Re-entry of mature T cells to the thymus: an epiphenomenon?

The unilateral involution in the thymus of a 96-year-old male leads to the preservation of structural integrity in one thymic lobe, as assessed by the expression of medullar and cortical antigens and the presence of CD3+ cells

The process of thymic involution begins soon after birth and continues through adult life. Although evolutionary conserved in all vertebrates, the thymic involution has no defined kinetics. Little is known about the pace of its regression in humans, except that there is a marked increase of thymic involution after puberty. This report describes the unusual structural findings in the thymus of a 96-year-old male. The morphological parameters of the organ were evaluated using H&E and immunohistochemistry (IHC) techniques. The macroscopic examination showed a typical organ's weight and size, except that the right thymic lobe presented a well-preserved organ and the left lobe was significantly adiposed. The H&E staining of the thymic sections from the left and right lobes confirmed advanced thymic adiposity in the left lobe and preserved thymic epithelial space containing hematoxylin-stained cells in the right lobe. The multiplex immunostaining of the right lobe sections with antibodies specific to cytokeratins -14 and -8, CD3, and CD4 revealed the presence of medullar and cortical epithelium and mix population of CD3+/CD4+ and CD3+/CD4- T cells. The T cells were associated with the medulla but not with the cortex of the thymus. The immunostaining with an antibody to FoxN1 showed that the protein was expressed in the thymic epithelium. Taken together, we provide evidence that the thymus of a 96-year-old man involuted different kinetics in each of the two thymic lobes. Furthermore, the presence of CD3+/CD4+ and CD3+/CD4-cells gives a hand to the hypothesis that a pool of T-cells may associate with this primary lymphatic organ for as long as there is the available thymic epithelium and be a source of lymphocytes aiding adaptive immune responses to old age.

The Thymus in Chagas Disease: Molecular Interactions Involved in Abnormal T-Cell Migration and Differentiation

Chagas disease, caused by the protozoan parasite T. cruzi, is a prevalent parasitic disease in Latin America. Presently, it is spreading around the world by human migration, thus representing a new global health issue. Chronically infected individuals reveal a dissimilar disease progression: while nearly 60% remain without apparent disease for life, 30% develop life-threatening pathologies, such as chronic chagasic cardiomyopathy (CCC) or megaviscerae. Inflammation driven by parasite persistence seems to be involved in the pathophysiology of the disease. However, there is also evidence of the occurrence of autoimmune events, mainly caused by molecular mimicry and bystander activation. In experimental models of disease, is well-established that T. cruzi infects the thymus and causes locally profound structural and functional alterations. The hallmark is a massive loss of CD4⁺CD8⁺ double positive (DP) thymocytes, mainly triggered by increased levels of glucocorticoids, although other mechanisms seem to act simultaneously. Thymic epithelial cells (TEC) exhibited an increase in extracellular matrix deposition, which are related to thymocyte migratory alterations. Moreover, medullary TEC showed a decreased expression of AIRE and altered expression of microRNAs, which might be linked to a disrupted negative selection of the T-cell repertoire. Also, almost all stages of thymocyte development are altered, including an abnormal output of CD4⁻CD8⁻ double negative (DN) and DP immature and mature cells, many of them carrying prohibited TCR-Vβ segments. Evidence has shown that DN and DP cells with an activated phenotype can be tracked in the blood of humans with chronic Chagas disease and also in the secondary lymphoid organs and heart of infected mice, raising new questions about the relevance of these populations in the pathogenesis of Chagas disease and their possible link with thymic alterations and an immunoendocrine imbalance. Here, we discuss diverse molecular mechanisms underlying thymic abnormalities occurring during T. cruzi infection and their link with CCC, which may contribute to the design of innovative strategies to control Chagas disease pathology.

