Shapiro, Michael J.

Abstract Inhibitory receptors have a critical role in the regulation of immunity. Siglecs are a family of primarily inhibitory receptors expressed by immune cells, which recognize specific sialic acid modifications on cell surface glycans. Sialyltransferases are the enzymes that catalyzing the addition of sialic acids onto glycoproteins and glycolipids. Siglec-E binds to α2,8-linked disialic acids, generated by the enzyme ST8Sia6 on O-linked glycoproteins and functions to inhibit activation of innate immune cells. Many tumors have increased sialic acid incorporation. Overexpression of the sialyltransferase ST8Sia6 on tumors led to altered macrophage polarization and increased tumor growth through engagement of ligands generated by ST8Sia6 with Siglec-E. Here, we examine the role of ST8Sia6 on immune cells in regulating anti-tumor immunity. The loss of ST8Sia6 in tumor bearing mice reduced tumor growth, increased survival, and altered the immune response as compared to ST8Sia6 sufficient tumor bearing mice. Using MC38, B16, and B16-OVA flank tumor mouse models, tumor growth was decreased in ST8Sia6 KO mice as compared to C57Bl/6 mice. Within the tumor microenvironment of tumors present in ST8Sia6 KO mice compared to tumors present in C57Bl/6 mice, macrophages and dendritic cells altered their phenotype towards enhanced immune activation. Tregs from tumors in ST8Sia6 KO mice also displayed an increase in RORγt and a reduction in TNFRII, suggesting a possible shift to a less suppressive Treg. These alterations in the immune response to tumors were due to the loss of ST8Sia6 in macrophages and dendritic cells and not T-cells. Thus, ST8Sia6 generates ligands for Siglecs that dampen anti-tumor immunity. Supported by grants from NIH (R01CA243545, 2T32AI007425) and Mayo Clinic Center of Biomedical Discovery

Abstract Sialic acids are the terminal modification on glycoproteins and glycolipids. The sialic acid transferase ST8Sia6 generates a2,8-linked disialic acids on O-linked glycoproteins, which bind to the inhibitory receptor Siglec-E on innate immune cells and suppress immune activation. Recently, our lab has shown that ST8Sia6 expression on tumor cells inhibits the immune response through Siglec-E, leading to increased tumor growth and decreased survival. Therefore, the products of ST8Sia6 can modulate the immune response to tumors. Our lab has generated ST8Sia6 knockout (KO) mice, and we seek to understand the role of ST8Sia6 in regulating the immune response to pathogens. We find that ST8Sia6 KO mice, when challenged with influenza virus, clear the infection significantly faster than wildtype controls, suggesting that absence of ST8Sia6 reduces inhibition of the innate compartment, thereby increasing immune recruitment to infection. This data suggests an important role for interactions between Siglec-E and ST8Sia6-generated disialic acids in immune regulation and recruitment to infection sites.

Abstract Cell surface hypersialylation is used by malignant cells as a survival mechanism, leveraging interactions with Siglec receptors to dampen the immune response. In this study, we aimed to harness this mechanism to prevent autoimmune rejection of beta cells in the context of type 1 diabetes. Mice engineered to overexpress the sialyltransferase ST8Sia6 in beta cell (BST) were generated and crossed to the non-obese diabetic (NOD) mouse for 10 generations. The blood glucose of these mice compared to non-overexpressing littermates was assessed twice weekly to determine diabetes incidence, defined as 3 consecutive blood glucose readings above 250 mg/dL. Analysis of female BST mice aged 15 weeks or greater showed no incidence of overt diabetes, while littermates developed diabetes. BST males aged 15 weeks or greater also had no disease incidence, while littermate controls also became diabetic. An insulin tolerance test demonstrated no significant difference between BST mice and littermates, though the BST had significantly higher blood glucose during a glucose challenge, possibly representing insulin hypersecretion by littermate islets in a stressed pre-diabetic state. Histologic analysis of young BST and littermate pancreata showed no differences, with similar levels of insulitis at 8 weeks. Thus, the overexpression of ST8Sia6 has been shown to mitigate autoimmune attack in the NOD model and presents a novel path forward for prevention of islet transplant rejection.

