Selection of mutant CHO clones resistant to murine gammaherpesvirus 68 infection

Glycoprotein K8.1A of Kaposi's Sarcoma-Associated Herpesvirus Is a Critical B Cell Tropism Determinant Independent of Its Heparan Sulfate Binding Activity

B lymphocytes are the major cellular reservoir in individuals infected with Kaposi's sarcoma-associated herpesvirus (KSHV), and the virus is etiologically linked to two B cell lymphoproliferative disorders. We previously described the MC116 human B cell line as a KSHV-susceptible model to overcome the paradoxical refractoriness of B cell lines to experimental KSHV infection. Here, using monoclonal antibody inhibition and a deletion mutant virus, we demonstrate that the KSHV virion glycoprotein K8.1A is critical for infection of MC116, as well as tonsillar B cells; in contrast, we confirm previous reports on the dispensability of the glycoprotein for infection of primary endothelial cells and other commonly studied non-B cell targets. Surprisingly, we found that the role of K8.1A in B cell infection is independent of its only known biochemical activity of binding to surface heparan sulfate, suggesting the possible involvement of an additional molecular interaction(s). Our finding that K8.1A is a critical determinant for KSHV B cell tropism parallels the importance of proteins encoded by positionally homologous genes for the cell tropism of other gammaherpesviruses.IMPORTANCE Elucidating the molecular mechanisms by which KSHV infects B lymphocytes is critical for understanding how the virus establishes lifelong persistence in infected people, in whom it can cause life-threatening B cell lymphoproliferative disease. Here, we show that K8.1A, a KSHV-encoded glycoprotein on the surfaces of the virus particles, is critical for infection of B cells. This finding stands in marked contrast to previous studies with non-B lymphoid cell types, for which K8.1A is known to be dispensable. We also show that the required function of K8.1A in B cell infection does not involve its binding to cell surface heparan sulfate, the only known biochemical activity of the glycoprotein. The discovery of this critical role of K8.1A in KSHV B cell tropism opens promising new avenues to unravel the complex mechanisms underlying infection and disease caused by this viral human pathogen.

Virally-induced upregulation of heparan sulfate on B cells via the action of type I IFN

Cell surface heparan sulfate (HS) is an important coreceptor for many cytokines, chemokines, and growth factors. In this study, we report that splenic murine B cells express very little HS and that upon infection with either gammaherpesvirus (murine gammaherpesvirus 68) or betaherpesvirus (murine cytomegalovirus), HS is rapidly upregulated at the surface of B cells. HS upregulation was not observed in mice deficient for the type I IFN (IFN-I) receptor. Additionally, treatment of wild-type mice with the IFN-I inducer polyinosine polycytidylic acid triggered HS expression at the B cell surface. Similarly, incubation of purified splenic B cells with IFN-I, TLR ligands, or BCR stimulators ex vivo resulted in a drastic increase in HS surface expression. We found that IFN-I induced an increase in the surface expression of HS-modified syndecan 4 as well as that of an unidentified heparan sulfate proteoglycan. Finally, IFN-I treatment increased B cell responsiveness to APRIL, a cytokine involved in B cell survival and T cell-independent B cell responses. Enzymatic removal of HS from IFN-I-treated B cells inhibited APRIL. Altogether, our results indicate that upon herpesvirus infection in mice, HS is rapidly upregulated at the surface of B cells due to the action of IFN-I, potentially increasing B cell responsiveness to cytokines. Induction of HS expression at the B cell surface by stimulators of the innate immune response likely plays a key role in the development of a robust immune response.

