Kaposi's Sarcoma-Associated Herpesvirus Forms a Multimolecular Complex of Integrins (αVβ5, αVβ3, and α3β1) and CD98-xCT during Infection of Human Dermal Microvascular Endothelial Cells, and CD98-xCT Is Essential for the Postentry Stage of Infection

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Abstract
Kaposi's sarcoma-associated herpesvirus (KSHV) interacts with cell surface heparan sulfate (HS) and alpha 3 beta 1 integrin during the early stages of infection of human dermal microvascular endothelial cells (HMVEC-d) and human foreskin fibroblasts (HFF), and these interactions are followed by virus entry overlapping with the induction of preexisting host cell signal pathways. KSHV also utilizes the amino acid transporter protein xCT for infection of adherent cells, and the xCT molecule is part of the cell surface heterodimeric membrane glycoprotein CD98 (4F2 antigen) complex known to interact with alpha 3 beta 1 and alpha V beta 3 integrins. KSHV gB mediates adhesion of HMVEC-d, CV-1, and HT-1080 cells and HFF via its RGD sequence. Anti-alpha V and -beta 1 integrin antibodies inhibited the cell adhesion mediated by KSHV-gB. Variable levels of neutralization of HMVEC-d and HFF infection were observed with antibodies against alpha V beta 3 and alpha V beta 5 integrins. Similarly, variable levels of inhibition of virus entry into adherent HMVEC-d, 293 and Vero cells, and HFF was observed by preincu-bating virus with soluble alpha 3 beta 1, alpha V beta 3, and alpha V beta 5 integrins, and cumulative inhibition was observed with a combination of integrins. We were unable to infect HT1080 cells. Virus binding and DNA internalization studies suggest that alpha V beta 3 and alpha V beta 5 integrins also play roles in KSHV entry. We observed time-dependent temporal KSHV interactions with HMVEC-d integrins and CD98/xCT with three different patterns of association and dissociation. Integrin alpha V beta 5 interaction with CD98/xCT predominantly occurred by 1 min post-infection (p.i.)and dissociated at 10 min p.i., whereas alpha 3 beta 1-CD98/xCT interaction was maximal at 10 min p.i. and dissociated at 30 min p.i., and alpha V beta 3-CD98/xCT interaction was maximal at 10 min p.i. and remained at the observed 30 min p.i. Fluorescence microscopy also showed a similar time-dependent interaction of alpha V beta 5CD98. Confocal-microscopy studies confirmed the association of CD98/xCT with alpha 3 beta 1 and KSHV. Preincubation of KSHV with soluble heparin and alpha 3 beta 1 significantly inhibited this association, suggesting that the first contact with HS and integrin is an essential element in subsequent CD98-xCT interactions. Anti-CD98 and xCT antibodies did not block virus binding and entry and nuclear delivery of viral DNA; however, viral-gene expression was significantly inhibited, suggesting that CD98-xCT play roles in the post-entry stage of infection, possibly in mediating signal cascades essential for viral-gene expression. Together, these studies suggest that KSHV interacts with functionally related integrins (alpha V beta 3, alpha 3 beta 1, and alpha V beta 5) and CD98/xCT molecules in a temporal fashion to form a multimolecular complex during the early stages of endothelial cell infection, probably mediating multiple roles in entry, signal transduction, and viral-gene expression.

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