A population dynamics analysis of the interaction between adaptive regulatory T cells and antigen presenting cells

Background

Regulatory T cells are central actors in the maintenance of tolerance of self-antigens or allergens and in the regulation of the intensity of the immune response during infections by pathogens. An understanding of the network of the interaction between regulatory T cells, antigen presenting cells and effector T cells is starting to emerge. Dynamical systems analysis can help to understand the dynamical properties of an interaction network and can shed light on the different tasks that can be accomplished by a network.

Methodology and Principal Findings

We used a mathematical model to describe a interaction network of adaptive regulatory T cells, in which mature precursor T cells may differentiate into either adaptive regulatory T cells or effector T cells, depending on the activation state of the cell by which the antigen was presented. Using an equilibrium analysis of the mathematical model we show that, for some parameters, the network has two stable equilibrium states: one in which effector T cells are strongly regulated by regulatory T cells and another in which effector T cells are not regulated because the regulatory T cell population is vanishingly small. We then simulate different types of perturbations, such as the introduction of an antigen into a virgin system, and look at the state into which the system falls. We find that whether or not the interaction network switches from the regulated (tolerant) state to the unregulated state depends on the strength of the antigenic stimulus and the state from which the network has been perturbed.

Conclusion/Significance

Our findings suggest that the interaction network studied in this paper plays an essential part in generating and maintaining tolerance against allergens and self-antigens.     

Regulatory T cells and immune tolerance

Regulatory T cells (Tregs) play an indispensable role in maintaining immunological unresponsiveness to self-antigens and in suppressing excessive immune responses deleterious to the host. Tregs are produced in the thymus as a functionally mature subpopulation of T cells and can also be induced from naive T cells in the periphery. Recent research reveals the cellular and molecular basis of Treg development and function and implicates dysregulation of Tregs in immunological disease.     

IL-9 production by regulatory T cells recruits mast cells that are essential for regulatory T cell-induced immune suppression

Both mast cells (MCs) and regulatory T cells (Tregs) have gained attention as immunosuppressive cell populations. To investigate a possible interaction, we used the Th1- and Th17-dependent model of nephrotoxic serum nephritis (NTS), in which both MCs and Tregs have been shown to play a protective role. Transfer of wild-type (wt) Tregs into wt recipients almost completely prevents development of NTS and leads to a profound increase of MCs in the renal draining lymph nodes (LNs). By contrast, transfer of wt Tregs into animals deficient in MCs, which are characterized by an exaggerated susceptibility to NTS, no longer exhibited protective effects. Blocking the pleiotropic cytokine IL-9, known to be involved in MC recruitment and proliferation, by means of a mAb in mice receiving Tregs abrogated protection from NTS. Moreover, transfer of IL-9-deficient Tregs also failed to protect from NTS. In the absence of Treg-derived IL-9, MCs fail to accumulate in the LNs, despite the fact that IL-9 deficiency does not alter the general suppressive activity of Tregs. In summary, to our knowledge, we provide the first direct in vivo evidence that the nephroprotective, anti-inflammatory effects of Tregs critically depend on IL-9-mediated attraction of MCs into kidney-draining LNs.     

A computational study of co-inhibitory immune complex assembly at the interface between T cells and antigen presenting cells

The activation and differentiation of T-cells are mainly directly by their co-regulatory receptors. T lymphocyte-associated protein-4 (CTLA-4) and programed cell death-1 (PD-1) are two of the most important co-regulatory receptors. Binding of PD-1 and CTLA-4 with their corresponding ligands programed cell death-ligand 1 (PD-L1) and B7 on the antigen presenting cells (APC) activates two central co-inhibitory signaling pathways to suppress T cell functions. Interestingly, recent experiments have identified a new cis-interaction between PD-L1 and B7, suggesting that a crosstalk exists between two co-inhibitory receptors and the two pairs of ligand-receptor complexes can undergo dynamic oligomerization. Inspired by these experimental evidences, we developed a coarse-grained model to characterize the assembling of an immune complex consisting of CLTA-4, B7, PD-L1 and PD-1. These four proteins and their interactions form a small network motif. The temporal dynamics and spatial pattern formation of this network was simulated by a diffusion-reaction algorithm. Our simulation method incorporates the membrane confinement of cell surface proteins and geometric arrangement of different binding interfaces between these proteins. A wide range of binding constants was tested for the interactions involved in the network. Interestingly, we show that the CTLA-4/B7 ligand-receptor complexes can first form linear oligomers, while these oligomers further align together into two-dimensional clusters. Similar phenomenon has also been observed in other systems of cell surface proteins. Our test results further indicate that both co-inhibitory signaling pathways activated by B7 and PD-L1 can be down-regulated by the new cis-interaction between these two ligands, consistent with previous experimental evidences. Finally, the simulations also suggest that the dynamic and the spatial properties of the immune complex assembly are highly determined by the energetics of molecular interactions in the network. Our study, therefore, brings new insights to the co-regulatory mechanisms of T cell activation.     