Expression of the RAE-1 family of stimulatory NK-cell ligands requires activation of the PI3K pathway during viral infection and transformation

Natural killer (NK) cells are lymphocytes that play a major role in the elimination of virally-infected cells and tumor cells. NK cells recognize and target abnormal cells through activation of stimulatory receptors such as NKG2D. NKG2D ligands are self-proteins, which are absent or expressed at low levels on healthy cells but are induced upon cellular stress, transformation, or viral infection. The exact molecular mechanisms driving expression of these ligands remain poorly understood. Here we show that murine cytomegalovirus (MCMV) infection activates the phosphatidylinositol-3-kinase (PI3K) pathway and that this activation is required for the induction of the RAE-1 family of mouse NKG2D ligands. Among the multiple PI3K catalytic subunits, inhibition of the p110α catalytic subunit blocks this induction. Similarly, inhibition of p110α PI3K reduces cell surface expression of RAE-1 on transformed cells. Many viruses manipulate the PI3K pathway, and tumors frequently mutate the p110α oncogene. Thus, our findings suggest that dysregulation of the PI3K pathway is an important signal to induce expression of RAE-1, and this may represent a commonality among various types of cellular stresses that result in the induction of NKG2D ligands.

Human and mouse cytomegaloviruses (HCMV and MCMV) are members of the Herpesvirus family. Both viruses cause disease in individuals with a compromised immune system, such as transplant patients and AIDS patients. Natural killer (NK) cells are essential players in the immune response against these viruses. NK cells recognize self-proteins, such as NKG2D ligands, that are poorly expressed on healthy cells but are upregulated on cells that are undergoing stress, such as infection and tumor development. The biological processes associated with NKG2D ligand expression in infected cells are unknown. The PI3K pathway, which controls many cellular processes, is activated by a variety of viruses to prime cells for efficient viral replication. We observed that MCMV activates the PI3K pathway and that this activation is required for NKG2D ligand expression. We also found that the expression of NKG2D ligands on cancer cell lines is dependent on this pathway. Our data suggest that NKG2D ligand expression, and thus recognition of infected and cancer cells by NK cells, is associated with a dysregulation in the PI3K pathway.

An unusual dependence of human herpesvirus-8 glycoproteins-induced cell-to-cell fusion on heparan sulfate

Human herpesvirus-8 (HHV-8) is known to interact with cell surface heparan sulfate (HS) for entry into a target cell. Here we investigated the role of HS during HHV-8 glycoproteins-induced cell fusion. Interestingly, the observed fusion demonstrated an unusual dependence on HS as evident from following lines of evidence: (1) a significant reduction in cell-to-cell fusion occurred when target cells were treated with heparinase; (2) in a competition assay, when the effector cells expressing HHV-8 glycoproteins were challenged with soluble HS, cell-to-cell fusion was reduced; and, (3) co-expression of HHV-8 glycoproteins gH-gL on target cells resulted in inhibition of cell surface HS expression. Taken together, our results indicate that cell surface HS can play an additional role during HHV-8 pathogenesis.

Lack of heparan sulfate expression in B-cell lines: implications for Kaposi's sarcoma-associated herpesvirus and murine gammaherpesvirus 68 infections

Kaposi's sarcoma-associated herpesvirus (KSHV) and its murine homolog, murine gammaherpesvirus 68 (MHV68), are lymphotropic viruses that establish latent infection in their host. Surprisingly, while B cells are the main viral reservoir in vivo, B-cell lines are poorly permissive to infection by either MHV68 or KSHV. Here, we report that most B-cell lines express very little to no cell surface heparan sulfate (HS), a glycosaminoglycan that is essential for infection by these viruses. We found that Ext1, a key enzyme in the biosynthesis of HS, was expressed at a low level in these cells. Transfection of B-cell lines with Ext1 restored high HS expression at the cell surface. Overexpression of Ext1 in murine A20 and M12 B-cell lines increased MHV68 surface binding and enhanced the efficiency of infection. Finally, although it was not sufficient to allow efficient infection, the expression of HS on BJAB cells promoted KSHV binding at the cell surface. Thus, our results indicate that MHV68 and KSHV cycles are blocked in B-cell lines at the binding step due to a lack of surface HS.