Emanuela Burchielli

Natural killer cell alloreactivity in allogeneic hematopoietic transplantation.

Purpose of review This review will focus on the translation of natural killer cell recognition of missing self into the clinical practice of allogeneic hematopoietic transplantation and discuss how it has opened innovative perspectives in the cure of leukemia. Allogeneic hematopoietic stem cell transplantation from a human leukocyte antigen-matched sibling can cure leukemia but 75% of patients do not have a matched donor, one alternative source of stem cells includes full haplotype mismatched family members. As haploidentical transplantation must be extensively T cell depleted to prevent lethal graft-versus-host disease, it cannot rely on donor T cells for the graft-versus-leukemia effect. Mismatched transplantation, however, triggers alloreactivity mediated by natural killer cells which is based upon ‘missing self recognition’. Recent studies Recent studies using preclinical murine models of haploidentical transplantation demonstrated that conditioning with alloreactive natural killer cells ablates the recipient immune system and leukemia cells. In the clinical setting of mismatched hematopoietic stem cell transplantation, donor versus recipient natural killer cell alloreactivity has been associated with better outcome, particularly in patients with acute myeloid leukemia who are transplanted in remission. Summary Given the benefits of natural killer cell alloreactivity, it is expected that it will encourage greater use of haploidentical transplants for the large numbers of leukemia patients without matched donors.

Natural killer cell alloreactivity and haplo-identical hematopoietic transplantation

In haplo-identical hematopoietic transplantation, donor vs. recipient natural killer (NK) cell alloreactivity derives from a mismatch between donor NK clones bearing inhibitory killer cell Ig-like receptors (KIR) for self-HLA class I molecules and their HLA class I ligands (KIR ligands) on recipient cells. When faced with mismatched allogeneic targets, these NK clones sense the missing expression of self-HLA class I alleles and mediate alloreactions. KIR ligand mismatches in the GvH direction trigger donor vs. recipient NK cell alloreactions, which improve engraftment, do not cause GvHD and control relapse in AML patients . The mechanism whereby alloreactive NK cells exert their benefits in transplantation has been elucidated in mouse models. The infusion of alloreactive NK cells ablates (i) leukemic cells, (ii) recipient T cells that reject the graft and (iii) recipient DC that trigger GvHD, thus protecting from GvHD.

Natural killer cell alloreactivity in haploidentical hematopoietic stem cell transplantation.

Natural killer (NK) cells are primed to kill by several activating receptors. NK cell killing of autologous cells is prevented because NK cells coexpress inhibitory receptors (killer cell immunoglobulin-like receptors [KIR]) that recognize groups of (self) major histocompatibility complex class I alleles. Because KIRs are clonally distributed, the NK cell population in any individual are constituted of a repertoire with a variety of class I specificities. NK cells in the repertoire mediate alloreactions when the allogeneic targets do not express the class I alleles that block them. After haploidentical hematopoietic transplantation, NK cell-mediated donor-versus-recipient alloresponses reduce the risk of relapse in acute myeloid leukemia patients while improving engraftment and protecting against graft-versus-host disease. High-resolution molecular HLA typing of recipient and donor, positive identification of donor KIR genes, and, in some cases, functional assessment of donor NK clones identify haploidentical donors who are able to mount donor-versus-recipient NK alloreactions.

Clinical impact of natural killer cell reconstitution after allogeneic hematopoietic transplantation.

Although the optimal donor for allogeneic hematopoietic stem cell transplantation is a human leukocyte antigen (HLA)-matched sibling, 75% of patients do not have a match and alternatives are matched unrelated volunteers, unrelated umbilical cord blood units, and full HLA-haplotype-mismatched family members. This review will focus on the open issues of allogeneic hematopoietic transplantation and on the benefits of natural killer (NK) cell alloreactivity and its underlying mechanisms. Donor-versus-recipient NK cell alloreactivity derives from a mismatch between inhibitory receptors for self major histocompatibility complex (MHC) class I molecules on donor NK clones and the MHC class I ligands on recipient cells. These NK clones sense the missing expression of the self MHC class I allele on the allogeneic targets (“missing self”) and mediate alloreactions. Here, we review the translation of NK cell allorecognition into the clinical practice of allogeneic hematopoietic transplantation and discuss how it has opened innovative perspectives in the cure of leukemia.

Optimizing a photoallodepletion protocol for adoptive immunotherapy after haploidentical SCT

In adults, one-haplotype-mismatched haematopoietic SCT (haploidentical HSCT) is associated with slow immune recovery due to decaying thymic function and extensive T-cell depletion of the graft. Although essential for preventing GVHD, T-cell depletion underlies the major reasons for transplant failure: leukemia relapse and infections, with infection-related mortality accounting for about 40% of non-leukemic deaths. Adoptive T-cell therapy would be helpful for these patients but to administer it without causing GVHD, alloreactive T cells need to be eliminated from donor T lymphocytes before infusion. In a preclinical study, to address this problem, we determined the efficacy of photodynamic purging of alloreactive T cells, by investigating combinations of parameters in order to achieve maximum allodepletion, preservation of T-regulatory cells and of pathogen and leukemia-specific T-cell responses in donor-vs-recipient MLR. We also needed to identify an optimal method to quantify the Ag-specific T-cell repertoires. Optimal procedures were identified. In particular, we compared limiting-dilution analyses (LDA) of proliferating T cells with H3-thymidine incorporation by bulk T cells and with flow cytometry CD25 expression, which is accepted as a T-cell activation marker. This study demonstrated that LDA is a reliable, predictable and sensitive method for measuring alloreactive, pathogen- and leukemia-specific T-cell frequencies.