Liver-derived, and serum-operative complement protecting our vascular space against pathogens through classically folded C3 that, upon activation, functions as an opsonin, contributes to C3/C5 convertases formation and initiates the lytic membrane attack complex. of the complosome in T cell metabolism. We further place complosome activities among the non-canonical roles of other intracellular innate danger sensing systems and argue that a location-centric view of complement evolution could logically justify its close connection with the regulation of basic cell physiology. T cell stimulation, induces cell death. Importantly, the provision of exogenous C3a to such CTSL inhibitor Sulfachloropyridazine treated cells does not rescue their survival (5, 48, 49). This novel role for the complosome in T cell homeostasis was exciting but was initially at odds with the fact that patients with serum-C3 deficiency have normal levels of circulating T cells. Interestingly, closer analysis of T cells isolated from these rare C3-deficient patients revealed that, in all cases analyzed so far, that while patient cells were unable to secrete C3 or C3 activation products, in contrast they contained normal levels of intracellular C3a protein (5, 50). Moreover, despite employing a range of technologies including CRISP-Cas9 we (and our collaborators) have to date been unable to generate a complete C3-deficient human (or mouse) CD4+ T cell (unpublished data). Thus, these data indicate that the complosome may indeed be critical to T cell survival and that complete (intra- and extracellular) C3 deficiency may hence not exist as it would be incompatible with life. Although it has not yet been Sulfachloropyridazine dissected how exactly the respective gene mutations in affected Sulfachloropyridazine individuals still allow the generation of the life-saving intracellular C3a, specific properties of intracellular C3, not shared by extracellular C3, may be at the heart of this (please see below and Figure 2A). Open in a separate window Figure 1. The Complosome as key driver of T cell metabolism.Circulating, non-activated CD4+ and CD8+ T cells generate continuously low levels of intracellular C3a via the cathepsin L-mediated cleavage of intracellular C3 stores (cleavage of C3H2O that can be taken up by cultured T cells is not depicted here). This C3a generation ensures homeostatic survival of T cells through tonic mTOR activation via C3aR engagement on lysosomes. In addition, CD46 surface expression sustains IL-7R expression also required for homeostatic survival of CD4+ T cells and prevents activating Notch1 stimulation. During TCR activation (and CD28 co-stimulation, not shown here) intracellular C3b translocates rapidly to the cell surface and actively engages CD46. CD46 signaling triggers three key metabolic events: the -secretase-processed intracellular domain of CD46 translocates to the LEPR nucleus (not shown) and induces gene and protein surface expression of critical nutrient transporters (GLUT1, LAT1) as well as LAMTOR5-driven mTORC1 assembly at the lysosomes. CD46 activation further induces increased expression of metabolic enzymes, including fatty acid synthase (specifically in CD8+ T cells), GAPDH, etc. CD46 also strongly augments activation of intracellular C5 pools with the intracellularly generated C5a engaging intracellular C5aR1 resulting in ROS production and NLRP3 inflammasome activation in CD4+ T cells. Together, these events drive the high levels of glycolysis, OXPHOS and ROS production needed specifically for the induction of IFN- production and granzyme B expression. Thus, autocrine complosome activity is an integral part of normal human CD4+ Th1 effector function and CD8+ effector CTL activity. The complosome also contributes to the safe metabolic shut-down of human Th1 activity as CD46 (via expression induction of a repressive CD46 isoform, not shown)-driven signals reduce glycolysis and OXPHOS while at the same time supporting cholesterol efflux and MAF expression. Autocrine generation of the des-Arginated form of C5a (C5a-desArg) engages Sulfachloropyridazine the repressive C5aR2 on the T cell surface, which reduces C5aR1 activity. Finally, C1q, taken up by the activated T cell hampers normal mitochondrial activity (in CD8+ T cells) via gC1qR via a yet unknown mechanism C together, these events lead to secession of IFN- production in T cells. Of note, whilst human CTLs harbor the intracellular C3/C5 systems a possible functional role during CTL homeostasis and/or contraction has not yet been explored. Further, although not formerly proven yet, current data strongly suggest that the intracellular C3 system is engaged during all life cycle stages of T cells, while the C5 system is mostly engaged during the Th1 and CTL effector phase. CTSL, cathepsin L; FAS, fatty acid synthase/synthesis; GLUT1; glucose transporter 1; LAT1, large neutral amino acid transporter 1; MAF, cMaf musculoaponeurotic fibrosarcoma oncogene homolog; mTOR, mechanistic target of rapamycin; mTORC1, mechanistic target of rapamycin complex 1; OXPHOS, oxidative phosphorylation; ROS, reactive oxidation species; TCR, T cell receptor. Open in a separate window Figure 2. Location-driven activities of key innate immune sensors.(A) Suggestion of bi-furcated evolution of our contemporary complement system. Evolutionary older C3 proteins analyzed in precursor organisms such as Porifera and Agnatha contained additional domains with homology to enzymes.