Evaluation of TCR repertoire diversity in patients after hematopoietic stem cell transplantation

Introduction

Hematopoietic stem cell transplantation (HSCT), including allogeneic HSCT and autologous HSCT, is performed to treat a broad spectrum of illnesses. Favorable outcome for HSCT depends on either complete hematopoietic or immune reconstitution. Moreover, immune reconstitution is one of the primary factors that determines long-term prognosis following transplantation, particularly T cell immune reconstitution, which is important for disease relapse and virus infection. While the recovery of peripheral T cells occurs in transplant recipients via thymus-dependent and thymus-independent pathways, the regeneration of a population of phenotypically naive T cells with a broad T cell antigen receptor (TCR) repertoire relies entirely on the de novo generation of T cells in the thymus (1), however, early T cell reconstitution also depends on the persistence and function of T cells that are adoptively transferred with grafts (2,3). Therefore, dynamic analysis of the TCR repertoires of T cells in patients after HSCT is important for estimating the immune reconstitution in different clinical situations.

The T cell receptor and its diversity

The TCR includes the α, β, γ and δ chains, which form α/β or γ/δ heterodimeric chains that are expressed on mature T cell surfaces. TCRs are specific for antigen recognition in conjunction with major histocompatibility complex (MHC) molecules, leading to T cell activation and proliferation. Each TCR chain, similar to the immunoglobulin (Ig) heavy and light chains, is encoded by multiple gene segments. The variable domains of TCRs are assembled by the somatic recombination of one variable (V), diversity (D, only for β or δ chain), and joining (J) segment each, and they comprise three hypervariable or complementarity-determining regions (CDR1, CDR2, CDR3) within the TCR. CDR3 is involved in the response to specific interactions with antigenic peptides. For example, in the TCR β chain, nucleotide transferases add or remove nucleotides at various Vβ-Dβ and Dβ-Jβ junctions during the recombination process, and the CDR3 region of the Vβ-Jβ combination may vary in length by as many as 6-8 amino acids (4). The mechanisms of such recombinations are mediated by a recombinase that recognizes recombination signal sequences (RSSs) and brings the exons together. Overall, such a TCR rearrangement makes up 10¹⁶ to 10¹⁸ diverse TCR repertoires.

Therefore, analysis of TCR repertoires could provide a global picture of the distribution and clonal expansion of TCR subfamilies in patients and normal individuals. This type of evaluation can help to characterize the features of the host T cell immune status and identify T cell populations of interest in cancer and peripheral immune reconstitution after HSCT (5-7).

Recovery of diversity of TCR repertoire in patients after HSCT

Since the early 1990s when the TCR repertoire analysis technique of using polymerase chain reaction (PCR)-based CDR 3 size spectratyping and DNA sequencing, or PCR-Genescan, was established, TCR repertoire analysis has been used to evaluate the recovery of immune repertoires after HSCT. Until now, TCR repertoire diversity analysis has been a common method for characterizing immune reconstitution after HSCT and analyzing the oligoclonal expansion of T cells during virus infection, graft-versus-leukemia (GVL) effects, and graft-versus-host disease (GVHD). In addition, this effective technique has been used to compare quantifications of TCR repertoire recovery between different types of HSCTs, such as allogeneic or autologous HSCT, cord blood (CB) or peripheral blood (PB) HSCT, unrelated donor (UD) or related donor HSCT, haploidentical HSCT (haplo-HSCT), and T-cell-depleted (TCD) HSCT (8-14).

Significant immune renewal example data have come from comparisons of TCR repertoires in patients with severe combined immunodeficiency (SCID) before and after HSCT. At one month after transplantation, there was progressive improvement in TCR Vβ repertoire diversity in all cases (15). A study by Wu et al. used an 8-point scoring system for each Vβ subfamily to quantify TCR repertoire changes over time and showed that, in contrast with 10 normal donors who display highly diverse and polyclonal spectratypes, the mean complexity score for patient samples with chronic myelogenous leukemia (CML) before and early after TCD allogeneic bone marrow transplantation (BMT) (3 months) was significantly lower. These data revealed markedly skewed repertoires consisting of absent, monoclonal, or oligoclonal profiles for the majority of Vβ subfamilies in all patient samples. Normalization of the repertoires began in patients at 6 months after BMT, but the majority of the patients continued to display abnormal repertoires up to 3 years after BMT (16).

