12/15-lipoxygenase orchestrates the clearance of apoptotic cells and maintains immunologic tolerance.

Noninflammatory clearance of apoptotic cells (ACs) is crucial to maintain self-tolerance. Here, we have reported a role for the enzyme 12/15-lipoxygenase (12/15-LO) as a central factor governing the sorting of ACs into differentially activated monocyte subpopulations. During inflammation, uptake of ACs was confined to a population of 12/15-LO-expressing, alternatively activated resident macrophages (resMΦ), which blocked uptake of ACs into freshly recruited inflammatory Ly6C(hi) monocytes in a 12/15-LO-dependent manner. ResMΦ exposed 12/15-LO-derived oxidation products of phosphatidylethanolamine (oxPE) on their plasma membranes and thereby generated a sink for distinct soluble receptors for ACs such as milk fat globule-EGF factor 8, which were essential for the uptake of ACs into inflammatory monocytes. Loss of 12/15-LO activity, in turn, resulted in an aberrant phagocytosis of ACs by inflammatory monocytes, subsequent antigen presentation of AC-derived antigens, and a lupus-like autoimmune disease. Our data reveal an unexpected key role for enzymatic lipid oxidation during the maintenance of self-tolerance.

Monocyte subsets responsible for immunoglobulin G-dependent effector functions in vivo.

Immunoglobulin G (IgG) antibodies confer protection against pathogenic microorganisms, serve as therapeutics in tumor therapy, and are involved in destruction of healthy tissues during autoimmune diseases. Understanding the molecular pathways and effector cell types involved in antibody-mediated effector functions is a prerequisite to modulate these activities. In this study we used two independent model systems to identify innate immune effector cells required for IgG activity in vivo. We first defined the precise repertoire of receptors for the IgG Fc fragment (FcγR) on innate immune effector cells in the blood and on tissue-resident macrophage populations. Despite expression of relevant activating FcγRs on various phagocyte populations, our data indicate that the majority of these cell types are dispensable for IgG activity in vivo. In contrast, IgG-dependent effector functions were selectively impaired in animals lacking the CX(3)CR1(hi)Ly6C(lo)CD11c(int) monocyte subset, which expressed the full set of FcγRs required for IgG activity.

B-cell inhibition by cross-linking CD79b is superior to B-cell depletion with anti-CD20 antibodies in treating murine collagen-induced arthritis.

Depletion of B cells with the anti-CD20 antibody rituximab is an established therapy for rheumatoid arthritis. However, rituximab has only moderate efficacy, most likely due to insufficient depletion of B cells in lymphoid organs and expansion of pathogenic B cells. We found that an antibody against mouse CD79b profoundly blocks B-cell proliferation induced via the B-cell receptor, CD40, CD180, and chondroitin sulfate, but not via TLR4 or TLR9. Treatment with anti-CD79b also induces death in resting and activated B cells. B-cell inhibition is mediated by cross-linkage of CD79b, but independent of Fc-receptor engagement. In the model of collagen-induced arthritis, an antibody against mouse CD20 depletes B cells very efficiently but fails to suppress the humoral immune response against collagen and the development of arthritis. In contrast, the antibody against CD79b, and a deglycosylated variant of this antibody, almost completely inhibits the increase in anti-collagen antibodies and the development of arthritis. In mice with established arthritis only the fully glycosylated antibody against CD79b is effective. Our data show that targeting B cells via CD79b is much more effective than B-cell depletion with anti-CD20 antibodies for therapy of arthritis. These findings may have important implications for treatment of B-cell-mediated autoimmune diseases.

B cells are critical for autoimmune pathology in Scurfy mice.

Impaired regulatory T-cell function results in a severe chronic autoimmune disease affecting multiple organs in Scurfy mice and humans with the immune dysregulation, polyendocrinopathy, enteropathy, X-linked (IPEX) syndrome. Previous studies have shown that T helper cells but not cytotoxic T cells are critical for the disease pathology. Whether this T-cell subset is responsible directly for tissue inflammation or rather indirectly via the interaction with B cells or myeloid cells is largely unknown. To study this and to identify potential therapeutic targets for this lethal disease we investigated the contribution of B cells to this complex autoimmune phenotype. We show that B cells and the production of autoantibodies plays a major role for skin, liver, lung, and kidney inflammation and therapeutic depletion of B cells resulted in reduced tissue pathology and in prolonged survival. In contrast, the absence of B cells did not impact systemic T-cell activation and hyperreactivity, indicating that autoantibody production by B cells may be a major factor for the autoimmune pathology in mice deficient for regulatory T cells.

B cells and CD22 are dispensable for the immediate antiinflammatory activity of intravenous immunoglobulins in vivo.

Intravenous immunoglobulins (IVIgs) efficiently suppress a variety of autoimmune diseases. Over the past few years several potential mechanisms underlying this antiinflammatory activity have become apparent. Among these, terminal sialic acid residues in the sugar moiety of the immunoglobulin G constant fragment have been shown to be critical for the antiinflammatory activity of IVIgs in models of rheumatoid arthritis and immunothrombocytopenia (ITP). More recently, B cells and the sialic acid-binding protein CD22 were suggested to be involved in this IVIg-dependent immunomodulatory pathway. To study whether B cells are directly involved in IVIg-mediated suppression of acute autoimmune diseases, we tested the activity of IVIgs in mice deficient in B cells or CD22. We show that neither B cells nor CD22 are critical for the immediate antiinflammatory activity of IVIgs in mouse models of rheumatoid arthritis and ITP.

The role of Fcgamma receptors in murine autoimmune thrombocytopenia.

Immune thrombocytopenia (ITP) can become a life-threatening condition that requires immediate medical attention. The loss in platelet numbers during ITP can be induced by a variety of triggers. Anti-platelet antibodies of several isotypes and subclasses are a major cause for ITP and are a hallmark of many complex autoimmune diseases such as systemic lupus erythematosus. Mouse models have been important to understand the effector pathways involved in antibody-mediated platelet depletion. Therapeutic interventions based on these results have been proven successful in treating human ITP, thus validating the use of these model systems. One major problem that remains to be answered is which cell populations are crucial for platelet removal. Targeting these cells directly might be a novel therapeutic strategy and will also be important to understand the underlying biological mechanisms.