Thymic peptides in immune reconstitution and clinical outcome after allogeneic hematopoietic cell transplantation

Abstract

Patients with hematologic malignancies or nonmalignant diseases may undergo allogeneic hematopoietic cell transplantation (HCT), which represents a potential curative treatment. However, they are still at risk of life-threatening complications, such as relapse, acute graft-versus-host disease, and opportunistic infections. These complications are more likely if T-cell reconstitution is delayed during the initial 3 to 4 months after HCT. Therefore, it is of clinical importance to advance early peripheral T-cell expansion. The thymus is the cradle of T-cell production, but it is extremely sensitive to conditioning drugs used in HCT. As a result, egress of T cells from the thymus is abrogated during the first 3 to 6 months after HCT. Instead, early T-cell reconstitution depends on peripheral expansion of engrafted donor T cells. However, besides its established function to produce T cells, the thymus also produces thymic peptides with hormone-like activity. These molecules play an essential part in the development and nature of immune responses and may have a role in modulating T-cell expansion and function after HCT. In this review, we investigate the role of thymic peptides in shaping the dynamics of immune reconstitution early after HCT. Furthermore, we summarize current reports on clinical application of thymic peptides post-HCT and discuss their potential use in improving patient outcomes.

Introduction

Patients with various hematologic malignancies or nonmalignant diseases may undergo allogeneic hematopoietic cell transplantation (HCT), which represents a potential curative treatment. The success of this therapy is affected by the high risk of life-threatening complications post-HCT, such as transplantation-related mortality, (opportunistic) infections, acute graft-versus-host disease (GVHD), and malignant relapse. These complications have been highly associated with a delayed immune reconstitution (IR).¹ In particular, delayed CD4 T-cell reconstitution in the first 3 months post-HCT predicts survival of patients with GVHD, viral reactivation, or relapse (in case of acute myeloid leukemia).¹ We demonstrated that achieving >50 CD4 T cells per μL before day 100 is a major predictor outcome after allogeneic HCT. Therefore, advancing early peripheral T-cell expansion should be of high interest, as it could improve HCT success.

Although the exact mechanism remains undisclosed, T-cell reconstitution in the first months post-HCT depends on peripheral expansion (PE) of infused T cells.² Only after thymic regeneration, T-cell reconstitution is supported further by naive T-cell formation through thymopoiesis.³ Thymopoiesis generally takes months to even years to occur,⁴ and it is important for replenishing T-cell receptor (TCR) diversity.⁵ Thymopoiesis and TCR rearrangement are possible due to the unique microenvironment of the human thymus. Besides immature T cells, the thymus hosts thymic epithelial cells (TECs) and other antigen-presenting cells, such as dendritic cells (DCs), macrophages, and B cells. The organ structurally consists of the following 4 layers: capsula, cortex, corticomedullary junction, and medulla, each with a different role in central tolerance induction and the overall T-cell development process.⁶ In the cortex, thymic progenitor cells commit to the T-cell lineage, become double negative, undergo TCR rearrangement, and progress to the double-positive stage. Interaction with TECs in the cortex leads to positive selection, after which thymocytes migrate toward the medulla. In the medulla, further interaction with TECs and other antigen-presenting cells leads to negative selection. This ensures survival of matured naive CD4 or CD8 T cells, which are then ready to egress the thymus.⁶

The human thymus is not only pivotal for T-cell development; it also produces thymic peptides with hormone-like activity which are important in normal T-cell homeostasis. Previous observations also pointed out that graft T-cell survival after infusion depends on TCR binding⁷ and that the T-cell proliferation may be stimulated by cytokines and hormones, such as interleukin-7 (IL-7) or thymosins.^3,4 Thymosins or thymic peptides, such as thymosin alpha-1 (Ta1) and its propeptide prothymosin alpha, are mainly produced by TECs, and they can stimulate expansion and regulation of T cells in the blood and tissues.⁸ Thymic peptides are molecules that are highly abundant in cellular nuclei, and they have rudimental intracellular functions.⁹ Moreover, they stimulate pathways that are shared by cytokines,¹⁰ suggesting a potential synergy in their effects. Owing to such rudimental roles, especially in normal T-cell homeostasis and expansion, they might stimulate and regulate PE early post-HCT. Because of their importance in T-cell homeostasis and immune modulation, some thymic peptides have recently gained renewed clinical interest.