Data from a prospective comparison of the immune reconstitutions of pediatric recipients of positively selected CD34⁺ peripheral blood stem cells from unrelated donors (UD) vs. recipients of unmanipulated bone marrow from matched sibling donors (MSD) indicated delayed early memory-type T cell recovery with reduced TCR diversity in the UD-HSCT group, and T cell reconstitution occurred greater than 100 days after for the UD-HSCT compared with MSD-BMT group. After UD-HSCT, the TCR repertoire becomes severely skewed and demonstrates significantly reduced diversity during the first year, but only minor abnormalities are observed after MSD-BMT. TCR diversity simultaneously increases with the number of naive T cells. In both of the groups, transient expansion of γδ T cells was observed (11). During the first year after transplantation, TCR repertoires are highly abnormal. Notably, two years after transplantation onward, TCR diversity was higher in CB recipients than bone marrow transplant recipients. These data indicate an efficient thymic regeneration pathway for CB lymphoid progenitors despite a low number of cells infused compared with that for bone marrow, suggesting complete clinical immune recovery after CB HSCT (14). In pediatric recipients of T cell depleted highly purified, CD34⁺ haploidentical HSCT during the first post-transplant year, early reconstituting T cells had a predominantly primed, activated phenotype with a severely skewed TCR repertoire complexity. Naive T cells emerged 6 months post transplantation paralleled with an increase in TCR repertoire diversity (10). Overall, broad T cell repertoire diversity in recipients indicates favorable long-term immune reconstitution after HSCT. Delayed T cell recovery and restricted TCR diversity after HSCT are associated with increased risks of infection and leukemia relapse after HSCT (17). In contrast, higher TCR diversity early after transplantation possibly implies lower risk for both GVHD and relapse following HSCT transplantation (18).

Previously, TCR repertoire diversity evaluations were based on analysis of the TCR Vβ repertoire, which represents the diversity of the TCRαβ⁺ T cell repertoire and occupies greater than 90% of the T cells in blood (5,7,8,19). Recently, it has been demonstrated that TCRγδ repertoire reconstitution is an important marker post HSCT (20,21), and TCRγδ⁺ T cells are effective antiviruses and GVLs after HSCT (22,23). In addition, the samples for TCR repertoire analysis are, in order, peripheral blood mononuclear cells, CD3⁺ cells, CD4⁺ cells, CD8⁺ cells, CD45RA⁺ cells, CD45RO⁺ cells, CD45RO⁺HLA-DR⁺ cells, and Treg cells, thereby providing differences in TCR diversity in different T cell subsets and characterizing their significance and contribution to immune reconstitution after HSCT (9,11,13,24).

TCR repertoire analysis using the traditional PCR-Genescan technique may also provide stable and comparable data for the clinic. In addition, TCR repertoire diversity analysis has increased utilization; however, little is known about the minimum number of sequences necessary for accurately and efficiently describing the composition of the TCR repertoire in patients after HSCT. Data from Packer et al. has reported that by analyzing 75 to 100 in-frame sequences, they were able to estimate the TCR diversity within 5.0% to 7.4% of the values obtained in endpoint analysis (213-312 sequences per sample) in patients with multiple sclerosis (MS) who underwent autologous HSCT (12). Moreover, technical challenges have also limited the faithful measurement of TCR diversity after HSCT, and novel approaches, such as TCRβ-based oligonucleotide microarrays, have also been reported and confirmed their specificity, clonal discrimination, sensitivity of detection, and feasibility for monitoring T cell population diversity in patients with HSCT (8). Recently, novel techniques such as 5' rapid amplification of complementary DNA ends PCR combined with deep sequencing and high-throughput TCR sequencing (TCR-seq) using next generation sequencing platforms were used to quantify TCR diversity, enabling quantification of T cell diversity at unprecedented resolution (13,18,25). TCR-seq studies have provided new insight into the healthy human T cell repertoire, such as revised estimates of repertoire size and the understanding that TCR specificities are more frequently shared among individuals than previously anticipated (25). For example, the use of this technique was capable of accurately comparing the frequency of individual TCRs between patients after CB or T cell-depleted PB HSCT. After 6 months, CB-graft recipients had approximately the same TCR diversity as healthy individuals, whereas recipients of T cell-depleted PB stem cell grafts had 28- and 14-fold lower CD4⁺ and CD8⁺ T cell diversities, respectively. After 12 months, these deficiencies improved for the CD4⁺ but not the CD8⁺ T cell compartment (13). This method improves the global view of T cell repertoire recovery after allo-HSCT and may identify patients at high risk for infection or relapse.

