British Journal of Cancer (2000) 83(12), 1755–1761 © 2000 Cancer Research Campaign doi: 10.1054/ bjoc.2000.1565, available online at http://www.idealibrary.com on http://www.bjcancer.com Anti-CD7 antibody and immunotoxin treatment of human CD7+ T-cell leukaemia is significantly less effective in NOD/LtSz-scid mice than in CB.17 scid mice DJ Flavell, SL Warnes, AL Noss and SU Flavell The Simon Flavell Leukaemia Research Unit, Division of Cancer Sciences, University of Southampton School of Medicine, Southampton General Hospital, Southampton, Hampshire SO16 6YD, UK Summary Groups of 8 to ten SCID (CB.17 scid/scid) or NOD/SCID (NOD/LtSz-scid/scid) mice were injected i.v. with two million human HSB- 2 T-ALL cells on day 1 (SCID-HSB-2 and NOD/SCID-HSB-2 mice) and treated later with 3 i.v. 10 µ g doses of the anti-CD7 antibody HB2 on days 7, 9 and 11 or with a single 10 µ g dose of HB2-SAPORIN or a 7.4 µ g dose of HB2-F(ab)2-SAPORIN immunotoxin (IT) on day 7. Treatment of SCID-HSB-2 mice with HB2-SAPORIN led to a significant prolongation in the time to development of signs and symptoms of disease compared with PBS sham-treated controls with 80% of animals surviving disease-free. In contrast treatment with HB2-F(ab)2- SAPORIN was significantly less effective in SCID-HSB-2 mice with 80% of animals in this treatment group developing leukaemia over the course of the study. HB2 antibody treatment of SCID-HSB-2 mice also led to a significant prolongation in time to leukaemia development compared with sham-treated controls with 37% of animals in this treatment group disease-free at termination of the study. In contrast HB2 antibody treatment of NOD/SCID-HSB-2 mice had no therapeutic effect in these animals and the therapeutic effectiveness of both HB2- SAPORIN and HB2-F(ab)2-SAPORIN ITs was similar and significantly reduced compared to the effect observed in SCID-HSB-2 mice. It was initially thought that the lack of therapeutic effect of antibody and IT in NOD-SCID-HSB-2 mice might relate to their putative lack of NK cells but flow cytometric and functional studies with NOD-SCID mouse splenocytes revealed that these animals do have some functional NK cells though fewer in number and possibly lower in functionality than those of SCID mice. We reason that the complete lack of therapeutic effect of HB2 antibody and the reduced effect of HB2-SAPORIN in NOD/SCID mice is due to the reduced cytolytic activity of NOD/SCID NK cells which is probably below a certain critical threshold value in these animals. We conclude from this that immunotherapeutics like HB2-SAPORIN would be more accurately assessed for intrinsic potency in NOD/SCID mice where the effects of NK cell and possibly other non-adaptive immune mechanisms would not have a significant influence. © 2000 Cancer Research Campaign http://www.bjcancer.com The discovery of the severe combined immune deficient (SCID) mouse by Bosma and co-workers (Bosma et al, 1983) opened up avenues that allowed researchers to exploit this strain of mouse for the xenografting of human tumours and normal tissues in ways that had not previously been possible. The SCID mouse proved of partic- ular value to workers interested in the leukaemias and in haematopoiesis, allowing for early engraftment experiments initially with cell lines (Kamel-Reid et al, 1989) and later with primary haematological malignant (Cesano et al, 1991; Kamel-Reid et al, 1991) and non-malignant cells (Lapidot et al, 1992; Kollmann et al, 1994). The relative ease with which SCID mice accept xenografts can be attributed to their virtual total lack of functional B- and T- cells. This is due to a defect in their recombinase system that prevents immunoglobulin and T-cell receptor (TCR) gene rearr- angements that would normally drive the diversity of the antigen recognition repertoire in normal mice (Kotloff et al, 1993). Subsequently progenitor B- and T-cells fail to develop to a fully functional mature state in SCID mice (Custer et al, 1985) and instead undergo apoptosis early in their life history. SCID mice do however possess a virtually fully functional non-adaptive immune Received 12 June 2000 Revised 25 August 2000 Accepted 5 October 2000 Correspondence to: DJ Flavell system with normal levels of tissue macrophages, NK cells, serum complement activity and normal myelopoiesis (Dorshkind et al, 1984, 1985). The presence of functional NK cells and tissue macrophages has in some instances created a barrier to successful engraftment with both normal and malignant primary haematolog- ical material. This problem has been at least partially overcome in the past by depleting SCID mice of their NK cells by treatment prior to xenografting with non-lethal doses of total body radiation, anti-asialo GM1 antibody (Shpitz et al, 1994) or cytotoxic drugs such as cyclophosphamide. NK cells are however only transiently eliminated by such methods and this makes the SCID mouse of limited value in long-term engraftment studies where the reappear- ance of NK cells might likely compromise the biological behaviour of the graft. Non-Obese Diabetic mice (NOD/Lt strain) have a significant deficit in their non-adaptive immunity with reductions in NK cell functionality (Kataoka et al, 1983; Serreze and Leiter, 1988), an absence of circulating complement components (Baxter and Cooke, 1993) and functional defects in antigen presenting cells (APC) (Serreze et al, 1993). NOD/Lt mice develop a high incidence of autoimmune (type 1) insulin-dependent diabetes mellitus (IDDM) mediated by auto reactive T-cells (Christianson et al, 1993). This led Shultz and co-workers (Shultz et al, 1995) to backcross the scid mutation onto the NOD/Lt strain back- ground, resulting in an immunodeficient mouse (NOD/LtSz- scid/scid) with multiple defects in adaptive as well as non-adaptive 1755