Immunological self/nonself discrimination: integration of self vs nonself during cognate T cell interactions with antigen-presenting cells

Tolerogenic vaccines: Targeting the antigenic and cytokine niches of FOXP3+ regulatory T cells

FOXP3+ regulatory T cells (Tregs) constitute a critical barrier that enforces tolerance to both the self-peptidome and the extended-self peptidome to ensure tissue-specific resistance to autoimmune, allergic, and other inflammatory disorders. Here, we review intuitive models regarding how T cell antigen receptor (TCR) specificity and antigen recognition efficiency shape the Treg and conventional T cell (Tcon) repertoires to adaptively regulate T cell maintenance, tissue-residency, phenotypic stability, and immune function in peripheral tissues. Three zones of TCR recognition efficiency are considered, including Tcon recognition of specific low-efficiency self MHC-ligands, Treg recognition of intermediate-efficiency agonistic self MHC-ligands, and Tcon recognition of cross-reactive high-efficiency agonistic foreign MHC-ligands. These respective zones of TCR recognition efficiency are key to understanding how tissue-resident immune networks integrate the antigenic complexity of local environments to provide adaptive decisions setting the balance of suppressive and immunogenic responses. Importantly, deficiencies in the Treg repertoire appear to be an important cause of chronic inflammatory disease. Deficiencies may include global deficiencies in Treg numbers or function, subtle 'holes in the Treg repertoire' in tissue-resident Treg populations, or simply Treg insufficiencies that are unable to counter an overwhelming molecular mimicry stimulus. Tolerogenic vaccination and Treg-based immunotherapy are two therapeutic modalities meant to restore dominance of Treg networks to reverse chronic inflammatory disease. Studies of these therapeutic modalities in a preclinical setting have provided insight into the Treg niche, including the concept that intermediate-efficiency TCR signaling, high IFN-β concentrations, and low IL-2 concentrations favor Treg responses and active dominant mechanisms of immune tolerance. Overall, the purpose here is to assimilate new and established concepts regarding how cognate TCR specificity of the Treg repertoire and the contingent cytokine networks provide a foundation for understanding Treg suppressive strategy.

Associations of ICOS and PD.1 Gene Variants with Colon Cancer Risk in The Iranian Population

Background:

Positive and negative co-stimulatory molecules are important factors determining the outcome of immune responses to the presence of tumors. Since co-stimulatory molecule expression may be affected by gene polymorphisms, we aimed to investigate associations between variants of PD.1 and ICOS and susceptibility to colon cancer.

Material and methods:

ICOS (-693A/G), ICOS (+1720C/T) and PD.1 (-538G/A) gene polymorphisms were evaluated by the PCR-RFLP method in 76 colon cancer patients and 73 healthy controls.

Results:

The frequencies of the GG genotype and the G allele at position -693 of the ICOS gene were significantly higher in the patient group (P=0.014 and p=0.0002), while the AA genotype was significantly more common in controls (P=0.0016). At position -538 of PD.1, GG genotype and G allele frequencies were higher in the patient group (P<0.0001and P<0.0001). Again, AA and also AG genotypes significantly predominated in controls (P<0.0001 and P=0.012). Regarding genotypes and alleles of ICOS at position +1720. Frequencies of GCG and GTG haplotypes were higher in patients compared to those of controls (P=0.016 and P<0.0001), while, frequencies of GTA, ATA and ATG haplotypes were higher in controls (P=0.0017, P<0.0001 and P=0.015). GTG/GTG and GTG/GCG double haplotypes were more frequent in patients compared to controls (P=0.0147 and P=0.0071).

Conclusion:

Our study clarified that PD.1 (-538G/A) and ICOS (-693A/G) gene polymorphisms can be considered as genetic risk factors for the development of colon cancer among Iranian patients.

Programmed death-1 gene polymorphism (PD-1.5 C/T) is associated with colon cancer

Programmed death-1 (PD-1), expressed by activated T cells, is a negative regulator of T lymphocytes. The associations of the immune response-related genes with cancer have been demonstrated. In this study, the PD-1.5 C/T (+7785) polymorphism was investigated in 200 colorectal cancer patients and 200 healthy individuals as controls by nested polymerase chain reaction-restriction fragment length polymorphism method. The genotype and allele frequencies at PD-1.5 position were not significantly different between control individuals and the overall colorectal cancer patients. However, subdivision of the patients by the location (175 colon cancer and 25 rectal cancer) revealed a significant difference between colon cancer patients and healthy individuals (p=0.026), and between colon and rectal cancer patients (p=0.017). The frequency of the CT genotype was significantly higher in colon cancer patients than in control individuals (58.3% vs. 44.8%, Bonferroni corrected p-value=0.024; OR=1.74; 95% CI=1.15-2.62), and in rectal cancer patients (58.3% vs. 28.0%, Bonferroni corrected p-value=0.012; OR=3.59; 95% CI=1.42-9.04). Characteristics of the patients including age, sex, tumor grade and stage were not associated with the PD-1.5 polymorphism. Our results show a significant association between PD-1.5 polymorphism and colon cancer. Larger numbers of patients are required to investigate comprehensively the association of rectal cancer with PD-1.5 polymorphism.