Despite this renewed clinical interest, there are currently no open clinical trials assessing thymic peptides in HCT. Besides, the current focus is on monitoring the thymic output, more specifically, by enumeration of recent thymic emigrants with flow cytometry or assessing the beta-signal joint TCR rearrangement excision circle ratio by quantitative polymerase chain reaction. Despite monitoring the thymic output, the overall thymic function remains to be fully addressed. Therefore, in this review, we discuss the immunoendocrine role of the thymus, more specifically thymic peptides, their biological effects, and their current applications as therapies (Table 1; Figure 1., Figure 2., Figure 3., Figure 4.). Specifically, we focus on the application of thymic peptide therapies to improve T-cell reconstitution and outcome post-HCT.

Table 1.

Tabular overview of thymic peptides, their current clinical applications, biological activity, and implications for use in the HCT setting

Thymic peptide	Clinical applications		Biological activity		Implications for use in HCT
Ta1	Treatment of viral infectious diseases such as: •chronic hepatitis B11, 12, 13 and C •AIDS14 •sepsis •COVID-19	14, 15, 16, 17	Promotes expansion of: •NK •Th1 •DC •cytotoxic T cells	8,18,19	Increases levels of: •IFN-γ •IL-2 •IL-10 •IL-12 with no difference in T-cell numbers in 8 patients with HCT who developed infection after transplantation	20
	Adjuvant to influenza vaccines in: •immunocompromised patients •older patients21,22	21,22	Enhances expression of proinflammatory: •IL-2 •IFN-α Stimulates: •macrophage phagocytic abilities •T-cell–dependent antibody production	8,18,23	Improves: •polymorphonuclear •DC functions Accelerates: •the rate at which pathogen-specific CD4+ T cells appear Lowers: •nonrelapse mortality in 30 adult recipients of human leukocyte antigen-matched sibling T cell-depleted HCT, conditioned with TBI or chemotherapy	24
	Treatment of: •primary immunodeficiencies	25	Enhances: •stem cell expansion and differentiation in immunosuppressed mice	8	Improves: •survival Lowers: •relapse rate Increases: •effector T cell counts (Th1 and CD3+CD4–CD8+ T cells) in patients with acute myeloid leukemia, when combined with epigenetic regulators decitabine and chidamide	26
	Treatment of: •psoriatic arthritis	27	Enhances: •the recognition of virally infected or tumor cells by directly upregulating the expression of MHC classes 1 and 2 and beta-2 microglobulin	18	Ta1 was rendered to be safe in clinical use: •no adverse events or episodes of GVHD were registered	24 26
	Treatment of: •metastatic melanoma •lung cancer •breast cancer •hepatocellular carcinoma	28	Induces: •immune tolerance through stimulation of Tregs and IL-10 production	18,19
	Treatment of: •lymphodepleted SARS-CoV-2 •HIV-infected patients	14,29
TMD	Treatment of: •atopic dermatitis •anergy •bronchial asthma •acute hepatitis B •chronic liver disease •perennial allergic rhinitis •HIV •RRIs •patients with rheumatoid arthritis and •ageing	30	Affects: •maturation and function of T cells	30, 31, 32	No clinical trials in HCT, but: •it is generally regarded as clinically safe •it has a longer half-life in comparison to most cytokines	33
	Demonstrated positive effects in: •acute hepatitis B •anergy •atopic dermatitis •bronchial asthma •chronic liver disease •perennial allergic rhinitis and •ageing	30,34,35	Stimulates: •T cells in vitro Increases release of: •TNF and •granulocyte-macrophage CSF in macrophage-lymphocyte cultures	30,34,36	Increases: •leukocyte levels and •uptake of 59Fe isotope by erythrocytes Enhances: •the DNA synthesis in the thymus and bone marrow cells resulting in an increased survival rate in cyclophosphamide-treated and TBI mice	30
			Stimulates production of: •bone marrow CSF by T cells and/or monocytes Indirectly affects bone marrow colony growth	30,33,34,36	Restores levels of: •thymic hormone activity in thymectomized mice which could be relevant for the conditioning-induced hypothymism in patients with HCT	30 31