TCR repertoire reconstitution in patients with autoimmune diseases after autologous HSCT

Autologous HSCT is commonly employed for hematologic and non-hematologic malignancies and autoimmune diseases. Clinical trials have indicated that immunoablation followed by autologous HSCT has the potential to induce clinical remission in patients with refractory systemic lupus erythematosus (SLE) (26). There are numerous reports showing that children with systemic juvenile idiopathic arthritis (sJIA) and patients with SLE, severe MS and poor-prognosis MS have been successfully controlled by autologous HSCT (24,26-29). The characteristics of the recovery of TCR diversity is similar to that for leukemia patients, and the T cell repertoire is skewed before and until one year after HSCT in MS patients with shared expansions before and after a transplant in a given individual (27,28). Significantly, TCR repertoire reconstitution could be followed up for long time after HSCT in these patients. For example, at the time of follow up (mean: 11.5 years), four patients with sJIA remained in complete remission, the CD8⁺ TCR Vβ repertoire was highly oligoclonal early during immune reconstitution, and the re-emergence of pre-transplant TCR Vβ CDR3 dominant peaks was observed after transplantation in certain TCR Vβ families. Furthermore, the re-emergence of pre-ASCT clonal sequences in addition to new sequences was identified after transplantation (29). In a phase II study of HSCT for poor-prognosis MS, high-throughput deep TCRβ chain sequencing was used to assess millions of individual TCRs per patient sample, and it was demonstrated that HSCT has distinctive effects on CD4⁺ and CD8⁺ T cell repertoires. In CD4⁺ T cells, dominant TCR clones present before treatment were undetectable following reconstitution and patients largely developed new repertoires, while dominant CD8⁺ T cell clones were not effectively removed, and the reconstituted CD8⁺ T cell repertoire was created by clonal expansion of the cells present before treatment. Importantly, patients who failed to respond to treatment had less diversity in their T cell repertoires early during the reconstitution process (24). Therefore, accurate TCR characterization in different T cell subsets may enable the monitoring of pathogenic or protective T cell clones following HSCT and cellular therapies.

Factors that impact TCR repertoire reconstitution after HSCT

It is well known that delayed T cell immune reconstitution after HSCT may increase risks for infection and leukemia relapse (17). In allo-HSCT, T cell differentiation of donor progenitors within a recipient thymus is required to generate naive recent T cell emigrants (RTEs). These cells account for durable T cell reconstitution, generate a diverse TCR repertoire and robust response to infections (30). The factors that impact delayed T cell repertoire reconstitution may be associated with the type of HSCT, the transplant regimen, the composition of grafts (e.g., T cell depletion and Treg addition), and the incidence and degree of GVHD (10,11,14,31,32). For example, adoptive transfer of Tregs with HSCT leads an accelerated diversity of TCR Vβ repertoire donor lymphoid reconstitution by preventing GVHD induced damage in the thymic and secondary lymphoid microenvironment (33). Moreover, assessment of the host immune status has become a key issue in allogeneic HSCT. In long-term follow-up of patients, pre-transplant recipient thymic function correlates with clinical outcome in terms of survival and the occurrence of severe infections due to persistent immune defects (30). Data reported by Wu et al. also indicated that reconstitution of a normal T cell repertoire from T cell progenitors in adults is influenced by interactions between recipient and donor hematopoietic cells because complete donor hematopoiesis after HSCT strongly correlates with the restoration likelihood of the T cell repertoire complexity (16).

