Long-term T-cell-mediated immunity to Epstein-Barr virus in man. III. Activation of cytotoxic T cells in virus-infected leukocyte cultures

Experiments have been conducted to determine the role played by immune T cells in the regression of EB-virus-induced transformation which is exclusively seen in leukocyte cultures from sero-positive donors. Kinetic studies suggest that, in virus-infected cultures from such donors, a population of T cells proliferates within the first 2 weeks apparently in response to the appearance of virus-infected B cells. This proliferation continues to some extent during the period of regression. Nonspecific induction of T-cell proliferation by PHA did not induce regression in virus-infected cultures from seronegative donors and acutally prevented the regression in seropositive donor cultures. T cells harvested from seropositive donor cultures 11-14 days post infection were generally much more inhibitory to the growth of the autologous EB-virus-transformed cell line than were T cells either freshly prepared from whole blood or harvested from corresponding uninfected cultures; this inhibitory activity was either absent or much diminished when assayed against allogeneic target cell lines. The results suggest that virus-specific memory T cells capable of mounting a cytotoxic response when properly challenged in vitro.

Long-term T-cell-mediated immunity to Epstein-Barr virus in man. I. Complete regression of virus-induced transformation in cultures of seropositive donor leukocytes

Peripheral blood mononuclear cells from donors of known serological status with respect to EB virus were exposed to the virus in vitro and then cultured at various cell concentrations. All cultures from nine seronegative adult and 12 foetal donors gave rise to cell lines following subculture 4 weeks post infection. In contrast, seropositive donor cultures seeded at the higher cell concentrations developed foci of proliferating EBNA-positive cells within the first 1--2 weeks but thereafter regressed completely and subcultures made after 4 weeks never gave rise to cell lines. Out of 18 seropositive donors tested, 15 showed regression in all cultures seeded at 10(6) cells/ml and above, and with the other three donors a proportion of replicate cultures regressed. T-cell depletion and reconstitution experiments showed that the effect was absolutely dependent upon the presence in the cultures of T cells from these seropositive donors. The results strongly suggest that the regression phenomenon is an in vitro expression of long-term T-cell-mediated immunity to EB virus which the large majority, if not all, infected individuals possess.

Sensitivity of the transforming and replicative functions of Epstein--Barr virus to inhibition by phosphonoacetate

Disodium, phosphonoacetate (PA), at concentrations of 50 to 200 microgram/ml, which still allowed continued growth of the EB virus-transformed B95-8 cell line on a routine culture regimen, was able to inhibit the production of virus capsid antigen and of virus particles by these cells down to very low but finite levels which persisted despite prolonged treatment. Further experiments measured the effects of these same drug concentrations on the EB virus-induced in vitro transformation of foetal cord blood lymphocytes and on the colony forming ability of already established EB virus-transformed foetal cell lines; in both types of culture, doses of PA up to and including 50 microgram/ml did not affect cell growth within the 8-week observation period, whereas doses of 100 microgram/ml and above were increasingly inhibitory. The cell lines established by EB virus-induced transformation in the continual presence of PA at 50 to 150 microgram/ml contained multiple copies of the virus genome per cell just as did the corresponding cell lines established in control medium. The results argue against the existence of any PA-sensitive event unique to the EB virus-induced transformation process.

Epstein-Barr (EB) virus genome-containing, EB nuclear antigen-negative B-lymphocyte populations in blood in acute infectious mononucleosis

Experiments have been performed to identify the type and size of cell infected by EB virus in the blood of acute infectious mononucleosis (IM) patients, and to investigate the nature of the infection. Virus-infected cells, recognized by their ability to give rise to lymphoblastoid cell lines when co-cultivated with foetal lymphocytes, were shown to be restricted to the B-lymphocyte population. Samples of this population from each of eight IM patients were found to be negative for EB nuclear antigen (EBNA) staining. Thereafter, fractions of IM B-lymphocytes prepared on the basis of cell size were assayed either by co-cultivation, for the incidence of virus-infected cells, or by immunofluorescence staining for the presence of cells expressing EBNA. The great majority of virus-infected cells were found in the fractions of normal sized B-lymphocytes and yet these fractions were unequivocally EBNA-negative B-cell populations in IM blood is discussed in terms of the type of infection established by EB virus in the circulation of IM patients.

Mechanism of the establishment of Epstein-Barr virus genome-containing lymphoid cell lines from infectious mononucleosis patients: studies with phosphonoacetate

A concentration of disodium phosphonoacetate (PA) has been defined which will reduce the synthesis of infectious EB virus in a producer cell line to 1% of control values but which will not affect the growth of EB virus-transformed cells in a 12-week colony-forming assay. When total mononuclear cells or T-lymphocyte-depleted mononuclear cells from the blood of acute IM patients were cultured in the presence of PA at the above concentration, the regular establishment of EB virus genome-containing cell lines seen in control cultures was almost totally abolished. In further experiments, when T-lymphocyte-depleted IM mononuclear cells were co-cultivated with foetal cells of the opposite sex in the presence and absence of PA, cell lines of mixed or of exclusively foetal origin were obtained not only from control co-cultures but also on those rare occasions when transformed foci developed in PA-treated co-cultures. The results suggest that all cell lines derived from the blood of IM patients are initiated in culture by a two-step process of virus release and secondary infection, and argue against the occurrence of any direct outgrowth of IM cells transformed by the virus in vivo.

