age, 62 [12] years; 18 men [62%] and 11 women [38%]). The most common subsite was oral tongue (n = 16). Baseline clinical staging included patients with T2 (n = 20) or greater (n = 9) T stage and 17 patients (59%) with node-positive disease. Median time from cycle 1 to surgery was 19 days (range, 7-21 days); there were no surgical delays. There were toxic effects at least possibly related to study treatment in 21 patients, including grade 3 to 4 events in 2 (N), and 5 (N+I) patients. One patient died of conditions thought unrelated to study treatment (postoperative flap failure, stroke). There was evidence of response in both the N and N+I arms (volumetric response 50%, 53%; pathologic downstaging 53%, 69%; RECIST response 13%, 38%; and pathologic response 54%, 73%, respectively). Four patients had major/complete pathologic response greater than 90% (N, n = 1; N+I, n = 3). With 14.2 months median follow-up, 1-year progression-free survival was 85% and overall survival was 89%.

Conclusions and Relevance

Treatment with N and N+I was feasible prior to surgical resection. We observed promising rates of response in both arms, supporting further neoadjuvant studies with these agents.

Trial Registration

ClinicalTrials.gov Identifier: NCT02919683.

Preparing Cell Smears for Staining

Increasing use is being made of cell smears for cell-staining studies. Suspension cells can be attached to slides by drying, and cell smears can also be prepared from biopsy samples, such as needle aspirates, tissue scrapings, or freshly dissected tissues. In these procedures, a thin layer of cells is deposited on a dry slide by physical methods. The most important factor in obtaining good staining patterns is that the smear be only a single cell thick. Tissue smears do not preserve tissue architecture, but are useful for identifying pathological changes and infectious organisms in tissue samples. Cell smears are easily prepared and can be fixed readily by any of the methods used for attached cells.

Attaching Suspension Cells to Slides for Staining

Suspension cells can be prepared for staining by several different methods. A simple method for detecting intracellular antigens in cells that grow in suspension is to attach the cells to a solid substrate before fixation. This can be achieved by the use of a cytocentrifuge. For surface staining, suspension cells can be attached to slides by cross-linking with poly-l-lysine. Lysine can be polymerized to any desired length, and poly-l-lysine will bind to most solid supports through its charged side chains. The positively charged polymer will provide a site for binding of cells (which carry an overall negative charge). Although this cross-link is not covalent, it is sufficiently strong for most cell-staining techniques.

Meta-Analysis of PD-L1 Expression As a Predictor of Survival After Checkpoint Blockade

PURPOSE

Programmed cell death receptor ligand 1 (PD-L1) expression is the most studied biomarker to predict the efficacy of immune checkpoint inhibitors (ICIs), but its clinical significance is controversial. We estimated the distribution of PD-L1 expression scores (ie, tumor proportion score or combined proportion score) and the relationship between PD-L1 levels and ICIs’ impact on overall survival (OS).

METHODS

We reconstructed, pooled, and analyzed individual-level data on 7,617 patients with cancer from 14 randomized clinical trials. The effects of ICIs were quantified using differences in 24-month restricted mean survival times (ΔRMSTs; ie, the increase in life expectancy truncated at 2 years associated with ICI therapy). In a simulation study, we compared standard randomized clinical trial designs with a trial design that leverages meta-analytic results like ours.

RESULTS

Approximately 93% of patients had a PD-L1 expression ≤ 5% (66% of patients) or > 50% (27% of patients). OS improves with ICIs regardless of PD-L1 expression level, which predicts the benefits’ magnitude. For patients with non–small-cell lung cancer (NSCLC), ΔRMSTs ranged from 1.4 months (95% probability interval [PI], 0.7 to 2.2 months) for PD-L1 expression ≤ 1% to 4.1 months (95% PI, 3.2 to 5.2 months) for PD-L1 expression > 80%. For patients with non-NSCLC tumors, ΔRMSTs ranged from 0.8 months (95% PI, −0.1 to 1.7 months) to 2.3 months (95% PI, 1.3 to 4.4 months), again for PD-L1 expression levels of ≤ 1% and > 80%, respectively. Simulations suggested that designs tailored to meta-analytic results can detect the effects of ICIs in PD-L1 subgroups with higher probability (> 15%) than standard designs.

