PERFORIN, FAS FAS LIGAND, AND TNF-ALPHA PATHWAYS AS SPECIFIC AND BYSTANDER KILLING MECHANISMS OF HEPATITIS-C VIRUS-SPECIFIC HUMAN CTL/

In chronic hepatitis C, Fas expression is up-regulated in the hepatocy tes, especially near liver-infiltrating lymphocytes, and Fas ligand is expressed on the lymphocytes, The presence of hepatitis C virus (HCV) -specific CTLs has been demonstrated both in peripheral blood and amon g liver-infiltrating lymphocytes of patients with chronic hepatitis C. We studied the killing mechanisms of HCV-specific human CTLs using ta rget cells that were sensitive or resistant to agonistic anti-fas Abs and TNF-alpha. We show that HCV-specific CTL clones kill non-Ag-bearin g bystander cells as well as Ag-bearing cells, although the bystander killing is less efficient than the specific target cell killing, and t he efficacy of the bystander killing of anti-Fas- and soluble TNF-alph a-sensitive cells is greater than that of resistant cells. We also sho w that the killing of Ag-presenting, sensitive cells is mediated by Fa s ligand and TNF-alpha as well as perforin, although the latter plays a major role in the killing at a low E:T ratio, and that the killing o f sensitive bystander cells is primarily mediated by Fas ligand and TN F-alpha on CTLs expressed upon specific Ag stimulation, which may be r elevant to the bystander lysis by HCV-specific CTLs of uninfected hepa tocytes, in which Fas expression is up-regulated. Activated CTLs also kill bystander cells by the perforin-based mechanism, although it requ ires a high E:T ratio. The effective bystander killing requires a clos e intercellular contact between CTLs and target cells, although TNF-al pha released from the CTLs mediates lysis of the bystander cells witho ut a close cell-cell contact.