Th1 Biased Progressive Autoimmunity in Aged Aire-Deficient Mice Accelerated Thymic Epithelial Cell Senescence

Although autoimmune diseases, such as rheumatoid arthritis and systemic lupus erythematosus, are frequently associated with premature aging of the thymus, a direct link is missing between autoimmunity and thymic atrophy. Here we monitored the progression of thymic involution in Aire-deficient mice, in which defective negative selection causes spontaneous and progressive development of autoimmunity. In young and middle-aged mice, Aire deficiency appeared to be protective as supported by the reduced β-gal⁺ epithelial cells and the enhanced thymic output. However, once the autoimmune phenotype was fully developed in aged Aire-deficient mice, their thymuses underwent accelerated involution. In comparison to the age-matched wildtype littermates, old Aire-deficient mice showed lower numbers of total thymocytes and recent thymic emigrants but more β-gal⁺ thymic epithelial cells. This phenomenon may partly be attributable to the increased number of activated Th1 cells homing to the thymus. This speculation was further supported by the enhanced thymic aging following repeated challenges with complete Freund’s adjuvant immunization. Taken together, the present study highlights a unique mechanism by which autoimmunity facilitates the senescence of thymic epithelial cells through returning Th1 cells.

Thymic expression of IL-4 and IL-15 after systemic inflammatory or infectious Th1 disease processes induce the acquisition of "innate" characteristics during CD8+ T cell development

Innate CD8⁺ T cells express a memory-like phenotype and demonstrate a strong cytotoxic capacity that is critical during the early phase of the host response to certain bacterial and viral infections. These cells arise in the thymus and depend on IL-4 and IL-15 for their development. Even though innate CD8⁺ T cells exist in the thymus of WT mice in low numbers, they are highly enriched in KO mice that lack certain kinases, leading to an increase in IL-4 production by thymic NKT cells. Our work describes that in C57BL/6 WT mice undergoing a Th1 biased infectious disease, the thymus experiences an enrichment of single positive CD8 (SP8) thymocytes that share all the established phenotypical and functional characteristics of innate CD8⁺ T cells. Moreover, through in vivo experiments, we demonstrate a significant increase in survival and a lower parasitemia in mice adoptively transferred with SP8 thymocytes from OT I—T. cruzi-infected mice, demonstrating that innate CD8⁺ thymocytes are able to protect against a lethal T. cruzi infection in an Ag-independent manner. Interestingly, we obtained similar results when using thymocytes from systemic IL-12 + IL-18-treated mice. This data indicates that cytokines triggered during the acute stage of a Th1 infectious process induce thymic production of IL-4 along with IL-15 expression resulting in an adequate niche for development of innate CD8⁺ T cells as early as the double positive (DP) stage. Our data demonstrate that the thymus can sense systemic inflammatory situations and alter its conventional CD8 developmental pathway when a rapid innate immune response is required to control different types of pathogens.

Murine innate CD8⁺ T cells demonstrate strong cytotoxic capacity during the early phase of certain bacterial and viral infections. Such cells have been reported to be present in both mice and humans but many questions remain as to their differentiation and maturation process. Innate CD8⁺ T cells arise in the thymus and depend on IL-4 and IL-15 for their development. A description of the cellular and molecular mechanisms involved during their thymic development has been obtained from KO mice that lack kinases and transcription factors important for TCR signaling. In these mice, SP8 thymocytes with an innate phenotype are highly enriched over the conventional SP8 cells. Our work describes, for the first time, that in WT mice, thymic IL-4 and IL-15 expression triggered by Th1 infectious processes induce an adequate niche for development of innate rather than conventional CD8⁺ T cells. Our data show that the thymus is able to sense a systemic inflammatory response (probably mediated by systemic IL-12 and IL-18 production) and alter its ontogeny when pathogen control is needed.

Role of Hormonal Circuitry Upon T Cell Development in Chagas Disease: Possible Implications on T Cell Dysfunctions

T cell response plays an essential role in the host resistance to infection by the protozoan parasite Trypanosoma cruzi, the causative agent of Chagas disease. This infection is often associated with multiple manifestations of T cell dysfunction, both during the acute and the chronic phases of disease. Additionally, the normal development of T cells is affected. As seen in animal models of Chagas disease, there is a strong thymic atrophy due to massive death of CD4⁺CD8⁺ double-positive cells by apoptosis and an abnormal escape of immature and potentially autoreactive thymocytes from the organ. Furthermore, an increase in the release of corticosterone triggered by T. cruzi-driven systemic inflammation is strongly associated with the alterations seen in the thymus of infected animals. Moreover, changes in the levels of other hormones, including growth hormone, prolactin, and testosterone are also able to contribute to the disruption of thymic homeostasis secondary to T. cruzi infection. In this review, we discuss the role of hormonal circuits involved in the normal T cell development and trafficking, as well as their role on the thymic alterations likely related to the peripheral T cell disturbances largely reported in both chagasic patients and animal models of Chagas disease.

