Immune-desert tumor microenvironment in SMARCA4-deficient thoracic sarcomas with limited efficacy of immune checkpoint inhibitors.

11552 Background: SMARCA4-deficient thoracic sarcomas (SDS) are rare and aggressive sarcomas characterized by inactivating SMARCA4 mutations, with no approved treatment to date. Previous data linking SWI/SNF deficiency with tumor immune microenvironment (TME) are contradictory. While an immunogenic microenvironment and efficacy of immune checkpoint inhibitors (ICI) were described in SMARCA4-deficient small cell carcinoma of the ovary, the TME phenotype of SDS is unknown; in addition, response of patients to ICI is lacking. Methods: All consecutive patients diagnosed with SDS between 2016 and 2019 in Strasbourg University Hospital were included and clinical outcomes collected. Immunostainings for immune cell markers, immune checkpoints and tertiary lymphoid structures (TLS) were assessed on available samples. Validation was performed using an independent transcriptomes dataset of SDS (n = 12), not otherwise specified (NOS) non-small cell lung cancer (NSCLC) with/without SMARCA4 mutations (n = 14) and undifferentiated thoracic sarcoma (n = 5). Finally, chemokines (CXL9 and CXCL10) and PD-L1 expressions were assessed in NSCLC and thoracic fibroblast cell lines, treated with/without interferon gamma (IFNG). Results: Nine patients were identified and had all metastatic disease at presentation, with a median overall survival of 1.8 months (0.3-NR). Among them, 4 received ICI as part of their treatment. Out of 11 evaluated tumors samples, all but one case showed no TLS, consistent with an immune desert TME phenotype charted by low densities of CD3+ T-cells, CD8+ T-cells, CD20+ B-cells from one side and high density of CD68+ macrophage-cells from the other side. Conversely, the unique tumor with TLS aggregate showed an immune-rich TME phenotype associated with high mutational tumor burden. While the patient with TLS harboring tumor showed an exceptional long-lasting response, the 3 remaining patients without TLS had progressive disease at best response to ICI. Using an independent cohort, unsupervised clustering using immune cell scores identified two clusters tightly associated with cell ontogeny of cancer subtypes and immunity; while cluster 1 (C1) was enriched for NOS NSCLC independently from SMARCA4 status (n = 9/10; 90%) (p = 0.001), C2 was enriched for SDS (n = 11/12; 91.7%) (p = 0.005) and undifferentiated thoracic sarcomas (n = 5/5; 100%) (p = 0.0005). Finally, SMARCA4 loss of function experiments revealed upregulation of chemokines (CXL9 and CXCL10) and PD-L1 expression in the NSCLC cell line with no effect on thoracic fibroblast cell line. Conclusions: SDS harbor an immune desert TME phenotype with limited efficacy to ICI, similar to other sarcomas. Our data suggest that TME of SMARCA4-driven tumors might vary according to the cell of origin. Further studies are needed to understand the interplay between SWI/SNF mutations, cell ontogeny and immunity.

Stereotypic expansion of Tregulatory and Th17 cells during infancy is disrupted by HIV exposure and gut epithelial damage.

Few studies have investigated immune cell ontogeny throughout the period of increased vulnerability to infections in early life. Here, we evaluated the dynamics of two critical T cell populations, regulatory T (Treg) cells and Th17 cells, over the first 9 months of life. We observed that Treg and Th17 cells developed in a synchronous fashion. Infants exposed to HIV in utero (iHEU), who are more likely to develop infections, had a lower frequency of Tregs at birth and 36 weeks compared to HIV unexposed infants. This increased Th17/Treg ratio in iHEU was associated with impaired gut integrity at birth. These findings suggest that gut damage disrupts the Th17/Treg ratio during infant immune development, likely by attracting Treg cells to regulate inflammation occurring in the gut, so revealing an immune-gut nexus influenced by HIV exposure.

Generation of specialized blood vessels through transdifferentiation of lymphatic endothelial cells

The lineage and developmental trajectory of a cell are key determinants of cellular identity. Yet, the functional relevance of deriving a specific cell type from ontologically distinct progenitors, remains an open question. In the case of the vascular system, blood and lymphatic vessels are composed of endothelial cells (ECs) that differentiate and diversify to cater the different physiological demands of each organ. While lymphatic vessels have been shown to originate from multiple cell sources, lymphatic ECs (LECs) themselves seem to have a unipotent cell fate. In this work we uncover a novel mechanism of blood vessel formation through transdifferentiation of LECs. Using advanced long-term reiterative imaging and lineage-tracing of ECs in zebrafish, from embryonic development through adulthood, we reveal a hitherto unknown process of LEC-to-BEC transdifferentiation, underlying vascularization of the anal fin (AF). Moreover, we demonstrate distinct functional implications for deriving AF vessels from either LECs or BECs, uncovering for the first time a clear link between cell ontogeny and functionality. Molecularly, we identify Sox17 as a negative regulator of lymphatic fate specification, whose specific expression in AF LECs suppresses its lymphatic cell fate. Finally, we show that akin to the developmental process, during adult AF regeneration the vasculature is re-derived from lymphatics, demonstrating that LECs in the mature fish retain both potency and plasticity for generating specialized blood vessels. Overall, our work highlights a novel mechanism of blood vessel formation through LEC trans-differentiation, and provides the first in vivo evidence for a link between cell ontogeny and functionality in ECs.

Polypyrimidine tract-binding proteins are essential for B cell development

Polypyrimidine tract-binding protein 1 (PTBP1) is a RNA-binding protein (RBP) expressed throughout B cell development. Deletion of Ptbp1 in mouse pro-B cells results in upregulation of PTBP2 and normal B cell development. We show that PTBP2 compensates for PTBP1 in B cell ontogeny as deletion of both Ptbp1 and Ptbp2 results in a complete block at the pro-B cell stage and a lack of mature B cells. In pro-B cells PTBP1 ensures precise synchronisation of the activity of cyclin dependent kinases at distinct stages of the cell cycle, suppresses S-phase entry and promotes progression into mitosis. PTBP1 controls mRNA abundance and alternative splicing of important cell cycle regulators including CYCLIN-D2, c-MYC, p107 and CDC25B. Our results reveal a previously unrecognised mechanism mediated by a RBP that is essential for B cell ontogeny and integrates transcriptional and post-translational determinants of progression through the cell cycle.