Identification of Four Mouse FcRn Splice Variants and FcRn-Specific Vesicles.

Research into the neonatal Fc receptor (FcRn) has increased dramatically ever since Simister and Mostov first purified a rat version of the receptor. Over the years, FcRn has been shown to function not only as a receptor that transfers immunity from mother to fetus but also performs an array of different functions that include transport and recycling of immunoglobulins and albumin in the adult. Due to its important cellular roles, several clinical trials have been designed to either inhibit/enhance FcRn function or develop of non-invasive therapeutic delivery system such as fusion of drugs to IgG Fc or albumin to enhance delivery inside the cells. Here, we report the accidental identification of several FcRn alternatively spliced variants in both mouse and human cells. The four new mouse splice variants are capable of binding immunoglobulins' Fc and Fab portions. In addition, we have identified FcRn-specific vesicles in which immunoglobulins and albumin can be stored and that are involved in the endosomal-lysosomal system. The complexity of FcRn functions offers significant potential to design and develop novel and targeted therapeutics.

Fc engineered anti-virus therapeutic human IgG1 expressed in plants with altered binding to the neonatal Fc receptor.

Production of therapeutic monoclonal antibody (mAb) in transgenic plants has several advantages such as large-scale production and the absence of pathogenic animal contaminants. However, mAb with high mannose (HM) type glycans has shown a faster clearance compared to antibodies produced in animal cells. The neonatal Fc receptor (FcRn) regulates the persistence of immunoglobulin G (IgG) by the FcRn-mediated recycling pathway, which salvages IgG from lysosomal degradation within cells. In this study, Fc-engineering of antirabies virus therapeutic mAb SO57 with the endoplasmic reticulum (ER)-retention peptide signal (Lys-Asp-Glu-Leu; KDEL) (mAbpK SO57) in plant cell was conducted to enhance its binding activity to human neonatal Fc receptor (hFcRn), consequently improve its serum half-life. Enzyme-linked immunosorbent assay (ELISA) and Surface plasmon resonance assay showed altered binding affinity of the Fc region of three different mAbpK SO57 variants [M252Y/S254T/T256E (MST), M428L/N434S (MN), H433K/N434F (HN)] to hFcRn compared to wild type (WT) of mAbpK SO57. Molecular modeling data visualized the structural alterations in these mAbpK SO57. All of the mAbpK SO57 variants had HM type glycan structures similar to the WT mAbpK SO57. In addition, the neutralizing activity of the three variants against the rabies virus CVS-11 was effective as the WT mAbpK SO57. These results indicate that the binding affinity of mAbpK SO57 variants to hFcRn can be modified without alteration of N-glycan structure and neutralization activity. Taken together, this study suggests that Fc-engineering of antirabies virus mAb can be applied to enhance the efficacy of therapeutic mAbs in plant expression systems.

Intranasal Nose-to-Brain Drug Delivery via the Olfactory Region in Mice: Two In-Depth Protocols for Region-Specific Intranasal Application of Antibodies and for Expression Analysis of Fc Receptors via In Situ Hybridization in the Nasal Mucosa.

A region-specific catheter-based intranasal administration method was successfully developed, established, and validated as reported previously. By using this method, drugs can be applicated specifically to the olfactory region. Thereby, intranasally administered drugs could be delivered via neuronal connections to the central nervous system. Here, we present a detailed protocol with a step-by-step procedure for nose-to-brain delivery via the olfactory mucosa.Fc receptors such as the neonatal Fc receptor (FcRn) and potentially Fcγ receptor IIb (FcγRIIb) are involved in the uptake and transport of antibodies via the olfactory nasal mucosa. To better characterize their expression levels and their role in CNS drug delivery via the nose, an in situ hybridization (ISH) protocol was adapted for nasal mucosa samples and described in abundant details.

Functions of IgM fc receptor (FcµR) related to autoimmunity.

Unlike Fc receptors for switched immunoglobulin (Ig) isotypes, Fc receptor for IgM (FcµR) is selectively expressed by lymphocytes. The ablation of the FcµR gene in mice impairs B cell tolerance as evidenced by concomitant production of autoantibodies of IgM and IgG isotypes. In this essay, we reiterate the autoimmune phenotypes observed in mutant mice, ie IgM homeostasis, dysregulated humoral immune responses including autoantibodies, and Mott cell formation. We also propose the potential phenotypes in individuals with FCMR deficiency and the model for FcµR-mediated regulation of self-reactive B cells.

