A novel primary class switch recombination immunodeficiency

Antibody maturation in activated B-lymphocytes is a critical process within the adaptive immune response in humans. A number of human immunodeficiencies involve defects of B- cell function where class switch recombination (CSR) is deficient and antibody production is impaired, including the group of so-termed hyper-IgM syndromes and the related common variable immunodeficiencies (CVIDs). Molecular characterisation for many of these patients is still lacking, therefore some of the most critical players specific to antibody maturation and their human deficiency phenotype are still to be determined. In this project, I will carry out the in-depth genetic, immunological and proteomic characterization of a hitherto unrecognised mutation in a critical gene within the NF-B signalling cascade that we recently identified in a B cell deficiency patient. Understanding the molecular pathophysiology underlying the defective B cell immunity and antibody maturation in this disorder through use of a wide range of functional-immunological assays will be one focus of my work. Additionally, I aim to assemble a network of key antibody maturation factors on the proteomic level in a systems biology approach, using our newly identified gene as an entry point for network analysis. Furthermore, I will screen uncharacterised B cell deficiency patients for recurrence of the novel immunodeficiency, or for related deficiencies of NF-B signaling by next generation sequencing techniques. Thus, I will ultimately put this knowledge into the systematical context of the function of NF-B signaling within adaptive immunity. Collectively, through my approach of analysing a novel cause of primary antibody deficiency, I expect to contribute novel, fundamental insight about the biology of the adaptive immune system and its molecular basis.

Primary immunodeficiencies are inherited disorders resulting in malfunction of the immune system. The identification of the molecular defects underlying these disorders is crucial, not only to diagnose patients and enable targeted therapies, but also to highlight essential components of the human immune system and how they work together in immunity in general. This work has now uncovered a novel, rare form of primary immunodeficiency, and identified a loss-of-function mutation in the gene NF-?B-inducing kinase (NIK) as the underlying genetic cause for this disorder. We here decipher the essential role of NIK, a central protein for an efficient immune response in humans. Usually, the signalling molecule NIK exists in certain white blood cells of the immune system, the lymphocytes. However, the variant we found in the patients is inactive and therefore unable to transmit signals necessary to mount an appropriate immune response. Human NIK deficiency not only leads to defective B-lymphocytes, so that they cannot produce antibodies appropriately. We discovered that also T-lymphocytes, which specialize in memorizing and recognizing specific pathogens, as well as the so-called natural killer cells, which specialize in killing virus infected body cells, were affected in their functions. Together, these defects have a profound and devastating effect on the defence mechanisms of the patients. By identifying the signalling molecule NIK as a crucial player in a functional immune defence, we uncovered a central molecular mechanism of the human immune system, which is also relevant to chronic infections, autoimmune disorders, allergies and inflammation. In a next step, we aimed to uncover in detail how the NIK protein interacts with its neighbouring proteins within the cell. We isolated both the healthy and the faulty version of NIK from human cells and analysed by mass spectrometric methods which proteins it would bind. By doing the same with several coworker proteins within NIKs pathway, we could establish a detailed map of interactions for the signalling pathway. This resource can now be used by researchers to investigate this pathway is more detail. These findings will not only help to understand signalling in the human immune system better but will, in the long run, also enable scientists to develop molecular therapeutic approaches.