CONCLUSION

The practice of dichotomizing the range of PD-L1 expression scores is inadequate for patient stratification. Meta-analytic estimates of the distribution of PD-L1 scores and subgroup-specific treatment effects can improve the designs of future trials of ICIs.

Axicabtagene Ciloleucel in the Non-Trial Setting: Outcomes and Correlates of Response, Resistance, and Toxicity

PURPOSE

Axicabtagene ciloleucel (axi-cel) was approved by the Food and Drug Administration for relapsed aggressive B-cell non-Hodgkin lymphoma in part on the basis of durable remission rates of approximately 40% in a clinical trial population. Whether this efficacy, and the rates of toxicity, would be consistent in a postcommercial setting, with relaxed eligibility criteria and bridging therapy, is unknown. This study describes the efficacy and safety correlates and outcomes in this setting.

PATIENTS AND METHODS

One hundred twenty-two patients from 7 medical centers in the United States were treated with axi-cel and were included in a modified intent-to-treat (mITT) analysis. Seventy-six patients (62%) were ineligible for the ZUMA-1 trial. Response and toxicity rates, duration of response (DOR), survival, and covariates are described on the basis of the mITT population. Correlative studies on blood and tumor samples were performed to investigate potential biomarkers of response and resistance.

RESULTS

Median follow-up was 10.4 months. In the mITT population, the best overall and complete response (CR) rates were 70% and 50%, respectively. Median DOR and progression-free survival (PFS) were 11.0 and 4.5 months in all patients and were not reached (NR) in CR patients. Median overall survival (OS) was NR; 1-year OS was 67% (95% CI, 59% to 77%). Although response rates were similar in the ZUMA-1-eligible and ZUMA-1-ineligible groups (70% v 68%), there was a statistically significant improvement in CR rate (63% v 42%, P = .016), DOR (median, NR v 5.0 months; P = .014), PFS (median, NR v 3.3 months; P = .020), and OS (1-year OS, 89% v 54%; P < .001) in patients who were ZUMA-1 eligible. Rates of grade ≥ 3 cytokine release syndrome and neurotoxicty were 16% and 35%, respectively.

CONCLUSION

Axi-cel yields similar rates of overall response and toxicity in commercial and trial settings, although CR rates and DOR were more favorable in patients eligible for ZUMA-1.

A peripheral immune signature of responsiveness to PD-1 blockade in patients with classical Hodgkin lymphoma

PD-1 blockade is highly effective in classical Hodgkin lymphomas (cHLs), which exhibit frequent copy-number gains of CD274 (PD-L1) and PDC1LG2 (PD-L2) on chromosome 9p24.1. However, in this largely MHC-class-I-negative tumor, the mechanism of action of anti-PD-1 therapy remains undefined. We utilized the complementary approaches of T cell receptor (TCR) sequencing and cytometry by time-of-flight analysis to obtain a peripheral immune signature of responsiveness to PD-1 blockade in 56 patients treated in the CheckMate 205 phase II clinical trial (NCT02181738). Anti-PD-1 therapy was most effective in patients with a diverse baseline TCR repertoire and an associated expansion of singleton clones during treatment. CD4⁺, but not CD8⁺, TCR diversity significantly increased during therapy, most strikingly in patients who had achieved complete responses. Additionally, patients who responded to therapy had an increased abundance of activated natural killer cells and a newly identified CD3^-CD68⁺CD4⁺GrB⁺ subset. These studies highlight the roles of recently expanded, clonally diverse CD4⁺ T cells and innate effectors in the efficacy of PD-1 blockade in cHL.