			Enhances: •NK activity of human cord blood lymphocytes Increases: •macrophage human leukocyte antigen-DR expression and •mitogen-induced T-cell proliferation in peripheral blood lymphocytes	34,36	Reduces incidence of: •low leukocyte counts in patients undergoing radiotherapy	34
			Induces appearance of: •surface B markers •myelopoiesis in human bone marrow granulocyte/macrophage precursors	30	Stimulates: •myelopoiesis in irradiated mice and patients with cancer treated with chemotherapy	33
			Increases: •IFN-γ production in spleen cells of aged mice	32
			Reverses: •T-cell unresponsiveness in Mycobacterium-infected mice when combined with IL-2	32
TM	Not tested in clinical setting, but: •it has been beneficial in multiple animal disease models	37,38	Has: •anti-inflammatory properties Can directly or indirectly: •modulate immune responses	37, 38, 39, 40, 41	No clinical trials in HCT
	Reduces: •physiological impairments •thermal hyperalgesia and paw edema in diabetic and CFA mice	38,39	Potential role in signaling between the: •immune system •endocrine system •nervous system	42, 43, 44	In athymic mice, neonatal TM gene therapy: •restores the serum TM levels •increases corticotrope cell density, volume density, and cell surface	45,46
			Inhibits proinflammatory cytokines: •TNF-a •IL-5 •IL-17 and •IFN-y Suppresses: •p38 •NF-κB and •JNK signaling pathways	37, 38, 39	Potentially preventing several: •endocrine alterations •metabolic alterations	40,41
			Poses as: •a potential central physiological mediator of thymus-pituitary gland communication	45,46
TMPO	Cancer therapy target and reliable biomarker for different cancer types, such as breast cancer	47,48	Efficient immune modulator: •in a number of pathologies, where it simultaneously activates different T-cell subtypes	49,50	No clinical trials in HCT, but: •was tested safe in animal models and human	49
	Treatment of: •atopic dermatitis •DiGeorge syndrome and •primary T-cell defect–diagnosed patients	51	Regulates: •cell cycle Has an effect on: •neuromuscular transmission	49,52,53	Increases: •T-cell counts and •T-cell variety in immunodeficient patients	51
			Mediates: •immune responses Enhances: •proliferative response of peripheral T cells from murine lymph nodes and spleen	54
THF	Treatment of: •immunodeficiencies •viral infections (such as chronic hepatitis B) and •cancer	55, 56, 57	Modulates: •clonal expansion •differentiation and •maturation of T cells in vitro	58, 59, 60	No clinical trials in HCT
	Potential use in: •sclerosing panencephalitis patients and •systemic lupus patients	61,62	Positively affects: •T-cell lectin response •antibody response to SRBC and •overall cytotoxic activity	58	Increases the size of: •CD4 and •CD8 cells that were initially decreased	63
			Reduces: •IL-6 levels Increases: •IL-10 levels Has protective effects on: •liver damage and •fibrosis in autoimmune rat models	64	Improves: •cellular immunocompetence Normalizes the: •Th and T suppressor/cytotoxic cells by modulating the T-cell differentiation in patients with AIDS	58,65
			Increases: •cytomegalovirus-neutralizing antibodies and Improves: •NK activity in early infection stages in mice	63	Recovers defective cell-mediated immunity in: •different types of neoplasms or •secondary immune deficiencies caused by chemotherapy and radiotherapy	58
			Upregulates: •thymocytes •peripheral blood cell counts Statistically reduces: •metastases •local tumor growth	66	May: •potentiate antitumor drug activity in chemotherapeutic regimens •repair conditioning-induced damage to T cells in patients with immunogenic and nonimmunogenic tumors	67
			Reconstitutes reduced erythrocyte rosette-forming cells in some: •patients with sclerosing panencephalitis •patients with systemic lupus	61,62

CFA, complete Freund’s adjuvant; MHC, major histocompatibility complex; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2; Th1, T helper 1.

Figure 1.

Visual overview of Ta1 therapy implementation in the HCT setting, including patient cohorts, dosage, and outcomes of the trials.

Figure 2.

Visual overview of TMD therapy implementation in the clinical trials and mechanistic studies, including patient cohorts or study models, dosage, and outcomes of the trials.

Figure 3.

