Direct tumor lysis by NK cells uses a Ras-independent mitogen-activated protein kinase signal pathway

Destruction of tumor cells is a key function of lymphocytes, but the molecu lar processes driving it are unclear. Analysis of signal molecules indicate d that mitogen-activated protein kinase (MAPK)/extracellular regulated kina se 2 critically controlled lytic function in human NK cells. We now have ev idence to indicate that target ligation triggers a Ras-independent MAPK pat hway that is required for lysis of the ligated tumor cell. Target engagemen t caused NK cells to rapidly activate MAPK within 5 min, and PD098059 effec tively blocked both MAPK activation and tumoricidal function in NK cells. T arget engagement also rapidly activated Ras, detected as active Ras-GTP bou nd to GST-Raf-RBD, a GST fusion protein linked to the Raf protein fragment containing the Ras-GTP binding domain. However, Ras inactivation by pharmac ological disruption with the farnesyl transferase inhibitor, FTI-277, had n o adverse effect on the ability of NK cells to lyse tumor cells or to expre ss MAPK activation upon target conjugation, Notably, MAPK inactivation with PD098059, but not Ras inactivation with FTI-277, could interfere with perf orin and granzyme B polarization within NK cells toward the contacted targe t cell. Using vaccinia delivery of N17 Ras into NK cells, we demonstrated t hat IL-2 activated a Ras-dependent MAPK pathway, while target ligation used a Ras-independent MAPK pathway to trigger lysis in NK cells.