Kinetics and activation requirements of contact-dependent immune suppression by human regulatory T cells

Naturally occurring regulatory T cells (Tregs) maintain self tolerance by dominant suppression of potentially self-reactive T cells in peripheral tissues. However, the activation requirements, the temporal aspects of the suppressive activity, and mode of action of human Tregs are subjects of controversy. In this study, we show that Tregs display significant variability in the suppressive activity ex vivo as 54% of healthy blood donors examined had fully suppressive Tregs spontaneously, whereas in the remaining donors, anti-CD3/CD2/CD28 stimulation was required for Treg suppressive activity. Furthermore, anti-CD3/CD2/CD28 stimulation for 6 h and subsequent fixation in paraformaldehyde rendered the Tregs fully suppressive in all donors. The fixation-resistant suppressive activity of Tregs operated in a contact-dependent manner that was not dependent on APCs, but could be fully obliterated by trypsin treatment, indicating that a cell surface protein is directly involved. By add-back of active, fixed Tregs at different time points after activation of responding T cells, the responder cells were susceptible to Treg-mediated immune suppression up to 24 h after stimulation. This defines a time window in which effector T cells are susceptible to Treg-mediated immune suppression. Lastly, we examined the effect of a set of signaling inhibitors that perturb effector T cell activation and found that none of the examined inhibitors affected Treg activation, indicating pathway redundancy or that Treg activation proceeds by signaling mechanisms distinct from those of effector T cells.     

Advantage of having regulatory T cells requires localized suppression of immune reactions

The immune system of vertebrates may attack its own body and cause autoimmunity diseases. To prevent autoimmunity, regulatory T cells suppress the activity of the autoreactive effector T cells, but they also interrupt normal immune reactions against foreign antigens. In this paper, we discuss the advantage of having some regulatory T cells by considering the host's ability of coping with foreign antigens and the harm of autoimmunity. Assumptions are as follows: the immature T cells reactive to abundant self-antigens are eliminated, those reactive to rare self-antigen will become regulatory T cells, and those that fail to interact with the antigens to which they are reactive will become effector T cells. Some self-reactive immature T cells may fail to interact with their own target antigens during the limited training period, and will later become effector T cells, causing autoimmunity. Analysis suggests that, having some regulatory T cells can never be advantageous to the host, if activated regulatory T cells suppress effector T cells at any location of the body (global suppression). In contrast, producing some regulatory T cells can be beneficial, if the body is composed of many compartments and regulatory T cells suppress the immune reactions only within the same compartment (localized suppression). This requires regulatory T cells to stop circulating once they are activated by their own target self-antigens.     

Regulatory T cells dynamically control the primary immune response to foreign antigen

The population dynamics that enable a small number of regulatory T (T(R)) cells to control the immune responses to foreign Ags by the much larger conventional T cell subset were investigated. During the primary immune response, the expansion and contraction of conventional and T(R) cells occurred in synchrony. Importantly, the relative accumulation of T(R) cells at peak response significantly exceeded that of conventional T cells, reflecting extensive cell division within the T(R) cell pool. Transfer of a polyclonal T(R) cell population before immunization antagonized both polyclonal and TCR transgenic responses, whereas blocking T(R) cell function enhanced those responses. These results define an inverse quantitative relationship between T(R) and conventional T cells that controls the magnitude of the primary immune response. The high frequency of dividing T(R) cells suggests degenerate TCR specificity enabling activation by a broad spectrum of Ags.     

