A stem cell allograft is the only way of curing CLL, but how does it work? It seems that there is immune response from the grafted immune system against the leukemia. Now a paper in this week's Blood suggests how this might work. Hitherto, it has been thought that post-transplant immunity against the leukemia was mediated by T cells, but Baskar and colleagues have asked whether it might be mediated by antibody.
They have looked at stored serial blood samples from two patients who remain in molecular remission respectively 6 and 7 years after their transplant. They have looked by flow cytometry to see whether any of the sera from these samples reacted with CLL cells. This is technically difficult because CLL cells already have immunoglobulin on their surface and they have a receptor that binds to immunoglobulin surrounding them in fluid, so both of these have first to be blocked. Nevertheless both cases had detectable anti-CLL cells antibody in their blood, one peaking at 6 months post-transplant and one peaking at 6 months and the other at 10 months post-transplant. Interestingly these coincided with the disappearance of minimal residual disease in each patient.
The next thing was to find the B cells circulating at the time that were circulating at the time that were responsible for the production of these antibodies. To do this a phage library was constructed. This is a complicated procedure in which the total RNA was extracted from the cryopreserved B cells and the RNA coding for the immunoglobulin fragment containing the antibody activity (known as Fab) was amplified by PCR using appropriate primers. This process also inserts appropriate RNA into bacteriophages (a bacteriophage is a virus that infects a bacterium). E.coli bacteria were then infected by the phage (this process makes the Fab stick out from the surface of the e.coli. This can be used to select those bacteriophage that have a code for an anti-CLL cell Fab. The selection process was dominated by one antibody that in all its manifestations could be predicted to react with the same antigen, which was present on nearly all CLL cells and more weakly on normal B cells, but not on other B-cell tumors or tumor cell lines.
The significance of this study is that it may identify an antigen on CLL cells which is extremely susceptible to killing by antibody, so that a new monoclonal that eliminates CLL cells completely may be produced, or if the antigen is fully identified, other small molecules might be derived that kill CLL cells. This paper also provides a method that could be applied to other leukemias that can be cured by stem cell transplantation.