Visual overview of TMPO therapy implementation in the clinical trials and mechanistic studies, including patient cohorts or study models, dosage, and outcomes of the trials.

Figure 4.

Visual overview of THF therapy implementation in the clinical trials and mechanistic studies, including patient cohorts or study models, dosage, and outcomes of the trials.

Thymic peptides and their clinical applications

Ta1

One of the thymic peptides that are gaining quite an interest in the context of HCT is Ta1. Ta1 is derived from prothymosin alpha and consists of only 28 amino acids with an acetylated N-terminal.⁸ Originally, it was identified in the TEC in the subcortical capsule,⁶⁸ but more sensitive radio-immunoassays also localized Ta1 in the thymic medulla.⁶⁹ In addition, Ta1 was measured in the serum of healthy individuals within the range of 0.1 to 1.0 ng/mL.^68,70 According to some reports, Ta1 levels are somewhat lower in women, aged patients,⁶⁸ and patients with dysregulated immune responses.^27,68 Furthermore, COVID-19 research reported Ta1 levels to be correlated with longevity of humoral responses.²³ Therefore, physiological Ta1 levels might be a key to normal functioning of the immune system.²⁷

Indeed, Ta1 was reported to have a broad range of biological activities ranging from immune-enhancing and immune-restorative to immune-attenuative effects.^8,18,19 Through interactions with Toll-like receptors 9 and 2, Ta1 promotes expansion of natural killer (NK), T helper 1, DCs, and cytotoxic T cells. It enhances expression of proinflammatory IL-2 and interferon alfa (IFN-α) and stimulates macrophage phagocytic abilities and T-cell–dependent antibody production.^8,18,23 Furthermore, Ta1 enhances stem cell expansion and differentiation in immunosuppressed mice.⁸ Ta1 also enhances the recognition of virally infected or tumor cells by directly upregulating the expression of major histocompatibility complex classes 1 and 2 and beta-2 microglobulin.¹⁸ However, Ta1 dampens the immune response and induces immune tolerance, averting a proinflammatory cytokine storm by positively affecting regulatory T cells (T_regs) and IL-10 production through activation of indoleamine-2,3-dioxygenase in plasmacytoid DCs.^18,19 Because of these various immunomodulating properties, Ta1 is under extensive evaluation for therapeutic use in immune-related diseases ranging from autoimmunity to cancer.

The synthetic form of Ta1, called thymalfasin, has already been evaluated in a variety of clinical conditions, such as infectious diseases, immunodeficiencies, aging, and cancer.⁸ In the last 3 decades, thymalfasin demonstrated promising potential in the treatment of viral infectious diseases, such as chronic hepatitis B and C, AIDS, sepsis, and more recently COVID-19.14, 15, 16, 17 When used as an adjuvant to influenza vaccines in immunocompromised and older patients, it successfully enhanced T-cell–mediated antibody responses.^21,22 In patients with primary immunodeficiencies, thymalfasin displayed an IL-2–mediated immune-restorative effect.²⁵ Similarly, encouraging results are expected in the treatment of psoriatic arthritis.²⁷ Recently, thymalfasin administration increased T-cell numbers and thymic output (increased TCR excision circles) in lymphodepleted severe acute respiratory syndrome coronavirus 2 and HIV-infected patients.^14,29 Furthermore, thymalfasin was effective in metastatic melanoma, lung cancer, breast cancer, and hepatocellular carcinoma treatments.²⁸ The latest comprehensive review, including >30 trials and >11 000 patients, reports that thymalfasin is a well-tolerated and effective immune modulator.⁷¹

Some time ago, thymalfasin was investigated as a potential therapeutic agent for quicker IR after HCT. In 8 patients with HCT who developed an infection post-HCT, subcutaneous administration of 3.2 mg/wk Ta1 for 4 weeks resulted in an increase of IFN-γ, IL-2, IL-10, and IL-12 levels, with no difference in T-cell numbers in comparison to the control group.²⁰ Another clinical trial with 30 adult recipients of human leukocyte antigen-matched sibling T-cell–depleted HCT, conditioned with total body irradiation (TBI) or chemotherapy, concluded that subcutaneous administration of 1.6 mg/d Ta1 for 16 weeks from the day of the transplant improved polymorphonuclear and DC functions, accelerated the rate at which pathogen-specific CD4⁺ T cells appeared, and lowered nonrelapse mortality.²⁴ In patients with acute myeloid leukemia, thymalfasin in combination with epigenetic regulators decitabine and chidamide resulted in higher survival, lower relapse rate, an increase in effector T-cell counts (T helper 1 and CD3⁺CD4⁻CD8⁺ T cells), and no apparent GVHD.²⁶ Overall, there were no adverse events or episodes of GVHD registered. Ta1 was rendered to be safe in clinical use²⁴ and may have a favorable effect on IR (Figure 1).

