Alloreactive T cell clonotype recruitment in a mixed lymphocyte reaction: Implications for graft engineering

Review of flow cytometry as a tool for cell and gene therapy

Quality control testing and analytics are critical for the development and manufacture of cell and gene therapies, and flow cytometry is a key quality control and analytical assay that is used extensively. However, the technical scope of characterization assays and safety assays must keep apace as the breadth of cell therapy products continues to expand beyond hematopoietic stem cell products into producing novel adoptive immune therapies and gene therapy products.  Flow cytometry services are uniquely positioned to support the evolving needs of cell therapy facilities, as access to flow cytometers, new antibody clones and improved fluorochrome reagents becomes more egalitarian. This report will outline the features, logistics, limitations and the current state of flow cytometry within the context of cellular therapy.

Qualification of a multidonor mixed lymphocyte reaction assay for the functional characterization of immunomodulatory extracellular vesicles

BACKGROUND AIMS

Extracellular vesicles (EVs), including exosomes and microvesicles, are released by almost all cells and found in all body fluids. Unknown proportions of EVs transmit specific information from their cells of origin to specific target cells and are key mediators in intercellular communication processes. Depending on their origin, EVs can modulate immune responses, either acting as pro- or anti-inflammatory. With the aim to analyze the immunomodulating activities of EV preparations, especially those from mesenchymal stromal cells (MSCs) in vitro, a multi-donor mixed lymphocyte reaction (mdMLR) assay was established and stressed for its reproducibility.

METHODS

To this end, human peripheral blood-derived mononuclear cells (PBMCs) of 12 different healthy donors were pooled warranting mutual allogeneic cross-reactivity, even following an optimized freezing and thawing procedure. After thawing, mixed PBMCs were cultured for 5 days in the absence or presence of EVs to be tested. Reflecting allogeneic reactions, in the absence of EVs, pooled PBMCs form characteristic satellite colonies whose appearance can be modulated by EVs. More quantifiable, the strength of the allogenic reaction is reflected by the content of activated CD4 and CD8 T cells being recognized by means of their CD25 and CD54 expression.

RESULTS

Of note, connected to the use of primary cells, independent multi-donor PBMC pools differed in their capability to activate their cultured T cells. Thus, throughout the study, only pooled PBMC batches were used whose activated T-cell contents exceeded 25% of the total T-cell population at culture day 5 and whose contents were reproducibly reduced in the presence of immunomodulatory active MSC-EVs. T-cell activation-suppressing effects of the MSC-EV preparations tested were in all cases accompanied by the impact on monocytes. In the presence of immunomodulatory active MSC-EVs, more monocytes were harvested from mdMLR cultures than in their absence. Furthermore, in the absence of immunomodulatory EVs, most monocytes appeared as non-classical (CD14+CD16+) monocytes, whereas immunomodulatory active MSC-EVs promoted the appearance of classical (CD14++CD16-) and intermediate (CD14++CD16+) monocyte subpopulations.

CONCLUSIONS

Overall, the obtained results qualify the mdMLR assay as a robust experimental tool for the evaluation of immunomodulatory potentials of given MSC-EV samples. However, further assay development is required to develop and qualify an authority-acceptable potency assay for clinically applicable MSC-EV products.

Alloimmune T cells in transplantation

Alloimmune T cells are central mediators of rejection and graft-versus-host disease in both solid organ and hematopoietic stem cell transplantation. Unique among immune responses in terms of its strength and diversity, the T cell alloresponse reflects extensive genetic polymorphisms between allogeneic donors and recipients, most prominently within the major histocompatibility complex (MHC), which encodes human leukocyte antigens (HLAs) in humans. The repertoire of alloreactive T cell clones is distinct for every donor-recipient pair and includes potentially thousands of unique HLA/peptide specificities. The extraordinary magnitude of the primary alloresponse and diversity of the T cell population mediating it have presented technical challenges to its study in humans. High-throughput T cell receptor sequencing approaches have opened up new possibilities for tackling many fundamental questions about this important immunologic phenomenon.

A New Window into the Human Alloresponse

Alloreactive T lymphocytes are the primary mediators of allograft rejection. The size and diversity of the HLA-alloreactive T cell repertoire has thus far precluded the ability to follow these T cells and thereby to understand their fate in human transplant recipients. This review summarizes the history, challenges, and recent advances in the study of alloreactive T cells. We highlight the historical development of assays to measure alloreactivity and discuss how high-throughput T cell receptor (TCR) sequencing-based assays can provide a new window into the fate of alloreactive T cells in human transplant recipients. A specific approach combining a classical in vitro assay, the mixed lymphocyte reaction, with deep T cell receptor sequencing is described as a tool to track the donor-reactive T cell repertoire for any specific HLA-mismatched donor-recipient pair. This assay can provide mechanistic insights and has potential as a noninvasive, highly specific biomarker for rejection and tolerance.

