AML: exposed and exploited?

AML is the most common adult and second most common childhood acute leukemia, but associates with compromised patient survival due to its high incidence of refractory and relapsed disease. Allogeneic HSCT remains the definitive cure for high-risk and relapsed AML in adult²  and pediatric³  patients, as allogeneic HSCT uses donor-derived immunity to eradicate leukemic cells in the transplant recipient (GVL). Natural killer (NK) and T cells have conventionally been regarded as the primary cell mediators of the GVL response, and their ex vivo expansion and engineering have proven useful in reducing malignant disease relapse in the context of allogeneic HSCT.⁴  However, the contribution of B cells to GVL activity is less established. Therefore, the work by Gillissen and colleagues is intriguing and begs the question: how can we exploit antibodies targeting neoantigens expressed by AML to reduce disease relapse and to improve disease cure following allogeneic HSCT?

The authors identified AML-specific antibodies using conventional laboratory techniques, including B-cell isolation and cell line immortalization, cell lysate immunoprecipitation, and mass spectroscopy and then confirmed target specificity using enzyme-linked immunosorbent assay and flow cytometry. The authors identified the spliceosome component, U5 snRNP200 complex, as the antibody-specific target for inducing oncosis in AML cells. Spliceosomes are responsible for removing introns from primary messenger RNA (mRNA) precursors for mature mRNA to be translated into protein.⁵  Interestingly, aberrant RNA splicing and spliceosome mutations may contribute to chemotherapy drug resistance and leukemogenesis in AML.⁶  So how did a large, multiprotein intracellular component of the spliceosome end up on the cell surface of AML blasts?

Given that cytotoxic antibodies were derived from allogeneic HSCT patients who experienced graft-versus-host disease (GVHD), GVHD itself could have induced cellular stress and damage to expose the U5 snRNP200 spliceosome complex, enabling subsequent donor-derived antibody formation via donor B-cell priming (see figure). HSCT-associated damage is well described and has been shown to instigate alloimmune reactions in transplant recipient, including acute GVHD. Specifically, conditioning regimen–induced damage-associated molecular patterns are recognized by pathogen recognition receptors like Toll-like receptors expressed on recipient antigen-presenting cells (APCs).⁷  Subsequent APC activation induces donor T-cell activation, which results in target tissue cytotoxicity in the HSCT recipient. However, the fact that not all AML patients who experienced GVHD developed anti-spliceosome complex antibodies suggests that GVHD itself may not be necessary to induce AML cellular stress and subsequent spliceosome complex surface expression. Therefore, other cellular stress–inducible factors as well as the process by which spliceosome complex surface expression occurs remain unknown.

Importantly, antibody specificity was confined to AML blasts expressing surface U5 snRNP200 complex, as the antibody did not bind to other hematopoietic (bone marrow–derived CD34⁺ hematopoietic progenitor cells, peripheral blood mononuclear cells, and monocytes) or nonhematopoietic cells (endothelial cells and fibroblasts). Therefore, subsequent antibody-mediated oncosis only affected AML blasts. Furthermore, the U5 snRNP200 complex–specific antibody was not found in healthy patients or in multiple myeloma patients undergoing allogeneic HSCT. Such target specificity is essential for using either vaccine- or cell-based immunotherapies to ensure the desired anti-tumor effects and to reduce off-target complications. Given these study findings, chimeric antigen receptor (CAR) T cells and NK cells could potentially be engineered to target AML cells expressing U5 snRNP200 complex. Furthermore, combining immune-directed cellular and pharmaceutical-based products like monoclonal antibodies might also be possible⁸  (see figure).

The Gillissen et al study does have apparent limitations. First, the mechanism by which the U5 snRNP200 complex becomes exposed on the surface of AML cells is unknown. Second, the Gillissen et al study does not prove that antibody development results from direct, donor-derived B-cell priming by leukemia cells themselves. Third, in vivo spliceosome complex antibody–mediated GVL activity was not demonstrated using a preclinical allogeneic HSCT model. Finally, clinical use of spliceosome complex–specific antibodies has yet to be studied and may even be restricted to certain AML subtypes in certain patients. However, these limitations do not necessarily diminish the study results, but rather open the door for further investigation in this area. As such, the work by Gillissen and colleagues exemplifies how screening for antibody repertoires in allogeneic HSCT patients may identify novel targets needed to exploit a potential cure for AML using immune-directed therapies.