Possibly, Ta1 could be implemented in the HCT treatment as a prevention tactic for life-threatening complications, such as GVHD, viral reactivations, and relapse. Owing to such potential, Ta1 deserves to be investigated into more detail. The field could benefit from more Ta1 clinical trials and interventions but also in vitro studies that would confirm its exact mechanism. In addition, by measuring endogenous Ta1 levels post-HCT, we could assess recovery of thymic function of each patient, which could further allow us in-time reaction and improve overall HCT success rate.

TMD

Partial acid lysis of the calf thymus produces thymic derivate called thymomodulin (TMD).³⁰ TMD, therefore, consists of multiple thymic peptides, which have a molecular weight range of 1 to 10 kDa. This thymic peptide derivate has a great biological activity with apparent effects on T, B, NK, and bone marrow stem cells.³⁰

Similarly to other thymic factors, it affects maturation and function of T cells.30, 31, 32 In vitro TMD treatments stimulated T cells and increased the release of tumor necrosis factor (TNF) and granulocyte-macrophage colony-stimulating factor (CSF) in macrophage-lymphocyte cultures.^30,34,36 TMD treatment also positively affected production of bone marrow CSF by T cells and/or monocytes and indirectly affected bone marrow colony growth.^30,33,34,36 Besides stimulating CSF production, TMD enhanced the NK activity of human cord blood lymphocytes and increased macrophage human leukocyte antigen-DR isotype (HLA-DR) expression and mitogen-induced T-cell proliferation in peripheral blood lymphocytes.^34,36 Furthermore, TMD induced appearance of surface B markers and myelopoiesis in human bone marrow granulocyte/macrophage precursors.³⁰ In another in vitro study with spleen cells of aged mice, TMD significantly increased IFN-γ production.³² In addition, in Mycobacterium-infected mice, treatment with TMD in combination with IL-2 reversed T-cell unresponsiveness.³²

TMD was also successfully tested in clinical setting, in patients with atopic dermatitis, chronic bronchitis, viral hepatitis, HIV, recurrent respiratory infections (RRIs), and rheumatoid arthritis. Cazzola et al³⁰ reveal that oral 3-month TMD treatment (2-3 mg/kg body weight per day) normalized patients’ low or high CD4/CD8 ratio. Similarly, in HIV-infected patients, oral TMD treatment (60 mg/d, 50 days) increased their CD4 T-cell levels and improved their symptoms.⁷² TMD treatment also reduced RRI symptoms, improved neutrophil functions, and increased CD3 and CD4 T-cell counts and IgA levels in saliva.^30,35,73 In addition, TMD (administration of 120-180 mg/d, for 2-3 months) prevented RRI relapse by enhancing phagocytic response of alveolar macrophages and serum Ig-s levels.³⁰ Overall, TMD demonstrated positive effects in acute hepatitis B, anergy, atopic dermatitis, bronchial asthma, chronic liver disease, perennial allergic rhinitis, and ageing.^30,34,35 Such potential might be due to TMD’s direct modulating impact on T and B function, but also stimulative effect on other phagocytic cells, such as macrophages.³⁴