Binding Antibodies to Attached Cells or Tissues in Preparation for Staining

Cells for staining are usually prepared from one of three sources: adherent cells, suspension cells, or whole tissues. Antibodies generally are applied directly to the area of the cells or tissues that is being studied. The antibodies can be labeled directly or they can be detected by using a labeled secondary reagent that will bind specifically to the primary antibody. Detection reagents for cell staining can be labeled with fluorochromes, enzymes, gold, or iodine.

Fixing Attached Cells for Staining

For cell staining, fixation methods decrease generally into two classes, organic solvents and cross-linking reagents. Organic solvents such as alcohols and acetone remove lipids and dehydrate the cells, precipitating the proteins on the cellular architecture. Cross-linking reagents such as paraformaldehyde form intermolecular bridges, normally through free amino groups, thus creating a network of linked antigens. Choosing between fixation in organic solvents or cross-linking agents is empirical. There are no general rules to decide between the two and both procedures are described here. Both methods may denature protein antigens, and for this reason, antibodies prepared against denatured proteins may be more useful for cell staining. In some instances, anti-denatured-protein antibodies are the only ones that can work. Fixation in protein cross-linking reagents such as paraformaldehyde or glutaraldehyde preserves cell structure better than organic solvents but may reduce the antigenicity of some cell components. Simple fixation with paraformaldehyde or glutaraldehyde does not allow the antibody to access the specimen and therefore is followed by a permeabilization step using an organic solvent or nonionic detergent. Using the organic solvent is easy, but it can destroy certain elements of the cell architecture, although prior fixation with paraformaldehyde does help to preserve the cellular structure. If preservation of cell structure is important, the best first choice would be to use a nonionic detergent.

Characterizing diversity in the tumor-immune microenvironment of distinct subclasses of gastroesophageal adenocarcinomas

BACKGROUND

Gastroesophageal adenocarcinomas (GEAs) are heterogeneous cancers where immune checkpoint inhibitors have robust efficacy in heavily inflamed microsatellite instability (MSI) or Epstein-Barr virus (EBV)-positive subtypes. Immune checkpoint inhibitor responses are markedly lower in diffuse/genome stable (GS) and chromosomal instable (CIN) GEAs. In contrast to EBV and MSI subtypes, the tumor microenvironment of CIN and GS GEAs have not been fully characterized to date, which limits our ability to improve immunotherapeutic strategies.

PATIENTS AND METHODS

Here we aimed to identify tumor-immune cell association across GEA subclasses using data from The Cancer Genome Atlas (N = 453 GEAs) and archival GEA resection specimen (N = 71). The Cancer Genome Atlas RNAseq data were used for computational inferences of immune cell subsets, which were correlated to tumor characteristics within and between subtypes. Archival tissues were used for more spatial immune characterization spanning immunohistochemistry and mRNA expression analyses.

RESULTS

Our results confirmed substantial heterogeneity in the tumor microenvironment between distinct subtypes. While MSI-high and EBV+ GEAs harbored most intense T cell infiltrates, the GS group showed enrichment of CD4+ T cells, macrophages and B cells and, in ∼50% of cases, evidence for tertiary lymphoid structures. In contrast, CIN cancers possessed CD8+ T cells predominantly at the invasive margin while tumor-associated macrophages showed tumor infiltrating capacity. Relatively T cell-rich 'hot' CIN GEAs were often from Western patients, while immunological 'cold' CIN GEAs showed enrichment of MYC and cell cycle pathways, including amplification of CCNE1.

CONCLUSIONS

These results reveal the diversity of immune phenotypes of GEA. Half of GS gastric cancers have tertiary lymphoid structures and are therefore promising candidates for immunotherapy. The majority of CIN GEAs, however, exhibit T cell exclusion and infiltrating macrophages. Associations of immune-poor CIN GEAs with MYC activity and CCNE1 amplification may enable new studies to determine precise mechanisms of immune evasion, ultimately inspiring new therapeutic modalities.