In contrast, the persistence and function of T cells that have been transferred with grafts also greatly contribute to peripheral reconstitution. Recently, T memory stem cells (TSCMs), which have demonstrated superior reconstitution capacity in preclinical models, have been proven in vivo and at the antigen-specific and clonal level to directly differentiate from naive precursors infused within grafts and contribute to peripheral reconstitution by differentiating into effectors in the early days following haploidentical transplantation combined with posttransplant cyclophosphamide (PT-Cy) (2,3). Thus, the abundance of naive T cells (TN) or TSCMs in grafts may influence the outcome of patients after HSCT.

Improvement in TCR repertoire recovery after HSCT

The thymus is crucial for reconstituting the T cell compartment following lymphodepletion by HSCT and establishing a normal, diverse TCR repertoire after immune responses to antigens. Thus, enhancing the thymic output function is one approach for improving the recovery of the TCR repertoire after HSCT, and cytokines that have been demonstrated to improve thymic function, such as IL-7 and IL-15, may be used (34-36). In addition, both keratinocyte growth factor (KGF) and sex steroid ablation (SSA) have shown promising effects in the improvement of thymic regeneration (37-41). Moreover, it has been demonstrated that G-CSF administration leads to higher CD4⁺ TCRβ diversity in donor T cells, and this is associated with lower reactivation of cytomegalovirus and Epstein-Barr virus after HSCT (42).

In contrast, donor lymphocyte infusion (DLI), particularly adoptive T cell transfer, is a direct approach for rapidly improving T cell immune function after HSCT to overcome virus infections and disease relapse (43,44). DLIs have been demonstrated to induce clinical responses in patients with relapsed CML and those with other relapsed hematologic malignancies after allogeneic HSCT, and the response and conversion to complete donor hematopoiesis in patients after DLI has been associated with the normalization of TCR complexity (45). The outcome of patients with high-risk/advanced-stage hematologic malignancies who received TCD haplo-HSCT combined with donor T lymphocytes pretreated with IL-10 has been recently reported (ALT-TEN trial). This study has indicated that IL-10-anergized donor T cells (IL-10-DLI) contain T regulatory type 1 (Tr1) cells specific for host alloantigens, limiting donor vs. host reactivity, and memory T cells capable of responding to pathogens. After the infusion of IL-10-DLI in 12 patients after haplo-HSCT, fast immune reconstitution with a progressively normalized TCR Vβ repertoire and T-cell function was detected in patients (46).

Conclusions and future perspectives

Evidence from over 25 years of TCR repertoire characterization indicates that it is a feasible and informative immune biomarker for evaluating the global T cell immune status and monitoring pathogenic or protective T cell clones following HSCT and cellular therapies; however, little is known about whether it is befitting when patients recover completely normalized TCR repertoires after HSCT or whether it is necessary to confirm that it should contain particular antigen-specific clonally expanded T cells directed against leukemia and viruses in patients after HSCT based on the normalized TCR repertoire background. A number of studies have indicated that lower antigen-specific clonally expanded T cells e.g., WT1+ CTL, are associated with disease relapse in patients after HSCT (47-49). Therefore, evaluation of the TCR diversity may also be combined to characterize and quantitate the leukemia- and virus-specific TCR repertoires to maximize the prediction value of immune reconstitution in patients after HSCT. Moreover, recent years, resident memory CD8⁺ T cells (T_RM) were described to be an important role in immune surveillance (50-52), whether the local T_RM display specific TCR repertoire, and whether they have any contribution to evaluation for the immunity in patients after HSCT, it may be interesting for further investigation.

Acknowledgements

Funding: This study was supported by grants from the National Natural Science Foundation of China (No. 81270604), the Guangdong Natural Science Foundation (No. S2013020012863), the Foundation for High-level Talents in Higher Education of Guangdong, China (No. [2013]246-54) and the Guangzhou Science and Technology Project Foundation (No. 201510010211).

Footnote

Conflicts of Interest: The authors have no conflicts of interest to declare.

