cytometer following indirect or direct staining of splenocytes with the appropriate antibody as described (Flavell et al, 1998). In vivo experiments On day one of study, groups of 10 SCID or NOD/SCID mice were injected into the tail vein with two million HSB-2 cells in a 200 µ l volume of R10 medium (termed SCID-HSB-2 or NOD/SCID-HSB- 2 animals). We have described the growth and dissemination of the human T-ALL cell line HSB-2 in SCID mice in detail previously (Morland et al, 1994). We have observed that the pattern of disease development and progression is virtually identical in NOD/SCID mice (unpublished observations). The appropriate groups of SCID- HSB-2 or NOD/SCID-HSB-2 mice were treated i.v. with three 10 µ g doses of HB2 antibody given on days 7, 9 and 11 or sham treated with three doses of PBS given at the same time points. Other groups of SCID-HSB-2 or NOD/SCID-HSB-2 animals received a single 10 µ g i.v. dose of HB2-SAPORIN or with a molar equivalent amount (7.4 µ g) of HB2-F(ab)2-SAPORIN on day 7 or were sham treated with a single i.v. injection of 200 µ l PBS on day 7. All animals were monitored twice daily for the duration of the study and strictly defined endpoints were enforced as per recommendations under the UKCCCR guidelines (Workman et al, 1998) to ensure that no animal suffered undue or unnecessary stress or pain during the course of the study. Thus, animals showing any identifiable early signs of being unwell were killed humanely and full post-mortem examinations conducted to confirm the presence of tumour. Statistical analysis Log-Rank analysis (Peto’s method) using the SOLO statistics soft- ware package (BMDP Statistical Software. Los Angeles, CA, USA) was used to determine if there were any significant differ- ences between the various therapy groups. P values of 0.05 or less were considered as statistically significant in these studies. RESULTS Immunophenotypic analysis of SCID and NOD/SCID splenocyte populations Comparative immunophenotypic analysis of splenocytes taken from SCID and NOD/SCID mice was undertaken by single colour flow cytometry and revealed the differences shown in Table 1. 32% of the splenic population of SCID mice were positive for the NK cell marker NK1.1 compared with only 20% of NOD/SCID splenocytes. Similarly there were fewer NOD/SCID splenocytes expressing FcgRII/RIII compared with SCID splenocytes (22% versus 37%, respectively) and also fewer macrophages identified Table 1 Immunophenotypic composition of splenocytes from CB.17 SCID and NOD/SCID mice as determined by flow cytometry % positive splenocytes Surface antigen NK 1.1 F4/80 CD16/32 CD2 CD19 SCID 32 17 37 5 2 NOD/SCID 20 12 22 2 2 Immunotherapy in immunodeprived mice 1757 by the F4/80 antibody in NOD/SCID spleens compared with SCID spleens (12% versus 17%). There were also marginally fewer CD2 expressing splenocytes in NOD/SCID mice but similar numbers of CD19 expressing cells. Functional characteristics of SCID and NOD/SCID splenocytes Constitutive cytotoxicity for YAC-1 and HSB-2 cells Splenocytes taken from SCID and NOD/SCID mice that had been treated 24 h previously with 100 µ g poly IC i.v. were tested for Figure 1 Lysis of (A) target YAC-1 cells or (B) HSB-2 cells expressed as a percentage of 51Cr release following incubation with either CB.17 SCID ( ) or NOD/SCID ( ) effector splenocytes at target:effector ratios ranging from 1:200 to 1:20. Bars, SD © 2000 Cancer Research Campaign British Journal of Cancer (2000) 83(12), 1755–1761