An engineered immunomodulatory IgG1 Fc suppresses autoimmune inflammation through pathways shared with i.v. immunoglobulin.

Immunoglobulin G (IgG) antibodies in the form of high-dose intravenous immunoglobulin (IVIG) exert immunomodulatory activity and are used in this capacity to treat inflammatory and autoimmune diseases. Reductionist approaches have revealed that terminal sialylation of the single asparagine-linked (N-linked) glycan at position 297 of the IgG1 Fc bestows antiinflammatory activity, which can be recapitulated by introduction of an F241A point mutation in the IgG1 Fc (FcF241A). Here, we examined the antiinflammatory activity of CHO-K1 cell-produced FcF241A in vivo in models of autoimmune inflammation and found it to be independent of sialylation. Intriguingly, sialylation markedly improved the half-life and bioavailability of FcF241A via impaired interaction with the asialoglycoprotein receptor ASGPR. Further, FcF241A suppressed inflammation through the same molecular pathways as IVIG and sialylated IgG1 Fc and required the C-type lectin SIGN-R1 in vivo. This contrasted with FcAbdeg (efgartigimod), an engineered IgG1 Fc with enhanced neonatal Fc receptor (FcRn) binding, which reduced total serum IgG concentrations, independent of SIGN-R1. When coadministered, FcF241A and FcAbdeg exhibited combinatorial antiinflammatory activity. Together, these results demonstrated that the antiinflammatory activity of FcF241A requires SIGN-R1, similarly to that of high-dose IVIG and sialylated IgG1, and can be used in combination with other antiinflammatory therapeutics that rely on divergent pathways, including FcAbdeg.

Physiologically based pharmacokinetic (PBPK) model that describes enhanced FcRn-dependent distribution of monoclonal antibodies (mAbs) by pI-engineering in mice.

We previously reported that monoclonal antibodies (mAbs) with a high isoelectric point (pI) value tended to exhibit fast plasma clearance (CL) and large steady-state volume of distribution (Vdss) in mice. However, the positive correlation between pI, CL, and Vdss cannot be described by the reported physiologically based pharmacokinetic (PBPK) models, in which FcRn-mediated transcytosis of mAbs is set to be minimal compared to convection-mediated transport. To address this issue, physiological parameters (lymph flow rate, reflection coefficient, endothelial uptake clearance, and FcRn concentration) were optimized based on the pharmacokinetic profiles of mAbs with various pI values in wild type and FcRn-deficient (beta-2-microglobulin knockout [KO]) mice. Simulations using the PBPK model developed in this study showed a positive correlation between pI, CL and Vdss observed in wild-type mice. Therefore, this model successfully characterized our hypothetical mechanism that an electrostatic positive interaction between mAbs and the endothelial membrane enhances FcRn-mediated transcytosis of mAbs, resulting in large Vdss. We sought to determine the right contribution of the two pathways of antibody distribution to the interstitial space and established a new model that could effectively capture the effect of pI on FcRn-mediated distribution of mAbs in the body.

FCRL1 immunoregulation in B cell development and malignancy.

Immunotherapeutic targeting of surface regulatory proteins and pharmacologic inhibition of critical signaling pathways has dramatically shifted our approach to the care of individuals with B cell malignancies. This evolution in therapy reflects the central role of the B cell receptor (BCR) signaling complex and its co-receptors in the pathogenesis of B lineage leukemias and lymphomas. Members of the Fc receptor-like gene family (FCRL1-6) encode cell surface receptors with complex tyrosine-based regulation that are preferentially expressed by B cells. Among them, FCRL1 expression peaks on naïve and memory B cells and is unique in terms of its intracellular co-activation potential. Recent studies in human and mouse models indicate that FCRL1 contributes to the formation of the BCR signalosome, modulates B cell signaling, and promotes humoral responses. Progress in understanding its regulatory properties, along with evidence for its over-expression by mature B cell leukemias and lymphomas, collectively imply important yet unmet opportunities for FCRL1 in B cell development and transformation. Here we review recent advances in FCRL1 biology and highlight its emerging significance as a promising biomarker and therapeutic target in B cell lymphoproliferative disorders.

The potential of FCRL genes as targets for cancer treatment: insights from bioinformatics and immunology.

