To investigate the role of signal transducer and activator of transcription (STAT) proteins in granulocyte colony-stimulating factor (G-CSF)-regulated biological responses, we generated transgenic mice with a targeted mutation of their G-CSF receptor (termed d715F) that abolishes G-CSF-dependent STAT-3 activation and attenuates STAT-5 activation. Homozygous mutant mice are severely neutropenic with an accumulation of immature myeloid precursors in their bone marrow. G-CSF-induced proliferation and granulocytic differentiation of hematopoietic progenitors is severely impaired. Expression of a constitutively active form of STAT-3 in d715F progenitors nearly completely rescued these defects. Conversely, expression of a dominant-negative form of STAT-3 in wild-type progenitors results in impaired G-CSF-induced proliferation and differentiation. These data suggest that STAT-3 activation by the G-CSFR is critical for the transduction of normal proliferative signals and contributes to differentiative signals.

Mycobacterium abscessus causes disease in patients with structural abnormalities of the lung, and it is an emerging pathogen in patients with cystic fibrosis. Colonization of the airways by nontuberculous mycobacteria is a harbinger of invasive lung disease. Colonization is facilitated by biofilm formation, with M. abscessus glycopeptidolipids playing an important role. M. abscessus can transition between a noninvasive, biofilm-forming, smooth colony phenotype that expresses glycopeptidolipid, and an invasive rough colony phenotype that expresses minimal amounts of glycopeptidolipid and is unable to form biofilms. The ability of this pathogen to transition between these phenotypes may have particular relevance to lung infection in cystic fibrosis patients since the altered pulmonary physiology of these patients makes them particularly susceptible to colonization by biofilm-forming bacteria. In this study we demonstrate that rough variants of M. abscessus stimulate the human macrophage innate immune response through TLR2, while smooth variants do not. Temperature-dependent loss or physical removal of glycopeptidolipid from the cell wall of one of the smooth variants leads to TLR2 stimulation. This response is stimulated in part through phosphatidyl-myo-inositol mannosides that are present in the cell wall of both rough and smooth variants. Mannose-binding lectins bind to rough variants, but lectin binding to an isogenic smooth variant is markedly reduced. This suggests that glycopeptidolipid in the outermost portion of the M. abscessus cell wall masks underlying cell wall lipids involved in stimulating the innate immune response, thereby facilitating colonization. Conversely spontaneous “unmasking” of cell wall lipids may promote airway inflammation.

Lin et al. show that Bhlhe40 expression identifies encephalitogenic CD4+ T helper cells and define a pertussis toxin–IL-1–Bhlhe40 pathway active in experimental autoimmune encephalomyelitis, a mouse model of multiple sclerosis.

B cells are increasingly regarded as integral to the pathogenesis of multiple sclerosis (MS) in part due to the success of B cell depletion therapy. Multiple B cell-dependent mechanisms contributing to inflammatory demyelination of the central nervous system (CNS) have been explored using experimental autoimmune encephalomyelitis (EAE), a CD4 T cell-dependent animal model for multiple sclerosis (MS). While B cell antigen presentation has been suggested to regulate CNS inflammation during EAE, direct evidence that B cells can independently support antigen-specific autoimmune responses by CD4 T cells in EAE is lacking. Using a newly developed murine model of in vivo conditional expression of MHCII, we previously reported that encephalitogenic CD4 T cells are incapable of inducing EAE when B cells are the sole antigen presenting cell. Herein we find that B cells cooperate with dendritic cells to enhance EAE severity resulting from myelin oligodendrocyte glycoprotein (MOG) immunization. Further, increasing the precursor frequency of MOG-specific B cells, but not addition of soluble MOG-specific antibody, is sufficient to drive EAE in mice expressing MHCII by B cells alone. These data support a model in which expansion of antigen-specific B cells during CNS autoimmunity amplifies cognate interactions between B and CD4 T cells and have the capacity to independently drive neuro-inflammation at later stages of disease.

The activation, differentiation and subsequent effector functions of CD4 T cells depend on interactions with a multitude of MHCII-expressing antigen presenting cells (APCs). To evaluate the individual contribution of various APCs to CD4 T cell function, we have designed a new murine tool for selective in vivo expression of MHCII in subsets of APCs. Conditional expression of MHCII in B cells was achieved using a cre-loxP approach. After intravenous or subcutaneous priming, partial proliferation and activation of CD4 T cells was observed in mice expressing MHCII only by B cells. Restricting MHCII expression to B cells constrained secondary CD4 T cell responses in vivo, as demonstrated in a CD4 T cell-dependent model of autoimmunity, EAE. These results highlight the limitations of B cell antigen presentation during initiation and propagation of CD4 T cell function in vivo using a novel system to study individual APCs by the conditional expression of MHCII.