No abstract

Macrophage migration inhibitory factor (MIF) is an upstream activator of innate immunity that regulates subsequent adaptive responses. It was previously shown that in macrophages, MIF binds to a complex of CD74 and CD44, resulting in initiation of a signaling pathway. In the current study, we investigated the role of MIF in B cell survival. We show that in B lymphocytes, MIF initiates a signaling cascade that involves Syk and Akt, leading to NF-B activation, proliferation, and survival in a CD74-and CD44-dependent manner. Thus, MIF regulates the adaptive immune response by maintaining the mature B cell population.An established paradigm divides the immunologic response into innate and adaptive components, with critical interactions between the two producing normal immunity or immunopathology. The innate response represents the earliest host response to invasive pathogens by cells such as monocytes/ macrophages, whereas the adaptive response includes the development of antigen-specific responses, antibody production, and immunologic memory. Significant attention is currently focused on the molecules that link innate and adaptive immunity, and that may critically regulate immune responses or the development of inflammatory/autoimmune diseases.CD74 is a non-polymorphic type II integral membrane protein that is expressed on antigen presenting cells including macrophages and B cells. It has a short N-terminal cytoplasmic tail of 28 amino acids, followed by a single 24-amino acid transmembrane region and an ϳ150-amino acid lumenal domain. The CD74 chain was considered initially to function mainly as a major histocompatibility complex class II chaperone, which promotes endoplasmic reticulum exit of major histocompatibility complex class II molecules, directs them to endocytic compartments, prevents peptide binding in the endoplasmic reticulum, and contributes to peptide editing in the major histocompatibility complex class II compartment (1). A small proportion of CD74 is modified by the addition of chondroitin sulfate (CD74-CS), and this form of CD74 is expressed on the surface of antigen presenting cells, including monocytes and B cells. Antibody blocking studies additionally have shown that CD74-CS interacts with CD44, which activates a Src-kinase dependent signaling pathway (2).It was previously shown that macrophage migration inhibitory factor (MIF) 3 binds to the CD74 extracellular domain on macrophages, a process that results in initiation of a signaling pathway (3). MIF accounts for one of the first cytokine activities to have been described (4). MIF promotes monocyte/macrophage activation and is required for the optimal expression of tumor necrosis factor, interleukin-1, and prostaglandin E 2 (5-7). MIF-activated macrophages are more phagocytic and better able to destroy intracellular pathogens, such as Leishmania (8, 9). These activating functions have been verified in MIF knock-out mice (6, 10, 11). The role of MIF in adaptive immunity is less well characterized, but neutralization of MIF using specific anti...

In a manner dependent on CD4 T cell help and Toll-like receptor signals, B cells lacking WASp induce autoantibody production and autoimmune disease in mice.

No abstract

To more precisely identify the B-cell phenotype in Wiskott-Aldrich syndrome (WAS), we used 3 distinct murine in vivo models to define the cell intrinsic requirements for WAS protein (WASp) in central versus peripheral B-cell development. Whereas WASp is dispensable for early bone marrow B-cell development, WASp deficiency results in a marked reduction in each of the major mature peripheral B-cell subsets, exerting the greatest impact on marginal zone and B1a B cells. Using in vivo bromodeoxyuridine labeling and in vitro functional assays, we show that these deficits reflect altered peripheral homeostasis, partially resulting from an impairment in integrin function, rather than a developmental defect. Consistent with these observations, we also show that: (1) WASp expression levels increase with cell maturity, peaking in those subsets exhibiting the greatest sensitivity to WASp deficiency; (2) WASp+ murine B cells exhibit a marked selective advantage beginning at the late transitional B-cell stage; and (3) a similar in vivo selective advantage is manifest by mature WASp+ human B cells. Together, our data provide a better understanding of the clinical phenotype of WAS and suggest that gene therapy might be a useful approach to rescue altered B-cell homeostasis in this disease.