Specialized proteasome subunits have an essential role in the thymic selection of CD8(+) T cells

The cells that stimulate positive selection express specialized proteasome β-subunits different from those expressed by all other cells, including those involved in negative selection. Mice that lack all four specialized proteasome β-subunits, and therefore express only constitutive proteasomes in all cells, had a profound defect in the generation of CD8(+) T cells. While a defect in positive selection would reflect an inability to generate the appropriate positively selecting peptides, a block at negative selection would point to the potential need to switch peptides between positive selection and negative selection to avoid the two processes' often cancelling each other out. We found that the block in T cell development occurred around the checkpoints of positive selection and, unexpectedly, negative selection as well.

Trypanosoma cruzi Experimental Infection Impacts on the Thymic Regulatory T Cell Compartment

The dynamics of regulatory T cells in the course of Trypanosoma cruzi infection is still debated. We previously demonstrated that acute murine T. cruzi infection results in an impaired peripheral CD4+Foxp3+ T cell differentiation due to the acquisition of an abnormal Th1-like phenotype and altered functional features, negatively impacting on the course of infection. Moreover, T. cruzi infection induces an intense thymic atrophy. As known, the thymus is the primary lymphoid organ in which thymic-derived regulatory T cells, known as tTregs, differentiate. Considering the lack of available data about the effect of T. cruzi infection upon tTregs, we examined tTreg dynamics during the course of disease. We confirmed that T. cruzi infection induces a marked loss of tTreg cell number associated to cell precursor exhaustion, partially avoided by glucocorticoid ablation- and IL-2 survival factor depletion. At the same time, tTregs accumulate within the CD4 single-positive compartment, exhibiting an increased Ki-67/Annexin V ratio compared to controls. Moreover, tTregs enhance after the infection the expression of signature markers (CD25, CD62L and GITR) and they also display alterations in the expression of migration-associated molecules (α chains of VLAs and chemokine receptors) such as functional fibronectin-driven migratory disturbance. Taken together, we provide data demonstrating profound alterations in tTreg compartment during acute murine T. cruzi infection, denoting that their homeostasis is significantly affected. The evident loss of tTreg cell number may compromise the composition of tTreg peripheral pool, and such sustained alteration over time may be partially related to the immune dysregulation observed in the chronic phase of the disease.

Dynamics of HIV infection in lymphoid tissue network

Human immunodeficiency virus (HIV) is a fast replicating ribonucleic acid virus, which can easily mutate in order to escape the effects of drug administration. Hence, understanding the basic mechanisms underlying HIV persistence in the body is essential in the development of new therapies that could eradicate HIV infection. Lymphoid tissues are the primary sites of HIV infection. Despite the recent progress in real-time monitoring technology, HIV infection dynamics in a whole body is unknown. Mathematical modeling and simulations provide speculations on global behavior of HIV infection in the lymphatic system. We propose a new mathematical model that describes the spread of HIV infection throughout the lymphoid tissue network. In order to represent the volume difference between lymphoid tissues, we propose the proportionality of several kinetic parameters to the lymphoid tissues' volume distribution. Under this assumption, we perform extensive numerical computations in order to simulate the spread of HIV infection in the lymphoid tissue network. Numerical computations simulate single drug treatments of an HIV infection. One of the important biological speculations derived from this study is a drug saturation effect generated by lymphoid network connection. This implies that a portion of reservoir lymphoid tissues to which drug is not sufficiently delivered would inhibit HIV eradication despite of extensive drug injection.

