Wild-Type Human Adenovirus (Serotype 5)

Human adenovirus 5 (HAdV-5) is used as a vector in gene therapy clinical trials, and therefore its interaction with the host immune system has been extensively studied. Previous studies have shown that HAdV-5 specifically binds to mouse coagulation factor X (mFX) and inhibits IgM- and complement-mediated neutralization. Here, researchers examined the physical binding of immune components to HAdV-5 by nanoparticle tracking analysis, neutralization assays, mass spectrometry, and in vivo experiments. They observed that purified mouse immunoglobulin M (IgM) antibodies bound to HAdV-5 only in the presence of complement components. Active serum components bound to HAdV-5 in the presence or absence of mFX, suggesting that immune molecules and mFX may bind to different sites. Because mFX binding to HAdV-5 blocks the neutralization cascade, these findings suggest that not all complement binding sites may be involved in virion neutralization. In addition, data obtained from serum neutralization experiments suggest that immune molecules other than IgM and IgG may also trigger activation of the complement cascade in vitro.

Here, the researchers investigated the binding of proteins present in mouse serum to HAdV-5 virions. Incubation of HAdV-5 with serum from immunocompetent (C57BL/6) mice showed that serum components bound to virions at similar levels in the presence or absence of the FX inhibitor X-bp (Figure 1A, compare "C57" condition to "C57 + X-bp"). X-bp is a protein isolated from the venom of Trimeresurus flavoviridis that belongs to the C-type lectin superfamily and binds to the GLA domain of FX, inhibiting its interaction with the HAdV-5 hexon. Neutralization assays performed in parallel confirmed that X-bp could indeed inhibit FX binding in the researchers' experimental setting. Furthermore, the size changes observed were complement-dependent, as the particle size of HAdV-5 incubated with heat-inactivated immunocompetent mouse serum was significantly reduced compared to untreated serum (Figure 1B). These results suggest that serum proteins may bind to virions in the presence or absence of FX, with a significant contribution from complement factors.

Figure 1. Human adenovirus 5 (HAdV-5) binds to murine serum components in a FX-independent manner, but in a complement-dependent way. (Doszpoly A, et al. 2019)