Circulating Leukemic Myeloid Dendritic Cells from Patient with Leukemia Elicit CDK2-Specific CTLs from Allogeneic HLA-A24+ Naive CD8+ T Cells.
Authors: Kondo Yukio , Ohata Kinya , Nakao Shinji 
Published: October 2019

Abstract
Aberrantly expressed self-antigens in leukemic cells are a kind of leukemia-associated-antigens (LAAs). Although such self-antigen-derived LAAs potentially induce high avidity CTLs in patients with leukemia, these CTLs usually do not persist in the patients because they undergo apoptosis upon encountering with leukemic cells. In allogeneic stem cell transplant (allo-SCT) recipients with leukemia, LAAs-specific high avidity CTL may be elicited from donor-derived naive T cells that are sensitized by residual leukemic cells. Cyclin-dependent kinase 2 (CDK2) is a cell cycle regulator protein that is aberrantly expressed by leukemia cells. We previously reported that two CDK2-derived peptides (CDK2 158-166, CDK2 178-186) avidly bound to HLA-A24 molecule to elicit each peptide-specific CTLs from HLA identical donor of a AML patient, and CDK2-specific CD8 + T cells preferentially killed the recipient AML cells which aberrantly expressed CDK2 protein (Blood.108 (11): a3173. 2006). When we assessed cryopreserved PBMCs obtained before and after allo-SCT from 16 HLA-A24+ patients (6 AML, 1 MDS, 1 CML, 2 ALL, 4 NHL and 2 RCC) using CDK2 158/A24 and CDK2 178/A24 pentamers, small populations (0.1–1.0%) of CDK2 158 and CDK2 178-specific CTL were detectable in 6 patients after SCT but not before SCT. All of the 6 patients had MRD at the time of SCT and achieved molecular CR after SCT. None of the 3 patients relapsed after SCT did not show CDK2 immunity. There is no relationship between the appearance of CDK2-specific CTLs and the development of GvHD (Figure). Leukemic myeloid dendritic cells (mDC) are known to present in vivo in patients with leukemia, and be able to trigger a protective antileukemic immune response by allogeneic T-cells. We hypothesized that residual circulating leukemic mDCs may sensitize donor-derived T cells by cross-presenting CDK2-peptides early after allo-SCT and elicit high avidity CTLs specific to leukemic cells. mDCs subset was identified with by three-color staining using mAbs against CD85k, CD33, CD14 and CD16. Circulating mDCs represented 5.4% of PBMCs in a patient with CML-CP at diagnosis. Leukemic mDCs from the patient were purified with CD1c mAb-conjugated maginetic beads and were assessed for their ability to stimulate allogeneic T cells to acquire specific cytotoxicity against CDK2-peptides. The purity of the mDCs as determined by flow cytometry was 92% of living isolated cells. Naive CD8 + T cells isolated from healthy individual were cultured with the leukemic mDCs for 12 days and subjected to pentamers staining. After the coculturing, the proportion of CDK2 158/A24 and CDK2 178/A24 pentamers + CD8 T cells increased 0.6% to 2.2% and 0.6% to 2.6%, respectively. These data suggest that CDK2-specific CTLs can be induced from donor-derived T cells due to in vivo sensitization of donor T cells by residual leukemic mDCs and this may be a mechanism responsible for the generation of CDK2-specific CTLs in allo-SCT recipients with MRD without vaccination of CDK2-peptides. Vaccination with CDK2-peptides after allo-SCT may be useful in both enhancing CDK2-specific immunity in patients with MRD and in generating CDK2 immunity in those without MRD.
Figure: An immune response to CDK2 is associated with GvL and MRD at the time of allo-SCT, but not with GvHD. Figure:. An immune response to CDK2 is associated with GvL and MRD at the time of allo-SCT, but not with GvHD.