Neoadjuvant and Adjuvant Pembrolizumab in Resectable Locally Advanced, Human Papillomavirus-Unrelated Head and Neck Cancer: A Multicenter, Phase II Trial

PURPOSE

Pembrolizumab improved survival in patients with recurrent or metastatic head and neck squamous-cell carcinoma (HNSCC). The aims of this study were to determine if pembrolizumab would be safe, result in pathologic tumor response (pTR), and lower the relapse rate in patients with resectable human papillomavirus (HPV)-unrelated HNSCC.

PATIENTS AND METHODS

Neoadjuvant pembrolizumab (200 mg) was administered and followed 2 to 3 weeks later by surgical tumor ablation. Postoperative (chemo)radiation was planned. Patients with high-risk pathology (positive margins and/or extranodal extension) received adjuvant pembrolizumab. pTR was quantified as the proportion of the resection bed with tumor necrosis, keratinous debris, and giant cells/histiocytes: pTR-0 (<10%), pTR-1 (10%-49%), and pTR-2 (≥50%). Coprimary endpoints were pTR-2 among all patients and 1-year relapse rate in patients with high-risk pathology (historical: 35%). Correlations of baseline PD-L1 and T-cell infiltration with pTR were assessed. Tumor clonal dynamics were evaluated (ClinicalTrials.gov NCT02296684).

RESULTS

Thirty-six patients enrolled. After neoadjuvant pembrolizumab, serious (grades 3-4) adverse events and unexpected surgical delays/complications did not occur. pTR-2 occurred in eight patients (22%), and pTR-1 in eight other patients (22%). One-year relapse rate among 18 patients with high-risk pathology was 16.7% (95% confidence interval, 3.6%-41.4%). pTR ≥10% correlated with baseline tumor PD-L1, immune infiltrate, and IFNγ activity. Matched samples showed upregulation of inhibitory checkpoints in patients with pTR-0 and confirmed clonal loss in some patients.

CONCLUSIONS

Among patients with locally advanced, HPV-unrelated HNSCC, pembrolizumab was safe, and any pathologic response was observed in 44% of patients with 0% pathologic complete responses. The 1-year relapse rate in patients with high-risk pathology was lower than historical.

Activation of CAR and non-CAR T cells within the tumor microenvironment following CAR T cell therapy

Mechanisms of chimeric antigen receptor (CAR) T cell-mediated antitumor immunity and toxicity remain poorly characterized because few studies examine the intact tumor microenvironment (TME) following CAR T cell infusion. Axicabtagene ciloleucel is an autologous anti-CD19 CAR T cell therapy approved for patients with large B cell lymphoma. We devised multiplex immunostaining and ISH assays to interrogate CAR T cells and other immune cell infiltrates in biopsies of diffuse large B cell lymphoma following axicabtagene ciloleucel infusion. We found that a majority of intratumoral CAR T cells expressed markers of T cell activation but, unexpectedly, constituted ≤5% of all T cells within the TME 5 days or more after therapy. Large numbers of T cells without CAR were also activated within the TME after axicabtagene ciloleucel infusion; these cells were positive for Ki-67, IFN-γ, granzyme B (GzmB), and/or PD-1 and were found at the highest levels in biopsies with CAR T cells. Additionally, non-CAR immune cells were the exclusive source of IL-6, a cytokine associated with cytokine release syndrome, and were found at their highest numbers in biopsies with CAR T cells. These data suggest that intratumoral CAR T cells are associated with non-CAR immune cell activation within the TME with both beneficial and pathological effects.

Detecting β-Galactosidase-Labeled Cells

β-Galactosidase has been used extensively both as a label in enzyme immunoassays and for immunocytochemistry. One good substrate is 5-bromo-4-chloro-3-indolyl-β-d-galactopyranoside (X-gal), which gives an intense blue product. The product is stable and insoluble in alcohol as well as H₂O.

The Society for Immunotherapy of Cancer statement on best practices for multiplex immunohistochemistry (IHC) and immunofluorescence (IF) staining and validation

OBJECTIVES

The interaction between the immune system and tumor cells is an important feature for the prognosis and treatment of cancer. Multiplex immunohistochemistry (mIHC) and multiplex immunofluorescence (mIF) analyses are emerging technologies that can be used to help quantify immune cell subsets, their functional state, and their spatial arrangement within the tumor microenvironment.

