SummaryLymphocytes circulate through lymph nodes (LN) in search for antigen in what is believed to be a continuous process. Here, we show that lymphocyte migration through lymph nodes and lymph occurred in a non-continuous, circadian manner. Lymphocyte homing to lymph nodes peaked at night onset, with cells leaving the tissue during the day. This resulted in strong oscillations in lymphocyte cellularity in lymph nodes and efferent lymphatic fluid. Using lineage-specific genetic ablation of circadian clock function, we demonstrated this to be dependent on rhythmic expression of promigratory factors on lymphocytes. Dendritic cell numbers peaked in phase with lymphocytes, with diurnal oscillations being present in disease severity after immunization to induce experimental autoimmune encephalomyelitis (EAE). These rhythms were abolished by genetic disruption of T cell clocks, demonstrating a circadian regulation of lymphocyte migration through lymph nodes with time-of-day of immunization being critical for adaptive immune responses weeks later.

Complement is an ancient danger-sensing system that contributes to host defense, immune surveillance and homeostasis. C5a and its G protein–coupled receptor mediate many of the proinflammatory properties of complement. Despite the key role of C5a in allergic asthma, autoimmune arthritis, sepsis and cancer, knowledge about its regulation is limited. Here we demonstrate that IgG1 immune complexes (ICs), the inhibitory IgG receptor FcγRIIB and the C-type lectin–like receptor dectin-1 suppress C5a receptor (C5aR) functions. IgG1 ICs promote the association of FcγRIIB with dectin-1, resulting in phosphorylation of Src homology 2 domain–containing inositol phosphatase (SHIP) downstream of FcγRIIB and spleen tyrosine kinase downstream of dectin-1. This pathway blocks C5aR-mediated ERK1/2 phosphorylation, C5a effector functions in vitro and C5a-dependent inflammatory responses in vivo, including peritonitis and skin blisters in experimental epidermolysis bullosa acquisita. Notably, high galactosylation of IgG N-glycans is crucial for this inhibitory property of IgG1 ICs, as it promotes the association between FcγRIIB and dectin-1. Thus, galactosylated IgG1 and FcγRIIB exert anti-inflammatory properties beyond their impact on activating FcγRs.

Loss of tolerance in systemic lupus erythematosus (SLE) leads to the generation of autoantibodies, which accumulate in end-organs where they induce disease. Here we show that immunoglobulin (Ig)G2a and 2b autoantibodies are the pathogenic isotypes by recruiting FcγRIV expressing macrophages. Class switching, but not development, of IgM anti-self B cells to these pathogenic subclasses requires the innate immune receptor Toll-like receptor (TLR)9 and MyD88 signaling. In their absence, switching of autoreactive B cells to the IgG2a and 2b subclasses is blocked, resulting in reduced pathology and mortality. In contrast, switching of anti-self B cells to IgG1 is not perturbed and generation of nonautoreactive IgG2a and 2b antibodies is not impaired in TLR9-deficient mice. Thus, the TLR9 pathway is a potential target for therapeutic intervention in SLE.

The final differentiation or maturation of dendritic cells (DCs) in response to environmental stimuli influences their ability to both initiate immunity and determine the quality of the response to antigens. Circulating immune complexes and cell-bound immunoglobulins present in normal human sera represent a potential stimulus for inadvertent DC activation in the steady state and during autoimmunity. Here, we show that selective blockade of the inhibitory Fc␥ receptor (Fc␥R) Fc␥RIIb with recently developed monoclonal antibodies leads to maturation of human monocytederived DCs, which depends on the presence of IgG in normal human plasma. Plasma, in the presence of an Fc␥RIIb blockade, caused the DCs to up-regulate the expression of costimulatory molecules and to produce the inflammatory mediator IL-12p70. Fc␥RIIb blockade of DCs loaded with tumor cells led to increased tumor-specific T cell immunity without the need for exogenous stimuli other than human plasma. Therefore, the activation status of DCs in the presence of normal human serum depends on the balance between activating and inhibitory Fc␥Rs and can be enhanced by new antibodies that react selectively with Fc␥RIIb. These data suggest an approach for modifying this balance to enhance immunity to immune complexes and antibody-coated tumor cells and to silence DC activation by immune complexes in autoimmune states.autoimmunity ͉ monoclonal antibody ͉ myeloma ͉ vaccination ͉ crosspresentation D endritic cells (DCs) are highly differentiated antigenpresenting cells that play a key role in the initiation and regulation of T cell immunity to pathogens and tumors while at the same time preventing immune responses against self-tissues or environmental antigens (1, 2). A critical property of DCs is that their ability to activate or inhibit immunity is linked to environmental stimuli, which determine their final differentiation or maturation status (3). Several stimuli, such as pathogens recognized by means of Toll-like receptors, CD40L, heat shock proteins, inflammatory cytokines, and innate lymphocytes, can lead to DC maturation and T cell immunity (2). However, under steady state, DCs must avoid inappropriate activation to prevent responses to self-antigens (''horror autotoxicus'') and harmless environmental antigens (4, 5). Specific pathways that prevent spontaneous DC activation are not as well understood as microbial and inflammatory stimuli.Circulating immune complexes and cell-bound immunoglobulins present in normal human sera represent a potential stimulus for DC activation in the steady state (6). The physiologic consequences of cell-bound IgG and immune complexes are modulated by a balance between activating and inhibitory Fc␥ receptors (Fc␥Rs) and include immune regulatory and inflammatory responses (7-10). Engagement of activating Fc␥Rs that contain an immune tyrosine-based activation motif on effector cells, including monocytes, neutrophils, natural killer cells, and mast cells, mediates phagocytosis, antibody-dependent cellmediated cytotoxicity, and r...