References

1. Krenger W, Holländer GA. The thymus in GVHD pathophysiology. Best Pract Res Clin Haematol 2008;21:119-28. [PubMed]
2. Cieri N, Oliveira G, Greco R, et al. Generation of human memory stem T cells after haploidentical T-replete hematopoietic stem cell transplantation. Blood 2015;125:2865-74. [PubMed]
3. Roberto A, Castagna L, Zanon V, et al. Role of naive-derived T memory stem cells in T-cell reconstitution following allogeneic transplantation. Blood 2015;125:2855-64. [PubMed]
4. Pannetier C, Even J, Kourilsky P. T-cell repertoire diversity and clonal expansions in normal and clinical samples. Immunol Today 1995;16:176-81. [PubMed]
5. Chen S, Huang X, Zheng H, et al. The evolution of malignant and reactive γδ + T cell clones in a relapse T-ALL case after allogeneic stem cell transplantation. Mol Cancer 2013;12:73. [PubMed]
6. Fozza C, Longinotti M. T-cell receptor repertoire usage in hematologic malignancies. Crit Rev Oncol Hematol 2013;86:201-11. [PubMed]
7. Li Y, Chen S, Yang L, et al. TRAV and TRBV repertoire, clonality and the proliferative history of umbilical cord blood T-cells. Transpl Immunol 2007;18:151-8. [PubMed]
8. Chen X, Hale GA, Neale GA, et al. A novel approach for the analysis of T-cell reconstitution by using a T-cell receptor beta-based oligonucleotide microarray in hematopoietic stem cell transplantation. Exp Hematol 2007;35:831-41. [PubMed]
9. Eyrich M, Croner T, Leiler C, et al. Distinct contributions of CD4(+) and CD8(+) naive and memory T-cell subsets to overall T-cell-receptor repertoire complexity following transplantation of T-cell-depleted CD34-selected hematopoietic progenitor cells from unrelated donors. Blood 2002;100:1915-8. [PubMed]
10. Eyrich M, Lang P, Lal S, et al. A prospective analysis of the pattern of immune reconstitution in a paediatric cohort following transplantation of positively selected human leucocyte antigen-disparate haematopoietic stem cells from parental donors. Br J Haematol 2001;114:422-32. [PubMed]
11. Eyrich M, Leiler C, Lang P, et al. A prospective comparison of immune reconstitution in pediatric recipients of positively selected CD34+ peripheral blood stem cells from unrelated donors vs recipients of unmanipulated bone marrow from related donors. Bone Marrow Transplant 2003;32:379-90. [PubMed]
12. Packer AN, Muraro PA. Optimized clonotypic analysis of T-cell receptor repertoire in immune reconstitution. Exp Hematol 2007;35:516-21. [PubMed]
13. van Heijst JW, Ceberio I, Lipuma LB, et al. Quantitative assessment of T cell repertoire recovery after hematopoietic stem cell transplantation. Nat Med 2013;19:372-7. [PubMed]
14. Talvensaari K, Clave E, Douay C, et al. A broad T-cell repertoire diversity and an efficient thymic function indicate a favorable long-term immune reconstitution after cord blood stem cell transplantation. Blood 2002;99:1458-64. [PubMed]
15. Okamoto H, Arii C, Shibata F, et al. Clonotypic analysis of T cell reconstitution after haematopoietic stem cell transplantation (HSCT) in patients with severe combined immunodeficiency. Clin Exp Immunol 2007;148:450-60. [PubMed]
16. Wu CJ, Chillemi A, Alyea EP, et al. Reconstitution of T-cell receptor repertoire diversity following T-cell depleted allogeneic bone marrow transplantation is related to hematopoietic chimerism. Blood 2000;95:352-9. [PubMed]
17. Toubai T, Hirate D, Shono Y, et al. Chimerism and T-cell receptor repertoire analysis after unrelated cord blood transplantation with a reduced-intensity conditioning regimen following autologous stem cell transplantation for multiple myeloma. Int J Lab Hematol 2008;30:75-81. [PubMed]