METHODS

The Society for Immunotherapy of Cancer (SITC) convened a task force of pathologists and laboratory leaders from academic centers as well as experts from pharmaceutical and diagnostic companies to develop best practice guidelines for the optimization and validation of mIHC/mIF assays across platforms.

RESULTS

Representative outputs and the advantages and disadvantages of mIHC/mIF approaches, such as multiplexed chromogenic IHC, multiplexed immunohistochemical consecutive staining on single slide, mIF (including multispectral approaches), tissue-based mass spectrometry, and digital spatial profiling are discussed.

CONCLUSIONS

mIHC/mIF technologies are becoming standard tools for biomarker studies and are likely to enter routine clinical practice in the near future. Careful assay optimization and validation will help ensure outputs are robust and comparable across laboratories as well as potentially across mIHC/mIF platforms. Quantitative image analysis of mIHC/mIF output and data management considerations will be addressed in a complementary manuscript from this task force.

Abstract A83: Subsets of exhausted CD8+ T cells differentially mediate tumor control and respond to checkpoint blockade

T-cell dysfunction in the tumor microenvironment (TME) is a hallmark of many cancers. Reinvigoration of T-cell function by PD-1 checkpoint blockade can result in striking clinical responses, but is only effective in a minority of patients. The mechanisms by which anti-PD-1 therapy acts on exhausted T cells are not fully understood. Here we show that anti-PD-1 therapy acts on a specific subpopulation of CD8+ tumor-infiltrating lymphocytes (TILs) in melanoma mouse models, which can also be found in patients with melanoma. Exhausted CD8+ TILs contain a subpopulation of “progenitor exhausted” T cells with critical functional attributes that are not shared by the majority “terminally exhausted” TILs: they retain more polyfunctionality, persist following transfer into tumor-bearing mice, and differentiate to repopulate terminally exhausted TILs in the TME. As a result, progenitor exhausted CD8+ TILs are better able to control tumor growth than terminally exhausted cells. Progenitor exhausted, but not terminally exhausted, CD8+ TILs can respond to anti-PD-1 therapy. Melanoma patients with a higher percentage of progenitor exhausted cells have a longer duration of response to checkpoint blockade therapy. Therefore, approaches to expand progenitor exhausted CD8+ T cells in the tumor microenvironment may be an important component of improving checkpoint blockade response.

Citation Format: Brian C. Miller, Debattama R. Sen, Rose Al Abosy, Kevin Bi, Yamini Virkud, Martin W. LaFleur, Kathleen B. Yates, Ana Lako, Kristen Felt, Girish S. Naik, Michael Manos, Evisa Gjini, Jeffrey J. Ishizuka, F. Stephen Hodi, Scott J. Rodig, Arlene H. Sharpe, W. Nicholas Haining. Subsets of exhausted CD8+ T cells differentially mediate tumor control and respond to checkpoint blockade [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2019 Nov 17-20; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(3 Suppl):Abstract nr A83.

Author Correction: Subsets of exhausted CD8+ T cells differentially mediate tumor control and respond to checkpoint blockade

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

CXCR4 upregulation is an indicator of sensitivity to B-cell receptor/PI3K blockade and a potential resistance mechanism in B-cell receptor-dependent diffuse large B-cell lymphomas