Abstract Type 1 diabetes is an autoimmune disease that destroys insulin-producing beta cells in the pancreatic islets. Although exogenous insulin replacement therapy is lifesaving, precise blood glucose management remains a significant challenge. Experimental islet transplantation restores normoglycemia, but the lack of available islets and graft rejection present major barriers to widespread adoption as standard therapy. The islet resident macrophage (IRM) is in close contact with beta cells, efficiently presents islet antigen to autoreactive T cells, and is critical to disease occurrence. We have demonstrated that murine IRMs express high levels of Siglec-E, an immune-regulatory receptor that preferentially binds α2,8-disialic acids in cis or in trans. When islets were exposed to streptozocin (STZ)-induced stress in culture, beta cells upregulated ligands for Siglec-E. To determine the biological significance of this, we generated a mouse model where ST8Sia6, the sialyltransferase that produces α2,8-disialic acids and thus Siglec-E ligands, is constitutively overexpressed in beta cells. This increased trans Siglec-E binding affinity to the beta cell surface, and mitigated hyperglycemia in the multiple low-dose STZ model of diabetes. Significant reduction in blood sugar was evident at 14 days post-initial injection of STZ, coinciding with T cell infiltration of islets. T cells from ST8Sia6+ islets were less activated when compared to littermates. This work reveals a novel and important role for Siglec-E in autoimmune diabetes and provides insight into how rational targeting of this immune receptor may instruct islet-residing antigen presenting cells to provide local tolerance signals in the setting of islet transplantation.

Abstract T-cell targeted immunotherapy has resulted in novel treatments for multiple cancers and has been the primary focus in cancer immunotherapy. However, many patients still fail checkpoint therapies due to immune suppression mechanisms driven by cells, such as tolerogenic M2-like macrophages, which support tumor growth. ST8Sia6, a sialyltransferase, generates α2,8-linked disialic acids on O-linked glycoproteins. Cancer patients with high ST8Sia6 expression exhibit reduced survival as compared to patients with low expression, but the mechanism is unknown. We generated MC38 and B16–F10 murine cancer cell lines that overexpress ST8Sia6. Expression of ST8Sia6 accelerated flank tumor growth in both single and dual flank injection models as compared to parental tumors. In vivo, macrophage polarization was altered in ST8Sia6 expressing tumors. At day 7, tumor associated macrophages (TAMs) from ST8Sia6 negative MC38 tumors were primarily M1-like, while TAMs from ST8Sia6 expressing MC38 tumors were predominately M2-like. Tumor growth and M2-like polarization was dependent Siglec-E. Siglec-E present on innate cells bind α2,8-linked disialic acids generated by ST8Sia6 and suppress immune activation. Therefore, ST8Sia6 overexpressing tumors engage Siglec-E, suppressing the immune response and increasing tumor growth. ST8Sia6 overexpression in a spontaneous murine colon cancer model decreased survival from over 6 months to 2–3 months. Therefore, ST8Sia6 expression in tumors may promote tumor growth by promoting tolerogenic immune responses.