Autoimmunity and asthma: The dirt on the hygiene hypothesis

Self peptides shape T-cell development through selectional processes in the thymus and secondary lymphoid organs to promote a diverse and balanced repertoire of conventional and regulatory T cells. Foreign proteins and their derivative peptides permeate our mucosal tissues to constitute another diverse array of peptides that may specify and diversify the mucosal T-cell repertoire. Indeed, the distinction between self peptides and environmental foreign peptides may be academic if both are present constantly within the body. The premise here is that the plethora of foreign peptides, present ubiquitously in our environment and body, form homeostatic niches to foster highly diversified repertoires of conventional and regulatory T cells that recognize persistent environmental peptides as self. Highly diversified repertoires that recognize myriads of self and environmental foreign peptides as homeostatic ligands may be critical for adaptive distinctions of friend or foe in mucosal tissues. The change from our agrarian past to the highly sterile environments of today may adversely impact the diversity and concentrations of foreign peptides that shape the mucosal T-cell repertoire. Various hygiene hypotheses postulate that the lack of factors such as infectious pathogens, innate receptor engagement or Th1 bias is key to the marked increase in immunological disease in modern society. In this version of the hygiene hypothesis, highly diverse constellations of innocuous environmental peptides are postulated to be the critical factor for immune balance and homeostasis.

The evolution of mathematical immunology

The types of mathematical models used in immunology and their scope have changed drastically in the past 10 years. Classical models were based on ordinary differential equations (ODEs), difference equations, and cellular automata. These models focused on the 'simple' dynamics obtained between a small number of reagent types (e.g. one type of receptor and one type of antigen or two T-cell populations). With the advent of high-throughput methods, genomic data, and unlimited computing power, immunological modeling shifted toward the informatics side. Many current applications of mathematical models in immunology are now focused around the concepts of high-throughput measurements and system immunology (immunomics), as well as the bioinformatics analysis of molecular immunology. The types of models have shifted from mainly ODEs of simple systems to the extensive use of Monte Carlo simulations. The transition to a more molecular and more computer-based attitude is similar to the one occurring over all the fields of complex systems analysis. An interesting additional aspect in theoretical immunology is the transition from an extreme focus on the adaptive immune system (that was considered more interesting from a theoretical point of view) to a more balanced focus taking into account the innate immune system also. We here review the origin and evolution of mathematical modeling in immunology and the contribution of such models to many important immunological concepts.

Models of anergy in the human Jurkat T cell line

We investigated two model systems to study anergy in a human T cell line. OKT3 or calcium ionophore stimulation of Jurkat cells, in the absence of costimulation, resulted in a steep reduction in the transcription and secretion of IL-2 in response to subsequent stimulation via CD3 and CD28. Treatment of anergic Jurkat cells with the combination of the phorbol ester, PMA, and ionomycin restored IL-2 production in cells rendered anergic by both mechanisms. However, hydrogen peroxide, which also stimulates kinases downstream of the proposed block that occurs in anergic murine cells, did not reverse the anergic state of these cells induced by either stimulus. The cause of unresponsiveness in these two models was found to differ. OKT3-induced anergy resulted in a substantial down-regulation of the CD3 on these cells. In contrast, anergy induced by treatment with a calcium ionophore did not result in CD3 down-regulation. These data indicate that the Jurkat cell line may serve as a suitable model for studying anergy in human T cells; however, the mechanism by which anergy is induced may vary dramatically in response to these two commonly used anergy-inducing strategies. Understanding the similarities and differences between these two models of anergy may lead to a better overall understanding of the anergic state of the T cell.

