Subset-specific co-stimulatory signals are required for IL-2 production but not growth inhibition responses by T cell hybrids specific for myelin basic protein.

Two distinct types of T cell hybridomas (designated THYB-1 and T-HYB-2) were derived by fusing BW5147 thymoma cells with encephalitogenic T helper cells from Lewis rats. Both subsets required MHC-restricted presentation of determinants within the 72-86 peptide sequence of myelin basic protein (MBP) as a requisite signal for IL-2 production. Unlike THYB-1 hybrids, however, THYB-2 hybrids required additional accessory cell activities that were mediated by radiosensitive nonadherent (RS-NAdh) splenocytes (SPL). In this study, we describe two observations indicating that RS-NAdh SPL enable MBP-specific responses of THYB-2 hybrids by providing subset-specific co-stimulatory signals that act independently of antigen recognition pathways. First, RS-NAdh SPL were required by THYB-2 hybrids for MBP-stimulated IL-2 production but were not needed when MBP-specific inhibition of hybrid growth was used as an alternative measure of cellular activation. Second, PMA and ionomycin induced optimal IL-2 production by both THYB-1 hybrids and BW5147 thymoma cells but only stimulated low or marginal levels of IL-2 production by THYB-2 hybrids. Together, these observations indicate that RS-NAdh SPL were required for the specific response of IL-2 production regardless of whether the response was stimulated by antigen or by mitogens that bypass initial antigen recognition events. This study thereby provides additional evidence that distinct stimulus-response relationships define two T-helper cell lineages in experimental autoimmune encephalomyelitis.