Cancer is a prevalent and dangerous disease that requires a multifaceted approach to treatment. The FCRL family gene has been linked to immune function and tumor progression. Bioinformatics may help unravel their role in cancer treatment. We conducted a comprehensive analysis of the FCRL family genes in pan-cancer using publicly available databases and online tools. Specifically, we examined gene expression, prognostic significance, mutation profiles, drug resistance, as well as biological and immunomodulatory roles. Our data were sourced from The Cancer Genome Atlas, Genotype-Tissue Expression, cBioPortal, STRING, GSCALite, Cytoscape, and R software. The expression of FCRL genes varies significantly across different tumor types and normal tissues. While high expression of most FCRL genes is associated with a protective effect in many cancers, FCRLB appears to be a risk factor in several types of cancer. Alterations in FCRL family genes, particularly through amplification and mutation, are common in cancers. These genes are closely linked to classical cancer pathways such as apoptosis, epithelial-mesenchymal transition (EMT), estrogen receptor (ER) signaling, and DNA damage response. Enrichment analysis indicates that FCRL family genes are predominantly associated with immune cell activation and differentiation. Immunological assays demonstrate a strong positive correlation between FCRL family genes and tumor-infiltrating lymphocytes (TILs), immunostimulators, and immunoinhibitors. Furthermore, FCRL family genes can enhance the sensitivity of various anticancer drugs. The FCRL family genes are vital in cancer pathogenesis and progression. Targeting these genes in conjunction with immunotherapy could enhance cancer treatment efficacy. Further research is required to determine their potential as therapeutic targets.

ChAdOx1 nCoV-19 (AZD1222) vaccine-induced Fc receptor binding tracks with differential susceptibility to COVID-19.

Despite the success of COVID-19 vaccines, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants of concern have emerged that can cause breakthrough infections. Although protection against severe disease has been largely preserved, the immunological mediators of protection in humans remain undefined. We performed a substudy on the ChAdOx1 nCoV-19 (AZD1222) vaccinees enrolled in a South African clinical trial. At peak immunogenicity, before infection, no differences were observed in immunoglobulin (Ig)G1-binding antibody titers; however, the vaccine induced different Fc-receptor-binding antibodies across groups. Vaccinees who resisted COVID-19 exclusively mounted FcγR3B-binding antibodies. In contrast, enhanced IgA and IgG3, linked to enriched FcγR2B binding, was observed in individuals who experienced breakthrough. Antibodies unable to bind to FcγR3B led to immune complex clearance and resulted in inflammatory cascades. Differential antibody binding to FcγR3B was linked to Fc-glycosylation differences in SARS-CoV-2-specific antibodies. These data potentially point to specific FcγR3B-mediated antibody functional profiles as critical markers of immunity against COVID-19.

Antimyeloperoxidase antibodies modulate inflammatory responses and activate profibrotic pathways in human monocytes.

Antimyeloperoxidase (anti-MPO) and antiproteinase 3 (anti-PR3) antibodies are found in anti-neutrophil cytoplasmic antibody-associated vasculitis (AAV). We investigated the effect of both anti-MPO and anti-PR3 IgG on human monocytes. Peripheral blood monocytes were cultured under a range of conditions that included TLR agonists, anti-MPO IgG and anti-PR3 IgG with appropriate controls. Experiments included whole transcriptome profiling and an assessment of the role of Fc receptors. When monocytes were stimulated with LPS or R848, anti-MPO but not anti-PR3 IgG, caused a reduction in IL-10 secretion and had a profound effect on cell-surface marker expression. Anti-MPO but not anti-PR3 IgG enhanced monocyte survival in the absence of TLR stimulation. These effects depended on the Fc receptor CD32a. With TLR stimulation, the effect of anti-MPO but not anti-PR3 IgG on the transcriptional response at 6 h was variable, but we identified a core set of transcripts likely to be important. Without TLR stimulation, there was a robust effect of anti-MPO but not anti-PR3 IgG on the transcriptional response at 24 h, and there was a highly significant enrichment of genes encoding extracellular matrix and extracellular matrix-associated proteins. Analysis with nCounter confirmed many of the differentially expressed transcripts and supported a role for CD32a. These data show that anti-MPO, but not anti-PR3 IgG, from patients with AAV has wide-ranging effects on monocytes which depend on CD32a. The activation of a profibrotic transcriptional response by anti-MPO but not anti-PR3 IgG may give insights into the differences in disease phenotype.

Enhanced Mott cell formation linked with IgM Fc receptor (FcµR) deficiency.