Bcl11b prevents the intrathymic development of innate CD8 T cells in a cell intrinsic manner

If Bcl11b activity is compromised, CD4(+)CD8(+) double-positive (DP) thymocytes produce a greatly increased fraction of innate CD8(+) single-positive (SP) cells highly producing IFN-γ, which are also increased in mice deficient of genes such as Itk, Id3 and NF-κB1 that affect TCR signaling. Of interest, the increase in the former two is due to the bystander effect of IL-4 that is secreted by promyelocytic leukemia zinc finger-expressing NKT and γδT cells whereas the increase in the latter is cell intrinsic. Bcl11b zinc-finger proteins play key roles in T cell development and T cell-mediated immune response likely through TCR signaling. We examined thymocytes at and after the DP stage in Bcl11b (F/S826G) CD4cre, Bcl11b (F/+) CD4cre and Bcl11b (+/S826G) mice, carrying the allele that substituted serine for glycine at the position of 826. Here we show that Bcl11b impairment leads to an increase in the population of TCRαβ(high)CD44(high)CD122(high) innate CD8SP thymocytes, together with two different developmental abnormalities: impaired positive and negative selection accompanying a reduction in the number of CD8SP cells, and developmental arrest of NKT cells at multiple steps. The innate CD8SP thymocytes express Eomes and secrete IFN-γ after stimulation with PMA and ionomycin, and in this case their increase is not due to a bystander effect of IL-4 but cell intrinsic. Those results indicate that Bcl11b regulates development of different thymocyte subsets at multiple stages and prevents an excess of innate CD8SP thymocytes.

T cells home to the thymus and control infection

The thymus is a target of multiple pathogens. How the immune system responds to thymic infection is largely unknown. Despite being considered an immune-privileged organ, we detect a mycobacteria-specific T cell response in the thymus following dissemination of Mycobacterium avium or Mycobacterium tuberculosis. This response includes proinflammatory cytokine production by mycobacteria-specific CD4(+) and CD8(+) T cells, which stimulates infected cells and controls bacterial growth in the thymus. Importantly, the responding T cells are mature peripheral T cells that recirculate back to the thymus. The recruitment of these cells is associated with an increased expression of Th1 chemokines and an enrichment of CXCR3(+) mycobacteria-specific T cells in the thymus. Finally, we demonstrate it is the mature T cells that home to the thymus that most efficiently control mycobacterial infection. Although the presence of mature T cells in the thymus has been recognized for some time, to our knowledge, these data are the first to show that T cell recirculation from the periphery to the thymus is a mechanism that allows the immune system to respond to thymic infection. Maintaining a functional thymic environment is essential to maintain T cell differentiation and prevent the emergence of central tolerance to the invading pathogens.

Immature CD4+CD8+ thymocytes are preferentially infected by measles virus in human thymic organ cultures

Cells of the human immune system are important target cells for measles virus (MeV) infection and infection of these cells may contribute to the immunologic abnormalities and immune suppression that characterize measles. The thymus is the site for production of naïve T lymphocytes and is infected during measles. To determine which populations of thymocytes are susceptible to MeV infection and whether strains of MeV differ in their ability to infect thymocytes, we used ex vivo human thymus organ cultures to assess the relative susceptibility of different subpopulations of thymocytes to infection with wild type and vaccine strains of MeV. Thymocytes were susceptible to MeV infection with the most replication in immature CD4⁺CD8⁺ double positive cells. Susceptibility correlated with the level of expression of the MeV receptor CD150. Wild type strains of MeV infected thymocytes more efficiently than the Edmonston vaccine strain. Thymus cultures from children ≥3 years of age were less susceptible to MeV infection than cultures from children 5 to 15 months of age. Resistance in one 7 year-old child was associated with production of interferon-gamma suggesting that vaccination may result in MeV-specific memory T cells in the thymus. We conclude that immature thymocytes are susceptible to MeV infection and thymocyte infection may contribute to the immunologic abnormalities associated with measles.

Incomplete TCR-β allelic exclusion accelerates spontaneous autoimmune arthritis in K/BxN TCR transgenic mice

Allelic exclusion of antigen receptor loci is a fundamental mechanism of immunological self-tolerance. Incomplete allelic exclusion leads to dual T-cell receptor (TCR) expression and can allow developing autoreactive αβ T lymphocytes to escape clonal deletion. Because allelic exclusion at the TCR-β locus is more stringent than at the TCR-α locus, dual TCR-β expression has not been considered a likely contributor to autoimmunity. We show here that incomplete TCR-β allelic exclusion permits developing thymocytes bearing the autoreactive, transgene-encoded KRN TCR to be positively selected more efficiently, thereby accelerating the onset of spontaneous autoimmune arthritis. Our findings highlight dual TCR-β expression as a mechanism that can enhance the maturation of autoreactive pathogenic T cells and lead to more rapid development of autoimmune disease.