18. Yew PY, Alachkar H, Yamaguchi R, et al. Quantitative characterization of T-cell repertoire in allogeneic hematopoietic stem cell transplant recipients. Bone Marrow Transplant 2015;50:1227-34. [PubMed]
19. Brewer JL, Ericson SG. An improved methodology to detect human T cell receptor beta variable family gene expression patterns. J Immunol Methods 2005;302:54-67. [PubMed]
20. Fujishima N, Hirokawa M, Fujishima M, et al. Skewed T cell receptor repertoire of Vdelta1(+) gammadelta T lymphocytes after human allogeneic haematopoietic stem cell transplantation and the potential role for Epstein-Barr virus-infected B cells in clonal restriction. Clin Exp Immunol 2007;149:70-9. [PubMed]
21. Hirokawa M, Horiuchi T, Kawabata Y, et al. Reconstitution of gammadelta T cell repertoire diversity after human allogeneic hematopoietic cell transplantation and the role of peripheral expansion of mature T cell population in the graft. Bone Marrow Transplant 2000;26:177-85. [PubMed]
22. Godder KT, Henslee-Downey PJ, Mehta J, et al. Long term disease-free survival in acute leukemia patients recovering with increased gammadelta T cells after partially mismatched related donor bone marrow transplantation. Bone Marrow Transplant 2007;39:751-7. [PubMed]
23. Lamb LS Jr, Musk P, Ye Z, et al. Human gammadelta(+) T lymphocytes have in vitro graft vs leukemia activity in the absence of an allogeneic response. Bone Marrow Transplant 2001;27:601-6. [PubMed]
24. Muraro PA, Robins H, Malhotra S, et al. T cell repertoire following autologous stem cell transplantation for multiple sclerosis. J Clin Invest 2014;124:1168-72. [PubMed]
25. Woodsworth DJ, Castellarin M, Holt RA. Sequence analysis of T-cell repertoires in health and disease. Genome Med 2013;5:98. [PubMed]
26. Alexander T, Thiel A, Rosen O, et al. Depletion of autoreactive immunologic memory followed by autologous hematopoietic stem cell transplantation in patients with refractory SLE induces long-term remission through de novo generation of a juvenile and tolerant immune system. Blood 2009;113:214-23. [PubMed]
27. Sun W, Popat U, Hutton G, et al. Characteristics of T-cell receptor repertoire and myelin-reactive T cells reconstituted from autologous haematopoietic stem-cell grafts in multiple sclerosis. Brain 2004;127:996-1008. [PubMed]
28. Farge D, Henegar C, Carmagnat M, et al. Analysis of immune reconstitution after autologous bone marrow transplantation in systemic sclerosis. Arthritis Rheum 2005;52:1555-63. [PubMed]
29. Wu Q, Pesenacker AM, Stansfield A, et al. Immunological characteristics and T-cell receptor clonal diversity in children with systemic juvenile idiopathic arthritis undergoing T-cell-depleted autologous stem cell transplantation. Immunology 2014;142:227-36. [PubMed]
30. Toubert A, Glauzy S, Douay C, et al. Thymus and immune reconstitution after allogeneic hematopoietic stem cell transplantation in humans: never say never again. Tissue Antigens 2012;79:83-9. [PubMed]
31. Chen X, Hale GA, Barfield R, et al. Rapid immune reconstitution after a reduced-intensity conditioning regimen and a CD3-depleted haploidentical stem cell graft for paediatric refractory haematological malignancies. Br J Haematol 2006;135:524-32. [PubMed]
32. Jin Z, Wu X, Chen S, et al. Distribution and clonality of the vα and vβ T-cell receptor repertoire of regulatory T cells in leukemia patients with and without graft versus host disease. DNA Cell Biol 2014;33:182-8. [PubMed]
33. Nguyen VH, Shashidhar S, Chang DS, et al. The impact of regulatory T cells on T-cell immunity following hematopoietic cell transplantation. Blood 2008;111:945-53. [PubMed]