The role of antigen in the activation of regulatory T cells by immune B cells

The transfer of B cells from mice immunized with Type III pneumococcal polysaccharide (SSS-III) results in the activation of suppressor and amplifier T cells that control the magnitude of the antibody response in recipient mice, immunized subsequently with SSS-III. Prior treatment of transferred B cells with an excess of enzyme (polysaccharide depolymerase) capable of hydrolyzing SSS-III, does not alter the capacity of these cells to activate regulatory T cells. These findings indicate that the activation of regulatory T cells by immune B cells is not mediated by residual antigen on the surface of transferred cells.     

Natural regulatory T cells: mechanisms of suppression

Natural FOXP3+CD25+CD4+ regulatory T cells (Tregs) actively suppress pathological and physiological immune responses, contributing to the maintenance of immunological self-tolerance and immune homeostasis. Various molecular and cellular events have been described to explain the mechanism(s) of Treg-mediated suppression. However, none of the proposed mechanisms can explain all aspects of suppression. It is probable that various combinations of several mechanisms are operating, depending on the milieu and the type of immune responses, although there might be a single key mechanism that has a predominant role. Further studies of suppression and search for Treg-specific cell surface molecules are required for potential clinical application to treat and prevent immunological diseases and to control immune responses for the benefit of the host.     

Immunomodulation of antigen presenting cells promotes natural regulatory T cells that prevent autoimmune diabetes in NOD mice

Progression towards type 1 diabetes (T1D) in susceptible patients is linked to a progressive decline in the capacity of regulatory T cells (Treg) to maintain tolerance. As such, therapies aimed at redressing the failing Treg compartment have been the subject of intense investigation. Treg dysfunction in T1D has recently been linked to a reduced capacity of antigen presenting cells (APCs) to maintain Treg function rather than Treg intrinsic defects. This suggests that therapies aimed simply at addressing the failing Treg compartment are unlikely to provide long-term protection. Here, we demonstrate that modulation of the inflammatory status of CD11b+CD11c- APCs favors the upregulation of protective Tregs in a mouse model of T1D. We further demonstrate that reduced expression of the costimulatory molecule CD40 plays a role in this increased immunoregulatory capacity. Strikingly, Treg upregulation resulted exclusively from an increase in natural Tregs rather than the peripheral conversion of conventional T cells. This suggests that modulation of CD11b+ CD11c- APCs inflammatory properties favors the establishment of natural Treg responses that, unlike adaptive Treg responses, are likely to maintain tolerance to a broad range of antigens. As such, modulation of this APC subset represents a potential therapeutic avenue to reestablish peripheral tolerance and protect from autoimmune diseases such as T1D.     

Regulatory T cells and mechanisms of immune system control

The immune system evolved to protect the host against the attack of foreign, potentially pathogenic, microorganisms. It does so by recognizing antigens expressed by those microorganisms and mounting an immune response against all cells expressing them, with the ultimate aim of their elimination. Various mechanisms have been reported to control and regulate the immune system to prevent or minimize reactivity to self-antigens or an overexuberant response to a pathogen, both of which can result in damage to the host. Deletion of autoreactive cells during T- and B-cell development allows the immune system to be tolerant of most self-antigens. Peripheral tolerance to self was suggested several years ago to result from the induction of anergy in peripheral self-reactive lymphocytes. More recently, however, it has become clear that avoidance of damage to the host is also achieved by active suppression mediated by regulatory T (T(reg)) cell populations. We discuss here the varied mechanisms used by T(reg) cells to suppress the immune system.     

Differential suppression of tumor-specific CD8+ T cells by regulatory T cells

In the CT26 BALB/c murine model of colorectal carcinoma, depletion of regulatory T cells (Tregs) prior to tumor inoculation results in protective immunity to both CT26 and other BALB/c-derived tumors of diverse histological origin. In this paper, we show that cross-protection can be conferred by adoptively transferred CD8(+) CTLs. Other schedules for inducing immunity to CT26 have been described, but they do not lead to cross-protection. We show that Treg ablation facilitates the development of new CTL specificities that are normally cryptic, and have mapped the root epitope of one of these responses. This work has allowed us to demonstrate how the specificity of CTL responses to tumor Ags can be controlled via differential suppression of CTL specificities by Tregs, and how this can result in very different physiological outcomes.     