Despite such clinical potential, up to this date, there are no published data on TMD clinical trials in the context of HCT. Nevertheless, other studies pointed toward TMD properties that could be advantageous in HCT. For instance, few studies found that TMD has protective benefits for radiation and chemotherapy. In mice, TMD increased leukocyte levels and exhibited radioprotective effect in cyclophosphamide-treated and TBI mice, respectively.³⁰ TMD treatment increased the uptake of ⁵⁹Fe isotope by erythrocytes and enhanced the DNA synthesis in the thymus and bone marrow cells, which further leads to increase in survival rate of chemotherapy and TBI mouse models.³⁰ In addition, TMD (5 mg per mouse) restored levels of thymic hormone activity in thymectomized mice,³⁰ which could be relevant for the conditioning-induced hypothymism in patients with HCT.³¹ In a study with patients undergoing radiotherapy, TMD treatment (20 mg/d) statistically reduced incidence of low leukocyte counts.³⁴ In irradiated mice and patients with cancer treated with chemotherapy, TMD stimulated myelopoiesis.³³ Overall, TMD boosts the host’s immune cells important for proper function, adequate defense, and general survival of the organism. It is generally regarded as clinically safe, due to its low toxicity exhibited in multiple studies³³ (Figure 2). TMD also has a longer serum half-life in comparison to most cytokines³³ which is yet another reason why it could be regarded as a great alternative for treatment of immunosuppressed patients undergoing HCT treatment. However, as most of the promising data are from 1980s, it is essential to confirm the findings and further evaluate mechanism of action and efficacy of TMD in prospective studies.

TM

Another thymic peptide exclusively produced by the thymus is thymulin (TM). TM activity was measured only in a few patient cohorts, and it was correlated with age and serum concentration of thyroid hormones T₃ and T₄.⁷⁴ Lower levels of circulating TM were found in malnourished or undernourished, nutrient-deficient, HIV-, and Trypanosoma cruzi-infected patients and murine models.75, 76, 77 Furthermore, patients with (severe) combined immunodeficiency had low to undetectable TM levels before HCT.⁷⁸ Lower TM serum levels were also measured in zinc-deficient patients.⁷⁹ Zinc is relevant in many thymic reactions, including regeneration of TEC and T-cell reconstitution,⁸⁰ and it is an essential component of TM peptide. TM has anti-inflammatory properties and can directly or indirectly modulate immune responses.37, 38, 39, 40, 41 It inhibits proinflammatory cytokines TNF-a, IL-5, IL-17, and IFN-y, suppresses p38,³⁸ and inhibits the NF-κB and JNK signaling pathways.^37,39

TM has been beneficial in multiple animal disease models.^37,39,38 In diabetic and complete Freund’s adjuvant mice, TM reduced physiological impairments and thermal hyperalgesia and paw edema, respectively.^39,38 In athymic mice, neonatal TM gene therapy restored the serum TM levels and increased corticotrope cell density, volume density, and cell surface.^45,46 TM gene therapy was proposed for prevention of some endocrine and metabolic alterations,^40,41 which can also be found in thymus-deficient in vivo models.^40,41 Overall, TM might play a role in signaling between the immune, endocrine, and nervous systems,42, 43, 44 and it could be a central physiological mediator of thymus-pituitary gland communication.^45,46 Owing to its anti-inflammatory properties and successful implementation in athymic mice, TM gene therapy could have a potential in HCT treatment. Furthermore, Incefy et al⁷⁸ monitored TM levels in immunodeficient patients after HCT and observed an increase at the time of lymphoid chimerism after engraftment, but before the restoration of immune function. This study demonstrates a potential of TM in immunomonitoring after HCT; however, more follow-up studies are needed to address the mechanism behind this increase.

TMPO

The 49 amino acid-long thymic peptide thymopoietin (TMPO) can be found in the plasma ranging from 0.25 to 1 ng/mL.⁸¹ TMPO (-α) genes regulate cell cycle⁵² and have an effect on neuromuscular transmission.⁴⁹ TMPO might also mediate immune responses and have an impact on peripheral T cells.^82,83 According to multiple reports, TMPO is upregulated in several types of cancer, including lung, breast, colorectal, gastric, ovarian, esophageal, Wilms, cervical, bladder, prostate, and thymus cancers, retinoblastoma, hepatocellular carcinoma, and osteosarcoma.^47,48 However, its antisense RNA competes with endogenous RNA and prevents it from inhibition of downstream oncogenes expression.⁴⁷ Therefore, TMPO could be a cancer therapy target and reliable biomarker for different cancer types, such as breast cancer.^47,48