Comparative studies on adult donor lymphocytes infected by EB virus in vivo or in vitro: origin of transformed cells arising in co-cultures with foetal lymphocytes

Co-cultures were set up between equal numbers of mononuclear cells from the blood of EB virus-infected individuals, either acute IM patients or healthy seropositive adult donors, and foetal cord blood mononuclear cells of the opposite sex. The cell lines arising in the co-cultures were of mixed origin, with foetal cells predominating in many cases. In contrast, when mononuclear cells from seronegative adult donors were first infected with EB virus in vitro and then 5 to 12 days later co-cultured with a large excess of foetal cells of the opposite sex, the cell lines which arose were almost exclusively derived from the adult donor despite the fact that a small minority of the virus-infected adult cells released infectious virus capable of transforming the co-cultivated foetal cells. The experiments suggest that EB virus-infected cells present in the blood of IM patients and seropositive donors do not possess the capacity for unlimited in vitro growth shown by seronegative adult donor lymphocytes experimentally infected with the virus.

Inhibition of the in vitro outgrowth of Epstein-Barr virus-transformed lymphocytes by thymus-dependent lymphocytes from infectious mononucleosis patients

Known numbers of thymus-dependent (T) lymphocytes, obtained by positive selection from the blood of acute infectious mononucleosis (IM) patients and from control donors, were added to target cultures of foetal mononuclear cells within 0-7 days of exposure of the target cells to one of a range of doses of Epstein-Barr (EB) virus. The subsequent outgrowth of virus-transformed foetal cells was markedly inhibited by the presence in the cultures of IM-derived T cells, whilst similar numbers of T cells prepared either from cord blood or from adult donors seronegative for EB virus had little or no inhibitory effect. Target foetal cells treated with papain to remove any viral envelope material remaining on the cell surface after infection, were just as sensitive as untreated cells to the addition of IM-derived T cells. It is concluded that the inhibition cannot be mediated through recognition either of viral envelope structures on the surface of infected cells or of the antigenically related virus-determined membrane antigen, MA, but must depend upon recognition of the lymphocyte-detected membrane antigen, LYDMA. The regularity with which IM-derived T cells block the outgrowth of virus-transformed foetal cells suggests that LYDMA consistently appears on the surface of infected foetal cells before the establishment of transformed foci, but is unlikely to be directly associated with the cells' existing histocompatibility antigens.

Observations on the type of infection by Epstein-Barr virus in peripheral lymphoid cells of patients with infectious mononucleosis

Transformation to continuous cell lines has been studied in cultures of peripheral leuckocytes from infectious mononucleosis (IM) patients and in co-cultures of IM leukocytes and foetal cord blood leukocytes of opposite sex. The transformed cells in the co-cultures were of mixed origin with foetal cells usually predominating. Neutralizing antisera to EB virus markedly reduced or abolished the incidence of transformation in IM leukocyte cultures. This effect was not due to cytotoxicity and followed the pattern seen with cultures where transformation was known to depend on the inter-cellular transfer of infectious EB virus. The findings suggest that EB virus is harboured in peripheral lymphocytes of IM patients as a non-productive unexpressed infection which is activated to produce virus in vitro, the particles released then infecting neighbouring cells to give transformed lines. The differences between this mechanism and the one whereby lines arise in culture from malignant cells of Burkitt's lymphoma are considered, and their significance is discussed.

Epstein-Barr virus nuclear antigens 1 and 2 in Burkitt lymphoma cell lines containing either 'A'- or 'B'-type virus

Epstein-Barr virus (EBV) has previously been classified into two different types according to the organization of the EB nuclear antigen 2 (EBNA2) gene region. Type A virus hybridizes with probes from B95-8 or M-ABA viruses and the B type virus with probes from the Jijoye virus strain. The substituted region in EBV type B codes for a different, but related EBNA2 antigen, named EBNA2B as opposed to EBNA2A. In this study Burkitt lymphoma cell lines, previously typed according to the EBV viral genomes they carry, as well as some matching lymphoblastoid cell lines were examined by immunoblotting for the expression of both EBNA1 and EBNA2 antigens. Variation in the molecular weight of EBNA1 indicated that both A and B virus types contained a variety of different virus isolates. EBNA2A was identified in all lines carrying A type viral genomes, but was not observed in any of the lines harboring B type virus. EBNA2B was identified in 4 of 10 Burkitt lymphoma lines carrying EBV type B.