Peripheral T cells re-enter the thymus and interfere with central tolerance induction

The thymus mainly contains developing thymocytes that undergo thymic selection. In addition, some mature activated peripheral T cells can re-enter the thymus. We demonstrated in this study that adoptively transferred syngeneic Ag-specific T cells can enter the thymus of lymphopenic mice, where they delete thymic dendritic cells and medullary thymic epithelial cells in an Ag-specific fashion, without altering general thymic functions. This induced sustained thymic release of autoreactive self-Ag-specific T cells suggested that adoptively transferred activated T cells can specifically alter the endogenous T cell repertoire by erasing negative selection of their own specificities. Especially in clinical settings in which adoptively transferred T cells cause graft-versus-host disease or graft-versus-leukemia, as well as in adoptive tumor therapies, these findings might be of importance, because the endogenous T cell repertoire might be skewed to contribute to both manifestations.

Cytokine crosstalk for thymic medulla formation

The medullary microenvironment of the thymus plays a crucial role in the establishment of self-tolerance through the deletion of self-reactive thymocytes and the generation of regulatory T cells. Crosstalk or bidirectional signal exchanges between developing thymocytes and medullary thymic epithelial cells (mTECs) contribute to the formation of the thymic medulla. Recent studies have identified the molecules that mediate thymic crosstalk. Tumor necrosis factor superfamily cytokines, including RANKL, CD40L, and lymphotoxin, produced by positively selected thymocytes and lymphoid tissue inducer cells promote the proliferation and differentiation of mTECs. In return, CCR7 ligand chemokines produced by mTECs facilitate the migration of positively selected thymocytes to the medulla. The cytokine crosstalk between developing thymocytes and mTECs nurtures the formation of the thymic medulla and thereby regulates the establishment of self-tolerance.

Key players for T-cell regeneration

PURPOSE OF REVIEW

The thymus provides a unique and essential microenvironment for T-cell precursors to develop into mature functionally competent T lymphocytes. Ageing causes architectural changes in the thymus resulting in a loss of thymic epithelial space required for thymopoiesis - a process known as thymic involution. Additionally, cytoablative regimens used to treat malignancies also destroy thymic architecture. The net result of both processes is diminished thymic output and function that may lead to impaired immunity. Thus, immunocompromised individuals would benefit from strategies aimed at enhancing T-cell reconstitution.

RECENT FINDINGS

Here we discuss strategies such as the use of sex steroid ablation, keratinocyte growth factor, interleukin-7, and in-vitro-generated progenitor T cells as candidates for restoring T-cell immunity. Using various animal models of ageing or hematopoietic stem cell transplantation, these strategies have been shown to restore thymic architecture and cellularity, resulting in increased output and T-cell function in the periphery.

SUMMARY

These candidate approaches are currently being tested in clinical trials, with preliminary evidence showing encouraging effects on T-cell reconstitution. Nevertheless, although these strategies show clear promise in animal models, and in early human trials, further data are needed to determine their efficacy in patients.

