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We have recently shown that the physiological mediator of granule-mediated apoptosis is a macromolecular complex of granzymes and perforin complexed with the chondroitin-sulfate proteoglycan, serglycin (Metkar, S. S., Wang, B., Aguilar-Santelises, M., Raja, S. M., Uhlin-Hansen, L., Podack, E., Trapani, J. A., and Froelich, C. J. (2002)Immunity16, 417–428). We now report our biophysical studies establishing the nature of granzyme B-serglycin (GrB·SG) complex. Dynamic laser light scattering studies establish that SG has a hydrodynamic radius of ∼140 ± 23 nm, comparable to some viral particles. Agarose mobility shift gels and surface plasmon resonance (SPR), show that SG binds tightly to GrB and has the capacity to hold 30–60 GrB molecules. SPR studies also indicate equivalent binding affinities (Kd∼ 0.8 μm), under acidic (granule pH) and neutral isotonic conditions (extra-cytoplasmic pH), for GrB·SG interaction. Finally, characterization of GrB·SG interactions within granules revealed complexes of two distinct molecular sizes, one held ∼4–8 molecules of GrB, whereas the other contained as many as 32 molecules of GrB or other granule proteins. These studies provide a firm biophysical basis for our earlier reported observations that the proapoptotic granzyme is exocytosed predominantly as a macromolecular complex with SG.

The expression of CD40L was investigated in HD involved lymph nodes by flow cytometry (FCM) and reverse transcriptase polymerase chain reaction (RT-PCR). Also an investigation of the role of CD40L in upregulation of the anti-apoptotic gene BclxL in a Hodgkin's disease (HD) derived cell line was undertaken. HD patients (n = 18) had significantly higher numbers of activated CD4+ and CD8+ T cells in the tumor microenvironment as compared to controls (n = 8). HD patients also demonstrated higher numbers of CD4+, CD8+ and CD19+ lymphocytes co-expressing CD40L as compared to controls. The CD40L signal was consistently and significantly upregulated in HD patients (n = 5) as compared to controls (n = 3) at the mRNA level. RT-PCR and FCM analysis revealed that soluble CD40L upregulated BclxL levels in the Fas-sensitive HD cell line HDLM2. We conclude that CD40L can act as an important anti-apoptotic molecule by upregulating BclxL expression in Reed-Sternberg cells of HD and may be partly responsible for their survival 'in-vivo'.

We investigated whether cell-permeable, synthetic ceramide (C6 ceramide) could induce apoptosis in Fas-resistant Hodgkin's disease (HD)-derived cell lines. Despite strongly expressing the Fas-receptor, two of three HD-derived cell lines were resistant to Fas-mediated apoptosis. This resistance to Fas could not be attributed to differential Fas isoform generation patterns between the Fas-resistant and the Fas-sensitive cell lines. The Fas-resistant cell lines did not demonstrate the presence of Fas exon 8 deletion. Bcl-2 and BclxL levels were comparable between the Fas-resistant and the Fas-sensitive cell lines. C6 ceramide could induce apoptosis in both Fas-resistant cell lines and this was associated with a decrease in BclxL level. Caspase-1, caspase-3, or pan-caspase inhibitors could not prevent ceramide-induced apoptosis. Furthur, ceramide treatment did not lead to cleavage of caspase 3 or poly(ADP-ribose) polymerase, but caused a loss in mitochondrial transmembrane potential which could not be prevented by caspase inhibitors. Thus, we conclude that ceramide-induced apoptosis in Fas-resistant HD cell lines is caspase independent.

Abstract: Modulation of Fas expression and function by CD40 ligation was investigated in the Fas-sensitive human Hodgkin's disease cell line HDLM2. The recombinant human trimeric soluble CD40L (sCD40L) protected this cell line from apoptosis induced by an agonistic Fas antibody at all concentrations tested. sCD40L also protected HDLM2 when added up to 2 h after Fas ligation. Apoptosis induced by a cell-permeable synthetic ceramide could not be prevented by sCD40L. Thus, CD40 ligation is likely to intervene in the early phases of the Fas signal transduction pathway. When CD40 ligation preceded Fas ligation, it rendered the cells refractory to Fas-induced apoptosis. sCD40L-mediated protection could not be attributed to reduction in surface Fas expression, increase in Bcl-2 levels or to increase in the levels of soluble Fas isoforms.