Tolerance is established in polyclonal CD4(+) T cells by distinct mechanisms, according to self-peptide expression patterns.

Studies of repertoires of mouse monoclonal CD4(+) T cells have revealed several mechanisms of self-tolerance; however, which mechanisms operate in normal repertoires is unclear. Here we studied polyclonal CD4(+) T cells specific for green fluorescent protein expressed in various organs, which allowed us to determine the effects of specific expression patterns on the same epitope-specific T cells. Peptides presented uniformly by thymic antigen-presenting cells were tolerated by clonal deletion, whereas peptides excluded from the thymus were ignored. Peptides with limited thymic expression induced partial clonal deletion and impaired effector T cell potential but enhanced regulatory T cell potential. These mechanisms were also active for T cell populations specific for endogenously expressed self antigens. Thus, the immunotolerance of polyclonal CD4(+) T cells was maintained by distinct mechanisms, according to self-peptide expression patterns.

Single naive CD4+ T cells from a diverse repertoire produce different effector cell types during infection.

A naive CD4(+) T cell population specific for a microbial peptide:major histocompatibility complex II ligand (p:MHCII) typically consists of about 100 cells, each with a different T cell receptor (TCR). Following infection, this population produces a consistent ratio of effector cells that activate microbicidal functions of macrophages or help B cells make antibodies. We studied the mechanism that underlies this division of labor by tracking the progeny of single naive T cells. Different naive cells produced distinct ratios of macrophage and B cell helpers but yielded the characteristic ratio when averaged together. The effector cell pattern produced by a given naive cell correlated with the TCR-p:MHCII dwell time or the amount of p:MHCII. Thus, the consistent production of effector cell subsets by a polyclonal population of naive cells results from averaging the diverse behaviors of individual clones, which are instructed in part by the strength of TCR signaling.

T cell receptor cross-reactivity between similar foreign and self peptides influences naive cell population size and autoimmunity.

T cell receptor (TCR) cross-reactivity between major histocompatibility complex II (MHCII)-binding self and foreign peptides could influence the naive CD4(+) T cell repertoire and autoimmunity. We found that nonamer peptides that bind to the same MHCII molecule only need to share five amino acids to cross-react on the same TCR. This property was biologically relevant because systemic expression of a self peptide reduced the size of a naive cell population specific for a related foreign peptide by deletion of cells with cross-reactive TCRs. Reciprocally, an incompletely deleted naive T cell population specific for a tissue-restricted self peptide could be triggered by related microbial peptides to cause autoimmunity. Thus, TCR cross-reactivity between similar self and foreign peptides can reduce the size of certain foreign peptide-specific T cell populations and might allow T cell populations specific for tissue-restricted self peptides to cause autoimmunity after infection.

The Neonatal CD4+ T Cell Response to a Single Epitope Varies in Genetically Identical Mice.

Neonatal infection is a major cause of morbidity and mortality worldwide. Increased susceptibility to infection in the neonate is attributed in part to defects in T cell-mediated immunity. A peptide:MHC class II tetramer-based cell enrichment method was used to test this hypothesis at the level of a single epitope. We found that naive T cells with TCRs specific for the 2W:I-A(b) epitope were present in the thymuses of 1-d-old CD57BL/6 mice but were barely detectable in the spleen, likely because each mouse contained very few total splenic CD4(+) T cells. By day 7 of life, however, the total number of splenic CD4(+) T cells increased dramatically and the frequency of 2W:I-A(b)-specific naive T cells reached that of adult mice. Injection of 2W peptide in CFA into 1-d-old mice generated a 2W:I-A(b)-specific effector cell population that peaked later than in adult mice and showed more animal-to-animal variation. Similarly, 2W:I-A(b)-specific naive T cells in different neonatal mice varied significantly in generation of Th1, Th2, and follicular Th cells compared with adult mice. These results suggest that delayed effector cell expansion and stochastic variability in effector cell generation due to an initially small naive repertoire contribute to defective peptide:MHC class II-specific immunity in neonates.

CD4+ T cell persistence and function after infection are maintained by low-level peptide:MHC class II presentation.

CD4(+) memory-phenotype T cells decline over time when generated in response to acute infections cleared by other components of the immune system. Therefore, it was of interest to assess the stability of CD4(+) T cells during a persistent Salmonella infection, which is typical of persistent phagocytic infections that are controlled by this lymphocyte subset. We found that CD4(+) T cells specific for Salmonella peptide:MHC class II (MHCII) ligands were numerically stable for >1 y after initial oral infection. This stability was associated with peptide:MHCII-driven proliferation by a small number of T cells in the secondary lymphoid organs that harbored bacteria. The persistent population consisted of multifunctional Th1 cells that induced PD-1 and became exhausted when transferred to hosts expressing the specific peptide:MHCII ligand in all parts of the body. Thus, persistent infection of phagocytes produced a CD4(+) T cell population that was stably maintained by low-level peptide:MHCII presentation.

Inborn Errors of the Immune System Associated With Atopy.

Atopic disorders, including atopic dermatitis, food and environmental allergies, and asthma, are increasingly prevalent diseases. Atopic disorders are often associated with eosinophilia, driven by T helper type 2 (Th2) immune responses, and triggered by disrupted barrier function leading to abnormal immune priming in a susceptible host. Immune deficiencies, in contrast, occur with a significantly lower incidence, but are associated with greater morbidity and mortality. A subset of atopic disorders with eosinophilia and elevated IgE are associated with monogenic inborn errors of immunity (IEI). In this review, we discuss current knowledge of IEI that are associated with atopy and the lessons these immunologic disorders provide regarding the fundamental mechanisms that regulate type 2 immunity in humans. We also discuss further mechanistic insights provided by animal models.