Abstract ABCB7 is a mitochondrial transporter that exports iron-sulfur (Fe-S) cluster intermediates from the mitochondria to the cytoplasm. Using CD2-iCre ABCB7 cKO mice, we found ABCB7 is required for early B cell development. These mice have a severe block in pro-B cell development at the Fr. C to Fr. C′ transition, with reduced expression of intracellular μ and pre-BCR. CD19-cre and CD23-cre ABCB7 cKO mice have normal B cell development and peripheral B cell populations, indicating ABCB7 is required only during early B cell development. We observed reduced expression of Runx1, Ikaros, Foxo1, E47 (E2A), and Pax5 in CD2-iCre ABCB7 cKO pro-B cells. Interestingly, we also observed decreased BACH2 expression, which indicates elevated heme synthesis is occurring in the absence of ABCB7. We hypothesize that in the absence of ABCB7, developing B cells enhance heme synthesis to dispose of accumulating mitochondrial iron. Using a hen egg lysozyme BCR transgene, we were able to rescue B cell development in CD2-iCre ABCB7 cKO mice. Intriguingly, at the age of 5–8 weeks old, CD2-iCre ABCB7 cKO mice become lethargic and hunched. CBCs reveal an increase in peripheral blood neutrophils. Both the spleen and bone marrow become highly infiltrated by CD11b+Ly6Glow/+Ly6C− cells that are immature and mature neutrophils. As other knockout mice with pro-B cell blocks do not have neutrophilia, we considered if increased iron or heme is responsible for our observation. As heme can stimulate TLR4, we propose that excess heme from developing ABCB7-deficent B cells activates macrophages. In support of this, hemin induced GM-CSF production in BMDM cultures. We are currently exploring the mechanism by which excess heme production can stimulate the production of neutrophils.

Abstract T cell development depends on the coordinated interplay between receptor signaling and transcriptional activation/repression. We performed a genetic complementation screen, and identified a novel transcriptional repressor, NKAP. NKAP associates with HDAC3 and is part of a DNA-binding complex, as shown by chromatin immunoprecipitation. NKAP also associates with CIR, a part of the Notch corepressor complex. The expression of NKAP during T cell development inversely correlates with the expression of Notch target genes, implying that NKAP may modulate Notch-mediated transcription. To examine the function of NKAP in T cell development, Lck-cre NKAP conditional knockout mice were generated. Interestingly, loss of NKAP blocks development of alpha/beta but not gamma/delta T cells. In addition, lck-cre NKAP cKO DP T cells express 8- to 20-fold higher levels of Hes1, Deltex1 and CD25, providing in vivo evidence that NKAP functions as a transcriptional repressor, acting at least in part, by repression of the Notch pathway.

Abstract During hematopoiesis, proper differentiation of the long-term, multi-potent, self-renewing hematopoietic stem cell (HSC) results in the cellular components of blood. The lineage fate decisions that occur during hematopoiesis result from the coordinated regulation of transcriptional activators, repressors, and chromatin modifiers. We have identified NKAP as a novel regulator of hematopoiesis, that functions at least in part as a transcriptional repressor. In this study we employ a combination of conditional and inducible mouse models of NKAP deletion to explore the role of NKAP in hematopoietic development starting at the HSC stage. Our models allow for both the pan-deletion of NKAP starting in the HSC stage by using a vav-cre transgenic, as well as temporally regulated deletion of NKAP in the hematopietic compartment through the use of the interferon-inducible mx1-cre model system. These complimentary approaches allow us to conclude that NKAP functions in a cell intrinsic manner, as a member of a unique group of transcriptional regulators, and is necessary in the earliest stage of definitive hematopoiesis.

Abstract Inhibitory receptors have a critical role in the regulation of immunity. Siglecs are a family of primarily inhibitory receptors expressed by immune cells that recognize specific sialic acid modifications on cell surface glycans. Many tumors have increased sialic acid incorporation. Overexpression of the sialyltransferase ST8Sia6 on tumors led to altered immune responses and increased tumor growth. In this study, we examined the role of ST8Sia6 on immune cells in regulating antitumor immunity. ST8Sia6 knockout mice had an enhanced immune response to tumors. The loss of ST8Sia6 promoted an enhanced intratumoral activation of macrophages and dendritic cells, including upregulation of CD40. Intratumoral regulatory T cells exhibited a more inflammatory phenotype in ST8Sia6 knockout mice. Using adoptive transfer studies, the change in regulatory T cell phenotype was not cell intrinsic and depended on the loss of ST8Sia6 expression in APCs. Thus, ST8Sia6 generates ligands for Siglecs that dampen antitumor immunity.