Homeostasis in the immune system is maintained by specialized regulatory CD4+ T cells (Treg) expressing transcription factor Foxp3. According to the current paradigm, high-affinity interactions between TCRs and class II MHC-peptide complexes in thymus “instruct” developing thymocytes to up-regulate Foxp3 and become Treg cells. However, the loss or down-regulation of Foxp3 does not disrupt the development of Treg cells but abrogates their suppressor function. In this study, we show that Foxp3-deficient Treg cells in scurfy mice harboring a null mutation of the Foxp3 gene retained cellular features of Treg cells including in vitro anergy, impaired production of inflammatory cytokines, and dependence on exogenous IL-2 for proliferation and homeostatic expansion. Foxp3-deficient Treg cells expressed a low level of activation markers, did not expand relative to other CD4+ T cells, and produced IL-4 and immunomodulatory cytokines IL-10 and TGF-β when stimulated. Global gene expression profiling revealed significant similarities between Treg cells expressing and lacking Foxp3. These results argue that Foxp3 deficiency alone does not convert Treg cells into conventional effector CD4+ T cells but rather these cells constitute a distinct cell subset with unique features.

Medullary thymic epithelial cells expressing the Aire gene play a critical role in the induction of tolerance to tissue-specific Ags (TSAs). It was postulated that recognition of Aire-controlled TSAs by immature thymocytes results in the selection of natural CD4+Foxp3+ regulatory T cells (Tregs) and enriches this repertoire in self-reactive receptors, contributing to its vast diversity. In this study, we compared the TCRs on individual Tregs in Aire+ and Aire− mice expressing a miniature TCR repertoire (TCRmini) along with GFP driven by the Foxp3 promoter (Foxp3GFP). The Treg TCR repertoires in Aire+ and Aire− TCRminiFoxp3GFP mice were similar and more diverse than their repertoires on CD4+ Foxp3− thymocytes. Further, TCRs found on potentially self-reactive T cells, with an activated phenotype (CD4+Foxp3−CD62Llow) in Aire− TCRminiFoxp3GFP mice, appear distinct from TCRs found on Tregs in Aire+ TCRminiFoxp3GFP mice. Lastly, we found no evidence that TSAs presented by medullary thymic epithelial cells in Aire+TCRmini mice are often recognized as agonists by Treg-derived TCR hybridomas or CD4+CD25+ thymocytes, containing both natural Tregs and precursors. Thus, positive selection and self-reactivity of the global Treg repertoire are not controlled by Aire-dependent TSAs.

SummaryFor most bacteria, adherence to human cells is achieved by bacterial lectins binding to mammalian surface glyconjugates. 6-Phosphogluconate dehydrogenase (6PGD) was identified by us as one of Streptococcus pneumoniae cell wall lectin proteins, which elicits an age-dependent immune response in humans. This study assesses the role of 6PGD in S. pneumoniae pathogenesis as an adhesin and its ability to elicit a protective immune response in mice. Recombinant 6PGD (r6PGD) was cloned from S. pneumoniae serotype 3 (strain WU2). r6PGD interference in adhesion of three genetically unrelated unencapsulated pneumococcal strains (3·8, 14·8 and R6) and two genetically unrelated encapsulated pneumococcal strains (WU2 and D39) to A549 type II lung carcinoma cell was tested. BALB/c mice were immunized with r6PGD and boosted after 3 weeks. Immunized mice were challenged intranasally with a lethal dose of S. pneumoniae . r6PGD inhibited 90% and 80% of pneumococcal adhesion to the A549 cells of three unencapsulated S. pneumoniae strains and two encapsulated S. pneumoniae strains, respectively, in a concentration-dependent manner ( P < < < < 0·05). Antibodies to r6PGD produced in mice significantly inhibited bacterial adhesion to A549 cell ( P < < < < 0·05). Immunization of mice with r6PGD protected 60% ( P < < < < 0·001) of mice for 5 days and 40% ( P < < < < 0·05) of the mice for 21 days following intranasal lethal challenge. We have identified 6PGD as a surface-located immunogenic lectin protein capable of acting as an adhesin. 6PGD importance to bacterial pathogenesis was demonstrated by the ability of r6PGD to elicit a protective immune response in mice.