T-helper lymphocytes specific for myelin basic protein: activation-induced refractoriness of IL-2 production pathways augments an anti-CD4-mediated proliferative deficit.

Cloned and uncloned lines of encephalitogenic rat T cells produce IL-2 when activated with myelin basic protein (MBP) in the presence of irradiated splenocytes (SPL). Although these T cells use IL-2 as a primary mediator of autocrine growth, regulatory mechanisms controlling production of IL-2 have yet to be fully defined. This study shows that T cells reactivated within approximately 7 days of a prior activation were refractory to the reinduction of MBP-stimulated IL-2 production. In contrast, T cells rested for > 7 days regained the ability to produce optimal levels of IL-2 during activation with MBP. Cultures containing both activated and resting T cells responded to MBP by producing levels of IL-2 that were similar to those obtained from control cultures of resting T cells. The lack of IL-2 production during this refractory phase was associated with lowered responsiveness to MBP in proliferative assays as evidenced by right-shifted dose-response curves. However, this refractory phase did not affect the magnitude of responses elicited by optimal concentrations of MBP. The dissociation of proliferation from IL-2 production suggested parallel pathways of autocrine growth. Indeed, anti-MBP-proliferative responses were mediated by two distinct mechanisms distinguished by differential susceptibility to the anti-CD4 mAb W3/25. The W3/25-sensitive proliferation was desensitized in chronically activated T cells as well as in T cells activated once in the presence of the anti-CD4 mAb W3/25. Conversely, MBP responsiveness of W3/25-insensitive proliferation was unchanged by both chronic activation and by a prior activation in the presence of W3/25. In cultures of T cells recently activated by MBP in the presence of W3/25, the use of nonirradiated SPL rather than irradiated SPL reversed W3/25-mediated tolerance but did not restore MBP-stimulated IL-2 production. In summary, this study reveals mechanisms whereby the engagement of TcR and CD4 negatively regulates subsequent responsiveness of IL-2 production pathways and thereby impairs restimulation of IL-2-dependent proliferation by MBP-specific T-helper cells.