In previous studies, Mott cells, an unusual form of plasma cells containing Ig-inclusion bodies, were frequently observed in peripheral lymphoid tissues in our IgM Fc receptor (FcµR)-deficient (KO) mouse strain. Because of discrepancies in the reported phenotypes of different Fcmr KO mouse strains, we here examined two additional available mutant strains and confirmed that such enhanced Mott-cell formation was a general phenomenon associated with FcµR deficiency. Splenic B cells from Fcmr KO mice clearly generated more Mott cells than those from WT mice when stimulated in vitro with LPS alone or a B-1, but not B-2, activation cocktail. Nucleotide sequence analysis of the Ig variable regions of a single IgMλ+ Mott-hybridoma clone developed from splenic B-1 B cells of Fcmr KO mice revealed the near (VH) or complete (Vλ) identity with the corresponding germline gene segments and the addition of six or five nucleotides at the VH/DH and DH/JH junctions, respectively. Transduction of an FcµR cDNA into the Mott hybridoma significantly reduced cells containing IgM-inclusion bodies with a concomitant increase in IgM secretion, leading to secreted IgM binding to FcµR expressed on Mott transductants. These findings suggest a regulatory role of FcµR in the formation of Mott cells and IgM-inclusion bodies.

Evidence for Extensive Duplication and Subfunctionalization of FCRL6 in Armadillo (Dasypus novemcinctus).

The control of infections by the vertebrate adaptive immune system requires careful modulation to optimize defense and minimize harm to the host. The Fc receptor-like (FCRL) genes encode immunoregulatory molecules homologous to the receptors for the Fc portion of immunoglobulin (FCR). To date, nine different genes (FCRL1-6, FCRLA, FCRLB and FCRLS) have been identified in mammalian organisms. FCRL6 is located at a separate chromosomal position from the FCRL1-5 locus, has conserved synteny in mammals and is situated between the SLAMF8 and DUSP23 genes. Here, we show that this three gene block underwent repeated duplication in Dasypus novemcinctus (nine-banded armadillo) resulting in six FCRL6 copies, of which five appear functional. Among 21 mammalian genomes analyzed, this expansion was unique to D. novemcinctus. Ig-like domains that derive from the five clustered FCRL6 functional gene copies show high structural conservation and sequence identity. However, the presence of multiple non-synonymous amino acid changes that would diversify individual receptor function has led to the hypothesis that FCRL6 endured subfunctionalization during evolution in D. novemcinctus. Interestingly, D. novemcinctus is noteworthy for its natural resistance to the Mycobacterium leprae pathogen that causes leprosy. Because FCRL6 is chiefly expressed by cytotoxic T and NK cells, which are important in cellular defense responses against M. leprae, we speculate that FCRL6 subfunctionalization could be relevant for the adaptation of D. novemcinctus to leprosy. These findings highlight the species-specific diversification of FCRL family members and the genetic complexity underlying evolving multigene families critical for modulating adaptive immune protection.

Toward systems-informed models for biologics disposition: covariates of the abundance of the neonatal Fc Receptor (FcRn) in human tissues and implications for pharmacokinetic modelling.

Biologics are a fast-growing therapeutic class, with intertwined pharmacokinetics and pharmacodynamics, affected by the abundance and function of the FcRn receptor. While many investigators assume adequacy of classical models, such as allometry, for pharmacokinetic characterization of biologics, advocates of physiologically-based pharmacokinetics (PBPK) propose consideration of known systems parameters that affect the fate of biologics to enable a priori predictions, which go beyond allometry. The aim of this study was to deploy a systems-informed modelling approach to predict the disposition of Fc-containing biologics. We used global proteomics to quantify the FcRn receptor [p51 and ß2-microglobulin (B2M) subunits] in 167 samples of human tissue (liver, intestine, kidney and skin) and assessed covariates of its expression. FcRn p51 subunit was highest in liver relative to other tissues, and B2M was 1-2 orders of magnitude more abundant than FcRn p51 across all sets. There were no sex-related differences, while higher expression was confirmed in neonate liver compared with adult liver. Trends of expression in liver and kidney indicated a moderate effect of body mass index, which should be confirmed in a larger sample size. Expression of FcRn p51 subunit was approximately 2-fold lower in histologically normal liver tissue adjacent to cancer compared with healthy liver. FcRn mRNA in plasma-derived exosomes correlated moderately with protein abundance in matching liver tissue, opening the possibility of use as a potential clinical tool. Predicted effects of trends in FcRn abundance in healthy and disease (cancer and psoriasis) populations using trastuzumab and efalizumab PBPK models were in line with clinical observations, and global sensitivity analysis revealed endogenous IgG plasma concentration and tissue FcRn abundance as key systems parameters influencing exposure to Fc-conjugated biologics.