34. Lévy Y, Sereti I, Tambussi G, et al. Effects of recombinant human interleukin 7 on T-cell recovery and thymic output in HIV-infected patients receiving antiretroviral therapy: results of a phase I/IIa randomized, placebo-controlled, multicenter study. Clin Infect Dis 2012;55:291-300. [PubMed]
35. Cieri N, Camisa B, Cocchiarella F, et al. IL-7 and IL-15 instruct the generation of human memory stem T cells from naive precursors. Blood 2013;121:573-84. [PubMed]
36. Alpdogan O, van den Brink MR. IL-7 and IL-15: therapeutic cytokines for immunodeficiency. Trends Immunol 2005;26:56-64. [PubMed]
37. Awong G, LaMotte-Mohs R, Zúñiga-Pflücker JC. Key players for T-cell regeneration. Curr Opin Hematol 2010;17:327-32. [PubMed]
38. Goldberg GL, King CG, Nejat RA, et al. Luteinizing hormone-releasing hormone enhances T cell recovery following allogeneic bone marrow transplantation. J Immunol 2009;182:5846-54. [PubMed]
39. Seggewiss R, Loré K, Guenaga FJ, et al. Keratinocyte growth factor augments immune reconstitution after autologous hematopoietic progenitor cell transplantation in rhesus macaques. Blood 2007;110:441-9. [PubMed]
40. Bernstein ID, Boyd RL, van den Brink MR. Clinical strategies to enhance posttransplant immune reconstitution. Biol Blood Marrow Transplant 2008;14:94-9. [PubMed]
41. Chang YJ, Zhao XY, Huang XJ. Immune reconstitution after haploidentical hematopoietic stem cell transplantation. Biol Blood Marrow Transplant 2014;20:440-9. [PubMed]
42. Ritter J, Seitz V, Balzer H, et al. Donor CD4 T Cell Diversity Determines Virus Reactivation in Patients After HLA-Matched Allogeneic Stem Cell Transplantation. Am J Transplant 2015;15:2170-9. [PubMed]
43. Mattsson J, Ringdén O, Storb R. Graft failure after allogeneic hematopoietic cell transplantation. Biol Blood Marrow Transplant 2008;14:165-70.
44. Lin R, Liu Q. Diagnosis and treatment of viral diseases in recipients of allogeneic hematopoietic stem cell transplantation. J Hematol Oncol 2013;6:94. [PubMed]
45. Orsini E, Alyea EP, Chillemi A, et al. Conversion to full donor chimerism following donor lymphocyte infusion is associated with disease response in patients with multiple myeloma. Biol Blood Marrow Transplant 2000;6:375-86. [PubMed]
46. Bacchetta R, Lucarelli B, Sartirana C, et al. Immunological Outcome in Haploidentical-HSC Transplanted Patients Treated with IL-10-Anergized Donor T Cells. Front Immunol 2014;5:16. [PubMed]
47. Rezvani K, Yong AS, Savani BN, et al. Graft-versus-leukemia effects associated with detectable Wilms tumor-1 specific T lymphocytes after allogeneic stem-cell transplantation for acute lymphoblastic leukemia. Blood 2007;110:1924-32. [PubMed]
48. Kapp M, Stevanović S, Fick K, et al. CD8+ T-cell responses to tumor-associated antigens correlate with superior relapse-free survival after allo-SCT. Bone Marrow Transplant 2009;43:399-410. [PubMed]
49. Tyler EM, Jungbluth AA, O'Reilly RJ, et al. WT1-specific T-cell responses in high-risk multiple myeloma patients undergoing allogeneic T cell-depleted hematopoietic stem cell transplantation and donor lymphocyte infusions. Blood 2013;121:308-17. [PubMed]
50. Gebhardt T, Mackay LK. Local immunity by tissue-resident CD8(+) memory T cells. Front Immunol 2012;3:340. [PubMed]
51. Mackay LK, Gebhardt T. Tissue-resident memory T cells: local guards of the thymus. Eur J Immunol 2013;43:2259-62. [PubMed]
52. Mueller SN, Gebhardt T, Carbone FR, et al. Memory T cell subsets, migration patterns, and tissue residence. Annu Rev Immunol 2013;31:137-61. [PubMed]