In murine spleen or bone marrow cells, TMPO selectively induced maturation of T thymocytes and was able to restore the balance regardless of initially enhanced or suppressed immune responsiveness.⁴⁹ In glioblastoma cells, lack of TMPO significantly inhibits cell proliferation, arrests cell cycle at G2/M phase, and promotes apoptosis.⁵³ TMPO administration (1.5 ng/mL) enhanced proliferative response in peripheral T cells from murine lymph nodes and spleen.⁵⁴

Its synthetic form, thymopentin (TP-5), was tested safe to use (absent toxicity) both in animal models (10 mg/kg for 4 weeks in rats and dogs) and human (1 mg/kg for 1 year, 100 patients).⁴⁹ It was an efficient immune modulator in a number of pathologies,⁴⁹ where it simultaneously activated different T-cell subtypes.⁵⁰ In patients with atopic dermatitis, TP-5 (3 times a week, 50 mg, 6 weeks) increase usually reduced cytotoxic T-cell subsets and improved cytotoxic T-cell function.⁸⁴ In addition, TP-5 demonstrated promising results in DiGeorge syndrome and primary T-cell defect-diagnosed patients, suggesting it could be used for treatment of immunodeficient patients.⁵¹ This increase of T-cell counts and T-cell variety in immunodeficient patients could be considered promising for the HCT setting (Figure 3). However, more studies focused on mechanism of action and evaluating TMPO and TP-5 in clinical setting are needed.

THF

Thymic humoral factor (THF) is an immunomodulatory octapeptide which endogenous levels in human serum have to date not been published. THF demonstrated potential in treatment of immunodeficiencies, viral infections such as chronic hepatitis B, and cancer.55, 56, 57 In vitro, THF affected clonal expansion, differentiation, and maturation of T cells observed on THF treatment.58, 59, 60

THF had a positive effect on T-cell lectin response, antibody response to sheep red blood cells (SRBC), and overall cytotoxic activity.⁵⁸ In autoimmune rat models, THF reduced IL-6 and increased IL-10 levels in the serum, but it also had beneficial effect on liver damage and fibrosis.⁶⁴ In a murine model, THF increased cytomegalovirus-neutralizing antibodies and improved NK activity in early infection stages.⁶³ In tumor mouse models, daily subcutaneous applications of 200 ng of THF per mouse for a week upregulated thymocytes and peripheral blood cell counts and statistically reduced metastases and local tumor growth.⁶⁶ Furthermore, THF was able to reconstitute reduced erythrocyte rosette-forming cells in some patients with sclerosing panencephalitis, which is why THF was suggested as an addition to immunotherapy in some individuals.⁶¹ Similarly, in patients with systemic lupus, THF treatment significantly increased erythrocyte rosette-forming cells of the peripheral blood lymphocytes.⁶²

THF has not been tested in the HCT setting, but other clinical studies revealed beneficial properties that could be exploited for patients with HCT. For instance, THF treatment increased the size of CD4 and CD8 cells that were initially decreased due to the infection.⁶³ It was proposed that this positive restoration of CD4 and CD8 cells might be partially due to the restoration of IL-2 levels.^58,63,65 Besides increased IL-2 production, THF treatment in patients with AIDS improved cellular immunocompetence⁵⁸ and normalized the Th and T suppressor/cytotoxic cells by modulating the T-cell differentiation.^58,85 In patients with different types of neoplasms or secondary immune deficiencies caused by chemotherapy and radiotherapy, THF treatment was able to recover defective cell-mediated immunity.⁵⁸ Adding THF to anticancer chemotherapeutic regimens could potentiate the antitumor drug activity, and possibly even repair conditioning-induced damage to T cells in patients with immunogenic and nonimmunogenic tumors.⁶⁷ In the same study, THF positively affected overall survival of immunogenic tumor-bearing mice and directly stimulated proliferation of myeloid stem cells.⁶⁷ To sum up, THF demonstrated HCT-advantageous properties (Figure 4), such as a potential to restore CD4 and CD8 cell levels, improved cellular immunocompetence and postconditioning immunity recovery, which would make it a great candidate to test in the clinical HCT setting.

Future perspectives and concluding remarks

Life-threatening complications post-HCT (relapse, GVHD, opportunistic infections) demand urgency in early T-cell reconstitution. As de novo T-cell production and diverse TCR repertoire maintenance depend on the thymus, it should be of great interest to further explore its role in IR. To possibly optimize and expand treatment and intervention options in the HCT setting, we evaluated current knowledge on thymic peptides with hormone-like activity.

