Role of nonspecific cytotoxic cells in the induction of programmed cell death of pathogenic protozoans: participation of the Fas ligand-Fas receptor system.

Numerous different species of parasites and pathogenic microorganisms produce programmed cell death (PCD) and apoptosis in eukaryotic targets. How ever, only a few studies have demonstrated that effector cells, cytokines, growth factors, or soluble apoptosis-inducing factors are capable of initiating apoptosis in protozoan parasites. Certain Tetrahymena spp. in teleosts are opportunistic pathogens. In the present study these pathogenic protozoans were developed as a model system to describe the potential role of the Fas ligand (FasL)-Fas receptor (FasR) system as a means of innate immunity in teleosts. Nonspecific cytotoxic cells (NCC) constitutively express soluble FasL (sFasL). Binding of the antigen receptor (i.e., NCCRP-1) on NCC to target cells caused the release of sFasL into the milieu. The presence of functional sFasL in these supernatants was determined by Western blot analysis and by demonstrating the lysis of FasR(+) HL-60 but not IM-9 (FasR(-)) targets. Soluble FasL containing supernatants generated by tumor cell-activated NCC also produced a reduction in 2 N DNA (i.e., DNA hypoploidy) of T. furgasoni. The induction of DNA hypoploidy by NCC supernatants could be neutralized by adsorption of the supernatants with anti-FasL antibody (but not with an isotype control). Experiments were next done to determine the expression of FasR on Tetrahymena and study the effects of anti-FasR monoclonal crosslinkage and treatment with soluble human recombinant FasL (huFasL) on initiation of PCD in Tetrahymena. Cell cycle analysis revealed that both crosslinkage and soluble huFasL binding to Tetrahymena produced DNA hypoploidy. The reduction in diploid DNA was confirmed by observing oligonucleosome fragmentation (DNA laddering) following anti-FasR treatment. Additional evidence for FasR expression on Tetrahymena was obtained using fluorescence microscopy and flow cytometry. Both methods showed that all Tetrahymena examined (three species consisting of four isolates) expressed membrane FasR. These studies demonstrated the potential of the FasL-FasR system in teleosts for initiation of antiparasite innate immunity. Effector NCC may initiate PCD of Tetrahymena that express a FasR-like protein. Induction of apoptosis may be a major mechanism of homeostatic control of protozoan parasite infestations/infections.