B-cell receptor (BCR) signaling pathway components represent promising treatment targets in multiple B-cell malignancies including diffuse large B-cell lymphoma (DLBCL). In in vitro and in vivo model systems, a subset of DLBCLs depend upon BCR survival signals and respond to proximal BCR/phosphoinositide 3 kinase (PI3K) blockade. However, single-agent BCR pathway inhibitors have had more limited activity in patients with DLBCL, underscoring the need for indicators of sensitivity to BCR blockade and insights into potential resistance mechanisms. Here, we report highly significant transcriptional upregulation of C-X-C chemokine receptor 4 (CXCR4) in BCR-dependent DLBCL cell lines and primary tumors following chemical spleen tyrosine kinase (SYK) inhibition, molecular SYK depletion or chemical PI3K blockade. SYK or PI3K inhibition also selectively upregulated cell surface CXCR4 protein expression in BCR-dependent DLBCLs. CXCR4 expression was directly modulated by fork-head box O1 via the PI3K/protein kinase B/forkhead box O1 signaling axis. Following chemical SYK inhibition, all BCR-dependent DLBCLs exhibited significantly increased stromal cell-derived factor-1α (SDF-1α) induced chemotaxis, consistent with the role of CXCR4 signaling in B-cell migration. Select PI3K isoform inhibitors also augmented SDF-1α induced chemotaxis. These data define CXCR4 upregulation as an indicator of sensitivity to BCR/PI3K blockade and identify CXCR4 signaling as a potential resistance mechanism in BCR-dependent DLBCLs.

PD-1 blockade with pembrolizumab for classical Hodgkin lymphoma after autologous stem cell transplantation

Autologous stem cell transplantation (ASCT) remains the standard of care for patients with relapsed/refractory (RR) classical Hodgkin lymphoma (cHL) who respond to salvage chemotherapy. However, relapse after ASCT remains a frequent cause of treatment failure, with poor subsequent prognosis. Because cHL is uniquely vulnerable to programmed cell death-1 (PD-1) blockade, PD-1 blockade given as consolidation after ASCT could improve ASCT outcomes. We therefore conducted a multicohort phase 2 study of pembrolizumab in patients with RR cHL after ASCT, hypothesizing that it would improve the progression-free survival (PFS) at 18 months after ASCT (primary end point) from 60% to 80%. Pembrolizumab was administered at 200 mg IV every 3 weeks for up to 8 cycles, starting within 21 days of post-ASCT discharge. Thirty patients were treated on this study. The median age was 33 years, and 90% were high-risk by clinical criteria. Seventy-seven percent completed all 8 cycles. Toxicity was manageable, with 30% of patients experiencing at least 1 grade 3 or higher adverse event (AE), and 40% at least 1 grade 2 or higher immune-related AE. Two patients were lost to follow-up in complete remission at 12 months. The PFS at 18 months for the 28 evaluable patients was 82%, meeting the primary end point. The 18-month overall survival was 100%. In conclusion, pembrolizumab was successfully administered as post-ASCT consolidation in patients with RR cHL, and resulted in a promising PFS in a high-risk patient cohort, supporting the testing of this strategy in a randomized trial. This trial was registered at www.clinicaltrials.gov as #NCT02362997.

Detecting Gold-Labeled Cells

Colloidal gold particles bind tightly but not covalently to proteins at pH values that are around the protein's pI. Colloidal gold particles conjugated with a wide range of anti-immunoglobulin antibodies, Protein A, or streptavidin are available commercially. Gold labels were developed originally for electron microscopic studies, but they also work well at the level of the light microscope. They give higher resolution than enzyme-based methods and avoid the problems of substrate preparation and endogenous enzyme activity. Until recently, the gold labels lacked sensitivity at the level of light microscopy, but the recent development of the photochemical silver method of amplification, described here, has overcome this problem. Unamplified gold labels can be detected under the light microscope using bright-field illumination in which the label ranges from pale pink to deep red, depending on the strength of the reaction. Nomarski differential interference contrast microscopy makes the label appear dark red to black. With the silver enhancement method, the gold particles become coated in metallic silver and yield a black-brown label, best visualized by bright-field optics. Gold labeling methods are compatible with many histochemical stains. Gold labeling reactions are very readily controlled, because the appearance of staining can be monitored directly and continuously under the microscope.

