Inhibition of EB virus transformation of non-adherent human lymphocytes by co-cultivation with adult fibroblasts

Inhibition of growth of certain human tumour cell lines by a factor derived from human fibroblast-like cell lines. I. Demonstration by mixed culture and by use of cell washings

The growth of 7/9 Burkitt's lymphoma (BL) cell lines and 5/6 human melanoma cell lines was inhibited by certain human fibroblast-like cell lines, especially human myofibroblast cell lines. In contrast to BL lines, 3/5 lymphoblastoid cell lines were less susceptible and 3/4 human leukaemic lines were resistant. The effect was time- and concentration-dependent. Co-cultivation led to inhibition of tumour-cell growth within 3 days, but after removal from the fibroblasts, washing, and resuspending in fresh medium some tumour cells recovered and growth improved. The inhibitory activity was also demonstrated with washings of fibroblast-like cell lines, and was sensitive to pronase. The biological significance of this factor remains to be defined.

Colonies of EBNA-positive cells in soft agar from peripheral leukocytes of infectious mononucleosis patients

Epstein-Barr virus (EBV)-associated nuclear antigen (EBNA)-positive lymphoblastoid cells grew as colonies in soft agar after seeding of leukocytes from the peripheral blood of four patients with infectious mononucleosis serologically determined to be caused by EBV. In individual cases more colonies were obtained from blood specimens during the acute phase of the disease than during the convalescent phase. Incorporation of human umbilical cord serum, which contained neutralizing antibody to EBV, into the agar medium did not reduce the number of colonies developing. Our observations indicate that colony-forming cells were originally present in the blood samples, and that they were not infected and subsequently transformed in vitro. Cells from less than 20% of the EBNA-positive colonies grew to form lymphoblastoid cell lines, which were EBNA-positive and had B lymphocyte surface markers. However, the majority (over 80%) of the EBNA-positive colonies failed to form immortalized cell lines. No colonies were obtained from 91 blood samples from healthy young adults and from five patients with an IM-like disease unrelated to EBV infections. The present results strongly suggest that already transformed cells or cells very easily transformed by EBV are present in the blood of IM patients.