Retrovirus-mediated immunosuppression. I. FeLV-UV and specific FeLV proteins alter T lymphocyte behavior by inducing hyporesponsiveness to lymphokines.

Murine splenocytes were used to study the in vitro immunosuppressive effects of UV-inactivated feline leukemia virus (FeLV-UV). FeLV-UV blocks both alloantigen (DBA/2)-induced and Con A-induced proliferation of C57BL/6 splenocytes in a dose-dependent manner. Furthermore, C57BL/6 anti-DBA/2 mixed lymphocyte cultures containing FeLV-UV fail to develop detectable DBA/2-specific cytolytic activity, although FeLV-UV has no effect on the cytolytic activity of preformed C57BL/6 anti-DBA/2 cytolytic T cells (CTL). Disruption of lymphocyte proliferation and CTL generation by FeLV-UV could not be overcome by the addition of exogenous lymphokines. These data suggest that FeLV-UV can interfere with the lymphokine reactivity of alloactivated lymphocytes. In fact, FeLV-UV blocks the lymphokine-induced proliferation of the murine IL 2-dependent cell line CTLL-20. The CTLL-20 cells were subsequently used to study the mechanism(s) by which retroviruses alter T lymphocyte function. Normally, CTLL-20 cells undergo significant proliferation when cultured in EL4 SN, an IL 2-containing culture supernatant from PMA-stimulated EL4 cells. This lymphokine-induced CTLL-20 proliferation is abrogated in a dose-dependent manner by UV-inactivated murine leukemia virus (MuLV-UV), FeLV-UV, and a purified 15,000 dalton viral protein, p15, derived from FeLV. Suppression of CTLL-20 proliferation requires only brief contact (6 hr) with FeLV-UV or with p15, but is most efficient after prolonged (24 hr) contact with these agents. Furthermore, suppression of CTLL-20 proliferation by FeLV-UV and p15 is reversible, because CTLL-20 cells which have been pretreated for 24 hr with FeLV-UV or p15 are equally as efficient at responding to EL4 SN as untreated CTLL-20. Additional studies indicate that CTLL-20 cells continue to remove IL 2 activity from EL4 SN in the presence of suppressive concentrations of FeLV-UV, and that suppressive concentrations of FeLV-UV do not remove IL 2 activity from EL4 SN. This suggests that FeLV does not block CTLL-20 proliferation by absorbing or inactivating IL 2, or by occluding IL 2 receptors, and that T lymphocytes develop an insensitivity to lymphokines after contact with FeLV-UV, which may be caused by a metabolic, rather than an immunologic, defect. Because lymphokines are requisite signals for T cell function, considerable immunosuppression would be associated with acquired lymphokine insensitivity.