PSGL-1 function in immunity and steady state homeostasis

The substantial importance of P-selectin glycoprotein ligand 1 (PSGL-1) in leukocyte trafficking has continued to emerge beyond its initial identification as a selectin ligand. PSGL-1 seemed to be a relatively simple molecule with an extracellular mucin domain extended as a flexible rod, teleologically consistent with its primary role in tethering leukocytes to endothelial selectins. The rolling interaction between leukocyte and endothelium mediated by this selectin-PSGL-1 interaction requires branched O-glycan extensions on specific PSGL-1 amino acid residues. In some cells, such as neutrophils, the glycosyltransferases involved in formation of the O-glycans are constitutively expressed, while in other cells, such as T cells, they are expressed only after appropriate activation. Thus, PSGL-1 supports leukocyte recruitment in both innate and adaptive arms of the immune response. A complex array of amino acids within the selectins engage multiple sugar residues of the branched O-glycans on PSGL-1 and provide the molecular interactions responsible for the velcro-like catch bonds that support leukocyte rolling. Such binding of PSGL-1 can also induce signaling events that influence cell phenotype and function. Scrutiny of PSGL-1 has revealed a better understanding of how it performs as a selectin ligand and yielded unexpected insights that extend its scope from supporting leukocyte rolling in inflammatory settings to homeostasis including stem cell homing to the thymus and mature T-cell homing to secondary lymphoid organs. PSGL-1 has been found to bind homeostatic chemokines CCL19 and CCL21 and to support the chemotactic response to these chemokines. Surprisingly, the O-glycan modifications of PSGL-1 that support rolling mediated by selectins in inflammatory conditions interfere with PSGL-1 binding to homeostatic chemokines and thereby limit responsiveness to the chemotactic cues used in steady state T-cell traffic. The multi-level influence of PSGL-1 on cell traffic in both inflammatory and steady state settings is therefore substantially determined by the orchestrated addition of O-glycans. However, central as specific O-glycosylation is to PSGL-1 function, in vivo regulation of PSGL-1 glycosylation in T cells remains poorly understood. It is our purpose herein to review what is known, and not known, of PSGL-1 glycosylation and to update understanding of PSGL-1 functional scope.

Vaccination with vif-deleted feline immunodeficiency virus provirus, GM-CSF, and TNF-alpha plasmids preserves global CD4 T lymphocyte function after challenge with FIV

Feline immunodeficiency virus (FIV) DNA vaccine approaches that included a vif-deleted FIV provirus (FIV-pPPRDeltavif) and feline cytokine expression plasmids were tested for immunogenicity and efficacy by immunization of specific pathogen free cats. Vaccine protocols included FIV-pPPRDeltavif plasmid alone; a combination of FIV-pPPRDeltavif DNA and feline granulocyte macrophage-colony stimulating factor (GM-CSF) and tumor necrosis factor (TNF)-alpha expression plasmids; or a combination of FIV-pPPRDeltavif and feline interleukin (IL)-15 plasmids. Cats immunized with FIV-pPPRDeltavif, GM-CSF and TNF-alpha plasmids demonstrated an increased frequency of FIV-specific T cell proliferation responses compared to other vaccine groups. Immunization with FIV-pPPRDeltavif and IL-15 plasmids was distinguished from other vaccine protocols by the induction of antiviral antibodies. Suppression of virus loads was not observed for any of the FIV-pPPRDeltavif DNA vaccine protocols after challenge with the FIV-PPR isolate. However, prior immunization with FIV-pPPRDeltavif, GM-CSF, and TNF-alpha plasmids resulted in preservation of CD4 T cell functions, including mitogen-induced cytokine expression and antigen-specific proliferation upon infection with FIV. These findings justify further examination of cytokine combinations as adjuvants for lentiviral DNA vaccines.

T-lymphocyte interaction with stromal, bone and hematopoietic cells in the bone marrow

Mature T cells in the bone marrow (BM) are in constant exchange with the blood pool. Within the BM, T-cell recognition of antigen presented by dendritic cell (DC) can occur, nevertheless it is thought that BM T cells mostly receive non-antigenic signals by either stimulatory, for example, interleukin (IL)-7, IL-15, tumor necrosis factor family members, or inhibitory molecules, for example, transforming growth factor-beta. The net balance is in favor of T-cell proliferation. Indeed, the percentage of proliferating T cells is higher in the BM than in spleen and lymph nodes, both within CD4 and CD8 T cells. High numbers of memory T cells proliferate in the BM, as they preferentially home to the BM and have an increased turnover as compared with naive T cells. I propose here that the BM plays an essential role in maintaining normal peripheral T-lymphocyte numbers and antigen-specific memory for both CD4 and CD8 T cells. I also discuss BM T-cell contribution to the homeostasis of bone metabolism as well as of hematopoiesis. It emerges that BM T cells play unexpected roles in several diseases, for example AIDS and osteoporosis. A better knowledge on BM T cells has implications for currently used clinical interventions, for example, vaccination, BM transplantation, mesenchymal stem cell-based therapies.