Abstract
We have previously shown that antibody-dependent cellular cytotoxicity (ADCC) cooperates with immunotoxin (IT)-mediated killing of human leukaemia cells in an severe combined immunodeficient (SCID) mouse model of human T-cell acute lymphoblastic leukaemia (SCID-HSB-2 mice), but not in an equivalent non-obese diabetic (NOD)/SCID mouse model. In these earlier studies, we reasoned that diminished ADCC due to the functional deficit in natural killer (NK) cell activity in NOD/SCID mice resulted in a failure of effective perforin/granzyme-mediated cytotoxicity necessary for the delivery of the augmentative effect. Poly-inosinic-cytidylic acid [poly (I:C)] is a synthetic dsRNA toll-like receptor 3 (TLR3) agonist that possesses a number of biological properties that includes the in vivo activation of NK cells. We show here that intravenous (i.v.) injection of SCID mice with [poly (I:C)] results in characteristic time-related changes in serum interleukin 2 (IL-2), IL-12, and interferon γ (INFγ) cytokine levels that are consistent with TLR3 driven activation of SCID mouse NK cells. Concomitantly, there are changes in the expression levels of CD2, CD16/32 (FcγRII/RIII), CD161 (NK1.1), and F4/80 in the bulk splenocyte population. These observed changes correlate with an increase in the in vitro lytic capabilities of putative NK cells from within the splenocyte population of [poly (I:C)] treated SCID mice. We demonstrate that the in vivo activation of NK cells with [poly (I:C)] in SCID mice bearing disseminated human T-cell leukaemia xenografts resulted in a significant improvement in the therapeutic activity exerted by an intact murine monoclonal antibody against human CD7. This was also seen for a saporin-based immunotoxin constructed with the same intact antibody (HB2-SAPORIN), but not with an F(ab’)2 derivative of the same antibody or of an IT constructed with the same F(ab’)2 HB2 antibody derivative. This study further demonstrates the previously reported reinforcing role of ADCC for the therapeutic activity of IT in an SCID mouse model of human T-ALL and the potential to significantly boost this further with [poly (I:C)]. Our study provides the rationale to justify the exploration of the clinical utility of IT based therapeutics in combination with TLR3 agonists, such as [poly (I:C)], for the treatment of haematological, and possibly other, malignancies.
Highlights
In pre-clinical studies, the evaluation of the in vivo therapeutic efficacy of immunotoxins (IT) and other antibody-drug conjugates (ADC) is often made in immunodeprived mouse xenograft models of human cancer [1]
In addition to demonstrating that the therapeutic reinforcement effect due to antibody-dependent cellular cytotoxicity (ADCC) is immunospecific for IT target specificity, we were able to show that the therapeutic activity of the same anti-CD7 IT in non-obese diabetic (NOD)/severe combined immunodeficient (SCID) mice bearing HSB-2 human leukaemia xenografts is significantly reduced in comparison to that seen in the SCID-HSB-2 model [5]
We show that in vivo activation of natural killer (NK) cells with poly (I:C)] in SCID mice xenografted with human T-cell acute lymphoblastic leukaemia (T-ALL) cells leads to complete cures of animals treated with the saporin-based anti-CD7
Summary
In pre-clinical studies, the evaluation of the in vivo therapeutic efficacy of immunotoxins (IT) and other antibody-drug conjugates (ADC) is often made in immunodeprived mouse xenograft models of human cancer [1]. Biomedicines 2019, 7, 13 constructed with an intact murine antibody, the Fc domain of the antibody is capable of recruiting host cytotoxic effector cells via antibody-dependent cellular cytotoxicity (ADCC) that acts co-operatively with the ribosome inactivating protein domain of the IT molecule to kill target leukaemia cells in vivo significantly more effectively [2].We were able to demonstrate that the systemic depletion of SCID mice of their natural killer cells (NK cells) with anti-asialo GM1 antibody [3] resulted in a significant reduction in the in vivo therapeutic activity of the anti-CD7 IT HB2-SAPORIN for CD7+ target HSB-2 cells. We speculate that this improvement is due to the increased efficiency with which [poly (I:C)] activated NK cells are able to deliver cytotoxic granzymes via ADCC to the target HSB-2 cell cytosol, which act in concert with the rip saporin to increase target cell killing
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