Murine polyclonal T-lymphocyte activation induced by phytohemmagglutinin; differential lymphokine requirements of two unusual activation pathways defined by resistance to blockade by barium and by cyclosporin A

We have recently demonstrated that polyclonal T-cell activation induced by PHA defines an activation pathway which is resistant to blockade by barium (Ba2+) ions. Other modes of T-cell activation, including ConA-induced responses, are completely blocked by Ba2+, which seems to affect an early Ca2+-dependent step of T-cell activation, as determined by kinetic and competition experiments. In the present study, we have analysed the lymphokine requirements of Ba2+-resistant pathway of PHA-induced T-cell activation by means of functional blocking experiments with monoclonal antibodies (mAbs) directed against mouse IL-2 (mAb S4B6) and against mouse IL-4 (mAb 11B11). We found that Ba2+-resistant T-cell activation can be blocked by either S4B6 or 11B11. Thus, both IL-2 and IL-4 participate in Ba2+-resistant T-cell growth induced by PHA. In addition, we found that cyclosporin A (CsA) completely blocks T-cell activation induced by either ConA or by PHA plus Ba2+, but not T-cell activation induced by PHA in the absence of Ba2+, which is reduced by less than 50% in most experiments. This CsA-resistant proliferative component of the PHA response is, thus, distinct from the Ba2+-resistant response, and is carried out by proliferating T-cells. Although mAbs S4B6 and 11B11 are potent blockers of ConA-induced responses, they failed to block CsA-resistant T-cell growth induced by PHA. At the doses of CsA employed, no IL-2 and/or IL-4 activity could be detected in the supernatants of CsA-treated, PHA-stimulated T-cell cultures. The data indicate that this CsA-resistant pathway is both IL-2 and IL-4-independent. The lymphokine involved in this T-cell activation pathway remains to be identified.