Co-Operativity between Antibody-Dependent Cellular Cytotoxicity (ADCC) and Immunotoxin-Mediated Killing of Human T-ALL Cells in a SCID Mouse Model: Simultaneous Delivery of Saporin Via CD7 and ADCC Via CD38 Is No Better Than the Simultaneous Delivery of Each Against Only CD7.

We have previously shown that the anti-CD7 saporin-based immunotoxin (IT) HB2-SAPORIN and host antibody-dependent cellular cytotoxicity (ADCC) co-operate in the killing of human T-ALL a SCID mouse model of this disease (Flavell et al 1998 Cancer Res 58, 5787). In this described model system it is the same anti-CD7 IT that is responsible for delivering both the ribosome inactivating protein saporin and eliciting ADCC by recruiting NK cells via binding of their FcγRIIIA to the Fc domain of target cell bound IT. There is however a conflict here in that for the IT to be effective it needs to internalize whilst for maximally effective ADCC, the IT needs to remain on the cell surface in order to recruit cytotoxic effector cells. Over 90% of HB2-SAPORIN IT is internalized by CD7⁺ HSB-2 target cells within 12h following bivalent ligation by this IT and yet the ten percent or less of CD7 remaining on the target cell surface appears sufficient to elicit adequate ADCC to achieve the co-operative effect in vivo. The question is if ADCC is performing sub-optimally due to the reduced amount of target cell expressed CD7 doe this mean that the subsequent co-operative effect is also sub-optimal and could this be improved upon to achieve a greater and more effective target cell kill?

To attempt to answer this question we have targeted saporin to the HSB-2 target cell via CD7 using a F(ab)₂ IT (HB2-F(ab)₂₋-SAPORIN) that is incapable of eliciting ADCC whilst simultaneously delivering ADCC via an intact IgG₁ anti-CD38 antibody (OKT10) that also binds to the target cell surface. The expression level of CD38 on HSB-2 cells is approximately ten-fold higher than CD7 and importantly the observed CD38 internalization rate and extent is considerably reduced compared with CD7 when ligated by cognate antibody. We therefore predicted that increased ADCC would be deliverable to HSB-2 target cells with OKT10 antibody and an in vitro chromium release assay confirmed this. However, the in vivo treatment of SCID-HSB-2 mice with a combination of HB2-F(ab)₂-SAPORIN and intact OKT10 antibody was not significantly better than treatment with HB2-SAPORIN intact IT alone. These preliminary data indicate that even though additional ADCC is apparently deliverable with the OKT10 antibody this does not result in a further enhancement of the co-operative effect in vivo suggesting that the optimum co-operativity threshold has already been reached in the HB2-SAPORIN mono-therapy system.