Molecular Immunology Unit
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| Mechanism of Genome Rearrangements in Lymphocyte Development: B- and T-lymphocytes are unique components of the adaptive immune system in higher vertebrates. They stand out by their abilities to recognize an enormous variety of pathogens and to initiate a highly specific response to fight the infection. In order to do so, each individual lymphocyte displays a unique antigen receptor molecule (named immunoglobulin, Ig, in B-cells, and T-cell receptor, TCR, in T-cells) on its surface. There are four processes that generate such a remarkable diversity within the genes encoding these antigen recognition proteins: V(D)J recombination, somatic hypermutation (SHM), class switch recombination (CSR) and immunoglobulin gene conversion (GCV). They share a common theme in that they involve an active modification of the antigen receptor gene loci in each developing lymphocyte. Thus they also impose a high risk to each cell (and the entire organism) as an error in any of these processes can result in chromosomal aberrations that could ultimately lead to cancer. |
| V(D)J Recombination: The initial Ig and TCR repertoire is generated by V(D)J recombination. In this tightly regulated cut-and-paste process, functional antigen receptor genes get assembled from individual V, D and J gene segments. The only lymphocyte specific factors are encoded by the recombination activating genes RAG1/2. Together with the ubiquitous DNA-bending protein HMG1/2, they form the recombinase complex that cleaves the DNA in the first phase of the process. In the second phase, joining, this complex acts together with many NHEJ family members (Ku70, Ku80, DNA-PKcs, artemis, xrcc4, DNA ligase IV) to religate the broken chromosome. One focus of our lab is to gain a detailed understanding of the mechanism and regulation of V(D)J recombination starting from the initial recombinase complex formation, its interactions with its cognate DNA substrate to the final resealing of the broken DNA. To address these questions, we are performing cell-based assays and biochemical experiments using purified (recombinant) proteins. |
| Evolution of Adaptive Immunity: We recently identified a gene pair in the genome of the purple sea urchin (Strongylocentrotus purpuratus) with striking similarity to the vertebrate RAG1/2 genes. The function of the encoded sea urchin Rag1/2-like proteins (spRag1L/spRag2L) is unknown, as this organism, to our current knowledge, shows no evidence of rearranging antigen receptor genes. We are currently pursuing in vitro and in vivo studies to characterize their molecular function, their role in sea urchin development and immunity, and the relationship to the classical vertebrate RAG1/2 proteins. |
| Immunoglobulin Gene Conversion and Somatic Hypermutation: While SHM occurs during immune responses in the germinal center of all jawed vertebrates, GCV has only been reported in chickens, rabbits and sheep (but not in humans or mice), and is thought to even contribute to the diversity of the naïve antibody repertoire. These processes are closely related, as activation-induced cytidine deaminase is the key enmzyme that initiates both processes by converting C to U during transcription of Ig genes. The resulting U:G mismatches are subsequently repaired by direct replication, error-prone DNA repair pathways, or homology-based repair in the case of GCV. We use the chicken DT40 B cell line as a model system to identify and characterize cis-regulatory elements and novel proteins complexes that are involved in GCV and SHM. We recently identified the first “targeting” element within an endogenous Ig locus that is essential to recruit AID-mediated mutagenesis to the chicken IGL locus. This element is evolutionarily conserved in sequence and function, and current projects are aimed at the characterization of the corresponding trans-acting factors and their mode of action. |
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Molecular Immunology Section |
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