Immortalization of Epstein-Barr virus-infected CD23-negative B lymphocytes by the addition of B cell growth factor

Epstein-Barr Virus- (EBV-) Immortalized Lymphoblastoid Cell Lines (LCLs) Express High Level of CD23 but Low CD27 to Support Their Growth

Epstein-Barr virus (EBV) is one of the common human herpesvirus types in the world. EBV is known to infect more than 95% of adults in the world. The virus mainly infects B lymphocytes and could immortalize and transform the cells into EBV-bearing lymphoblastoid cell lines (LCLs). Limited studies have been focused on characterizing the surface marker expression of the immortalized LCLs. This study demonstrates the generation of 15 LCLs from sixteen rheumatoid arthritis (RA) patients and a healthy volunteer using B95-8 marmoset-derived EBV. The success rate of LCL generation was 88.23%. All CD19+ LCLs expressed CD23 (16.94-58.9%) and CD27 (15.74-80.89%) on cell surface. Our data demonstrated two distinct categories of LCLs (fast- and slow-growing) (p<0.05) based on their doubling time. The slow-growing LCLs showed lower CD23 level (35.28%) compared to fast-growing LCLs (42.39%). In contrast, the slow-growing LCLs showed higher percentage in both CD27 alone and CD23+CD27+ in combination. Overall, these findings may suggest the correlations of cellular CD23 and CD27 expression with the proliferation rate of the generated LCLs. Increase expression of CD23 may play a role in EBV immortalization of B-cells and the growth and maintenance of the EBV-transformed LCLs while CD27 expression might have inhibitory effects on LCL proliferation. Further investigations are warranted to these speculations.

Identification of a sub-population of B cells that proliferates after infection with Epstein-Barr virus

Background

Epstein-Barr virus (EBV)-driven B cell proliferation is critical to its subsequent persistence in the host and is a key event in the development of EBV-associated B cell diseases. Thus, inquiry into early cellular events that precede EBV-driven proliferation of B cells is essential for understanding the processes that can lead to EBV-associated B cell diseases.

Methods

Infection with high titers of EBV of mixed, primary B cells in different stages of differentiation occurs during primary EBV infection and in the setting of T cell-immunocompromise that predisposes to development of EBV-lymphoproliferative diseases. Using an ex vivo system that recapitulates these conditions of infection, we correlated expression of selected B cell-surface markers and intracellular cytokines with expression of EBV latency genes and cell proliferation.

Results

We identified CD23, CD58, and IL6, as molecules expressed at early times after EBV-infection. EBV differentially infected B cells into two distinct sub-populations of latently infected CD23⁺ cells: one fraction, marked as CD23^hiCD58⁺IL6^- by day 3, subsequently proliferated; another fraction, marked as CD23^loCD58⁺, expressed IL6, a B cell growth factor, but failed to proliferate. High levels of LMP1, a critical viral oncoprotein, were expressed in individual CD23^hiCD58⁺ and CD23^loCD58⁺ cells, demonstrating that reduced levels of LMP1 did not explain the lack of proliferation of CD23^loCD58⁺ cells. Differentiation stage of B cells did not appear to govern this dichotomy in outcome either. Memory or naïve B cells did not exclusively give rise to either CD23^hi or IL6-expressing cells; rather memory B cells gave rise to both sub-populations of cells.

Conclusions

B cells are differentially susceptible to EBV-mediated proliferation despite expression of viral gene products known to be critical for continuous B cell growth. Cellular events, in addition to viral gene expression, likely play a critical role in determining the outcome of EBV infection. By indentifying cells predicted to undergo EBV-mediated proliferation, our study provides new avenues of investigation into EBV pathogenesis.

Epstein-Barr virus nuclear antigen EBNA3C/6 expression maintains the level of latent membrane protein 1 in G1-arrested cells

The Epstein-Barr virus in the Burkitt lymphoma-derived cell line Raji has a deletion in the EBNA3C gene. When Raji cells are allowed to grow to high density and most of the cells become growth arrested in the G1 phase of the cell cycle, the level of detectable latent membrane protein 1 (LMP1) is substantially reduced. After dilution of the cells with fresh growth medium, within 8 h, there is a large increase in LMP1 mRNA, and by 12 h, LMP1 is expressed to a high level (H. Boos, M. Stoehr, M. Sauter, and N. Mueller-Lantzch, J. Gen. Virol. 71:1811-1815, 1990). Here we show that in Raji cells which constitutively express a transfected EBNA3C gene, the down-regulation of LMP1 in growth-arrested cells does not take place. Furthermore, we show that in wild-type Raji cells, low-level LMP1 expression occurs when most of the cells are arrested at a point(s) early in G1 (or G0) when the product of the retinoblastoma gene, pRb, is hypophosphorylated. The dramatic synthesis of LMP1 coincides with the progression of these cells to late G1 when pRb becomes hyperphosphorylated. Thus, in Raji cells, the LMP1 gene is apparently regulated in a cell cycle- or proliferation-dependent manner, but when EBNA3C is present, sustained LMP1 expression occurs as it does in a lymphoblastoid cell line. EBNA3C appears to either relieve the apparent repression of LMP1 in cells progressing through early G1 or possibly alter the stage at which the cells growth arrest to one where they are permissive for LMP1 expression.