Fc Receptor Expression as a Prognostic Factor in Patients With Non-small-cell Lung Cancer.

BACKGROUND/AIM: The neonatal Fc receptor (FcRn) is a major histocompatibility class I-like molecule responsible for the transfer of passive humoral immunity from a mother to her newborn. Recent research revealed that FcRn is involved in antigen-presentation, humoral immunity and antitumor immunity of various types of cancer, such as lung, colon and breast. Lung cancer is the leading cause of cancer-related death and non-small cell lung cancer (NSCLC) accounts for 85% of all lung cancer. NSCLC is a highly heterogeneous disease and this affects the prognosis. Therefore, many studies have tried to identify factors that are associated with prognosis. The lungs are a major organ expressing FcRn. We aimed to evaluate FcRn expression in surgical specimens of NSCLC and determine its correlation with patient prognosis. MATERIALS AND METHODS: We analyzed 140 NSCLC surgical specimens for FcRn expression using immunohistochemistry and correlated positivity with clinicopathology and survival of these patients. A chi-squared test and Kaplan-Meier analysis with log-rank tests were performed for statistical evaluation. RESULTS: The FcRn-positive group had a significantly higher disease-free survival and a tendency towards increased disease-specific survival in patients with tumor-node-metastasis stage I NSCLC. CONCLUSION: Our study supports the hypothesis that FcRn down-regulation is associated with NSCLC progression.

Prospective validation of a biomarker-driven response prediction model to romiplostim in lower-risk myelodysplastic neoplasms - results of the EUROPE trial by EMSCO.

The EUROPE phase 2 trial investigated the predictive value of biomarkers on the clinical efficacy of single agent romiplostim (ROM) treatment in patients with lower-risk myelodysplastic neoplasms (LR-MDS) and thrombocytopenia within the 'European Myelodysplastic Neoplasms Cooperative Group' (EMSCO) network. A total of 77 patients with LR-MDS and a median platelet count of 25/nl were included, all patients received ROM at a starting dose of 750 µg by SC injection weekly. Thirty-two patients (42%) achieved a hematologic improvement of platelets (HI-P) with a median duration of 340 days. Neutrophil (HI-N) and erythroid (HI-E) responses were observed in three (4%) and seven (9%) patients, respectively. We could not confirm previous reports that HI-P correlated with baseline endogenous thrombopoietin levels and platelet transfusion history, but SRSF2 mutation status and hemoglobin levels at baseline were significantly linked to HI-P. Sequential analysis of variant allelic frequency of mutations like SRSF2 did not reveal an impact of ROM on clonal evolution in both responders and non-responders. In summary, our study confirms the safety and efficacy of ROM in LR-MDS patients and may allow to better define subgroups of patients with a high likelihood of response.

Biophysical differences in IgG1 Fc-based therapeutics relate to their cellular handling, interaction with FcRn and plasma half-life.

Antibody-based therapeutics (ABTs) are used to treat a range of diseases. Most ABTs are either full-length IgG1 antibodies or fusions between for instance antigen (Ag)-binding receptor domains and the IgG1 Fc fragment. Interestingly, their plasma half-life varies considerably, which may relate to how they engage the neonatal Fc receptor (FcRn). As such, there is a need for an in-depth understanding of how different features of ABTs affect FcRn-binding and transport behavior. Here, we report on how FcRn-engagement of the IgG1 Fc fragment compare to clinically relevant IgGs and receptor domain Fc fusions, binding to VEGF or TNF-α. The results reveal FcRn-dependent intracellular accumulation of the Fc, which is in line with shorter plasma half-life than that of full-length IgG1 in human FcRn-expressing mice. Receptor domain fusion to the Fc increases its half-life, but not to the extent of IgG1. This is mirrored by a reduced cellular recycling capacity of the Fc-fusions. In addition, binding of cognate Ag to ABTs show that complexes of similar size undergo cellular transport at different rates, which could be explained by the biophysical properties of each ABT. Thus, the study provides knowledge that should guide tailoring of ABTs regarding optimal cellular sorting and plasma half-life.

