Complement component C1q is an immunological rheostat that regulates Fc:Fc[Formula: see text]R interactions.

Though binding sites for the complement factor C1q and the canonical fragment crystallizable (Fc) gamma receptors (Fc[Formula: see text]Rs) on immunoglobulin G (IgG) molecules overlap, how C1q decoration of immune complexes (ICs) influences their ability to engage Fc[Formula: see text]Rs remains unknown. In this report, we use recombinant human Fc multimers as stable IC mimics to show that C1q engagement of ICs directly and transiently inhibits their interactions with Fc[Formula: see text]RIII (CD16) on human natural killer (NK) cells. This inhibition occurs by C1q engagement alone as well as in concert with other serum factors. Furthermore, the inhibition of Fc[Formula: see text]RIII engagement mediated by avid binding of C1q to ICs is directly associated with IC size and dependent on the concentrations of both C1q and Fc multimers present. Functionally, C1q-mediated Fc blockade limits the ability of NK cells to induce the upregulation of the cosignaling molecule, 4-1BB (CD137), and to mediate antibody-dependent cell-mediated cytotoxicity (ADCC). Although C1q is traditionally viewed as a soluble effector molecule, we demonstrate that C1q may also take on the role of an "immunologic rheostat," buffering Fc[Formula: see text]R-mediated activation of immune cells by circulating ICs. These data define a novel role for C1q as a regulator of immune homeostasis and add to our growing understanding that complement factors mediate pleiotropic effects.

Tumour antigen targeted monoclonal antibodies incorporating a novel multimerisation domain significantly enhance antibody dependent cellular cytotoxicity against colon cancer.

Tumour antigen targeted antibodies (mAbs) can induce natural killer (NK) cells to kill tumours through antibody dependent cellular cytotoxicity (ADCC) upon engagement of NK cell expressed FcγRIIIa. FcγRIIIa polymorphisms partially dictate the potency of the ADCC response. The high affinity FcγRIIIa-158-valine (V) polymorphism is associated with more potent ADCC response than the low affinity FcγRIIIa-158-phenylalanine (F) polymorphism. Because approximately 45% of patients are homozygous for the FcγRIIIa-158-F polymorphism (FF genotype), their ability to mount ADCC is impaired. We investigated whether a novel mAb capable of binding multiple antigen specific targets and engaging multiple low affinity FcγRIIIa receptors could further enhance ADCC against colon cancer in vitro. Specifically, we generated a novel anti-epidermal growth factor receptor (EGFR) antibody (termed a stradobody) consisting of an unmodified Fab sequence and two Immunoglobulin G, subclass 1 (IgG1) Fc domains separated by an isoleucine zipper domain and the 12 amino-acid IgG2 hinge. The stradobody framework induced multimerisation and was associated with increased binding to the EGFR and FcγRIIIa. From a functional perspective, when compared to an unmodified anti-EGFR mAb with a sequence identical to cetuximab (a commercially available anti-EGFR mAb), stradobodies significantly enhanced ADCC. These effects were observed using both KRAS wild type HT29 and KRAS mutant SW480 colon cancer cells as targets, and by NK cells obtained from healthy donors and a cohort of patients with colon cancer. These data suggest that high avidity cross-linking of multiple tumour surface antigens and multiple NK cell associated FcγRIIIa molecules can enhance ADCC and partially overcome impaired ADCC by FF genotype individuals in vitro.

The N-terminal peptide of PSGL-1 can mediate adhesion to trauma-activated endothelium via P-selectin in vivo.

P-selectin glycoprotein ligand-1 (PSGL-1) is present on leukocytes and is the major ligand for endothelial expressed P-selectin. A variety of studies strongly suggests that the N-terminal region of PSGL-1 contains the binding site for P-selectin. We hypothesized that this relatively small N-terminal peptide of PSGL-1 is sufficient to support adhesion to P-selectin in vivo. To test this hypothesis, we coated 2 microm-diameter microspheres with a recombinant PSGL-1 construct, termed 19.ek.Fc. The 19.ek.Fc construct consists of the first 19 N-terminal amino acids of mature PSGL-1 linked to an enterokinase cleavage site that, in turn, is linked to human immunoglobulin G Fc. The 19.ek.Fc-coated microspheres were injected into the jugular vein of mice. Intravital microscopy of postcapillary venules within the cremaster muscle of mice revealed that a significantly greater number of 19.ek.Fc microspheres rolled compared with control microspheres. The number of rolling 19.ek.Fc microspheres was significantly diminished by pretreatment of the mice with a monoclonal antibody to P-selectin or by pretreatment of the 19.ek.Fc microspheres with a monoclonal antibody to PSGL-1. Combined, the results indicate that the N-terminal peptide of PSGL-1 can mediate adhesion to trauma-activated microvascular endothelium via P-selectin in vivo.