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Immune effector cell (IEC) therapies have revolutionized our approach to relapsed B-cell malignancies, and interest in the investigational use of IECs is rapidly expanding into other diseases. Current challenges in the analysis of IEC therapies include small sample sizes, limited access to clinical trials and a paucity of predictive biomarkers of efficacy and toxicity associated with IEC therapies. Retrospective and prospective multi-center cell therapy trials can assist in overcoming these barriers through harmonization of clinical endpoints and correlative assays for immune monitoring, allowing additional cross-trial analysis to identify biomarkers of failure and success. The Consortium for Pediatric Cellular Immunotherapy (CPCI) offers a unique platform to address the aforementioned challenges by delivering cutting-edge cell and gene therapies for children through multi-center clinical trials. Here the authors discuss some of the important pre-analytic variables, such as biospecimen collection and initial processing procedures, that affect biomarker assays commonly used in IEC trials across participating CPCI sites. The authors review the recent literature and provide data to support recommendations for alignment and standardization of practices that can affect flow cytometry assays measuring immune effector function as well as interpretation of cytokine/chemokine data. The authors also identify critical gaps that often make parallel comparisons between trials difficult or impossible.

The bone marrow is a frequent location of primary relapse after conventional cytotoxic drug treatment of human B-cell precursor acute lymphoblastic leukemia (BCP-ALL). Because stromal cells have a major role in promoting chemotherapy resistance, they should be included to more realistically model in vitro drug treatment. Here we validated a novel application of the xCELLigence system as a continuous co-culture to assess long-term effects of drug treatment on BCP-ALL cells. We found that bone marrow OP9 stromal cells adhere to the electrodes but are progressively displaced by dividing patient-derived BCP-ALL cells, resulting in reduction of impedance over time. Death of BCP-ALL cells due to drug treatment results in re-adherence of the stromal cells to the electrodes, increasing impedance. Importantly, vincristine inhibited proliferation of sensitive BCP-ALL cells in a dose-dependent manner, correlating with increased impedance. This system was able to discriminate sensitivity of two relapsed Philadelphia chromosome (Ph) positive ALLs to four different targeted kinase inhibitors. Moreover, differences in sensitivity of two CRLF2-drivenBCP-ALL cell lines to ruxolitinib were also seen. These results show that impedance can be used as a novel approach to monitor drug treatment and sensitivity of primary BCP-ALL cells in the presence of protective microenvironmental cells.

Summary The challenges in recapitulating in vivo human T cell development in laboratory models have posed a barrier to understanding human thymopoiesis. Here, we used single-cell RNA sequencing (sRNA-seq) to interrogate the rare CD34+ progenitor and the more differentiated CD34– fractions in the human postnatal thymus. CD34+ thymic progenitors were comprised of a spectrum of specification and commitment states characterized by multilineage priming followed by gradual T cell commitment. The earliest progenitors in the differentiation trajectory were CD7– and expressed a stem-cell-like transcriptional profile, but had also initiated T cell priming. Clustering analysis identified a CD34+ subpopulation primed for the plasmacytoid dendritic lineage, suggesting an intrathymic dendritic specification pathway. CD2 expression defined T cell commitment stages where loss of B cell potential preceded that of myeloid potential. These datasets delineate gene expression profiles spanning key differentiation events in human thymopoiesis and provide a resource for the further study of human T cell development.

