WT1 Peptide Vaccine Is Able to Induce WT1-Specifc Immune Response with TCR Clonal Enrichment to Control Minimal Residual Disease in Patients with Myeloid Leukemia

Background: Immunotherapy has the potential for clinical efficacy in patients with myeloid leukemia, especially in the setting of minimal residual disease. WT-1, aberrantly expressed in both myeloid and lymphoid leukemia, is associated with adverse risk in AML. WT1 is highly antigenic and is an attractive target for immunotherapy. The optimal strategy for vaccination to induce CD8+ T cell responses against WT1 is not known.

Methods: We performed a pilot randomized study of HLA-A02+ patients to receive vaccination with WT1 126-134 peptide (RMFPNAPYL) in Montanide or in poly ICLC (Hiltonol from Oncovir, a TLR3 agonist) to explore the novel immune adjuvant in patients with myeloid leukemia (NCT01842139). The vaccine was administered q 2 weeks X 6 during the induction phase followed by monthly booster vaccinations X 6 months.

Enrollment: Seven patients (4 males, 3 female ages 39 to 73) were randomized. Four patients received WT1 in Montanide (3 AML, 1 CML myeloid blast phase, 2 s/p allo-SCT), and three with WT1 in poly ICLC (2 AML, one MDS RAEB2 s/p allo-SCT). Five patients were in morphologic remission (3 in CR1) and two had very low burden of residual morphologic disease at study entry.

Toxicities: All patients finished the induction phase without any major toxicity except mild transient local injection reaction. One patient post allo-SCT on the Montanide arm developed transverse myelitis with evidence of bacterial meningitis following the first monthly booster vaccination. Another patient on the Montanide arm developed aseptic ulceration at the 12^th vaccine site followed by inflammation at the 11^th WT1 vaccine site, and persistent erythema at the 1^st induction vaccine site about 4 weeks after the completion of all 12 WT1 vaccinations. The aseptic ulcers eventually healed with wound care without antibiotics.

Efficacy: Three of 4 patients on the Montanide arm had decease of WT1 qRT-PCR levels after WT1 vaccination, and two of them demonstrated generation of WT1-specific cytotoxic CD8+ T cell responses with biased TCR beta chain enrichment. Three patients from who cells were available for TCR alpha and beta CDR3 sequencing had TCR clonal enrichment after WT1 vaccination. In contrast, no obvious WT1-specific immune responses were detected in 2 patients on the poly ICLC arm, nor was there clonal enrichment by TCR alpha/beta sequencing; however, these patients did have a decrease in WT1 qRT-PCR levels and remained in remission 3 years after the initiation of WT1 vaccination. Thus, WT1 peptide in poly ICLC may induce anti-leukemia immune response not detected by our current assays. The third patient on the poly ICLC arm was later found to have A0202 instead of A0201, and thus could serve as negative control. Not surprisingly, this patient did not have a decrease of WT1 qRT-PCR levels nor TCR clonal evolution during vaccination. The patient tolerated the vaccine well without injection reactions and had stable AML for 12 weeks, but the disease progressed before the first monthly WT1 vaccination.

Conclusions: WT1 peptide vaccine with Montanide as an adjuvant induces WT1-specific CD8+ T cell responses with TCR clonal and specific TCR beta CDR3 enrichment, which may be capable of controlling leukemia recurrence in the setting of minimal residual disease. Future investigation to combine checkpoint inhibitors with peptide vaccination might further enhance efficacy in patients with myeloid leukemia.