SUMMARY A naïve 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 naïve T cells. Different naïve 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 naïve 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 naïve cells results from averaging the diverse behaviors of individual clones, which are instructed in part by the strength of TCR signaling.

BACKGROUND. Pediatric 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. METHODS.We 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). RESULTS.Twenty-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. CONCLUSION. MIS-C encompasses a broad phenotypic spectrum with clinical and laboratory features distinct from KD and MAS.

Studies of mouse monoclonal CD4+ T cell repertoires have revealed several mechanisms of self-tolerance, however, which mechanisms operate in normal repertoires is unclear. Here, polyclonal CD4+ T cells specific for green fluorescent protein expressed in different organs were studied, allowing determination of 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 thymus-excluded peptides were ignored. Peptides with limited thymic expression induced partial clonal deletion and impaired effector but enhanced regulatory T cell potential. These mechanisms were also active for T cell populations specific for endogenously expressed self-antigens. Thus, immune tolerance of polyclonal CD4+ T cells is maintained by distinct mechanisms, according to self-peptide expression patterns.

SUMMARY T cell receptor (TCR) cross-reactivity between major histocompatibility complex II (MHCII)-binding self and foreign peptides could influence the naïve 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 since systemic expression of a self peptide reduced the size of a naïve cell population specific for a related foreign peptide by deletion of cells with cross-reactive TCRs. Reciprocally, an incompletely deleted naïve 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 may allow T cell populations specific for tissue-restricted self peptides to cause autoimmunity after infection.

CD4+ memory-phenotype T cells decline over time when generated in response to acute infections cleared by other components of the immune system. It was therefore 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:MHCII ligands were numerically stable for greater than a year 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 multi-functional 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.

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 Thrombin is a Na ϩ -activated serine protease (1) that is responsible for the progression and regulation of blood coagulation (2). As in other monovalent cation-activated enzymes (3), the role of Na ϩ in thrombin function is to lower energy barriers for substrate binding in the ground and transition states so that physiologic substrates like PAR1 and fibrinogen can be hydrolyzed efficiently during platelet activation and formation of a blood clot. Other members of the vitamin K-dependent family of clotting proteases to which thrombin belongs are endowed with Na ϩ activation (4). In fact, the activity of activated protein C (5) and coagulation factors VIIa (6), IXa (7), and Xa (8) is influenced significantly by the presence of Na ϩ .Details of the molecular mechanism of Na ϩ activation in thrombin have emerged recently from mutagenesis and structural analysis (9). Na ϩ binding is severely compromised (Ͼ30-fold increase in K d ) upon mutation of Asp-189, Glu-217, Asp-222, and Tyr-225. Asp-189 fixes the orientation of one of the four water molecules ligating Na ϩ and provides an important link between the Na ϩ site and the P1 residue of substrate (10). Glu-217 makes polar contacts with Lys-224 and Thr-172, which helps to stabilize the intervening 220-loop in the Na ϩ site. The ion pair between Arg-187 and Asp-222 latches the 186-loop onto the 220-loop to stabilize the Na ϩ site and the pore of entry of the cation to its binding site (11). Tyr-225 plays a crucial role in determining the Na ϩ -dependent allosteric nature of serine proteases (4) by allowing the correct orientation of the backbone oxygen of residue 224 (12), which contributes directly to the coordination of Na ϩ . The side chain of Tyr-225 also secures the integrity of the water channel embedding the primary specificity pocket (12), and the backbone around Tyr-225 is oriented like the selectivity filter of the KcsA K ϩ channel (3, 13). Of these four residues, Glu-217 and Tyr-225 are conserved in thrombin from all species sequenced to date, from hagfish to human (14). Asp-189 is a Ser in the sturgeon, and the D189S mutant of human thrombin has impaired Na ϩ binding and substrate recognition (10). Asp-222 is the least conserved residue among the four. It is Ser in the sturgeon, Lys in the mouse, and Asn in the rat (14). The substitution in the mouse is particularly interesting, as it involves a charge reversal in the 220-loop that has the potential to destabilize the Na ϩ binding environment.Does the presence of Lys-222 in murine thrombin preclude Na However, the mutant has functional properties interm...

Table 1 of this paper published in the January 2015 issue contains several errors. The Derp1 peptide comes from the Der p1 protein of Dermatophagoides pteronyssinus not the Der f1 protein of Dermatophagoides farinae. The sequence of the MOG peptide in the I-A b tetramer used in the study was GWYRSPFSRVV not GWYRSPFSRVVVHLY. The most likely 9 amino acid core register for the OVA2C peptide in the I-A b tetramer used in the study is VHAAHAEIN not AVHAAHAEI, which is the second most likely. The most likely register has a different number of predicted self peptide homologs than the second most likely one, which changed the x axis value for the OVA2C peptide in Figure 4G. However, neither the level of statistical significance of the correlation between the number of self peptide homologs and the number of naive cells specific for foreign peptides shown in Figure 4G nor the conclusion that ''TCR cross-reactivity on self-peptide homologs plays some role in determining the size of MHCII-bound foreign peptide-specific CD4 + T cell populations'' were changed. Thus, none of these errors affect any of the conclusions of the paper. The authors deeply regret these errors and apologize for them. Corrected versions of the table and figure appear here.

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 COVID-19 immunity, we conducted a longitudinal study of SARS-CoV-2-specific CD4 + T cell and antibody responses in convalescent subjects who seroconverted during the first wave of the pandemic in Boston, Massachusetts, United States, 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 (peptide:MHCII) tetramers demonstrated expanded populations of T cells recognizing the different SARS-CoV-2 epitopes in most subjects compared to pre-pandemic 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 following viral clearance also revealed an increased proportion of memory cTfh cells. Our findings indicate efficient early disease control also predicts favorable long-term adaptive immunity.