Interaction between antigen presenting cells and autoreactive T cells derived from BXSB mice with murine lupus

Systemic lupus erythematosus （SLE） is a typical autoimmune disease involving multiple systems and organs. Ample evidence suggests that autoreactive T cells play a pivotal role in the development of this autoimmune disorder. This study was undertaken to investigate the mechanisms of interaction between antigen presenting cells （APCs） and an autoreactive T cell （ATLI） clone obtained from lupus-prone BXSB mice. ATLI cells, either before or after 7-ray irradiation, were able to activate naive B cells, as determined by B cell proliferation assays. Macrophages from BXSB mice were able to stimulate the proliferation of resting ATL 1 cells at a responder/stimulator （R/S） ratio of 1/2.5. Dendritic cells （DCs） were much more powerful stimulators for ATLI cells on a per cell basis. The T cell stimulating ability ofmacrophages and B cells, but not DCs, was sensitive to T-ray irradiation. Monoclonal antibodies against mouse MHC-Ⅱ and CD4 were able to block DC-mediated stimulation of ATL 1 proliferation, indicating cognate recognition between ATL 1 and APCs. Our data suggest that positive feedback loops involving macrophages, B cells and autoreactive T cells may play a pivotal role in keeping the momentum of autoimmune responses leading to autoimmune diseases.     

Regulatory T cells induced by B cells: a novel subpopulation of regulatory T cells

Regulatory T cells play a crucial role in the homeostasis of the immune response. In addition to CD4⁺Foxp3⁺ regulatory T cells, several subsets of Foxp3^- regulatory T cells, such as T helper 3 (Th3) cells and type 1 regulatory T (Tr1) cells, have been described in mice and human. Accumulating evidence shows that naïve B cells contribute to tolerance and are able to promote regulatory T cell differentiation. Naïve B cells can convert CD4⁺CD25^- T cells into CD25⁺Foxp3^- regulatory T cells, named Treg-of-B cells by our group. Treg-of-B cells express LAG3, ICOS, GITR, OX40, PD1, and CTLA4 and secrete IL-10. Intriguingly, B-T cell-cell contact but not IL-10 is essential for Treg-of-B cells induction. Moreover, Treg-of-B cells possess both IL-10-dependent and IL-10-independent inhibitory functions. Treg-of-B cells exert suppressive activities in antigen-specific and non-antigen-specific manners in vitro and in vivo. Here, we review the phenotype and function of Foxp3⁺ regulatory T cells, Th3 cells, Tr1 cells, and Treg-of-B cells.     

Regulatory T cells inhibit protein kinase C theta recruitment to the immune synapse of naive T cells with the same antigen specificity

The precise mechanisms by which regulatory T cells operate, particularly their effect on signaling pathways leading to T cell activation, are poorly understood. In this study we have used regulatory T (Treg) cells of known Ag specificity, generated in vivo, to address their effects on early activation events occurring in naive T cells of the same Ag specificity. We found that the Treg cells need to be present at the moment of priming to suppress activation and proliferation of the naive T cell. Furthermore, the Treg cells significantly inhibit the recruitment of protein kinase Ctheta (PKCtheta) to the immune synapse of the naive T cell as long as both T cells are of the same Ag specificity and are contacting the same APC. Finally, naturally occurring CD4(+)25(+) T cells seem to have the same effect on PKCtheta recruitment in CD25(-) T cells of the same Ag specificity. These results suggest that although additional mechanisms of regulation are likely to exist, inhibition of PKCtheta recruitment in the effector T cell may be a common regulatory pathway leading to the absence of NF-kappaB activation and contributing to the block of IL-2 secretion characteristic of immune suppression.     

Testicular immune privilege promotes transplantation tolerance by altering the balance between memory and regulatory T cells

