B cells and the Antibody-Dependent Immune Response in Cancer and Infection
B cells and humoral immunity are critical components of an effective immune response. However, B cells are also a significant driver of a variety of autoimmune diseases and can also become malignant. Antibody-mediated B cell depletion is now regularly used in the clinic to treat both B cell-derived cancers and B-cell driven autoimmunity, and while depletion itself is effective in some patients, removal of B cells is not often curative for patients and may present additional, unforeseen risks. The overall goal of this dissertation was therefore to determine the impact of B cell depletion on T cell homeostasis and function during infection and to elucidate the genetic factors that determine the effectiveness of antibody-mediated therapy.
In Chapter 3 of this dissertation, the role of B cells in promoting T cell homeostasis was investigated by depleting mature B cells using CD20 monoclonal antibody (mAb). Acute B cell depletion in adult mice significantly reduced spleen and lymph node T cell numbers, including naïve, activated, and cytokine-producing cells, as well as Foxp3+ regulatory T cells, whereas chronic B cell depletion in aged mice resulted in a profound decrease in activated and cytokineproducing T cell numbers. To determine the significance of this finding, B cell-depleted adult mice were infected with acute lymphocytic choriomeningitis virus (LCMV). Despite their expansion, activated and cytokine-producing T cell numbers were still significantly reduced one week later. Moreover, viral peptide-specific T cell numbers and effector cell development were significantly reduced in mice lacking B cells, while LCMV titers were dramatically increased. Thus, B cells are required for optimal T cell homeostasis, activation, and effector development in vivo, particularly during acute viral infection.
In Chapter 4 of this dissertation, lymphoma genetic changes that conferred either sensitivity or resistance to CD20 mAb therapy were examined in a preclinical mouse lymphoma model. An examination of primary lymphomas and extensive lymphoma families demonstrated that sensitivity to CD20 mAb was not regulated by differences in CD20 expression, prior exposure to CD20 mAb, nor serial in vivo passage. An unbiased forward genetic screen of CD20 mAb-resistant and -sensitive lymphomas identified galectin-1 as a significant factor driving CD20 mAb therapy resistance. As lymphomas acquired therapy resistance following serial in vivo passage, galectin-1 expression also increased. Furthermore, inducing lymphoma galectin-1 expression within the tumor microenvironment ablated lymphoma sensitivity to CD20 mAb. Therefore, lymphoma acquisition of galectin-1 expression confers CD20 mAb therapy resistance.
In Chapter 5 of this dissertation, the distinct germline components that control the efficacy of host CD20 mAb-dependent B cell and lymphoma depletion were evaluated using genetically distinct lab mouse strains. Variations in B cell depletion by CD20 mAb among several lab mouse strains were observed, where 129 mice had significantly impaired mAb-dependent depletion of endogenous B cells and primary lymphomas relative to B6 mice. An unbiased forward genetic screen of mice revealed that a 1.5 Mbp region of Chromosome 12 that contains mycn significantly altered CD20 mAb-dependent lymphoma depletion. Elevated mycn expression enhanced mAb-dependent B cell depletion and lymphoma phagocytosis and correlated with higher macrophage numbers. Thus, host genetic variations in mycn expression in macrophages alter the outcome of Ab-dependent depletion of endogenous and malignant cells.
These studies collectively demonstrate that B cells are required for effective cellular immune responses during infection and identified factors that alter the effectiveness of mAb-dependent B cell depletion. This research also established and validated an unbiased forward genetics approach to identify the totality of host and tumor-intrinsic factors that influence mAb therapy in vivo. The findings of these studies ultimately urge careful consideration in the clinical application of B cell depletion therapies.
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