Matthias Wabl, PhD
It is surprising that, in light of our understanding of the immune system, there are very few acquired autoimmune diseases for which the proximal cause(s) is known. It is reasonable to assume that, in the complicated balance between self and non-self reactivity of the adaptive immune system, any wrong step in the sequence of lymphocyte development, selection and activation may result in auto-aggression. But what causes these missteps, and which ones are important for which autoimmune disease? Our working hypothesis postulates that the highly variable antigens of endogenous retroelements provide mimetopes to the adaptive immune system, and that the RNA and cDNA of replicating retroelements trigger pathways of the innate immune system. An indication that this may be the case is provided by a deficiency in Trex1, a cellular DNase that digests retroelement cDNA. Trex1-deficient humans and mice suffer from various inflammatory diseases. Our lab investigates ways to inhibit retroelement expression and its effect on autoimmune disease in mice prone to lupus erythematosus, hemolytic anemia, or autoimmune diabetes.
Although it is not an accepted fact, many observations made by several laboratories over the years have strengthened a retroelement hypothesis of autoimmune disease. Exogenous and endogenous retroviruses and retrotransposons are classified as retroelements. The retroelements establish additional copies in host cell genomes at new insertion sites via an RNA transcript that is copied into double-stranded DNA by reverse transcription. They are present in the genomes of all eukaryotes and are thus also ancient companions of the human genome, making up almost half of our genomic sequence.
There are several reasons why Koch’s postulates for the definitive proof of “infectious disease” etiology are difficult to fulfill for endogenous retroelements. These postulates have been important in defining our understanding of acute infectious disease caused by exogenous pathogens, but a broader framework is required for assessing the importance of a candidate pathogen in a complex multifactorial disease such as autoimmune disease. Thus, differences in MHC complex, for example, clearly play an important role, but one of the biggest obstacles to obtaining proof in our case is the nature of the pathogen—the endogenous retroelement. First, the large number and variety of retroelements, together with their high rates of mutation and recombination, make it difficult to identify the causative one(s); second, its being endogenous makes adding a pathogen less practical, as this only increases the load by an unknown percentage; and third, clearing the retroelement in experimental settings has been a challenge. Because the roles of Toll-like receptors and the interferon stimulatory DNA pathway in autoimmunity have recently become clear, our lab examines the linkage of retroelements to these pathways.
APOBEC enzymes as protectors from autoimmunity. An innate immunity different from the interferon response or the one triggered by the Toll-like receptors is mediated by the cytidine deaminases of the APOBEC3 family (abbreviated A3). These enzymes disrupt reverse transcription of retroelements by converting cytosine to uracil in the nascent single-stranded cDNA. Besides exogenous and endogenous retroviruses, human A3 enzymes can also inhibit the replication of retrotransposons, which do not leave their cell of origin. While humans have seven A3 family members, the mouse has only one. The mouse ortholog is also much less effective: mouse leukemia virus (MLV) infection, for example, is not inhibited. In contrast to mouse A3, human A3 enzymes are very effective inhibitors of MLV and retrotransposons. We have generated mice that express all seven human A3 enzymes, and we are now studying the effect of these enzymes on autoimmune disease in mice that are Trex1 deficient, prone to lupus, or prone to diabetes.
AID, a mutator enzyme and member of the APOBEC family. Activation-induced cytidine deaminase (AID) is a B-cell-specific DNA mutator that plays a key role in the formation of the secondary antibody repertoire in germinal center B cells. AID is the master protein for somatic hypermutation and the class switch of immunoglobulin genes. It is still a mystery how AID mediates two such distinct processes, but it is thought that specific cofactors interact with AID for control. However, the known proteins that interact with AID do not have the specificity needed for these functions.
Contrary to expectation from its nuclear activity on DNA, AID is found predominantly in the cytoplasm, where it also associates with RNA. We found that most AID in the cytoplasm of mice is part of a large complex that contains RNA, similar to the other Apobec enzymes. We are currently analyzing the difference between the AID complexes in switching and non-switching cells and defining the RNA that binds to AID.