Chimeric antigen receptors (CARs) targeting CD19 have mediated dramatic anti-tumor responses in hematologic malignancies, but tumor regression has rarely occurred using CARs targeting other antigens. It remains unknown whether the impressive effects of CD19 CARs relate to greater susceptibility of hematologic malignancies to CAR therapies, or superior functionality of the CD19 CAR itself. We discovered that tonic CAR CD3ζ phosphorylation, triggered by antigen-independent clustering of CAR scFvs, can induce early exhaustion of CAR T cells that limits anti-tumor efficacy. Such activation is present to varying degrees in all CARs studied, with the exception of the highly effective CD19 CAR. We further identify that CD28 costimulation augments, while 4-1BB costimulation ameliorates, exhaustion induced by persistent CAR signaling. Our results provide biological explanations for the dramatic anti-tumor effects of CD19 CARs and for the observations that CD19.BBz CAR T cells are more persistent than CD19.28z CAR T cells in clinical trials.

Data availabilityAll data presented in this manuscript are available from the corresponding author upon reasonable request. Bulk tumour cell RNA sequencing has been deposited at the Gene Expression Omnibus (GEO) under accession number https://www.ncbi.nlm.nih.gov/geo/ query/acc.cgi?acc=GSE110708. Single-cell RNA sequencing of tumour cells were also deposited at the GEO under accession numberhttps://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE110746.

We explored the utility of targeting anaplastic lymphoma kinase (ALK), a cell surface receptor overexpressed on pediatric solid tumors, using chimeric antigen receptor (CAR)-based immunotherapy. T cells expressing a CAR incorporating the single-chain variable fragment sequence of the ALK48 mAb linked to a 4-1BB-CD3ζ signaling domain lysed ALK-expressing tumor lines and produced interferon-gamma upon antigen stimulation but had limited anti-tumor efficacy in two xenograft models of human neuroblastoma. Further exploration demonstrated that cytokine production was highly dependent upon ALK target density and that target density of ALK on neuroblastoma cell lines was insufficient for maximal activation of CAR T cells. In addition, ALK CAR T cells demonstrated rapid and complete antigen-induced loss of receptor from the T cell surface via internalization. Using a model that simultaneously modulated antigen density and CAR expression, we demonstrated that CAR functionality is regulated by target antigen and CAR density and that low expression of either contributes to limited anti-tumor efficacy of the ALK CAR. These data suggest that stoichiometric relationships between CAR receptors and target antigens may significantly impact the anti-tumor efficacy of CAR T cells and that manipulation of these parameters could allow precise tuning of CAR T cell activity.

Genetically engineered T cells expressing CD19-specific chimeric antigen receptors (CARs) have shown impressive activity against B cell malignancies, and preliminary results suggest that T cells expressing a first generation disialoganglioside (GD2)-specific CAR can also provide clinical benefit in patients with neuroblastoma. We sought to assess the potential of GD2-CAR therapies to treat pediatric sarcomas. We observed that 18/18 (100%) of osteosarcomas, 2/15 (13%) of rhabdomyosarcomas, and 7/35 (20%) of Ewing sarcomas expressed GD2. T cells engineered to express a third generation GD2-CAR incorporating the 14g2a-scFv with the CD28, OX40, and CD3ζ signaling domains (14g2a.CD28.OX40.ζ) mediated efficient and comparable lysis of both GD2+ sarcoma and neuroblastoma cell lines in vitro. However in xenograft models, GD2-CAR T cells had no antitumor effect against GD2+ sarcoma, despite effectively controlling GD2+ neuroblastoma. We observed that pediatric sarcoma xenografts, but not neuroblastoma xenografts, induced large populations of monocytic and granulocytic murine myeloid-derived suppressor cells (MDSCs) that inhibited human CAR T-cell responses in vitro. Treatment of sarcoma-bearing mice with all-trans retinoic acid (ATRA) largely eradicated monocytic MDSCs and diminished the suppressive capacity of granulocytic MDSCs. Combined therapy using GD2-CAR T cells plus ATRA significantly improved antitumor efficacy against sarcoma xenografts. We conclude that retinoids provide a clinically accessible class of agents capable of diminishing the suppressive effects of MDSCs, and that co-administration of retinoids may enhance the efficacy of CAR therapies targeting solid tumors.

