Susceptibility of KSHV-Infected PEL Cell Lines to the Human Complement System

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

Kaposi’s sarcoma-associated herpesvirus (KSHV) modulates the immune response to allow the virus to establish persistent infection in the host and facilitate the development of KSHV-associated cancer. The complement system has a central role in the defense against pathogens. Hence, KSHV has adopted an evasion strategy for complement attack using the viral protein encoded by KSHV open reading frame 4. However, despite this defense mechanism, the complement system appears to become activated in KSHV-infected cells as well as in the region surrounding Kaposi’s sarcoma tumors. Given that the complement system can affect cell fate as well as the inflammatory microenvironment, complement activation is likely associated with KSHV pathogenesis. A better understanding of the interplay between KSHV and the complement system may, therefore, translate into the development of novel therapeutic interventions for KSHV-associated tumors. In this review, the mechanisms and functions of complement activation in KSHV-infected cells are discussed.

Activation of the complement system in an osteosarcoma cell line promotes angiogenesis through enhanced production of growth factors

There is increasing evidence that the complement system is activated in various cancer tissues. Besides being involved in innate immunity against pathogens, the complement system also participates in inflammation and the modulation of tumor microenvironment. Recent studies suggest that complement activation promotes tumor progression in various ways. Among some cancer cell lines, we found that human bone osteosarcoma epithelial cells (U2-OS) can activate the alternative pathway of the complement system by pooled normal human serum. Interestingly, U2-OS cells showed less expression of complement regulatory proteins, compared to other cancer cell lines. Furthermore, the activated complement system enhanced the production of growth factors, which promoted angiogenesis of human endothelial cells. Our results demonstrated a direct linkage between the complement system and angiogenesis using the in vitro model, which suggest the complement system and related mechanisms might be potential targets for cancer treatment.

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

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.