The cytokine thymic stromal lymphopoietin (TSLP) drives immature B cell development in vitro and may regulate T helper type 2 responses. Here we analyzed the involvement of TSLP in B cell development in vivo with a doxycycline-inducible, keratin 5-driven transgene encoding TSLP (K5-TSLP). K5-TSLP-transgenic mice given doxycycline showed an influx of immature B cells into the periphery, with population expansion of follicular mature B cells, near-complete loss of marginal zone and marginal zone precursor B cells, and 'preferential' population expansion of peritoneal B-1b B cells. These changes promoted cryoglobulin production and immune complex-mediated renal disease. Identical events occurred in mice without T cells, in alternative TSLP-transgenic models and in K5-TSLP-transgenic mice with undetectable systemic TSLP. These observations suggest that signals mediating localized TSLP expression may modulate systemic B cell development and promote humoral autoimmunity.

Quite a few regulatory proteins, including transcription factors, are normally maintained in a dormant state to be activated after internal or environmental cues. Recently, a novel strategy, requiring proteolytic cleavage, was described for the mobilization of dormant transcription factors. These transcription factors are initially synthesized in an inactive form, whereas "nesting" in integral membrane precursor proteins. After a cleavage event, these new active factors are released from the membrane and can migrate into the nucleus to drive regulated gene transcription. This mechanism, regulated intramembrane proteolysis (RIP), controls diverse biological processes in prokaryotes and eukaryotes in response to a variety of signals. The MHC class II chaperone, CD74 (invariant chain, Ii), was previously shown to function as a signaling molecule in several pathways. Recently, we demonstrated that after intramembranal cleavage, the CD74 cytosolic fragment (CD74-ICD) is released and induces activation of transcription mediated by the NF-B p65/RelA homodimer and the B-cell-enriched coactivator, TAF II 105. Here, we add CD74 to the growing family of RIP-processed proteins. Our studies show that CD74 ectodomain must be processed in the endocytic compartments to allow its intramembrane cleavage that liberates CD74 intracellular domain (CD74-ICD). We demonstrate that CD74-ICD translocates to the nucleus and induces the activation of the p65 member of NF-B in this compartment. INTRODUCTIONQuite a few regulatory proteins, including transcription factors, are normally maintained in a dormant state and are activated by internal or environmental cues. Recently, a novel strategy (regulated intramembrane proteolysis [RIP]), based on proteolytic cleavage, was discovered for the mobilization of dormant transcription factors. These transcription factors are synthesized initially as inactive, membranebound precursors. Once triggered, RIP proteins are cleaved within the plane of the membrane, and their cytosolic fragment migrates into the nucleus to drive transcription.In general, in most RIP cases reported, cleavage proceeds through a two-step sequential proteolytic process. The first step involves the cleavage of the extracytoplasmic segment to shorten the ectodomain to Ͻ30 aa. This seems to be a requirement for the second proteolytic event, which occurs at the transmembrane domain. The initial shedding prepares the substrate for the intramembrane proteolysis such that the transmembrane region becomes accessible to the second protease, which then releases the product from the lipid bilayer. The released cytosolic fragment then migrates into the nucleus. Intramembrane cleaving proteases (I-CLiPs) catalyze peptide bond hydrolysis in the plane of cellular membranes. These proteases are thought to mediate RIP (Brown et al., 2000), and the reactions they catalyze are, in most cases, part of highly controlled processes. The family of I-CLiPs is growing, and at present, three distinct protease families have been shown to catalyze intramem...

CD74 is a cell-surface receptor for the cytokine macrophage migration inhibitory factor. Macrophage migration inhibitory factor binding to CD74 induces its intramembrane cleavage and the release of its cytosolic intracellular domain (CD74-ICD), which regulates cell survival. In the present study, we characterized the transcriptional activity of CD74-ICD in chronic lymphocytic B cells. We show that following CD74 activation, CD74-ICD interacts with the transcription factors RUNX (Runt related transcription factor) and NF-κB and binds to proximal and distal regulatory sites enriched for genes involved in apoptosis, immune response, and cell migration. This process leads to regulation of expression of these genes. Our results suggest that identifying targets of CD74 will help in understanding of essential pathways regulating B-cell survival in health and disease.CD74 | transcription | CLL | NF-κB | RUNX