SARS-CoV-2 epitope-specific CD4+ memory T cell responses across COVID-19 disease severity and antibody durability.

CD4+ T cells are central to long-term immunity against viruses through the functions of T helper 1 (TH1) and T follicular helper (TFH) cell subsets. To better understand the role of these subsets in coronavirus disease 2019 (COVID-19) immunity, we conducted a longitudinal study of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-specific CD4+ T cell and antibody responses in convalescent individuals who seroconverted during the first wave of the pandemic in Boston, MA, USA, across a range of COVID-19 disease severities. Analyses of spike (S) and nucleocapsid (N) epitope-specific CD4+ T cells using peptide and major histocompatibility complex class II (pMHCII) tetramers demonstrated expanded populations of T cells recognizing the different SARS-CoV-2 epitopes in most individuals compared with prepandemic controls. Individuals who experienced a milder disease course not requiring hospitalization had a greater percentage of circulating TFH (cTFH) and TH1 cells among SARS-CoV-2-specific cells. Analysis of SARS-CoV-2-specific CD4+ T cells responses in a subset of individuals with sustained anti-S antibody responses after viral clearance also revealed an increased proportion of memory cTFH cells. Our findings indicate that efficient early disease control also predicts favorable long-term adaptive immunity.

Distinct clinical and immunological features of SARS-CoV-2-induced multisystem inflammatory syndrome in children.

BACKGROUNDPediatric SARS-CoV-2 infection can be complicated by a dangerous hyperinflammatory condition termed multisystem inflammatory syndrome in children (MIS-C). The clinical and immunologic spectrum of MIS-C and its relationship to other inflammatory conditions of childhood have not been studied in detail.METHODSWe retrospectively studied confirmed cases of MIS-C at our institution from March to June 2020. The clinical characteristics, laboratory studies, and treatment response were collected. Data were compared with historic cohorts of Kawasaki disease (KD) and macrophage activation syndrome (MAS).RESULTSTwenty-eight patients fulfilled the case definition of MIS-C. Median age at presentation was 9 years (range: 1 month to 17 years); 50% of patients had preexisting conditions. All patients had laboratory confirmation of SARS-CoV-2 infection. Seventeen patients (61%) required intensive care, including 7 patients (25%) who required inotrope support. Seven patients (25%) met criteria for complete or incomplete KD, and coronary abnormalities were found in 6 cases. Lymphopenia, thrombocytopenia, and elevation in inflammatory markers, D-dimer, B-type natriuretic peptide, IL-6, and IL-10 levels were common but not ubiquitous. Cytopenias distinguished MIS-C from KD and the degree of hyperferritinemia and pattern of cytokine production differed between MIS-C and MAS. Immunomodulatory therapy given to patients with MIS-C included intravenous immune globulin (IVIG) (71%), corticosteroids (61%), and anakinra (18%). Clinical and laboratory improvement were observed in all cases, including 6 cases that did not require immunomodulatory therapy. No mortality was recorded in this cohort.CONCLUSIONMIS-C encompasses a broad phenotypic spectrum with clinical and laboratory features distinct from KD and MAS.FUNDINGThis work was supported by the National Institutes of Health, National Institute of Arthritis and Musculoskeletal and Skin Diseases; the National Institute of Allergy and Infectious Diseases; Rheumatology Research Foundation Investigator Awards and Medical Education Award; Boston Children's Hospital Faculty Career Development Awards; the McCance Family Foundation; and the Samara Jan Turkel Center.

En Face Imaging of Epiretinal Membranes and the Retinal Nerve Fiber Layer Using Swept-Source Optical Coherence Tomography.

BACKGROUND AND OBJECTIVE: To describe en face swept-source optical coherence tomography (SS-OCT) imaging of epiretinal membranes (ERMs) and the underlying retinal nerve fiber layer (RNFL). PATIENTS AND METHODS: SS-OCT images were captured in nine eyes with ERMs. These SS-OCT en face images were qualitatively compared to spectral-domain OCT (SD-OCT) en face images. Using SS-OCT, en face images of the RNFL were obtained, if possible. RESULTS: En face SS-OCT images of ERMs were obtained in all nine eyes of eight individuals with clinically diagnosed ERMs that were superior to SD-OCT images. Clear en face images of the RNFL were acquired in seven of nine eyes (78%) using SS-OCT. CONCLUSIONS: SS-OCT is a novel method for generating en face images of ERMs. Compared with SDOCT en face images, SS-OCT could more clearly identify the plaques and folds of ERMs and underlying defects in the RNFL. Such images could be useful for surgical planning and assessment of the integrity of the underlying RNFL. [Ophthalmic Surg Lasers Imaging Retina. 2016;47:730-734.].

Murine thrombin lacks Na+ activation but retains high catalytic activity.

Human thrombin utilizes Na+ as a driving force for the cleavage of substrates mediating its procoagulant, prothrombotic, and signaling functions. Murine thrombin has Asp-222 in the Na+ binding site of the human enzyme replaced by Lys. The charge reversal substitution abrogates Na+ activation, which is partially restored with the K222D mutation, and ensures high activity even in the absence of Na+. This property makes the murine enzyme more resistant to the effect of mutations that destabilize Na+ binding and shift thrombin to its anticoagulant slow form. Compared with the human enzyme, murine thrombin cleaves fibrinogen and protein C with similar k(cat)/K(m) values but activates PAR1 and PAR4 with k(cat)/K(m) values 4- and 26-fold higher, respectively. The significantly higher specificity constant toward PAR4 accounts for the dominant role of this receptor in platelet activation in the mouse. Murine thrombin can also cleave substrates carrying Phe at P1, which potentially broadens the repertoire of molecular targets available to the enzyme in vivo.