Extracellular vesicles from KSHV-infected endothelial cells activate the complement system

Abstract

Extracellular vesicles (EVs), released by cells, are associated with cell-to-cell communication and regulate various cellular processes. EVs draw parallels with viruses for their similar structures and functions. Increasing evidences from recent studies indicate that cells infected with viruses release a variety of EVs. Delineating the functions and mechanisms of EVs released during virus infection is essential for understanding the molecular basis of viral infection and replication as well as associated pathogenesis. The most challenging obstacle for these studies is the separation of EVs from viruses. In this study, we successfully isolated the EVs from de novo Kaposi’s sarcoma-associated herpesvirus (KSHV) infected-human endothelial cells during the period between virus entry and production. Intriguingly, a proteomics analysis of these EVs has revealed alterations of the complement system. Additionally, we have discovered that the EVs from KSHV-infected endothelial cells are potent activators of an alternative pathway of the complement system via exploitation of the endogenous C3 complement protein and properdin. Furthermore, we have found that complement activation promotes KSHV persistent latent infection by activating the NF-κB pathway, which enhances the survival of KSHV-infected cells and inhibits viral lytic replication. Our work identifies a novel role of EVs induced by KSHV during de novo infection and the underlying mechanism of complement activation by EVs.

Kaposi’s Sarcoma-Associated Herpesvirus and Host Interaction by the Complement System

“…A previous study found C3d and C5b-9 deposition in human KS tissue, especially around spindle cells [11]. In vitro studies also reported C5b-9 deposition in various KSHV-infected cells treated with normal human serum [49,50]. These results may provide evidence for KSHV infection serving as a trigger for complement activation.…”

“…Since apoptotic cells acquire the capacity to bind complement initiation molecules and lose complement regulatory proteins on the cell surface [56], C5b-9 can be deposited in KSHV-infected HUVECs by lytic replication. Interestingly, C5b-9 deposition was detected on KSHV-infected HUVECs even 24 h postinfection without lytic replication or apoptosis [49]. At this time point, apoptosis or cell death by lytic replication was not detected.…”

“…Since KSHV-infected HUVECs are affected by EVs via autocrine effects and the production of EVs cannot be completely abolished, it is difficult to determine whether other intracellular factors in KSHV-infected cells also contribute to complement system activation. Considering that the deposition of C5b-9 induced by EVs from KSHV-infected cells increases in a dose-dependent manner and that the suppression of EV production by GW4869 decreases complement activation, the EVs from KSHV-infected HUVECs likely contribute to complement activation [49]. Although the underlying mechanisms are unclear, properdin has been reported to be an initiation factor for the activation of the alternative pathway [57,58].…”

“…As shown in Section 4, de novo infection of HUVEC by KSHV induces the activation of the complement system. The formation of MAC does not kill the KSHV-infected HUVECs but rather increases their survival, which is due to the sublytic MAC-mediated activation of the NF-κB pathway, which subsequently suppresses the lytic replication of KSHV [49]. Collectively, the complement system confers survival benefits to KSHV-infected cells in myriad ways (Figure 3).…”

Mitochondrial DNA: A Key Regulator of Anti-Microbial Innate Immunity

“…Hepatocytes of non-alcoholic steatohepatitis patients have reported leakage of mitochondrial DNA in the form of microparticles [35], indicating the release of mitochondrial DNA through extracellular vesicles is an effective strategy for cellular homeostasis. Some virus infected cells have also been shown to form extracellular vesicles in large numbers and reported to be implicated anti-viral immune responses [95,96]. To date, various biological mechanisms have been reported regarding the formation of these extracellular vesicles after virus infection.…”