Specific removal of alloreactive T-cells to prevent GvHD in hemopoietic stem cell transplantation: rationale, strategies and perspectives

Hemopoietic stem cell transplantation (HSCT) is a standard procedure for treatment of malignant and non-malignant hematological diseases. HSCT donors include HLA-identical siblings, matched or mismatched unrelated donors and haploidentical related donors. Graft-versus-host disease (GvHD), mediated by donor alloreactive T-cells in the graft, can be triggered by minor histocompatibility antigens in HLA-identical pairs, by alleles at loci not considered for MUD-matching or by the mismatched haplotype in haplo-HSCT. Therefore, removal of donor T-cells, that contain the alloreactive precursors, is required, but T-cell depletion associates with opportunistic infections and with reduced graft-versus-leukemia effect. Selective T-cell depletion strategies have been introduced, like removal of αβ T-lymphocytes and of naive T-cells, two subsets including the alloreactive precursors, but the ultimate goal is specific removal of alloreactive T-cells. Here we review the different approaches to deplete alloreactive T-cells only and discuss pros and cons, specificity, efficiency and efficacy. Combinations of different methods and innovative approaches are also proposed for depleting specific alloreactive T-cells with high efficiency.

Nonhuman primate transplant models finally evolve: detailed immunogenetic analysis creates new models and strengthens the old

Nonhuman primate (NHP) models play a critical role in the translation of novel therapies for transplantation to the clinic. However, although MHC disparity significantly affects the outcome of transplantation, until recently, experiments using NHP models were performed without the ability to rigorously control the degree of MHC disparity in transplant cohorts. In this review, we discuss several key technical breakthroughs in the field, which have finally enabled detailed immunogenetic data to be incorporated into NHP transplantation studies. These advances have created a new gold-standard for NHP transplantation research, which incorporates detailed information regarding the degree of relatedness and the degree of MHC haplotype disparity between transplant pairs and the precise MHC alleles that both donors and recipients express. The adoption of this new standard promises to increase the rigor of NHP transplantation studies and to ensure that these experiments are optimally translatable to patient care.

Selectively T cell-depleted allografts from HLA-matched sibling donors followed by low-dose posttransplantation immunosuppression to improve transplantation outcome in patients with hematologic malignancies

We evaluated a photodepletion technique to selectively deplete host-reacting T cells from human leukocyte antigen (HLA)-matched sibling stem cell transplantations with the goal of reducing posttransplantation immunosuppression to improve antimalignancy effects postallografting. Donor lymphocytes were stimulated with irradiated expanded recipient T lymphocytes in an ex vivo mixed lymphocyte reaction. Alloactivated T cells preferentially retaining the photosensitizer 4,5-dibromorhodamine 123 (TH9402) were eliminated by exposure to visible light. Twenty-four patients with hematologic malignancies (16 high risk) conditioned with fludarabine, cyclophosphamide, and totalbody irradiation received a CD34-selected stem cell allograft from an HLA-matched sibling along with 5 × 10(6)/kg selectively depleted donor T cells. Low-dose cyclosporine was used for posttransplantation immunosuppression. Eleven patients survived at a median of 30 months. Probabilities (± SEM) for overall and disease-free survival are 39% ± 12% and 30% ± 12%, respectively, whereas grade III-IV acute graft-versus-host disease (aGVHD) was 13% ± 7%. Six patients relapsed, with a relapse probability of 27% ± 10%. These results suggest that selectively photodepleted allografts in matched sibling transplantations followed by low-dose immunosuppression may protect against severe aGVHD but is associated with delayed immune recovery.

