Friedman, David J.

Abstract Description Interactions between ST6GAL1-produced sialic acids and Siglec-2 (CD22) result in immune inactivation. CD22 is a well-characterized regulator of B cell function. We demonstrate that conventional type-2 dendritic cells (cDC2s) and hybrid macrophages express CD22 across the spleen and tumor microenvironment. To understand the function of CD22 on antigen-presenting cells, we generated novel CD22-floxed mice and crossed them with a) Zbtb46-Cre ones (those lacking CD22 on cDC2s and hybrid macrophages) as well as b) CX3CR1-Cre mice (without CD22 on hybrid macrophages nor microglia). From the former cross, we generated subcutaneous flank tumor and colitis-associated colorectal cancer models. CD22 expression on myeloid cells did not impact anti-tumor responses when tumors expressed low CD22 ligand levels. From here, we assess the impact of tumor cell CD22 ligand overexpression on tumor responses. Also, to evaluate CD8 T cell response to CD22, we challenged mice with Theiler’s murine encephalomyelitis virus (TMEV), modeling multiple sclerosis via CD8 T cell hyperactivation. CD22 did not mediate CD8 activation nor infiltration in brains of the latter cross. Thus, CD22 may be functionally redundant across myeloid and B cells. Topic Categories Innate Immune Responses and Host Defense: Cellular Mechanisms (INC)

Type 1 diabetes is characterized by the autoimmune destruction of pancreatic β cells, resulting in permanent loss of glucose homeostasis. Islet transplantation is a promising potential cure that remains hindered by immune rejection. We previously showed that ST8Sia6 expression on tumors reduced immune surveillance and hypothesized that this sialyltransferase could protect β cells from autoimmune destruction. Here, we demonstrate that ectopic expression of ST8Sia6 in β cells of female nonobese diabetic mice (NOD βST) decreased the spontaneous incidence of diabetes by 90% and preserved β cell mass. NOD βST mice had comparable insulitis at 8 weeks of age that did not progress over time compared with littermate controls. β Cell–autoreactive B and T cells were present in NOD βST mice, indicating a peripheral rather than central mechanism of immune tolerance. The islets of NOD βST mice displayed a dampened type 1 immune response and reduced IL-12p35 expression in dendritic cells compared with those of littermate controls. The peripheral protection persisted even after removal of ST8Sia6 expression at 20 weeks of age, indicating that transient expression was sufficient for establishment of tolerance. These results demonstrate that ST8Sia6 protects β cells from immune-mediated attack and rejection, highlighting its therapeutic potential for autoimmune disorders.

Abstract At the terminal position of many glycan chains are unique sugars known as sialic acids. Sialic acids bind sialic acid immunoglobin-like lectins (Siglecs). Our lab is interested in the role of CD22 (Siglec-2), which binds to α2,6-linked sialic acid generated by ST6GalI. CD22 can associate with itself or other molecules on the same cells in a “cis” configuration or bind in “trans” with ligands on other cells. CD22 contains four ITIMs within its cytoplasmic tail and is primarily known to function as an inhibitory co-receptor of the B cell receptor (BCR). CD22-ligand interactions restrain CD22 and BCR association which dampens BCR signaling. While CD22 is usually characterized as a B cell specific protein, we found that CD22 is highly expressed in hybrid macrophages (CD11b+ CD11c+ F4/80+) and expression decreases with activation, implying that CD22 may modulate myeloid cell responses. Using novel macrophage and dendritic cell-specific CD22 cKO mouse models, we will explore how CD22 impacts myeloid cell activation. We propose that CD22 impacts myeloid cell function and modulating this interaction may improve immune responses.

Abstract Colorectal cancer (CRC) is one of the most common cancers in the world, yet it remains one of the most difficult to treat. Immunotherapy of CRC using checkpoint inhibitors is only efficacious in approximately 4% of patients with high microsatellite instability (MSI-H). In earlier studies we showed that in patients with inflammatory bowel diseases (IBD) and CRC as well as in the APCΔ468 murine polyposis model, Tregs overexpress β-catenin. Furthermore, we demonstrated that expression of a dominant active β-catenin in Tregs induces RORγt, compromising suppressive activity and promoting TH17-like properties. This results in a scurfy-like phenotype. By contrast loss of TCF-1 upregulated RORγt, but only compromised Treg anti-inflammatory functions. These observations suggested cooperation of β-catenin and TCF-1 in regulating Treg plasticity. We show here that in healthy mice, β-catenin expression is significantly higher in Tregs as compared to conventional CD4+ T-cells. In Tregs that express a dominant active β-catenin, ablation of TCF-1 or HEB provide partial rescue, while ablation of both TCF-1 and HEB recovers Treg numbers and prevents the scurfy-like phenotype. These observations suggest that TCF-1, HEB and β-catenin cooperate to determine Treg functions. In search for additional partners, we show that loss of TCF-1 upregulates SATB-1, a chromatin modifier that is known to determine T-cell development and function. Upregulation of SATB-1 is enhanced with the loss of both TCF-1 and HEB. All three transcription factors upregulate RORγt suggesting that they cooperatively determine Treg function and plasticity. Elucidating this cooperation will enhance our ability to control Treg plasticity in health and disease. NIH/NIAID - R01 AI108682-0A1

