Approximately 80% of B-cell non-Hodgkin's lymphomas are derived from germinal center (GC) B-cells. Somatic hypermutation within GC B-cells is necessary for antibody affinity maturation, but somatic mutations can also drive epigenetic lesions that enhance survival, maintain the hyperproliferative state of GC B-cells, and promote lymphomagenesis. Given the susceptibility of GC B-cells to malignant transformation, it is not surprising that the natural process of selection within the GC is thought to help protect against the selection of malignant clones. Accordingly, a deeper understanding of factors that restrain the GC are needed. Siglec- 10/G (human/mouse ortholog) are members of the Siglec family of inhibitory receptors that recognize sialic acid-containing glycan ligands. Siglec-10/G dampen B-cell activation through the B-cell receptor. In mice, the role of Siglec-G in inhibiting the initial events of B-cell activation have been described, but it is unknown if Siglec-G plays a role at later stages, such as in the GC. Interestingly, an expanded GC compartment is observed in Siglec-G-/- mice, Siglec-G-/- mice develop B-cell lymphomas, and Siglec-10 expression is lost on GC-derived B-cell lymphomas. Taken together, we hypothesize that Siglec-G restrains the GC reaction under healthy conditions and that loss of Sigelc-10/G expression promotes the development of GC-derived B-cell lymphomas due to a poorly controlled GC reaction. Preliminary results support a role for Siglec-G in restraining GC B-cells, as B-cell responses in mixed bone marrow chimeras reveal that Siglec-G-/- GC B-cells gradually outcompete WT GC B-cells. Additional preliminary evidence demonstrates a unique phenotype (increased ICOS expression) in GC T-follicular helper (GC-Tfh) cells within Siglec-G-/- mice, supporting other lines of evidence for a role for Siglec-G in modulating B-T cell interactions. This project aims to investigate a causal relationship between loss of Siglec-G and the development of GC-derived B-cell lymphomas and assess a potential role for Siglec-G in regulating B-T cell interactions. Two independent approaches will be used to test causality between loss of Siglec-G-/-, a dysregulated GC, and the development of GC-derived B- cell lymphomas. In the first approach, Tfh cells will be ablated in Siglec-G-/- mice. In the second approach, Siglec-G expression will be maintained transgenically in an independent mouse model that develops GC- derived B-cell lymphomas and loose Siglec-G expression. To examine a role for Siglec-G as a negative regulator of B-T cell interactions, the ligand specificity of Siglec-G will be dissected in reference to glycans on nave GC-Tfh CD4+ cells, and glycan ligands on CD4+ T-cells will be manipulated to examine what impact this has on B-T cell interactions. The overall objective of this project is to test the role of Siglec-G as a tumor suppressor in GC B-cells and examine one potential mechanism by which Siglec-G restrains the GC. This knowledge will lead to new insights into the etiology of GC-derived B-cell lymphomas, and potentially offer clues for new treatment strategies, which are especially needed for the incurable FL.
The germinal center is critical for immunity, enabling antibodies to mature and adapt to their targets in order to help protect against pathogens. Nevertheless, the germinal center is also a hotspot for development of B-cell lymphomas, and tight control of the germinal center reaction is critical. In the proposed project, we will test the hypothesis that loss of Siglec-10/G in B-cells uncouples selection in the germinal center, leading to B-cell lymphomas.
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