Complement-mediated immune mechanisms in allergy

Allergic conditions are associated with canonical and noncanonical activation of the complement system leading to the release of several bioactive mediators with inflammatory and immunoregulatory properties that regulate the immune response in response to allergens during the sensitization and/or the effector phase of allergic diseases. Further, immune sensors of complement and regulator proteins of the cascade impact on the development of allergies. These bioactive mediators comprise the small and large cleavage fragments of C3 and C5. Here, we provide an update on the multiple roles of immune sensors, regulators, and bioactive mediators of complement in allergic airway diseases, food allergies, and anaphylaxis. A particular emphasis is on the anaphylatoxins C3a and C5a and their receptors, which are expressed on many of the effector cells in allergy such as mast cells, eosinophils, basophils, macrophages, and neutrophils. Also, we will discuss the multiple pathways, by which the anaphylatoxins initiate and control the development of maladaptive type 2 immunity including their impact on innate lymphoid cell recruitment and activation. Finally, we briefly comment on the potential to therapeutically target the complement system in different allergic conditions.

Alternative translation and retrotranslocation of cytosolic C3 that detects cytoinvasive bacteria

Complement C3 was originally regarded as a serum effector protein, although recent data has emerged suggesting that intracellular C3 can also regulate basic cellular processes. Despite the growing interest in intracellular C3 functions, the mechanism behind its generation has not been demonstrated. In this study we show that C3 can be expressed from an alternative translational start site, resulting in C3 lacking the signal peptide, which is therefore translated in the cytosol. In contrast to the secreted form, alternatively translated cytosolic C3 is not glycosylated, is present mainly in a reduced state, and is turned over by the ubiquitin–proteasome system. C3 can also be retrotranslocated from the endoplasmic reticulum into the cytosol, structurally resembling secreted C3. Finally, we demonstrate that intracellular cytosolic C3 can opsonize invasive Staphylococcus aureus within epithelial cell, slowing vacuolar escape as well as impacting bacterial survival on subsequent exposure to phagocytes. Our work therefore reveals the existence and origin of intracellular, cytosolic C3, and demonstrates functions for cytosolic C3 in intracellular detection of cytoinvasive pathogens.

Interaction of Serum-Derived and Internalized C3 With DNA in Human B Cells-A Potential Involvement in Regulation of Gene Transcription

Beside its classical role as a serum effector system of innate immunity, evidence is accumulating that complement has an intracellular repertoire of components that provides not only immune defense, but also functions to maintain cellular homeostasis. While complement proteins, mainly the central component C3, have been detected in B cells, their exact function and source remain largely unexplored. In this study, we investigated the expression and origin of intracellular C3 in human B cells together with its role in B cell homeostasis. Our data provide evidence that endogenous expression of C3 is very low in human B cells and, in accordance with the recent publication, the main origin of intracellular C3 is the serum. Interestingly, we found that both serum-derived and purified C3 are able to enter the nucleus of viable B cells, suggesting its potential involvement in regulation of gene transcription. ELISA, gel shift assay, confocal microscopy, and chromatin immunoprecipitation proved that C3 and C3a strongly bind to nuclear DNA, and among the interacting genes there are key factors of lymphocyte development and differentiation. The strong interaction of C3 with histone proteins and its potential ability to induce chromatin rearrangement suggest that C3/C3a might regulate DNA transcription via chromatin remodeling. Our data reveal a novel, hitherto undescribed role of C3 in immune cell homeostasis, which further extends the repertoire how complement links innate and adaptive immunity and regulates basic processes of the cells.