Abstract 2701: Functionally specialized subsets of exhausted CD8+ T cells mediate tumor control and differentially respond to checkpoint blockade

T cell dysfunction in the tumor microenvironment (TME) is a hallmark of many cancers. Reinvigoration of T cell function by PD-1 checkpoint blockade can result in striking clinical responses, but is only effective in a minority of patients. The basis for T cell dysfunction in the TME, as well as the mechanisms by which anti-PD-1 therapy acts on dysfunctional T cells are not fully understood. Here we show that anti-PD-1 therapy acts on a specific subpopulation of CD8+ tumor-infiltrating lymphocytes (TILs) in melanoma mouse models, which can also be found in patients with melanoma. We find that dysfunctional CD8+ TILs possess canonical epigenetic and transcriptional features of T cell exhaustion, mirroring those seen in chronic viral infection. Similar to chronic viral infection, exhausted CD8+ TILs contain a subpopulation of “progenitor exhausted” T cells that have a distinct regulatory state. Progenitor exhausted TILs also have critical functional attributes that are not shared by the majority “terminally exhausted” TILs: they retain more polyfunctionality, persist following transfer into tumor-bearing mice, and differentiate to repopulate terminally exhausted TILs in the TME. As a result, progenitor exhausted CD8+ TILs are better able to control tumor growth than terminally exhausted cells. Progenitor exhausted, but not terminally exhausted, CD8+ TILs can respond to anti-PD-1 therapy but this occurs without reversion of their exhausted epigenetic state. Human melanomas contain CD8+ T cells with a progenitor exhausted phenotype and patients with a higher fraction of this subpopulation in their tumors have a significantly longer duration of response to combination checkpoint blockade therapy. Therefore, approaches to expand progenitor exhausted CD8+ T cells in the tumor microenvironment may be an important component of improving checkpoint blockade response.

Citation Format: Brian C. Miller, Debattama R. Sen, Rose Al Abosy, Kevin Bi, Yamini V. Virkud, Martin W. LaFleur, Kathleen B. Yates, Ana Lako, Kristen Felt, Girish S. Naik, Michael Manos, Evisa Gjini, F. Stephen Hodi, Scott J. Rodig, Arlene H. Sharpe, W. Nicholas Haining. Functionally specialized subsets of exhausted CD8+ T cells mediate tumor control and differentially respond to checkpoint blockade [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2701.

Detecting Iodine-Labeled Cells

The binding of radiolabeled reagents to cells is detected by coating the specimen with a photographic emulsion. When the emulsion is developed, silver grains can be seen at the location of the radiolabel. The silver grains appear as black dots in bright-field microscopy but are most readily detected by dark-field illumination, in which they are seen as intense silver reflections on a black background. The detection of iodine-labeled reagents is extremely sensitive, and the reaction is readily controlled by setting up duplicate samples and varying the exposure time. The method is compatible with histological stains and can be used in double-labeling procedures with enzyme- or fluorochrome-labeled reagents. When iodine-labeled and enzyme-labeled reagents are used together for double labeling, the enzyme-labeled reagents must be developed before the photographic emulsion is added.

Cooperation between Constitutive and Inducible Chemokines Enables T Cell Engraftment and Immune Attack in Solid Tumors

We investigated the role of chemokines in regulating T cell accumulation in solid tumors. CCL5 and CXCL9 overexpression was associated with CD8⁺ T cell infiltration in solid tumors. T cell infiltration required tumor cell-derived CCL5 and was amplified by IFN-γ-inducible, myeloid cell-secreted CXCL9. CCL5 and CXCL9 coexpression revealed immunoreactive tumors with prolonged survival and response to checkpoint blockade. Loss of CCL5 expression in human tumors was associated with epigenetic silencing through DNA methylation. Reduction of CCL5 expression caused tumor-infiltrating lymphocyte (TIL) desertification, whereas forced CCL5 expression prevented Cxcl9 expression and TILs loss, and attenuated tumor growth in mice through IFN-γ. The cooperation between tumor-derived CCL5 and IFN-γ-inducible CXCR3 ligands secreted by myeloid cells is key for orchestrating T cell infiltration in immunoreactive and immunoresponsive tumors.