DNA-protein nanoparticles in blood and their association with autoimmunity

What is it about?

How would you determine the size of a protein in an unfractionated blood sample? This question becomes significant because we have many reasons to believe that protein size is influenced by disease from processes such as proteolysis and disulfide bond reduction, as known from textbooks in biochemistry. Another source of size alterations is oligomerization, where the same unit of protein is included multiple times in a complex. Here, we investigate mannose (or mannan)-binding lectin (MBL), a vital part of the innate immune system. From previous work, it was known that it might bind DNA. We show that such binding leads to the formation of huge complexes, some with sizes of at least 200-300 nm, or approximately 35 times larger than serum albumin. Since these structures were made from protein complexes, which are already oligomeric with sizes of about 40 nm, we termed the formed structures “superoligomers.” We studied these species in diluted blood plasma samples, that is, with no prior fraction. Fraction sometimes is a source of artifacts, including protein aggregation, which would cloud any attempt to understand such processes in vivo. We used a reporter system for our studies where a high-quality monoclonal antibody was conjugated to quantum dots (QD). The monoclonal antibody binds to MBL with high specificity and affinity. QD are among the most significant novel tools in fluorescence detection. Unlike certain organic compounds, which may also emit fluorescence, QD do not bleach when exposed to UV radiation. This property enabled us to follow the QD in nanoparticle tracking analysis, thereby determining the size of particles with QD bound and eliminating from the analysis those QD, which were unbound. If it sounds straightforward, it should be added that this is the equivalent of counting and tracing single molecules – and here, Avogadro’s number should tell you that even high dilution leaves behind quite a lot of molecules! Even so, we now show that the MBL superoligomers increase with disease score in systemic lupus erythematosus (SLE or lupus). Lupus belongs to autoimmune diseases, where the immune system develops a reaction to the body’s own molecules. In lupus, there is a reaction to parts of the cell nucleus. However, we now argue that the formation of superoligomeric MBL may cause inflammation in the blood vessel walls, a known and significant morbidity of the disease. Furthermore, because these superoligomers may also contain DNA, we think that they are potent stimulators of an immune response towards DNA. In turn, as shown by our colleagues, such an autoimmune response may spread to develop other autoreactivities, further increasing the cataclysmic events of lupus. How would you determine

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Photo by Dan-Cristian Pădureț on Unsplash

Photo by Dan-Cristian Pădureț on Unsplash

Why is it important?

Autoimmune diseases are on the rise, but we known little about their origin or the molecular mechanisms, which explain disease manifestations. It has long been clear that events at the nanometer scale in protein structure can be a determinant in inflammatory diseases. Still, our possibilities of studying these processes have been limited. In this sense, our work opens up new technological possibilities. It also tells a vital aspect about the impact of nanoparticles on the immune system. Because such particulates often are made bottom-up from more minor, identical constituents, they may form strong, polyvalent interactions with cells of the immune system, especially B cells. Such interactions are undoubtedly helpful in fulfilling some of the roles of these cells in, for instance, fighting microbial infection. But we find that the complexes with DNA may cause overstimulation that, in turn, leads to autoimmunity. As it is becoming clear that cell-free DNA is a part of several autoinflammatory responses, it seems likely that our findings can be relevant in several diseases.

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