No abstract

Lineage specificity and temporal ordering of immunoglobulin (Ig) and T-cell receptor (TCR) gene rearrangement are reflected in the accessibility of recombination signal sequences (RSSs) within chromatin to in vitro cleavage by the V(D)J recombinase. In this report, we investigated the basis of this regulation by testing the ability of purified RAG1 and RAG2 proteins to initiate cleavage on positioned nucleosomes containing RSS substrates. We found that nicking and double-strand DNA cleavage of RSSs positioned on the face of an unmodified nucleosome are entirely inhibited. This inhibition was independent of translational position or rotational phase and could not be overcome either by addition of the DNA-bending protein HMG-1 or by the use of hyperacetylated histones. We suggest that the nucleosome could act as the stable unit of chromatin which limits recombinase accessibility to potential RSS targets, and that actively rearranging gene segments might be packaged in a modified or disrupted nucleosome structure.

Objective To assess the use of Helios in combination with FoxP3 as a superior method for identifying non–cytokine-producing human Treg cells in patients with systemic lupus erythematosus (SLE) and to determine if FoxP3+Helios+ Treg cells are maintained at normal levels in patients with clinically active disease. Methods Peripheral blood mononuclear cells (PBMCs) were purified from the blood of healthy volunteer donors and from 52 consecutive patients with SLE of varying clinical activity (Systemic Lupus Erythematosus Disease Activity Index scores of 0, 2–4, and ≥5). PBMCs (either fresh or after 4 hours of stimulation for cytokine production) were then analyzed by flow cytometry for the expression of cell surface markers (CD4, CD25, CD127, and CD45RA) and transcription factors (FoxP3 and Helios), as well as for the production of cytokines (interleukin-2 and interferon- γ). Results FoxP3+Helios+ Treg cells were found to be non–cytokine producing in both SLE patients and healthy controls. Patients with clinically active SLE had higher percentages of FoxP3+Helios+ Treg cells than did patients with inactive SLE or healthy controls. When corrected for the total CD4 cell count, the absolute numbers of FoxP3+Helios+ Treg cells in patients with moderately-to-highly active SLE were normal. Conclusion Previous reports of a deficiency in Treg cell number or function in SLE are limited by their use of CD25, either alone or in combination with other markers, to identify human Treg cells. Helios in combination with FoxP3 is a superior method for detecting all non–cytokine-producing Treg cells, irrespective of CD25 or CD45RA expression. Using this method, we showed that FoxP3+Helios+ Treg cell numbers are not reduced in patients with clinically active SLE.

Summary An adequate effector response against pathogens and its subsequent inactivation after pathogen clearance are critical for the maintenance of immune homeostasis. This process involves an initial phase of T‐cell effector (Teff) activation followed by the expansion of regulatory T cells (Tregs), a unique cell population that limits Teff functions. However, significant questions remain unanswered about the mechanisms that regulate the balance between these cell populations. Using an in vitro system to mimic T‐cell activation in human peripheral blood mononuclear cells (PBMC), we analysed the patterns of Treg and Teff activation, with special attention to the role of type I interferon (IFN‐I). Interestingly, we found that IFN‐α, either exogenously added or endogenously induced, suppressed the generation of CD4+ FoxP3HI IFN‐γNeg activated Tregs (aTregs) while simultaneously promoting propagation of CD4+ FoxP3Low/Neg IFN‐γPos activated Teffs (aTeffs). We also showed that IFN‐α‐mediated inhibition of interleukin (IL)‐2 production may play an essential role in IFN‐α‐induced suppression of aTregs. In order to test our findings in a disease state with chronically elevated IFN‐α, we investigated systemic lupus erythematosus (SLE). Plasma from patients with SLE was found to contain IFN‐I activity that suppressed aTreg generation. Furthermore, anti‐CD3 activated SLE PBMCs exhibited preferential expansion of aTeffs with a very limited increase in aTreg numbers. Together, these observations support a model whereby a transient production of IFN‐α (such as is seen in an early antiviral response) may promote CD4 effector functions by delaying aTreg generation, but a chronic elevation of IFN‐α may tip the aTeff:aTreg balance towards aTeffs and autoimmunity.