Backgroud: Despite advances in therapy and improved survival, relapsed and refractory B-cell precursor acute lymphoblastic leukemia (r/r BCP-ALL) in pediatric and adult patients still remains a problem. Chimeric antigen receptor T cells against CD19 (CD19 CAR T) show promising results in patients with r/r BCP-ALL. However, relapse of the disease still occurs with appreciable frequency even with this novel therapy. As a significant number of relapses post-CAR T lack surface CD19 expression, further CD19-directed therapy is not an option for these cases. Hypothesis: Sometimes despite CAR T engraftment and establishment of B-cell aplasia, relapse still occurs. We hypothesized that, similarly to cell adhesion mediated chemotherapeutic drug resistance (CAM-DR), cell adhesion mediated CAR T-cell resistance (CAM-CART-R) can contribute to relapse of ALL. Results: To test our hypothesis, primary ALL cells were treated with CD19 CAR T cells either with murine calvaria-derived bone marrow stromal cells, OP9, or cultured only with media in short term cultures. We observed B-ALL cells treated with CD19 CAR T on OP9 has 10-20% higher viability compared to B-ALL and CD19 CAR T co-culture in medium alone, supporting the notion of CAM-CART-R. We also determined that soluble factors in OP9 primed medium may contribute to CAM-CART-R. However, the direct stromal contact mediated significant protection again CAR T induced apoptosis of B-ALL cells. To determine the molecular mechanisms underlying the survival promoting effects of stromal cells on CD19-, these cells were starved in serum-free media for 4hours and then treated with PI3Kδ inhibitor CAL-101 or DMSO and co-cultured with OP9 cells for 1 hour. We found that p-Akt is upregulated by stromal contact in CD19-negative B-ALL cells post-CAR T therapy and that PI3Kδ inhibition using can downregulate p-Akt in CD19-negative B-ALL patients. Critically, we investigated whether CD19 CAR T cells were functional under these conditions. For this purpose, we determined if stromal contact of ALL cells or stromal contact of CAR T cells changes the intracellular cytokine milieu of CD19 CAR T cells and found that intracellular IL-6, TNF- α and IFN-γ were reduced upon stromal contact supporting our hypothesis of a role of stromal cells in CAM-CART-R. We also determined that immune checkpoints molecules on T cells are unaffected by OP9 cells. Despite the reduction of cytokine level in T cells upon co-culture with B-ALL cells on OP9, PD-1, TIM-3 and LAG3 expression on CD19 CAR T cells after 2 days of co-culture was not altered as determined by flow cytometry. Resistance of ALL cells to CD19 CART cells was not mediated through checkpoint inhibition, since the PD-1/PD-L1 inhibitor Nivolumab failed to enhance ALL killing. Phenotypic profiling of thirteen cases of primary ALL relapse post-CD19 CAR T cell therapy showed high expression of adhesion molecules including integrin α4. Phenotypic analysis also revealed high expression of integrins is retained in primary ALL cells after CD19 knockout in one case. To explore possible solutions to overcome CAM-CART-R, we examined a strategy of blocking specifically integrin α4. We have previously shown that blocking integrin α4 can de-adhere CD19-negative B-ALL relapse post-CAR T cell therapy from their respective counter-ligands in vitro and that these cells can benefit from integrin blocking therapy in vivo. We have now confirmed this in NSG mice injected with CD19-negative B-ALL cells from a patient with post-CAR T cell relapse. Mice were treated intraperitoneally (n=6/group) with total immunoglobulin (Ig) control or humanized anti-human integrin α4 antibody Natalizumab (NZM). As a result, Natalizumab monotherapy significantly prolonged survival of leukemic mice compared to control Ig group (66 days (Ig) vs 85 days (NZM) p Conclusion: In summary, our data indicate that similarly to CAM-DR, CAM-CART-R can occur resulting in relapse of ALL. Targeting adhesion molecules may be a new approach to treat or prevent relapse following CD19 CAR T cell therapy for . Disclosures Ahmed: CellMedica: Other: Royalties; Celgene: Other: Royalties; Adaptimmune: Membership on an entity's Board of Directors or advisory committees. Babak:Simurx. Inc: Other: Founder . Pulsipher:Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Jazz: Other: Education for employees; Adaptive: Membership on an entity's Board of Directors or advisory committees, Research Funding; CSL Behring: Membership on an entity's Board of Directors or advisory committees; Amgen: Other: Lecture; Bellicum: Consultancy; Miltenyi: Research Funding; Medac: Honoraria. Wayne:AbbVie: Consultancy; Kite, a Gilead Company: Consultancy, Research Funding; Servier: Consultancy; Spectrum Pharmaceuticals: Consultancy, Research Funding. Abdel-Azim:Adaptive: Research Funding.