Expression of RHAMM and WT1 As Well As T Cell Responses to These Antigens Before and After Allogeneic Stem Cell Transplantation in Patients with Leukemia

Patients with leukemia undergo chemotherapy as first treatment. Approximately 70–80% of patients with acute myeloid leukemia (AML) reach complete remission. However, most of them will relapse and only 25% survive more than five years. Therefore, there is a need for novel approaches in the treatment of leukemia, such as immunotherapy. Leukemic blasts have an aberrant expression of antigens. They are called leukemia-associated antigens (LAAs) like the receptor for hyaluronan acid-mediated motility (RHAMM) and the Wilms’ tumor gene 1 product (WT1). Epitopes of these LAAs can be recognized by CD8+ T cells.

In the present study, we analyzed the correlation between the clinical course of 18 patients suffering from leukemia (10 AML, 5 MDS, 1 ALL and 2 B-CLL) with the expression of RHAMM and WT1 transcripts before and after allogeneic stem cell transplantation (allo-SCT). Gene transcripts were measured by quantitative real time PCR (RQ-PCR) from RNA of peripheral blood mononuclear cells (PBMC) and bone marrow mononuclear cells (BMMC) samples. Antigen specific T cells were enriched in a mixed lymphocyte-peptide culture (MLPC) and antigen specific T cell responses were measured by enzyme-linked immunosorbent spot (ELISPOT).

We observed a reduction in WT1 transcripts in both PBMC and BMMC after transplantation in all of the WT1 positive patients (6/18 patients: 33%). Four of these six WT1+ patients (67%) remained in complete remission (CR) with low transcripts of WT1 (PBMC: lower than 14 WT1 copies/104 ABL copies, BMMC: lower than 202 WT1 copies/104 ABL copies). In contrast, 2 of 6 WT1+ patients (33%) showed an increase (PBMC: up to 98 WT1 copies/104 ABL copies, BMMC: up to 920 WT1 copies/104 ABL copies) of WT1 transcripts eventually resulting in a relapse. Specific T cell responses were detected against WT1 in two of three WT1+ patients in the presence of blasts (before transplantation or in relapse). However, these specific responses vanished while the patients reached a CR.

Furthermore, RHAMM+ patients (12/18: 67%) showed different patterns when correlated with clinical status. Five patients (42%) showed gradually increased levels of RHAMM transcripts during CR. No RHAMM specific T cells could be detected in this group (2/2 MLPCs). Four patients (33%) showed a decrease in the transcripts of RHAMM when they reached a CR. One of these patients developed a T cell response to RHAMM three months after allo-SCT (2/2 MLPCs).

One patient showed high transcripts of RHAMM and WT1 during the diagnosis, WT1 transcripts were reduced after allo-SCT. Both RHAMM and WT1 transcripts gradually increased until the patients died. We could detect in this patient both WT1 and RHAMM-specific T cells before transplantation. After allo-SCT the T cell response vanished.

Taken together, WT1 is a suitable marker for minimal residual disease after allo-SCT. One might speculate that T cells specific for WT1 vanished during the CR due to the absence of the antigen to stimulate the proliferation of specific T cell population. Moreover, the presence of RHAMM-specific T cells may help to maintain a CR. In both cases vaccination with RHAMM and WT1 derived peptide might enhance T cell responses in the patient leading to a better outcome of the patient.

Freund: Medac: Honoraria, Research Funding.