Overall, the described thymic peptides (Table 1; Figure 1., Figure 2., Figure 3., Figure 4.) have been linked to PE of T cells and have been used in multiple clinical trials to drive T-cell production, maturation, and activation.⁸⁶ So far, the only peptide that has been evaluated in the HCT setting is Ta1. However, other thymic peptides were rendered as safe for administration in various other patient cohorts,55, 56, 57 and their properties could be beneficial to patients with HCT. Namely, these thymic peptides restored T-cell counts,^{24,38,42,81,82} modulated function,^64,86 potentiated antitumor activity,^66,67 and enhanced overall thymic output.^14,29 Besides, they increased cytomegalovirus-neutralizing antibodies,⁶³ stimulated myeloid stem cell proliferation,⁶⁷ and mediated thymus-pituitary gland communication.^45,46

Not only could thymic peptides expedite early T-cell reconstitution and improve overall IR, they also have a potential to suppress GVHD and relapse. Namely, they can enhance T_reg expansion and promote graft-versus-tumor effect and graft-versus-leukemia effect. Although T_reg expansion can be enhanced by THF or Ta1 through immunostimulatory IL-2 and IL-10,^87,88 T_reg suppressive function can be improved by TMD by increased release of TNF.⁸⁹ Increased release of TNF can also promote graft-versus-tumor effect,⁸⁹ that is mediated by NK cells,^90,91 which activity could be enhanced by Ta1, TMD, TM, and THF. However, inhibition of TNF-α with TM could be used in the treatment of steroid-refractory GVHD.^89,92,93 Besides, graft-versus-leukemia effect can be promoted by IFN-γ,^94,95 which is increased by Ta1 and TMD. Furthermore, thymic peptides could be beneficial for bone marrow recovery and overall survival after ionizing irradiation. TMD can stimulate granulocyte-macrophage CSF, which was besides granulocyte CSF found to decrease the duration of neutropenia and shorten hospitalization in patients with AML in the HCT setting.96, 97, 98 Overall, thymic peptides have many beneficial properties and are important for T-cell homeostasis and immune modulation. Therefore, this immune-modulating potential of thymic peptides should be investigated during IR after HCT.

In general, IR mechanisms post-HCT have not been thoroughly investigated and sufficiently described. Similarly, there are still many unknowns about thymic function and regeneration after thymic injury that is caused by conditioning regimens, prophylaxis, and other therapies applied in HCT treatment. Investigating the association between endogenous thymic peptides and T-cell recovery and function, including outcome, could provide further rationale for clinical application of these molecules in HCT. In addition, the endogenous levels of thymic peptides could be a quantitative representation of thymic damage and could even elucidate the mechanistic differences between pediatric and adult IR.⁷⁸ This knowledge not only enables the identification of patients who might benefit from thymic hormonal interventions, but in combination with other thymic output measures, such as recent thymic emigrants or TCR excision circles, could allow us to understand how preserving thymic function might affect hormone levels.

We hypothesize that thymic peptides can positively affect both developing and peripheral T cells. They might have a great impact on restoring the thymic microenvironment that is essential for T-cell development.⁷⁸ In addition, they modulate T-cell differentiation and proliferation and affect overall homeostasis. Most thymic peptide therapies (Ta1, TMD, TMPO, and THF) have an established safety profile and have been tested specifically in immunocompromised patients. This merits testing them in patients with poor IR post-HCT (not reaching CD >50 at 100 days) before also implementing them as prophylactic compounds from the day of transplant, in a clinical trial with all types of HCT recipients, if proven safe. By including all HCT recipients, one could explore how to successfully adopt thymic peptide therapy and determine which patient group would benefit the most from it. Thymic peptide therapy recipients should be closely monitored for the incidence of viral reactivations and conditioning-related side effects. Furthermore, it is of great importance to monitor patients’ thymic output and investigate the effect that thymic peptides have on reconstituting cells to establish patterns determining successful early T-cell recovery and possibly recognize novel intervention opportunities. Novel insights from these trials could promote more mechanistic studies and prospective trials that focus on the preservation of thymic function in specific patients.

Conflict-of-interest disclosure: The authors declare no competing financial interests.