Regulation of cytotoxic lymphocyte effector functions

Abstract

Studies of patients with inborn errors of immunity provide an important opportunity to understand the human immune system in a natural context. The articles in this thesis are a compilation of such investigations. With our work, we aimed to identify the cause of primary hemophagocytic lymphohistiocytosis (HLH) in an unexplained pediatric patient (paper I), explore the modulation of SAMD9 and SAMD9L gain-of-function (GOF) mutants (paper II), and understand natural killer (NK) cell biology in the context of DEF6 deficiency (paper III).

In paper I, we uncovered biallelic loss-of-function variants in RHOG in a 4-monthold patient presenting with HLH and displaying defective lymphocyte exocytosis. Deletion of RHOG in a human NK cell line abrogated exocytosis that could be rescued by constructs expressing wild type RHOG protein. Moreover, we found that MUNC13-4, associated with autosomal recessive familial HLH type 3, required RHOG interactions for recruitment to the plasma membrane during cytotoxic granule exocytosis. Thus, we demonstrated that RHOG is essential in cytotoxic granule exocytosis by human T and NK cells and proposed that biallelic loss-offunction mutations in RHOG are a novel cause for familial HLH.

In paper II, we examined a variety of pathogenic SAMD9 and SAMD9L GOF variants associated with syndromes encompassing bone marrow failure, autoinflammation or selective loss of NK cells, B cells, and monocytes. We sought to understand whether viral host range factors, which are known to antagonize wild-type SAMD9 and SAMD9L proteins, could counteract the anti-proliferative and anti-translational activities of these pathogenic variants. SAMD9 or SAMD9L variants and viral factors were overexpressed in a cell line, followed by biochemical and functional analyses. Vaccinia virus K1 exhibited the highest inhibitory capacity but could not antagonize a truncated variant of SAMD9L that lacked the K1 binding site. The other factors (M062, C7 and KI) could interact with all the SAMD9 and SAMD9L mutants but displayed low capacity in antagonizing the anti-translational and anti-proliferative action of SAMD9/9L mutants. This study provided some novel insights into the structure and the regulation of SAMD9/9L proteins. Additionally, it showed that targeting pathogenic GOF SAMD9/L mutants via viral host range factors or, more broadly speaking, by protein-protein interaction is possible.

Autosomal recessive mutations in DEF6 are associated with autoimmunity and severe herpes virus infections. While the latter is a hallmark of NK cell deficiencies, this cell type has not been thoroughly studied in DEF6-deficient patients. In paper III, we surveyed the role of DEF6 in NK cell development and function. We analyzed NK cell phenotypes in DEF6-deficient patients from two unrelated families and found a reduction in canonical CD56dimPLZF+ NK cells concomitant with the expansion of CMV-induced adaptive CD56dimPLZF– NK cells. Deletion of DEF6 in induced pluripotent stem cells gave rise to mature NK cells in an in vitro culture system. In primary human NK cells, deletion of DEF6 reduced cell viability, degranulation and IFN-γ production. We therefore postulate that impaired canonical NK cell survival and function may contribute to the viral susceptibility of DEF6 deficient individuals.

To conclude, these studies have shed light on several cellular and molecular mechanisms behind IEI. These mechanisms enlarge our understanding of cytotoxic lymphocytes and of the immune system in general. Moreover, they have inspired us to pursue new directions and experimental approaches in our research.