Immune responses are suppressed in immunologically privileged sites, which may provide a unique opportunity to prolong allograft survival. However, it is unknown whether testicular immune privilege promotes transplantation tolerance. Mechanisms underlying immune privilege are also not well understood. Here we found that islet transplantation in the testis, an immunologically privileged site, generates much less memory CD8(+) T cells but induces more Ag-specific CD4(+)CD25(+) regulatory T cells than in a conventional site. These CD4(+)CD25(+) cells exhibited the suppression of alloimmune responses in vivo and in vitro. Despite the immune regulation, intratesticular islet allografts all were rejected within 42 days after transplantation although they survived longer than renal subcapsular islet allografts. However, blocking CD40/CD40L costimulation induced the tolerance of intratesticular, but not renal subcapsular, islet allografts. Tolerance to intratesticular islet allografts spread to skin allografts in the non-privileged sites. Either transfer of memory CD8(+) T cells or deletion of CD25(+) T cells in vivo broke islet allograft tolerance. Thus, transplantation tolerance requires both costimulatory blockade, which suppresses acute allograft rejection, and a favorable balance between memory and regulatory T cells that could favorably prevent late allograft failure. These findings reveal novel mechanisms of immune privilege and provide direct evidence that testicular immune privilege fosters the induction of transplantation tolerance to allografts in both immunologically privileged and non-privileged sites.     

Murine melanoma-infiltrating dendritic cells are defective in antigen presenting function regardless of the presence of CD4CD25 regulatory T cells

Tumor-infiltrating dendritic cells are often ineffective at presenting tumor-derived antigen in vivo, a defect usually ascribed to the suppressive tumor environment. We investigated the effects of depleting CD4⁺CD25⁺ “natural” regulatory T cells (Treg) on the frequency, phenotype and function of total dendritic cell populations in B16.OVA tumors and in tumor-draining lymph nodes. Intraperitoneal injection of the anti-CD25 monoclonal antibody PC61 reduced Treg frequency in blood and tumors, but did not affect the frequency of tumor-infiltrating dendritic cells, or their expression of CD40, CD86 and MHCII. Tumor-infiltrating dendritic cells from PC61-treated or untreated mice induced the proliferation of allogeneic T cells in vitro, but could not induce proliferation of OVA-specific OTI and OTII T cells unless specific peptide antigen was added in culture. Some proliferation of naïve, OVA-specific OTI T cells, but not OTII T cells, was observed in the tumor-draining LN of mice carrying B16.OVA tumors, however, this was not improved by PC61 treatment. Experiments using RAG1^−/− hosts adoptively transferred with OTI and CD25-depleted OTII cells also failed to show improved OTI and OTII T cell proliferation in vivo compared to C57BL/6 hosts. We conclude that the defective presentation of B16.OVA tumor antigen by tumor-infiltrating dendritic cells and in the tumor-draining lymph node is not due to the presence of “natural” CD4⁺CD25⁺ Treg.     

MHC-linked immune response suppression mediated by T cells bearing I-A-encoded determinants

The immune response to chicken egg-white lysozyme (HEL) is actively and specifically regulated by antigen-specific T cell-mediated suppression in mice bearing the H-2b haplotype; the suppression is therefore MHC-linked. In this report, we propose a possible mechanism for MHC-linked suppression of HEL-helper T cells based on expression of I region-encoded cell surface determinants. We determined whether inhibition of anti-HEL antibody responses correlated with expression of serologically detectable I-A-encoded cell surface determinants by antigen-specific helper, suppressor-inducer, or suppressor-effector T cells. It was observed that HEL-suppressor-effector T cells, but not helper or suppressor-inducer T cells, were eliminated after treatment with anti-I-Ab antibody and complement. Furthermore, suppressor-effector T cells co-express Thy-1, Lyt-2, and I-A cell surface antigens. These results raise the possibility that HEL-specific helper T cells become functionally inhibited after recognition of HEL and I-A alloantigen displayed by suppressor-effector T cells. Thus, the interaction between helper and suppressor T cells may be analogous to the mechanism of T cell-B cell interaction.     

B cells as antigen presenting cells

Several characteristics confer on B cells the ability to present antigen efficiently: (1) they can find T cells in secondary lymphoid organs shortly after antigen entrance, (2) BCR-mediated endocytosis allows them to concentrate small amounts of specific antigen, and (3) BCR signaling and HLA-DO expression direct their antigen processing machinery to favor presentation of antigens internalized through the BCR. When presenting antigen in a resting state, B cells can induce T cell tolerance. On the other hand, activation by antigen and T cell help converts them into APC capable of promoting immune responses. Presentation of self antigens by B cells is important in the development of autoimmune diseases, while presentation of tumor antigens is being used in vaccine strategies to generate immunity. Thus, detailed understanding of the antigen presenting function of B cells can lead to their use for the generation or inhibition of immune responses.