Siglec-15 is required for evasion of immune response in acute lymphoblastic leukemia Open Access

Pillsbury, Claire (Summer 2023)

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Relapsed/refractory pediatric B cell acute lymphoblastic leukemia (B-ALL) remains one of the leading causes of cancer-related death in children. Immunotherapies have shown promise in treatment of this disease, suggesting further study of the immune evasion mechanisms utilized by B-ALL can yield new therapeutic targets and improve patient outcomes. The sialic acid-binding molecule, Siglec-15 (Sig15), has recently emerged as a novel immunomodulatory molecule across a breadth of solid tumors, yet its role in blood cancers and its function in immunosuppression remains incomplete.

We began by characterizing the expression of Sig15 across blood cancers using in silico data, immortalized cell line models, and primary B-ALL patient samples, demonstrating pathological overexpression of Sig15 compared to healthy controls. This overexpression was found to be regulated by NF-κB, which also enhanced Sig15 localization to the cell surface and, notably, the release of a secreted/soluble form of Sig15 (sSig15) which circulates at elevated levels in the plasma of pediatric B-ALL patients. sSig15 was found to correlate with an overall more immunosuppressive circulating cytokine profile, and the protein in its recombinant form was successfully able to abrogate activation in a chimeric antigen receptor (CAR)-expressing Jurkat model. Sig15 ablation in a murine model of B-ALL significantly promoted leukemia clearance in immunocompetent recipients, accompanied by increases in CD8+ T cell expansion, activation, and effector profiles as well as a decrease in leukemia-promoting bone marrow cytokines. Thus Sig15 targeting was able to successfully reverse the bone marrow tumor microenvironment (TME) and augment leukemia control.

Further study of Sig15 in B-ALL demonstrated the expression of multiple alternatively spliced isoforms, one of which (Sig15-204) was demonstrated to be expressed at significantly lower levels in B-ALL and lymphoma cells as compared to healthy peripheral blood mononuclear cells (PBMCs). Investigation into the regulation of canonical Sig15 demonstrated sensitivity of Sig15 expression to active cell cycling, wherein Sig15 expression was increased in both B-ALL and healthy PBMCs upon proliferation and was found to be highest in G2/M phase in B-ALL cells. The extracellular domain of Sig15 was also found to be present in the nucleus of both B-ALL and osteosarcoma cells, where in the case of the former, nuclear Sig15 was found to localize to perinucleolar compartments with implications towards regulating RNA metabolism.

This dissertation thus provides a complex profile of Siglec-15 as a potent immunosuppressive molecule with unique regulation and subcellular localization in B-ALL. It plays an active role in immune evasion in leukemia and may be targeted therapeutically to activate T lymphocytes against leukemia cells.

Table of Contents

Chapter 1: Introduction   Pages 1-11

1.1 Introduction  1-2

1.2 Leukemia                                                             

1.2.1      Acute lymphoblastic leukemia (ALL)  2-3

1.2.2      B cell acute lymphoblastic leukemia (B-ALL)  3-5

1.3 Immune evasion in B-ALL

1.3.1      Mechanisms of immune escape in B-ALL  5-7

1.3.2      Calcineurin and IL-12 regulate B-ALL immune escape  7-8

Chapter 2: Siglec-15 and immunosuppression  Pages 12-29

2.1 Siglec family proteins

2.1.1      Sialic acid and self-immunity  12-14

2.1.2      Siglecs in overview  14-15

2.1.3      Siglecs in cancer  15-18

2.1.4      Siglecs in leukemia  18-19

2.2 Siglec-15 as a highly conserved immunomodulator

2.2.1      Siglec-15 in osteoclasts and osteoporosis  21

2.2.2      Siglec-15 and immunosuppression  22

2.2.3      Siglec-15 in cancer  22-23

2.3 Rationale and Research Goals  23-24

Chapter 3: Siglec-15 promotes evasion of adaptive immunity in B-cell acute lymphoblastic leukemia Pages 30-58 

3.1 Abstract  31

3.2 Introduction  31-33

3.3 Materials and Methods  33-37

3.4 Results  37-42

3.5 Discussion  42-45

Chapter 4: Siglec-15 possesses unique localization and alternative isoform expression in lymphoblastic cancers  Pages 60-77

4.1 Siglec-15 isoform expression differs between normal and malignant leukocytes  60-63

4.2. Siglec-15 expression changes through the cell cycle and is highest in mitosis in leukocytes  63-64

4.3 The extracellular domain of Sig15 has the capacity to localize to the nucleus  65-66

Chapter 5: General Discussion and Closing Remarks  Pages 78-86

5.1 Sig15 is pathologically expressed in blood cancers  78-79

5.2 NF-κB and calcineurin activation drives Sig15 pathology in B-ALL  79-80

5.3 Sig15 mediates immunosuppression in B-ALL  80-82

5.4 Secreted/soluble and nuclear Sig15 may demonstrate unique roles in Sig15 pathology  82

5.5 Future Directions  83-85

References  Pages 87-102

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