Epstein-barr virus-infected resting memory B cells, not proliferating lymphoblasts, accumulate in the peripheral blood of immunosuppressed patients.

When Epstein-Barr virus (EBV) infects B cells in vitro, the result is a proliferating lymphoblast that expresses at least nine latent proteins. It is generally believed that these cells are rigorously controlled in vivo by cytotoxic T cells. Consistent with this, the latently infected cells in the peripheral blood of healthy carriers are not lymphoblasts. Rather, they are resting memory B cells that are probably not subject to direct immunosurveillance by cytotoxic T lymphocytes (CTLs). When patients become immunosuppressed, the viral load increases in the peripheral blood. The expansion of proliferating lymphoblasts due to the suppressed CTL response is believed to account for this increase and is considered to be a major risk factor for posttransplant lymphoproliferative disease (PTLD) and AIDS-associated B cell lymphoma. Here we show that there is an increase in the numbers of latently infected cells in the peripheral blood of immunosuppressed patients. However, the cells are not proliferating lymphoblasts. They are all latently infected, resting, memory B cells-the same population of infected cells found in the blood of healthy carriers. These results are discussed in the context of a model for EBV persistence that explains why PTLD is usually limited to the lymph nodes.

EBV persistence in memory B cells in vivo.

Epstein-Barr virus establishes latency in vitro by activating human B cells to become proliferating blasts, but in vivo it is benign. In the peripheral blood, the virus resides latently in resting B cells that we now show are restricted to the sIgD memory subset. However, in tonsils the virus shows no such restriction. We propose that EBV indiscriminately infects B cells in mucosal lymphoid tissue and that these cells differentiate to become resting memory B cells that then enter the circulation. Activation to the blastoid stage of latency is an essential intermediate step in this process. Thus, EBV may persist by exploiting the mechanisms that produce and maintain long-term B cell memory.

The Kaposi sarcoma-associated herpesvirus (KSHV) is present as an intact latent genome in KS tissue but replicates in the peripheral blood mononuclear cells of KS patients.

Short DNA sequences have been identified, originally in association with Kaposi's sarcoma (KS) biopsies, that are highly homologous to oncogenic, lymphotropic herpesviruses. Recently a virus, Kaposi sarcoma associated herpesvirus (KSHV) or human herpesvirus-8 (HHV-8), bearing these sequences has been identified in a cell line derived from a body cavity-based lymphoma. In this report, we show that the same sequences are present in KS biopsies as DNA molecules of a form and size characteristic of latent herpesviruses-large, covalently closed, circular episomes. The genomes migrate with an apparent size larger than the herpesvirus Epstein-Barr virus (172 kb). This form of the viral genome was found in four of four biopsies and three of five peripheral blood samples from KS patients. Linear forms of the viral genome, characteristic of viral replication, were not detected in the biopsies, but were present in the peripheral blood of three out of five patients. The sequences for KSHV/HHV-8 were also detected in the blood of four of five allograft patients and three of five healthy donors without KS suggesting that the virus is widespread throughout the human population.

Detection of the latent form of Epstein-Barr virus DNA in the peripheral blood of healthy individuals.

Epstein-Barr virus infects resting B cells in vitro and activates them to continuously proliferating lymphoblasts. Activation is essential for the virus to convert its linear genome to the covalently closed circular episomal form in which it persists in proliferating cells. However, in vivo, Epstein-Barr virus persists in resting B cells. We found that in these cells also the virus is present as an episome, suggesting that the cells must, at some time, have been activated and then returned to a resting state. This is the first direct demonstration, for any herpesvirus, of this form of the viral genome in normal persistently infected tissue. Since no linear viral DNA was detected, we estimate that fewer than 1 in 40 cells replicates the virus in the peripheral blood of healthy donors.