Ryan W. Nelson

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.

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.

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 kcat/Km values but activates PAR1 and PAR4 with kcat/Km 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.

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-Ab 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-Ab–specific naive T cells reached that of adult mice. Injection of 2W peptide in CFA into 1-d-old mice generated a 2W:I-Ab–specific effector cell population that peaked later than in adult mice and showed more animal-to-animal variation. Similarly, 2W:I-Ab–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.