Understanding cellular and cytoskeletal dysregulation in immunodeficiency diseases

Abstract

The immune system is a complex network of cells and molecules interacting to preserve the integrity and survival of the host by distinguishing self from non-self, and dysregulation of immune cells can lead to immunodeficiency, autoimmunity, and cancer. The cytoskeleton is essential for several important immune cell functions, including proliferation, movement, signaling, and vesicle trafficking. The overall aim of this thesis was to investigate the importance of the cytoskeleton for lymphocytes in primary immunodeficiencies.

In paper I, we investigated the role of the actin-sensing protein MKL1 in B cells, by examining samples from three monozygotic triplets with an intronic deletion in MKL1, out of whom two had developed Hodgkin lymphoma. MKL1 deficiency has previously been described as causing primary immunodeficiency with dysregulation of actin. The intronic deletion in MKL1 led to increased MKL1 expression, increased MKL1-dependent gene transcription, hyperproliferation, dysregulated cytoskeleton, genomic instability, and formation of tumors in vivo. Inhibition of MKL1 led to reversion of the phenotype. These results suggest a role of MKL1 in the development of Hodgkin lymphoma. In paper II, our aim was to investigate the function of LRBA in lymphocytes from a patient with LRBA deficiency, presenting with immunodeficiency and autoimmunity. The function of LRBA has not been fully clarified previously. LRBA-deficient patient cells accumulated vesicles including endosomes, lysosomes, autophagosomes, and lipid bodies. Lipid metabolism was altered in patient cells in response to serum starvation. These results suggest that LRBA is involved in the response to cell stress, and that the accumulation of vesicles in LRBA deficiency may impair the survival and function of immune cells. In paper III, we examined the anti-tumor responses of NK cells and T cells in X-linked neutropenia, a primary immunodeficiency caused by activating mutations in the actin-regulatory protein WASp, with increased risk of myelodysplasia. We studied samples from patients and mouse models and found that NK cells and T cells in X-linked neutropenia displayed normal to increased tumor killing, decreased expression of the exhaustion marker KLRG1, increased granzyme B expression, and polarization of actin without stimuli. These results suggest that tumor immunosurveillance is not deficient in X-linked neutropenia and that the increased risk of malignancy rather is caused by cell-intrinsic aberrations. In paper IV, we investigated the development of the T cell receptor repertoire in mouse models for Wiskott-Aldrich syndrome, a primary immunodeficiency caused by loss-of-function mutations in the actin-regulating protein WASp. We examined thymocytes and spleen T cells from young and older mice and found that the thymocyte T cell receptor repertoire was intact in young mice but skewed in older mice. The spleen T cell receptor repertoire displayed signs of clonal expansions in older mice, suggesting development of an autoimmune response. These results suggest that the dependence on WASp for thymic output and the generation of a diverse T cell receptor repertoire increases with age.

In summary, the works presented in this thesis provide evidence for the role of dysregulated cytoskeletal responses in the development of primary immunodeficiency and cancer. Furthermore, potential drug targets are identified for diseases with increased actin responses or lipid dysregulation, and proteins identified as involved in cancer development may also be used as markers to screen for in the assessment of disease severity and prognosis.