Figure 4 Kaplan-Meyer plots for groups of (A) SCID mice and (B) NOD/SCID mice treated with a single i.v. injection of 10 µ g HB2-SAPORIN IT ( ), 7.4 µ g HB2-F(ab)2-SAPORIN ( ) or sham treated at the same time with 200 µ l PBS ( ) (compared with a zero survival rate in the NOD/SCID controls) the therapeutic effect was not found to be significant by log-rank analysis (P = 0.296). This was probably because the rapid rate at which the majority of animals developed disease early in the experi- ment skewed the statistical test. In this particular instance SCID and NOD/SCID PBS sham-treated animals died at a very similar rate and thus there was no significant difference between them. We also conducted an experiment to compare the therapeutic effects of HB2-SAPORIN and HB2-F(ab)2-SAPORIN immuno- toxins in SCID-HSB-2 and NOD/SCID-HSB-2 mice and the results we obtained are shown in Figure 4. In SCID-HSB-2 mice the HB2-F(ab)2-SAPORIN IT performed significantly less well therapeutically than the HB2-SAPORIN IT constructed with intact HB2 antibody (Figure 4A). In this instance the HB2-F(ab)2- SAPORIN-treated animals developed signs and symptoms of leukaemia at a more rapid rate than animals treated with HB2- SAPORIN and moreover only 20% of these animals were alive and leukaemia-free upon elective termination of the experiment at 170 days, in contrast to the 43% of animals treated with HB2- SAPORIN IT. Figure 3B shows that there was no significant difference in the therapeutic performance of either IT in NOD/SCID-HSB-2 mice. In this instance both HB2-SAPORIN and HB2-F(ab)2-SAPORIN were almost completely ineffective, © 2000 Cancer Research Campaign Immunotherapy in immunodeprived mice 1759 with animals from both IT therapy groups developing signs and symptoms of leukaemia only at a slightly slower rate than the PBS control group. There was one single animal from within the HB2- F(ab)2-SAPORIN treatment group that only developed signs and symptoms of HSB-2 leukaemia growth after 151 days, otherwise all other animals developed disease at a similar rate to HB2- SAPORIN-treated animals. The differences between the HB2- SAPORIN and HB2-F(ab)2-SAPORIN-treated animals and PBS sham-treated controls were not significant. Therefore in this partic- ular experiment the lack of therapeutic effect of HB2-SAPORIN IT in NOD/SCID-HSB-2 mice was even more pronounced than in the previously described experiment illustrated in Figure 2B. DISCUSSION The two major findings to emerge from this study are (a) that the therapy of NOD/SCID mice bearing the human CD7+ T-cell acute leukaemia cell line HSB-2 with the murine IgG1 antibody HB2 directed against the human CD7 molecule is therapeutically totally ineffective in contrast to the significant therapeutic effect that is achieved with this antibody in SCID mice and (b) that the thera- peutic effectiveness of the anti-CD7 IT HB2-SAPORIN is signifi- cantly reduced in NOD/SCID-HSB-2 mice compared with that obtained in SCID-HSB-2 mice. The in vivo experiments showing this, have been repeated on three separate occasions with similar results each time and two of these have been reported here. On each occasion the therapeutic effect of HB2 antibody or HB2-SAPORIN IT has been found to be significantly reduced in NOD/SCID-HSB-2 mice compared with SCID-HSB-2 mice. In one experiment reported here there was virtually no observable therapeutic effect of HB2- SAPORIN in NOD/SCID-HSB-2 mice (see Figure 4B) and in the other two studies, one of which is reported here (see Figure 3B), there was a therapeutic effect in a proportion of the NOD/SCID- HSB-2 mice but overall this effect was significantly reduced. We have previously shown that the antibody-mediated anti-leukaemia effect that is exerted by HB2 antibody in SCID-HSB-2 mice is very likely mediated by SCID mouse NK cells (Flavell et al, 1998) which we and others have shown to be functionally present in SCID mice (Dorshkind et al, 1985; Flavell et al, 1998). We were also able to show in the present study that NOD/SCID mice possess functionally intact NK cell-like activities that were capable of lysing target YAC- 1 cells and of also participating in ADCC against antibody coated target HSB-2 cells. However, it would appear that there are fewer FcγRII/RIII and NK1.1+ NK cells present in the spleens of NOD/SCID mice. Furthermore our study also suggests that NOD/SCID NK cells are probably functionally less active than the equivalent SCID mouse cell counterparts. Thus we were able to demonstrate in the present study a diminishing lytic capability of NOD/SCID mouse splenocytes for YAC-1 target cells at lower T:E ratios that was not paralleled by SCID splenocytes when used at the same ratios and furthermore a reduced lytic capacity of NOD/SCID splenocytes against HSB-2 cells in an ADCC assay compared with that seen with SCID splenocytes. Despite this, it is quite surprising that splenocytes from NOD/SCID mice, which do appear to possess some in vitro lytic NK cell-like activities, albeit at reduced levels, fail to elicit an effective in vivo ADCC anti-leukaemia response with HB2 antibody. One possibility is that this is because the functional quality of the relevant NOD/SCID splenocyte population is below the threshold value to be effective in vivo. An alternative explana- tion might be that the extra-splenic tissue distribution of functional NK cells is sparse in NOD/SCID mice and/or that the migration of British Journal of Cancer (2000) 83(12), 1755–1761