Biphasic effect of transforming growth factor-beta on Epstein-Barr virus-induced activation of human tonsillar B cells

Transforming growth factor-beta (TGF-beta) is known to inhibit mitogen-induced proliferation of human B lymphocytes. Earlier results showed that activation of B cells by Epstein-Barr virus (EBV) was also inhibited by TGF-beta. On the other hand, TGF-beta could enhance the transformation of EBV-infected B-cell cultures. In the present set of experiments, we have confirmed the inhibitory effect of TGF-beta on the EBV-induced blastogenesis and found lower expression of CD23 in the treated cultures. However, cells which escaped inhibition and entered in the blast stage expressed a higher level of CD23 molecules. The elevation of CD23 in the TGF-beta-treated cultures was more marked at a time when the cell size profiles of the control and treated cultures were similar. In view of the function of the CD23 molecule as an autocrine growth factor, its increased expression is consistent with previous findings on TGF-beta-mediated enhancement of the transformation of B-cell cultures. The occurrence of growth inhibitory and growth stimulatory effect of TGF-beta on the same cell type has been observed in several other systems as well.

Defective expression of CD23 and autocrine growth-stimulation in Epstein-Barr virus (EBV)-transformed B cells from patients with Wiskott-Aldrich syndrome (WAS)

WAS is an X-linked, recessive, immune deficiency syndrome, characteristically associated with lymphocyte and platelet dysfunction. Peripheral B lymphocytes from WAS patients are nonresponsive to polysaccharide antigens and show reduced numbers of cells expressing the integral membrane glycoprotein, CD23. The release of CD23 proteolytic fragments, so-called soluble CD23 (sCD23), by B lymphoblasts and EBV-transformed B cell lines has previously been described, and these fragments have been shown to stimulate autocrine growth of these cells. We have found that the surface expression of CD23 is reduced on WAS compared with control EBV-B cells. Surface CD23 levels were reduced two-fold in four WAS cell lines (group I) and nine-fold in four other lines (group II). Group II WAS cell lines also showed reduced growth rates in serum-free medium when compared with group I cell lines and EBV-B cell lines from eight normal subjects. In contrast to the group II WAS lines, group I and EBV-B cells from normal individuals produced an autocrine-growth factor activity which could be absorbed by anti-CD23 antibodies. Immunoprecipitation of sCD23 from culture supernatants confirmed that group I WAS cell lines produced less sCD23, particularly the 37K fragment which was prevalent in control EBV-B cells. Northern analysis showed that CD23 mRNA levels were increased three-fold in group I and unchanged in group II WAS compared with normal EBV-B cell lines, suggesting that decreased surface expression in WAS EBV-B cells reflects post-transcriptional events. Together these results suggest that reduced cell surface expression and aberrant proteolysis of CD23 occurs in WAS patients' B lymphocytes and may contribute to impaired immune function in these patients.

Stereotactic brain biopsy in AIDS

In the hope of finding a treatable condition, the need for rapid diagnosis in HIV-seropositive patients with brain lesions is apparent. In order to evaluate the efficacy of stereotactic brain biopsy in AIDS patients, we retrospectively studied 25 HIV-infected patients undergoing stereotactic biopsy. Brain lesions were identified with gadolinium-enhanced MRI and/or contrast CT. Brain biopsy was performed using the system of Riechert. From 8 up to 15 small tissue samples from one or two targets were obtained in every patient. The biopsy material was examined cytologically, histologically (including electron microscopy), immunohistochemically and, in part, by animal test and polymerase chain reaction (PCR). A definite diagnosis was achieved in 92%. Diagnosis included primary central nervous system lymphoma (PCNSL) (10), toxoplasmosis (10), progressive multifocal leukoencephalopathy (2) and one case of co-existing toxoplasmosis and cytomegalovirus infection. Two biopsies were non-diagnostic. All PCNSLs showed polymorphic B-cell populations of high malignancy; accurate classification according to the Kiel classification was not possible. In 3 lymphomas Epstein-Barr nuclear antigen (EBNA) 2-mRNA could be detected by PCR and confirmed immunohistochemically by EBNA 2 expression. In 6 cases autopsy confirmed the biopsy diagnosis. Conventional histology was not sufficiently decisive for toxoplasmosis and progressive multifocal leukoencephalopathy, so that immunohistochemistry and animal tests became very important for a final diagnosis. With the help of different morphological and molecular biological techniques stereotactic brain biopsy appears to be an effective method in the diagnosis of HIV-associated brain lesions. In view of the marked radio- and chemosensitivity of PCNSLs it is mandatory to establish an early and accurate histological diagnosis for adequate treatment.

When Epstein-Barr virus persistently infects B-cell lines, it frequently integrates

In this study we used Gardella gel analysis of intact DNA, Southern blotting of digested DNA, and fluorescence in situ hybridization to provide complementary and unequivocal information on the state of the Epstein-Barr virus (EBV) genome in persistently infected cells. The fluorescence in situ hybridization technique allowed us to directly visualize both integrated and episomal EBV DNA at the single-cell level. We show here that circularization of the EBV genome is rarely detected upon infecting activated normal B cells. The virus can persist upon infection of a different proliferating B-cell target, EBV-negative Burkitt's lymphoma tumor cell lines. Analysis of 16 such lines reveal again, that the virus infrequently persists as covalently closed episomes; rather, the virus preferentially persists by integrating into the host DNA (10 of 16 clones). The integrated virus is linear and usually intact, although 3 of 10 isolates have deletions from the left-hand end including the latent origin of replication. At the level of our analysis, no obvious relationship was seen between the integration sites. These studies provide, for the first time, a reproducible in vitro model system to study integration by EBV.