EHA 2016

Killer antibodies against AML

Dr Mette Hazenberg - AMC, Amsterdam, The Netherlands

I’d like to share with you some data on killer antibodies in AML which is a pretty new and exciting field, I would like to say, I think so. The purpose of our research, as you just heard, acute myeloid leukaemia which is a malignancy, a cancer, of the bone marrow often has a very poor prognosis, of course in elderly patients but also younger patients can actually still die from this. So we really need novel approaches to treat this disease and there are many developments, as we just heard, but we decided to look at it from a different angle.

So nowadays most patients with AML need a bone marrow transplantation and allogeneic stem cell transplantation to be cured from their AML. What happens is you can see that here patients get a healthy bone marrow, healthy stem cells from a healthy donor. Those stem cells home to the bone marrow of the patient and grow out into a healthy normal haematopoietic system. In case the leukaemia relapses, which you see now in the body of the little figure are leukemic cells, in case the leukaemia relapses the idea is that the immune system of the donor mounts a potent anti-leukaemia response. So it’s a form of immunotherapy but by nature.

So we decided to study this more in detail because we thought we could learn from nature. So it has been shown that allogeneic stem cell transplantations can mount these potent immune responses and we wondered whether antibodies could be involved in this. So what we did was we selected three patients with high risk leukaemia that nevertheless survived after receiving an allogeneic stem cell transplant. We isolated B-cells from these patients and B-cells are the cells in our bodies that produce antibodies. Antibodies, what they normally do is they bind to bacteria or viruses or tumour cells and help the immune system killing them. So we immortalised these B-cells and we cultured them and then we took the supernatant of these cultures that contains the antibodies and screened that supernatant for binding to AML. By doing this and by cloning the B-cells that did bind we were able to retrieve 17 monoclonal antibodies from these three patients that very specifically bind AML. So on your left you can see THP1 which is an AML cell line and AML M0, that’s an example of one of the patients in our clinic. You can see that the curve shifted to the right, indicating that the antibodies bind to those malignant cells. The other curves show that there’s no binding to healthy cells.

Of these 17 antibodies 7 recognised the same target and that target was very unexpected, was very surprising because that target is the U5 SNRNP200 protein that is a large protein, 250kD, and it’s part of the spliceosome complex. The spliceosome is normally located inside the cell where it cleaves pre-RNA. So every cell has it but normally it’s inside the cell and what we found was that in AML cells this protein is actually expressed on the surface of the AML cell and then recognised by these antibodies. So that was novel and we screened a few more patients and we found actually that 4 out of 5 patients had mounted such antibodies so apparently it’s a target that is often recognised by the immune system of the donor.

The second striking observation that we did was that these antibodies when they interact with that SNRNP protein on the cell membrane actually kill the leukaemic cells. So I have this clip and I hope it works. What you can see is these are two leukaemic blasts, I’m not sure if you can see it but they are a little pinkish and that is the antibody that is coloured in pink that binds to these blasts. What you can see is that they get very nervous because of this and you see that first this one is dying, it actually spits out its nucleus, and then the second one does the same, you can see it happening here. So these two cells are now dead. So this is nucleus and this is cell debris surrounding it.

So these antibodies kill leukemic blasts when they bind to it and they also do that in vivo, so we tested this in a mouse model. They do that via a very specific way, it’s called oncosis, it’s a non-apoptotic pathway. That’s why we call these antibodies cytotoxic antibodies because they kill blasts really on their own without any help.

So what we learned from nature, from studying patients that through an allogeneic stem cell transplantation cleared their leukaemia is that these patients make leukaemia-specific antibodies to really unexpected targets. Now with immunotherapy these days investigators decide which targets should be attacked by the immune system and we try to help the immune system by making antibody drug conjugates, for example. But here we let nature decide what the target would be and the target was, amongst others, this SNRNP200 complex and interestingly when these antibodies bind to this complex they actually kill the target cells, they kill the leukaemic cells. We think that this is a novel phenomenon, we didn’t know this before and we think we can develop this further into novel therapies.

I told you 7 out of 17 antibodies recognised the SNRNP complex, this afternoon I will present data on one of the other antibodies that we found that is also extremely interesting. If you have nothing to do perhaps this is a schedule suggestion. So thank you very much for your attention.