I was very excited when I saw first time a research article about novel function of complement C3. It was published in September 5 issue of journal Science and as the title suggested it showed as C3 having intracellular signaling capacity.
But after reading it, I am a little bit confused. Data presented in this paper do not necessarily support the authors conclusions, at least unequivocally. Surprisingly, the results in this paper are generated fully from the in vitro studies, that is by itself very rare, almost unheard of, for a Science paper with the focus on immunology.
Moreover, if one considers an in vitro experiment, one expects to see an experimental design that incorporates specific gene knockout (KO) cells, minimum. However, not a single KO cells are employed. For specific gene inhibition or specific protein depletion, the authors have used either (a) siRNA or (b) chemical inhibitors or (3) serum depleted of specific factors or antibody. However, none of these treatments provide the "clean" results.
Still, few results are worth mentioning, though with limitations discussed above.
First, using NF-kB-driven luciferase reporter HEK293T cell line, the authors showed that incubation of cells with adenovirus type 5 vector (AdV) in presence of serum (but not AdV alone) could activate NF-kB. Activation of NF-kB was equally diminished when serum was either (a) heat inactivated or (b) antibody depleted. However, activation of NF-kB was completely abolished when heat inactivated antibody depleted serum was used. The authors speculated that both antibodies and complement have unique functions in NF-kB activation when exposed to AdV.
Next, the authors showed that transfection of cells with beads incubated with C3fBfD (alternative complement activation), but not with beads or C3fBfD alone, could activate NF-kB.
Interestingly, using AdV-GFP construct, the authors showed that AdV+serum combination generated factors that inhibited AdV replication.
Mechanistically, using siRNA technology, the authors speculated that serum complement effect was mediated via MAVS (RNA recognizing intracellular molecule). However, the authors did not show whether C3 could interact directly with MAVS (for example, co-immunoprecipitation).
In summary, this paper tries to suggest that C3 attached to viruses or bacteria are internalized by the cells and signal through MAVS to activate NF-kB and produce cell autonomous defense state.