IL-4 responsive CD4+ T cells specific for myelin basic protein: IL-2 confers a prolonged postactivation refractory phase

This study compared myelin basic protein-specific T cells from Lewis rats that were derived in the presence of either rat IL-4 or IL-2. Interleukin-4 was a maintenance factor that enabled derivation of long-term T cell lines. When activated, IL-4 dependent lines were lacking in IL-2 production capacity but maintained high levels of responsiveness to IL-2 and recognized IL-2 as a dominant growth factor. Activated IL-4 dependent T cells rapidly reverted to a quiescent phenotype in the presence of IL-4 and rapidly regained myelin basic protein reactivity. In contrast, activated IL-2 dependent T cells that were propagated in IL-2 had a more persistent blastogenic phenotype and a prolonged refractory phase. Interleukin-4 dependent lines that were propagated in IL-2 up-regulated the capacity to produce IL-2 and also acquired prolonged postactivation refractoriness. Thus, IL-2 was a dominant growth factor that conferred prolonged activation-dependent non-responsiveness. The coupling of dominant growth factor activity with prolonged postactivation refractoriness may be associated with the requisite role of IL-2 in homeostatic self-tolerance.

Acquisition of functional MHC class II/peptide complexes by T cells during thymic development and CNS-directed pathogenesis

This study provides evidence that both rat and mouse thymic and splenic T cells express significant levels of MHC class II glycoproteins (MHCII) in vivo. Derivation of rat and mouse chimeras revealed that a major source of MHCII on thymic T cells was acquired from radioresistant host APC. Expression of MHC on thymic T cells appeared physiologically relevant because presentation of rat myelin basic protein (RMBP) by nonadherent, radiosensitive thymic T cells was associated with the adoptive transfer of tolerance. Mature MBP-specific effector T cells isolated from the CNS in both rat and mouse models of EAE also expressed significant levels of MHCII. Adoptive transfer of activated B10.PL MBP/I-A(u)-restricted TCR transgenic T cells into F1(C57BL/6 x B10.PL) mice revealed acquisition of allogeneic I-A(b) on encephalitogenic CNS-derived T cells. Overall, this study indicates that immature and mature T cells in rats and mice acquire functional MHCII in vivo during thymic development and pathogenic inflammation.

Interleukin-2 promotes antigenic reactivity of rested T cells but prolongs the postactivational refractory phase of activated T cells

IL-2 is a principal autocrine growth factor that promotes T-cell activation and proliferation. However, IL-2 has also been implicated as a key intermediate in the induction and maintenance of self-tolerance in vivo. The purpose of this study was to assess whether the differential regulatory activity of IL-2 was related to the activation status of responder T cells. In cultures of rested myelin basic protein (MBP)-specific T cells, IL-2 not only induced IL-2R alpha but also augmented surface expression of several other activation-associated glycoproteins including OX40, LFA-1, B7.1, B7.2, TCR, and CD4. Pretreatment of T cells with IL-2 also up-regulated subsequent antigen reactivity in assays of MBP-stimulated proliferation and IL-2 production and also promoted proliferative responsiveness to IL-2. In cultures of activated T cells, however, IL-2 inhibited subsequent reactivity to antigen or IL-2 and thereby prolonged a phase of postactivational refractoriness. Exposure of preactivated T cells to IL-2 also inhibited subsequent responses to the mitogenic combination of PMA, ionomycin, and IL-2 without enhancing cell death. These data support the concept that the inhibitory activity of IL-2 is dependent upon the activation status of T cells and is manifest as impaired cell cycle progression in response to a variety of IL-2-dependent stimuli.

Class II MHC/Peptide Complexes Are Released from APC and Are Acquired by T Cell Responders During Specific Antigen Recognition

T cell expression of class II MHC/peptide complexes may be important for maintenance of peripheral self-tolerance, but mechanisms underlying the genesis of class II MHC glycoproteins on T cells are not well resolved. T cell APC (T-APC) used herein were transformed IL-2-dependent clones that constitutively synthesized class II MHC glycoproteins. When pulsed with myelin basic protein (MBP) and injected into Lewis rats, these T-APC reduced the severity of experimental autoimmune encephalomyelitis, whereas unpulsed T-APC were without activity. Normal MBP-reactive clones cultured without APC did not express class II MHC even when activated with mitogens and exposed to IFN-γ. However, during a 4-h culture with T-APC or macrophage APC, recognition of MBP or mitogenic activation of responder T cells elicited high levels of I-A and I-E expression on responders. Acquisition of class II MHC glycoproteins by responders was resistant to the protein synthesis inhibitor cycloheximide, coincided with transfer of a PKH26 lipophilic dye from APC to responders, and resulted in the expression of syngeneic and allogeneic MHC glycoproteins on responders. Unlike rested I-A− T cell clones, rat thymic and splenic T cells expressed readily detectable levels of class II MHC glycoproteins. When preactivated with mitogens, naive T cells acquired APC-derived MHC class II molecules and other membrane-associated proteins when cultured with xenogeneic APC in the absence of Ag. In conclusion, this study provides evidence that APC donate membrane-bound peptide/MHC complexes to Ag-specific T cell responders by a mechanism associated with the induction of tolerance.