Abstract T-cell Acute lymphoblastic leukemia (T-ALL) is an aggressive disease with only a 10% 5-year survival rate for patients over 60, due to genomic instability. We have shown that β-catenin and TCF-1 cooperate in DP thymocytes to promote genomic instability leading to lymphomagenesis. We have also established that TCF-1 and HEB cooperate in the DP stage. Now, we aim to determine the molecular underpinnings of T-ALL and thymic development, by elucidating the interplay of all three factors, β-catenin, TCF-1, and HEB. Aberrant activation of β-catenin promotes the development of Lymphoma/Leukemia equivalent to the human disease T-ALL and blocks thymocyte development in the early DP stage. Ablation of TCF-1 in thymocytes with activated β-catenin rescues from lymphomagenesis, but the developmental block persists. Antithetically, developmental progression is achieved only when HEB is conditionally knocked out. In an effort to explain this phenomenon, we found that genes associated with the transition of DP thymocytes from DPsmall to CD69+ are downregulated by activating β-catenin (e.g. Runx-3, Ccr-7, TOX-2, etc.) and revert upon deletion of HEB. This is associated with changes in the binding of TCF-1 to DNA. Genomic sites that acquire TCF-1 binding upon β-catenin activation forfeit this binding when HEB is deleted, while at the same time novel TCF-1 binding sites emerge. These new binding sites are significantly more enriched for the conserved TCF-1 motif than TCF-1-bound sites in WT thymocytes. This observation promotes the notion that HEB guides TCF-1 towards alternative binding sites, thus altering gene expression. These findings highlight the complex interactive relationship of TCF-1 and HEB in both normal development and disease. NIH/NIAID R01 AI147652-01A1

Abstract The lifetime risk for colorectal cancer in the United States is approximately 4%. Individuals with Inflammatory Bowel Disease, including Ulcerative Colitis and Crohn’s Disease, have a substantially increased risk of developing colorectal cancer. The mechanisms for accelerated tumorigenesis due to enhanced inflammation are not fully characterized. Siglecs (sialic acid immunoglobulin lectin-like proteins) are a family of inhibitory receptors that are negative regulators of the immune response. Siglec-E is an inhibitory receptor that is expressed by innate immune cells, including monocytes, macrophages, neutrophils and dendritic cells. Interestingly, mice deficient in Siglec-E have accelerated development of tumors and reduced survival in a spontaneous mouse model of colorectal cancer (TS4-cre LSL-KRasG12D APClox468/wt). While tumors develop at approximately six months in mice with Siglec-E, tumors develop at approximately two months in Siglec-E knockout mice. Initial results indicate that Siglec-E knockout mice also have accelerated gut inflammation using the DSS colitis model as compared to WT mice. Current studies are examining inflammation that develops during tumorigenesis in Siglec-E knockout mice. Center for Biomedical Discovery NIH R01 CA243545-01A1 to VSS Mayo Clinic Graduate School of Biomedical Sciences Deans fellowship: Initiative for maximizing student development (IMSD) R25 GM055252-26 Doyon Foundation

Abstract Glycans decorate most proteins and lipids on the cell surface, and glycoproteins serve a critical role in cell-cell communication. The terminal modification of many glycans is a sialic acid. There are twenty different mammalian sialyltransferases, each with a unique specificity. Recently, we demonstrated that the sialyltransferase ST8Sia6, which adds a terminal alpha-2,8 disialic acid to a preexisting sialic acid on cell surface proteins, generates ligands for the inhibitory receptor Siglec-E. In patients, ST8Sia6 overexpression is associated with worse outcome. We have previously shown that ST8sia6 overexpression in a mouse tumor model accelerates tumor growth, dependent upon Siglec-E expression by immune cells. To understand the basis for ST8Sia6 function in tumors, proteomic analysis of murine tumor lines was performed to identify proteins modified by ST8Sia6. Here, we demonstrate that ST8Sia6 sialylates CD44. Siglecs vary significantly between species, with 5 non-conserved Siglecs in mice compared to 10 non-conserved Siglecs in humans. The closest ortholog to Siglec-E in mice is Siglec-7 in humans. We also demonstrate that ST8Sia6 sialylates CD44, generating ligands for Siglec-7. Thus, ST8sia6 has a conserved role in both mice and humans in modifying glycans on CD44 to generate ligands for Siglecs. As CD44 expression on tumors is associated with increased metastasis, we predict that ST8Sia6 may also have a role in this process which is under investigation. Supported by grants from 2T32AI007425-23 to DJF and NIH Glycobiology Common Fund supplement to R01CA243545-01A1 to VSS.