No abstract

Canine transmissible venereal tumor (CTVT) is a parasitic cancer clone that has propagated for thousands of years via sexual transfer of malignant cells. Little is understood about the mechanisms that converted an ancient tumor into the world's oldest known continuously propagating somatic cell lineage. We created the largest existing catalog of canine genome-wide variation and compared it against two CTVT genome sequences, thereby separating alleles derived from the founder's genome from somatic mutations that must drive clonal transmissibility. We show that CTVT has undergone continuous adaptation to its transmissible allograft niche, with overlapping mutations at every step of immunosurveillance, particularly self-antigen presentation and apoptosis. We also identified chronologically early somatic mutations in oncogenesis- and immune-related genes that may represent key initiators of clonal transmissibility. Thus, we provide the first insights into the specific genomic aberrations that underlie CTVT's dogged perseverance in canids around the world.

SummaryCostimulation mediated by the CD28 molecule plays an important role in optimal activation of T cells. However, CD28-deficient mice can mount effective T cell-dependent immune responses, suggesting the existence of other costimulatory systems. In a search for other costimulatory molecules on T cells, we have developed a monoclonal antibody (mAb) that can costimulate T cells in the absence of antigen-presenting cells (APC). The molecule recognized by this mAb, 9D3, was found to be expressed on almost all mature T cells and to be a protein of ,'-~24 kD molecular mass. By expression cloning, this molecule was identified as CD9. 9D3 (anti-CD9) synergized with suboptimal doses of anti-CD3 mAb in inducing proliferation by virgin T cells. Costimulation was induced by independent ligation of CD3 and CD9, suggesting that colocalization of these two molecules is not required for T cell activation. The costimulation by anti-CD9 was as potent as that by anti-CD28. Moreover, anti-CD9 costimulated in a CD28-independent way because anti-CD9 equally costimulated T cells from the CD28-deficient as well as wild-type mice. Thus, these results indicate that CD9 serves as a molecule on T cells that can deliver a potent CD28-independent costimulatory signal. Full activation of the T cell has been shown to require two independent signals (1). The first signal is provided by antigen-specific T cell receptor (TCR) interacting with processed antigen peptides plus major histocompatibitity complex (MHC) molecules on APC. This signal leads to an effective T cell response only when accompanied by a second costimulatory signal(s) presented by the APC. The lack of costimulation not only prevents activation but also induces tolerance called anergy (1). Identifying molecules capable of delivering costimulatory signals has been the subject of a large number of recent investigations (2-5). CD28 expressed on T cells was found to be a receptor for the costimulatory molecules CD80 and CD86 on APC (6). CD28 engagement, by either anti-CD28 mAb or ligands (CD80/ CD86), has been shown to costimulate T cells in the absence of APC, resulting in T cell activation (7-9). Conversely, the blocking of CD28-1igand interactions induced substantial inhibition of T cell activation (2). These observations indicated that the CD28-CD80/CD86 interaction functions as a critical pathway of T cell costimulation. Nevertheless, recent studies have revealed that CD28-deficient mice can develop normal in vivo immune responses (10) and that T cells from these mice mount APC-dependent responses for T cell activa~:ion in vitro although the response is reduced compared to T cells from wild-type mice (10, 11). Thus, these results strongly suggest that there may exist other molecules capable of providing costimulatory activity.In this report, we have developed a rat IgG mAb (9D3) by immunization with cells of a murine thymic stromal clone (12). This mAb recognized a protein of"-,24 kD that is expressed on immunizing thymic stromal cells as well as murine T cells. By cDNA expr...

The role of immune checkpoint inhibitors (ICIs) in the treatment of pediatric cancers continues to evolve. Such therapies function by augmenting existing antitumor T-cell responses that have been rendered ineffective by inhibitory pathways. Although ICIs have proven highly effective for adult cancers, initial phase I/II clinical trials using single-agent ICIs against unselected pediatric cancers have been overall disappointing. With the exception of pediatric classic Hodgkin lymphoma, responses to ICIs have been infrequent, likely stemming from an inherent difference in the immunogenicity of childhood cancers, which, on average, have far fewer neoantigens than adult cancers. Recently, however, hope has reemerged that certain subsets of children with cancer may benefit from ICI therapies. In preliminary studies, patients with both pediatric hypermutated and SMARCB1-deficient cancers have had impressive responses to ICI therapies, likely as a result of underlying biologies that enhance neoantigen expression and tumoral inflammation. Dedicated trials are ongoing to fully evaluate the efficacy of ICIs for patients with these subsets of pediatric cancer.