Loss of Functionally Redundant p38 Isoforms in T Cells Enhances Regulatory T Cell Induction.

The evolutionarily conserved protein kinase p38 mediates innate resistance to environmental stress and microbial infection. Four p38 isoforms exist in mammals and may have been co-opted for new roles in adaptive immunity. Murine T cells deficient in p38α, the ubiquitously expressed p38 isoform, showed no readily apparent cell-autonomous defects while expressing elevated amounts of another isoform, p38ß. Mice with T cells simultaneously lacking p38α and p38ß displayed lymphoid atrophy and elevated Foxp3+ regulatory T cell frequencies. Double deficiency of p38α and p38ß in naïve CD4+ T cells resulted in an attenuation of MAPK-activated protein kinase (MK)-dependent mTOR signaling after T cell receptor engagement, and enhanced their differentiation into regulatory T cells under appropriate inducing conditions. Pharmacological inhibition of the p38-MK-mTOR signaling module produced similar effects, revealing potential for therapeutic applications.

Impaired suppressive capacity of activation-induced regulatory B cells in systemic lupus erythematosus.

OBJECTIVE: B cells with immunoregulatory properties (Breg cells) have been described in mice, but their role in the control of human immune responses is not well defined. We recently identified a human population of activated FSC(high) B cells that exhibited regulatory activity toward T helper cells. The aim of the present study was to test such induced Breg (iBreg) cells in patients with autoimmune disease. METHODS: Purified CD19+FSC(high) B cells derived from patients with systemic lupus erythematosus (SLE) or from healthy donors, which were activated via their B cell receptor, were cocultured with CD3-stimulated CD4+ T helper cells from SLE patients or healthy donors. (3) H-thymidine incorporation, flow cytometry, and enzyme-linked immunosorbent assay (ELISA) were used to analyze proliferation, cytokine secretion, and surface marker expression. RESULTS: Although under costimulatory conditions, FSC(high) SLE B cells supported the proliferation of healthy donor T cells to a similar extent as donor B cells, their regulatory function was significantly diminished in B cell suppressor assays. Similar effects were seen when SLE T cells were used, confirming that SLE T cells were equally susceptible to iBreg cell signals as healthy donor T cells and that SLE iBreg cell defects were independent of T cell origin. B cell viability and expression of surface markers (CD25, CD80, and B7-H1) or cytokines (interleukin-6 [IL-6], tumor necrosis factor α, and IL-10) were comparable in the two B cell populations. There was no correlation between the extent of iBreg cell-induced inhibition and disease activity. CD19+FSC(high) B cells from patients with another systemic autoimmune disease, granulomatosis with polyangiitis (Wegener's) (GPA), exhibited no regulatory defects, which suggests that the iBreg cell defects were SLE-specific and not a general consequence of autoimmunity or inflammation. CONCLUSION: Induced Breg cells from SLE patients, but not GPA patients, are less effective in the control of T helper cell proliferation, which supports the reported skewed B cell repertoire in SLE. The malfunctioning SLE iBreg cells might allow the overstimulation of immune responses and contribute to the initiation and/or perpetuation of disease.

PDK1 regulates VDJ recombination, cell-cycle exit and survival during B-cell development.

Phosphoinositide-dependent kinase-1 (PDK1) controls the activation of a subset of AGC kinases. Using a conditional knockout of PDK1 in haematopoietic cells, we demonstrate that PDK1 is essential for B cell development. B-cell progenitors lacking PDK1 arrested at the transition of pro-B to pre-B cells, due to a cell autonomous defect. Loss of PDK1 decreased the expression of the IgH chain in pro-B cells due to impaired recombination of the IgH distal variable segments, a process coordinated by the transcription factor Pax5. The expression of Pax5 in pre-B cells was decreased in PDK1 knockouts, which correlated with reduced expression of the Pax5 target genes IRF4, IRF8 and Aiolos. As a result, Ccnd3 is upregulated in PDK1 knockout pre-B cells and they have an impaired ability to undergo cell-cycle arrest, a necessary event for Ig light chain rearrangement. Instead, these cells underwent apoptosis that correlated with diminished expression of the pro-survival gene Bcl2A1. Reintroduction of both Pax5 and Bcl2A1 together into PDK1 knockout pro-B cells restored their ability to differentiate in vitro into mature B cells.

RNA-binding proteins as a point of convergence of the PI3K and p38 MAPK pathways.

Understanding the mechanisms by which signal transduction pathways mediate changes in RNA abundance requires the examination of the fate of RNA from its transcription to its degradation. Evidence suggests that RNA abundance is partly regulated by post-transcriptional mechanisms affecting RNA decay and this in turn is modulated by some of the same signaling pathways that control transcription. Furthermore, the translation of mRNA is a key regulatory step that is influenced by signal transduction. These processes are regulated, in part, by RNA-binding proteins (RBPs) which bind to sequence-specific RNA elements. The function of RBPs is controlled and co-ordinated by phosphorylation. Based on the current literature we hypothesize that RBPs may be a point of convergence for the activity of different kinases such as phosphoinositide-3-kinase and mitogen-activated protein kinase which regulate RBP localization and function.

Polyubiquitin binding to ABIN1 is required to prevent autoimmunity.

The protein ABIN1 possesses a polyubiquitin-binding domain homologous to that present in nuclear factor κB (NF-κB) essential modulator (NEMO), a component of the inhibitor of NF-κB (IκB) kinase (IKK) complex. To address the physiological significance of polyubiquitin binding, we generated knockin mice expressing the ABIN1[D485N] mutant instead of the wild-type (WT) protein. These mice developed all the hallmarks of autoimmunity, including spontaneous formation of germinal centers, isotype switching, and production of autoreactive antibodies. Autoimmunity was suppressed by crossing to MyD88(-/-) mice, demonstrating that toll-like receptor (TLR)-MyD88 signaling pathways are needed for the phenotype to develop. The B cells and myeloid cells of the ABIN1[D485N] mice showed enhanced activation of the protein kinases TAK, IKK-α/ß, c-Jun N-terminal kinases, and p38α mitogen-activated protein kinase and produced more IL-6 and IL-12 than WT. The mutant B cells also proliferated more rapidly in response to TLR ligands. Our results indicate that the interaction of ABIN1 with polyubiquitin is required to limit the activation of TLR-MyD88 pathways and prevent autoimmunity.

Induction of CD4+ T-cell anergy and apoptosis by activated human B cells.

B cells are well-known mediators of humoral immunity and serve as costimulators in the generation of T cell-mediated responses. In several mouse models, however, it was observed that B cells can also down-regulate immune reactions, suggesting a dual role for B cells. Due to this discrepancy and so far limited data, we directly tested the effects of primary human B cells on activated CD4(+) T helper cells in vitro. We found that under optimal costimulation large, activated CD25(+) B cells but not small CD25(-) B cells induced temporary T-cell anergy, determined by cell division arrest and down-regulation of cytokine production. In addition, large CD25(+) B cells directly induced CD95-independent apoptosis in a subpopulation of activated T cells. Suppression required direct B-T-cell contact and was not transferable from T to T cell, excluding potential involvement of regulatory T cells. Moreover, inhibitory effects involved an IL-2-dependent mechanism, since decreasing concentrations of IL-2 led to a shift from inhibitory toward costimulatory effects triggered by B cells. We conclude that activated CD25(+) B cells are able to costimulate or down-regulate T-cell responses, depending on activation status and environmental conditions that might also influence their pathophysiological impact.