Characterization of a low molecular weight suppressor of lymphocyte proliferation from guinea pig L2C leukemia cells

Novel lymphocyte growth-inhibiting tripeptide: N-acetyl-glu-ser-GlyNH(2)

The limited and predetermined number of cells that constitutes an organ or specialized cell population is to all appearances regulated according to a negative feedback principle involving growth inhibitors with sufficient tissue specificity. To find growth-inhibiting factors in lymphoid cells, we followed established purification procedures and assays. We found a single-peak fraction in water extracts of dog spleen homogenates that inhibited proliferation of Molt (T cell) lymphoma cells at low concentrations in vitro, with no significant effect on a B cell lymphoma cell line (Ramos). C-terminal amino acid sequencing and MS analysis showed the factor to be a tripeptide: N-acetyl-Glu-Ser-GlyNH(2). Treatment with a synthetic tripeptide with the structure N-acetyl-Glu-Ser-GlyNH(2) decreased the number of cell doublings of Molt cells. The peptide also delayed cell flux at the G(2)-M transition of the cell cycle, while incorporation of tritiated thymidine was not altered at the examined time points in this cell line. However, DNA synthesis in PPD-stimulated normal human lymphocytes was significantly inhibited and with a bell-shaped dose-response curve.

Production of a suppressor of lymphocyte proliferation by two human oral carcinoma cell lines

This study examined the production of an immunosuppressive factor by the KB and H191 human oral squamous carcinoma cell lines. Conditioned media (CM) from both cell lines markedly inhibited mitogen- and alloantigen-induced proliferation of normal human and rat peripheral blood lymphocytes. By contrast, the proliferation of an exponentially-growing fibroblast cell line remained unchanged by CM. The immunosuppressive factor appeared to act after lymphocyte commitment as indicated by continued blast cell formation, the failure of CM to suppress resting lymphocytes and the fact that CM caused maximum inhibition of lymphocyte proliferation 72 h after the addition of PHA. The addition of exogenous IL-2 did not counteract lymphocyte suppression. Inclusion of indomethacin and isoniazid during cell culture did not significantly alter the degree of suppressive activity. Mycoplasma contamination was absent and CM did not act directly with the thymidine or mitogen. The factor was heat stable at 50 degrees C, acid labile and had a molecular weight in excess of 300 kDa. The results demonstrate that human oral squamous carcinoma cell lines produce an immunosuppressive factor that may have a role in tumour evasion of the host immune response.