Immune reconstitution in recipients of photodepleted HLA-identical sibling donor stem cell transplantations: T cell subset frequencies predict outcome

We evaluated an ex vivo photodepletion (PD) technique to selectively deplete graft-versus-host disease (GVHD) alloreacting T cells given to 24 human leukocyte antigen (HLA)-identical sibling stem cell transplantation (SCT) recipients. Donor lymphocytes were activated by 72-hour exposure to irradiated in vitro expanded recipient T lymphocytes and pulsed with a TH9402 photosensitizer. Alloactivated T cells preferentially retaining the photosensitizer were eliminated by light exposure. The PD product showed an inverted CD4(+)/CD8(+) ratio with greatest depletion occurring in the CD4(+) naive and central memory populations. In contrast, the CD8(+) naive and effector cells were relatively conserved, reflecting the differential extrusion of TH9402 by T cell subsets. Cytomegalovirus reactive T cells were reduced in the PD product and in recipient blood 100 days after SCT when compared with contemporaneous HLA-identical sibling donor T cell-depleted SCT recipients. Although PD SCT recipients experienced similar absolute lymphocyte counts during the first 100 days after SCT, they achieved 100% donor T cell chimerism more rapidly and had higher CD8(+) naive T cell counts early after SCT. SCT recipients of PD products with the lowest CD4 central memory content had the highest risk of developing chronic GVHD (cGVHD) (P = .04) and a poorer survival (P = .03). Although the persistence of CD8(+) naive T cells may have contributed to important antileukemia responses resulting in a relatively low relapse rate, our findings emphasize the role of donor memory T cells and CD4 cells in establishing immune competence post-SCT. Although PD is associated with excellent outcomes in the haploidentical setting, the low frequency of alloactivations in HLA-matched pairs makes the PD approach used by our group for allodepletion in HLA-matched sibling transplantations an inefficient technique.

Measurement of proliferative responses of cultured lymphocytes

Measurement of proliferative responses of human lymphocytes is a fundamental technique for the assessment of their biological responses to various stimuli. Most simply, this involves measurement of the number of cells present in a culture before and after the addition of a stimulating agent. This unit contains several different prototype protocols to measure the proliferative response of lymphocytes following exposure to mitogens, antigens, allogeneic or autologous cells, or soluble factors. Each of these protocols can be used in conjunction with an accompanying support protocol which contains methods for pulsing cultures with [3H]thymidine and determining incorporation of [3H]thymidine into DNA or assessing cell proliferation by nonradioactive methods, e.g., reduction of tetrazolium salts (MTT). The protocols described here provide an estimate of DNA synthesis and cell proliferation in an entire cell population, but do not provide information on the proliferation of individual cells. A protocol for CFSE labeling allows specific subpopulations of cells to be separated viably for further analysis.

In vitro methotrexate as a practical approach to selective allodepletion

Graft-versus-host disease (GVHD) is a major cause of transplant-related morbidity and mortality in recipients of allogeneic hematopoietic stem cell transplantation. As GVHD is mediated predominantly by alloreactive donor T cells, selective allodepletion from the graft may alleviate GVHD, whereas potentially maintaining other advantages conferred by donor T cells, such as graft survival, antiviral immunity, and graft-versus-leukemia effect. In this study, we evaluated the ability of methotrexate, a clinically approved antimetabolite drug, to deplete alloreactive T cells in HLA-mismatched mixed lymphocyte reactions (MLR). We observed that methotrexate could inhibit the proliferation of alloreactive T cells in primary in vitro MLR. On reexposure of methotrexate-treated cells to the same allostimulus, a significant reduction in the alloreactive immune response was observed, whereas responses to third-party allostimuli and viral antigens were preserved. Thus, our results provide preclinical evidence that in vitro methotrexate treatment results in specific allodepletion and may be used as an effective agent for preventing GVHD.

Effective graft depletion of MiHAg T-cell specificities and consequences for graft-versus-host disease

Minor histocompatibility antigen (MiHAg) differences between donor and recipient in MHC-matched allogeneic hematopoietic stem cell transplantation (allo-HSCT) often result in graft-versus-host disease (GVHD). While MiHAg-specific T-cell responses can in theory be directed against a large number of polymorphic differences between donor and recipient, in practice, T-cell responses against only a small set of MiHAgs appear to dominate the immune response, and it has been suggested that immunodominance may predict an important contribution to the development of GVHD. Here, we addressed the feasibility of graft engineering by ex vivo removal of T cells with 1 or more defined antigen specificities in a well-characterized experimental HSCT model (B6 → BALB.B). We demonstrate that immunodominant H60- and H4-specific CD8+ T-cell responses can be effectively suppressed through MHC class I tetramer–mediated purging of the naive CD8+ T cell repertoire. Importantly, the development of GVHD occurs unimpeded upon suppression of the immunodominant MiHAg-specific T-cell response. These data indicate that antigen-specific graft engineering is feasible, but that parameters other than immunodominance may be required to select T-cell specificities that are targeted for removal.