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A. Laghmouchi et al.
Since the level of recognition of the different primary acute myeloid leukemia samples by these clones was rather heterogeneous, flow cytometric analysis was per- formed to measure the surface expression of HLA-DP and adhesion molecule CD54 on the leukemic blasts (Online Supplementary Figures S7 and S9). For some samples (e.g. samples 37 and 40, Online Supplementary Figure S8D,F) the absence of recognition was correlated with the lack of proper HLA-DP surface expression. However, sample 35 (Online Supplementary Figure S8B) was not recognized despite high surface HLA-DP expression. Moreover, the maturation state (e.g. co-expression of maturation mark- ers) of this specific acute myeloid leukemia sample was not found to be different from that of other samples that were properly recognized (data not shown). These data indicate that the level of recognition is likely to be deter- mined by a combination of factors, including the level of HLA-DP expression, the expression of adhesion molecules (e.g. CD54), and the expression of the respective antigen.
These data illustrate that the allo-HLA-DP-specific T- cell repertoire comprises T cells with the capacity to rec- ognize primary malignant cells, without recognition of non-hematopoietic tissue cells.
Discussion
In this study we demonstrate that the allo-HLA-DP-spe- cific donor T-cell repertoire provoked by stimulation with HLA-DP-mismatched DC contains a variety of cell-lineage specificities. We found T cells that recognize all cells expressing the targeted mismatched HLA-DP allele, irre- spective of the cell lineage, T cells with the capacity to rec- ognize hematopoietic cells, including primary malignant cells without recognition of non-hematopoietic tissue cell lines, and also T cells that show restricted recognition of myeloid cells, including DC and primary acute myeloid leukemia samples, but not of cells of other hematopoietic and non-hematopoietic cell lineages. Similar cell-lineage- specific recognition patterns were found for allo-HLA- DQ-restricted T-cell clones isolated from a 9/10 HLA- matched pair, containing an additional HLA-DQ mis- match. Although the amplitude of the T-cell response is lower in the permissive HLA-DP responses compared to the non-permissive HLA-DP responses, a variety of cell- lineage-specific reactivity patterns was also found in the allo-HLA-DP-specific T-cell repertoire for the permissive HLA-DP responses.
It has been shown in patients receiving alloSCT and/or donor lymphocyte infusion from HLA-DP-mismatched donors, that the mismatched allo-HLA-DP alleles were targeted by CD4 T cells, resulting in the induction of GvHD in different organs (e.g. skin GvHD, and gut GvHD) indicating an upregulation of HLA-class II on inflamed non-hematopoietic tissues.23 However, in other patients allo-HLA-DP-reactive CD4 donor T cells induced specific GvL reactivity without coinciding GvHD, sug- gesting a hematopoietic-restricted HLA-DP response without cross-reactivity against non-hematopoietic tis- sues and/or absence of expression of HLA-class II on non- hematopoietic tissues in these cases.22,24 These clinical observations can be explained by different factors, the first being that the tissue restricted reactivity is dictated by the specificities present in the allo-HLA-DP T-cell repertoire.
The allo-HLA-DP-specific response is expected to be ini-
tiated in secondary lymphoid organs, most likely upon stimulation with patient-derived HLA-DP-expressing anti- gen-presenting cells of hematopoietic origin (DC).33,34 The allo-HLA-DP-restricted T cells activated by DC probably recognize antigens expressed by hematopoietic cells and presented in the mismatched HLA-DP molecule.14 This process could lead to an immune response skewed towards restricted recognition of hematopoietic cells, explaining the clinical observation in some patients of GvL reactivity without coinciding GvHD mediated by allo- HLA-DP-reactive CD4 donor T cells.22,24 In this study we showed that the allo-HLA-DP-restricted T-cell repertoire provoked by in vitro stimulation of donor T cells with HLA-DP-mismatched DC contains a broad spectrum of T- cell specificities. The restricted recognition of hematopoi- etic cells (e.g. DC and EBV-LCL) could indicate that in vivo T cells with comparable recognition profiles could con- tribute to a GvL effect in patients with HLA-DP-express- ing myeloid or B-cell malignancies.24,39 On the other hand, the allo-HLA-DP-specific immune response can also be initiated by DC residing in inflamed HLA-DP-expressing non-hematopoietic tissues. If the DC in inflamed tissues are cross-presenting antigens from the damaged surround- ing environment, allo-HLA-DP-restricted T cells provoked by these DC are more likely to be directed against anti- gens also expressed by non-hematopoietic cells and pre- sented in the mismatched HLA-DP molecule.35 Most like- ly, the magnitude of the allo-HLA-DP response and, there- by, the absolute number of allo-reactive T cells as well as the recognition profile of the induced T cells will deter- mine the balance between GvL and GvHD induction.
It has been shown in vivo that the magnitude of the allo- HLA-DP response is affected by the specific HLA-DP allele(s) expressed in the donor and patient.27,28 In the case of permissive HLA-DP mismatches it has been demon- strated in vivo29-32 and in vitro,40 in line with the results of our current study, that the HLA-DP response is of a lower amplitude and therefore could result in a more tolerable response than in the non-permissive HLA-DP-mis- matched combinations. However, in vitro HLA-DP-specific T-cell responses showed immunogenicity of HLA-DP alle- les in both permissive and non-permissive mismatched pairs.39,41 If the HLA-DP alleles and the peptidomes pre- sented in the HLA-DP alleles are similar between donor and patient, a large proportion of the allo-HLA-DP-specif- ic T cells is likely to be deleted during negative selection of self-reactive T cells in the thymus of the donor.42 This may explain the lower magnitude of the allo-HLA-DP-specific immune responses in permissive HLA-DP-mismatched donor/patient pairs.
Donor allo-HLA-DP-restricted CD4 T cells that target peptides expressed in non-self HLA-DP molecules in hematopoietic malignancies may specifically contribute to the GvL response after alloSCT. However, the occurrence of GvHD after HLA-DP-mismatched alloSCT and donor lymphocyte infusion remains a challenge, also in permis- sive HLA-DP-mismatched donor/patient pairs. The aim of our study was to elucidate the HLA-DP response in the donor T-cell repertoire when exposed to HLA-DP-mis- matched antigen-presenting cells, similar to the clinical setting in HLA-DP-mismatched alloSCT. New strategies resulting in the enrichment of hematopoiesis-restricted allo-HLA-DP-specific T cells from the donor repertoire for adoptive T-cell therapy may increase the likelihood of inducing a GvL effect without provoking severe GvHD.
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haematologica | 2019; 104(1)