Immunogenicity of Childhood T-ALL.

Background: Acute lymphoblastic leukemia (ALL) with a T cell immunophenotype accounts for about 15% of childhood ALL. While contemporary treatment protocols have improved the relapse free survival for children with T-ALL, it has for a long time been considered a malignancy with a dismal prognosis. In fact, this type of leukemia is still at a higher risk for treatment failure and early relapses than B cell precursor ALLs. Further intensification of conventional treatment or stem cell transplantation may increase the cure rate for only some subtypes of ALL but is also associated with considerable treatment-related mortality and morbidity. Thus, new treatment strategies that are less toxic and, at the same time, more effective for patients with a resistant disease, are needed. The aim of the study was to evaluate whether a specific immune reaction is triggered in children with T-ALL and to identify the respective targets. We chose SEREX (serological identification of leukemia derived antigens by recombinant expression cloning) to screen a T-ALL expression library with plasma from 18 children with T-ALL.

Results: Thirteen antigens with homology to known genes that are involved in critical cellular processes were detected. Some of them have already been implicated in the induction of an immune response in a variety of cancer types. From four of these genes novel isoforms were identified and further analyzed. mRNA expression of three isoforms (HECTD1_Δ, CX-ORF-15_Δ and hCAP-E_Δ) was restricted to more than 70% of T-ALLs (n=22) and specific antibodies against these isoforms were detected in up to 30% of patients (n=16) with the highest frequency for HECTD1_Δ. The alternative splicing leads to the deletion of one glutamic acid located in a putative PEST domain thereby decreasing the PEST score suggesting a higher stability of the protein. Interestingly, the HECTD1 protein was present at high abundance in T-ALLs while is was not detectable in normal hematopoietic tissues. Since the leukemia-associated antigens detected in this study have an intracellular localization, a feature that is shared by the majority of SEREX defined antigens, the generation of immune effector responses most likely requires antigen presentation. To test this assumption, dendritic cells were loaded with HECTD1_Δ protein and used for T cell stimulation. A specific T cell response was induced in vitro in all three donors, including a former T-ALL patient.

Conclusion: Leukemia-associated antigens, identified by SEREX, appear to be capable of inducing both a humoral and cellular immune response in children with T-ALL. Thus, these data support further studies to establish new approaches for immunotherapy.