Abstract Checkpoint blockade has improved patient outcome through reinvigorating T cells against cancer. Despite this success, many patients don’t respond, leaving them with few options. Tumors utilize many immunosuppressive mechanisms, including altering the function of innate immune cells. Cancer cells can promote differentiation of macrophages into tolerogenic “M2-like” macrophages, which support tumor growth. Macrophage differentiation by cancer cells is influenced by many receptors, including Siglec-E. 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. In some tumor types, higher expression of ST8Sia6 correlates with poorer survival. Therefore, ST8Sia6 overexpressing tumors may engage Siglec-E, leading to immune suppression and accelerating tumor growth. Mice injected with ST8Sia6 overexpressing MC38 and B16-F10 tumors had increased tumor burden, reduced survival and increased M2 macrophage polarization, which suppresses the immune response. This acceleration of tumor growth was abrogated when injected into Siglec-E knockout mice. Thus, the primary function of ST8Sia6 in tumors is an extrinsic alteration of the immune response, rather than an intrinsic effect on cell growth. Similarly, in a spontaneous colon cancer model, ST8Sia6 overexpression in a murine colon cancer model accelerated the development of tumors, decreasing survival to 2.5 months instead of 6 months. The overexpression of ST8Sia6 leads to increased Arg-1 expression by colonic tumor macrophages. Therefore, ST8Sia6 expression in tumors inhibits the immune response to promote tumor growth in a Siglec-E dependent manner.

Abstract Inflammatory bowel disorders (IBD), including Crohn’s disease and colitis, have a prevalence between 10–20%. IBD patients experience a reduced quality of life due to symptoms such as diarrhea, abdominal cramps, anal bleeding. The exact etiology of IBD is incompletely understood, but the immune response plays a critical role. Innate immune cells integrate the sum of stimulatory and inhibitory signals to determine overall activation. Solidifying the roles of pertinent proteins and their interactions is critical in mapping the immune response and understanding the implications of clinical treatment of IBD. ST8Sia6 is a sialic acid transferase which adds terminal a2,8 disialic acids to cell surface glycoproteins, and we have found that ST8Sia6 generates ligands for Siglec-2 and Siglec-E on innate immune cells, including colonic macrophages and neutrophils. These siglecs are inhibitory sialic acid-binding membrane receptors expressed on several immune cell subsets. Restraining ST8Sia6 should therefore enhance the immune response by reducing siglec-mediated inhibition. To examine this, our lab has generated a novel ST8Sia6 knockout (KO) mouse. At baseline these mice are healthy, but we have found that ST8Sia6 KO mice challenged with dextran sodium sulfate (DSS)-induced colitis lose weight and succumb to colonic inflammation significantly faster than their wildtype (WT) counterparts, particularly at low doses. ST8Sia6 KO mice exhibit increased histological damage and immune infiltration of the colon and small bowel as compared to WT controls. This data demonstrates a role for ST8Sia6 in regulating the hose immune response to inflammatory bowel disorders.

Abstract Glycans decorate almost all proteins and lipids on the cell surface, and they play a critical role in cell-cell interactions. Terminal sialic acid addition to glycans helps the immune system distinguish between “self” and “non-self.” The enzyme ST8sia6 adds alpha-2,8 disialic acids to cell surface receptors, and this sialic acid addition promotes binding to the inhibitory receptor Siglec-E. We have previously shown ST8sia6 overexpression in tumors accelerates tumor growth in a Siglec-E dependent manner. Proteomic analysis in murine tumor lines identified CD44 as a target for ST8sia6. CD44 expression on tumors is associated with metastasis and chemoresistance. Since humans and mice have different sialic acid structures, we examined whether ST8sia6 makes CD44 a ligand to the ortholog of Siglec-E in human cells, Siglec-7. When HEK293T cells overexpress ST8sia6 and CD44 together, Siglec-7 binding was significantly increased compared to ST8sia6 or CD44 overexpression alone. Thus, ST8sia6 has a conserved role from mouse to human in decorating CD44 with sialic acid. Our study proposes that ST8sia6 sialylation of CD44 could suppress immune responses and enables cancer progression in human. Supported by grant: 2T32AI007425-23 to DJF and R01 CA243545-01S1 to V.S.S.