SUMMARY B lymphocytes have critical roles as positive and negative regulators of immunity. Their inhibitory function has so far been associated primarily with interleukin (IL)-10 because B cell-derived IL-10 can protect against autoimmune disease and increase susceptibility to pathogens1,2. Here, we identify IL-35-producing B cells as novel key players in the negative regulation of immunity. Mice in which only B cells did not express IL-35 lost their ability to recover from the T cell-mediated demyelinating autoimmune disease experimental autoimmune encephalomyelitis (EAE). In contrast, these mice displayed a strikingly improved resistance to infection with the intracellular bacterial pathogen Salmonella typhimurium, as shown by their superior containment of the bacterial growth and their prolonged survival both after primary infection, and upon secondary challenge after vaccination, compared to control mice. The increased immunity found in mice lacking IL-35 production by B cells was associated with a higher activation of macrophages and inflammatory T cells, as well as an enhanced stimulatory function of B cells as antigen-presenting cells (APC). During Salmonella infection IL-35- and IL-10-producing B cells corresponded to two largely distinct sets of surface-IgM+CD138hiTACI+CXCR4+CD1dintTim1int plasma cells expressing the transcription factor Blimp1. During EAE CD138+ plasma cells were also the major source of B cell-derived IL-35 and IL-10. Collectively, our data unravel the importance of IL-35-producing B cells in regulation of immunity, and highlight IL-35 production by B cells as a novel therapeutic target for autoimmune and infectious diseases. More generally, this study emphasizes the central role of activated B cells, particularly plasma cells, and their production of cytokines in the regulation of immune responses in health and disease.

It has recently been proposed that experimental autoimmune encephalomyelitis, once considered the classical Th1 disease, is predominantly Th17 driven. In this study we show that myelin-reactive Th1 preparations devoid of contaminating IL-17+ cells are highly pathogenic. In contrast, Th17 preparations lacking IFN-γ+ cells do not cause disease. Our key observation is that only Th1 cells can access the noninflamed CNS. Once Th1 cells establish the experimental autoimmune encephalomyelitis lesion, Th17 cells appear in the CNS. These data shed important new light on the ability of Th1 vs Th17 cells to access inflamed vs normal tissue. Because the IL-17-triggered release of chemokines by stromal cells could attract many other immune cells, allowing Th17 cells to access the tissues only under conditions of inflammation may be a key process limiting (auto)immune pathology. This has major implications for the design of therapeutic interventions, many of which are now aiming at Th17 rather than Th1 cells.

Allergic airway inflammation (AAI) is characterized by airway hyperreactivity, eosinophilia, goblet cell hyperplasia, and elevated serum IgE, however, it is unclear what mediates natural resolution after cessation of allergen exposure. This is important because the outcome of subsequent allergen challenge may depend on the concurrent inflammatory milieu of the lung. Using a murine AAI model, we demonstrate that after exposure to a defined natural protein allergen, Der p1, the response in lungs and draining mediastinal lymph nodes (dMLN) peaks between 4 and 6 days then declines until resolution by 21 days. Der p1-specific serum IgE follows the same pattern while IgG1 continues to increase. Resolution of AAI is mediated by CD4+CD25+Foxp3+ regulatory T cells (Tregs), which appear in lungs and dMLN following airway challenge. Treg depletion exacerbated lung eosinophilia, increased dMLN IL-5 and IL-13 but not IL-10 secretion, and increased allergic Ab responses. Most convincingly, transfer of CD4+CD25+Foxp3+ T cells from Ag naive mice (natural Tregs) abolished AAI, decreased dMLN IL-5 and IL-13 secretion, increased dMLN IL-10 secretion, abolished IgE, and decreased IgG1 Abs. Blocking IL-10 receptor function in vivo did not block the anti-inflammatory function of transferred natural Tregs but did restore dMLN IL-5 and IL-13 secretion. Thus natural Tregs can control AAI in an IL-10 independent manner.