Improving Antibody Therapeutics by Manipulating the Fc Domain: Immunological and Structural Considerations.

Interactions between the crystallizable fragment (Fc) domain of antibodies and a plethora of cellular Fc receptors (FcRs) or soluble proteins form a critical link between humoral and innate immunity. In particular, the immunoglobulin G Fc domain is critical for the clearance of target cells by processes that include (a) cytotoxicity, phagocytosis, or complement lysis; (b) modulation of inflammation; (c) antigen presentation; (d) antibody-mediated receptor clustering; and (e) cytokine release. More than 30 Fc-engineered antibodies aimed primarily at tailoring these effects for optimal therapeutic outcomes are in clinical evaluation or have already been approved. Nonetheless, our understanding of how FcR engagement impacts various immune cell phenotypes is still largely incomplete. Recent insights into FcR biology coupled with advances in Fc:FcR structural analysis, Fc engineering, and mouse models that recapitulate human biology are helping to fill in existing knowledge gaps. These advances will provide a blueprint on how to fine-tune the Fc domain to achieve optimal therapeutic efficacy.

A Charge Variant of Bevacizumab Offers Enhanced FcRn-Dependent Pharmacokinetic Half-Life and Efficacy.

BACKGROUND: Lysine variants of monoclonal antibodies (mAbs) result from incomplete clipping of the C-terminal lysine residues of the heavy chain. Although the structure of the lysine variants has been determined for several mAb products, a detailed study that investigates the impact of lysine charge variants on PK/PD and preclinical safety is yet to be published. OBJECTIVE: An in-depth investigation of the impact of C- terminal lysine clipping of mAbs on safety and efficacy for bevacizumab charge variants. METHOD: Charge variant isolation using semi-preparative chromatography is followed by a comparative analysis of FcRn binding, pharmacokinetics, and pharmacodynamics in relevant animal models. RESULTS: K1 variant exhibited improved FcRn binding affinity (4-fold), half-life (1.3-fold), and anti-tumor activity (1.3-fold) as compared to the K0 (main) product. However, the K2 variant, even though exhibited higher FcRn affinity (2-fold), displayed lower half-life (1.6-fold) and anti-tumor activity at medium and low doses. Differential proteomic analysis revealed that seven pathways (such as glycolysis, gluconeogenesis, carbon metabolism, synthesis of amino acids) were significantly enriched. Higher efficacy of the K1 variant is likely due to higher bioavailability of the drug, leading to complete downregulation of the pathways that facilitate catering of the energy requirements of the proliferating tumor cells. On the contrary, the K2 variant exhibits a shorter half-life, resulting only in partial reduction in the metabolic/energy requirements of the growing tumor cells. CONCLUSION: Overall, we conclude that the mAb half-life, dosage, and efficacy of a biotherapeutic product are significantly impacted by the charge variant profile of a biotherapeutic product.

Model-Based Assessment of the Contribution of Monocytes and Macrophages to the Pharmacokinetics of Monoclonal Antibodies.

PURPOSE: We have hypothesized that a high concentration of circulating monocytes and macrophages may contribute to the fast weight-based clearance of monoclonal antibodies (mAbs) in young children. Exploring this hypothesis, this work uses modeling to clarify the role of monocytes and macrophages in the elimination of mAbs. METHODS: Leveraging pre-clinical data from mice, a minimal physiologically-based pharmacokinetic model was developed to characterize mAb uptake and FcRn-mediated recycling in circulating monocytes, macrophages, and endothelial cells. The model characterized IgG disposition in complex scenarios of site-specific FcRn deletion and variable endogenous IgG levels. Evaluation was performed for predicting IgG disposition with co-administration of high dose IVIG. A one-at-a-time sensitivity analysis quantified the role of relevant cellular parameters on IgG elimination in various scenarios. RESULTS: The plasma AUC of mAbs was highly sensitive to endothelial cell parameters, but had near-nil sensitivity to monocyte and macrophage parameters, even in scenarios with 90% loss of FcRn expression/activity. In mice with normal FcRn expression, simulations suggest that less than 2% of an IV dose is eliminated in macrophages, while endothelial cells are predicted to dominate mAb elimination. CONCLUSIONS: The model suggests that the role of monocytes and macrophages in IgG homeostasis includes extensive uptake and highly efficient FcRn-mediated protection, but not appreciable degradation when FcRn is present. Therefore, it is very unlikely that a high concentration of circulating monocytes can contribute to explaining the fast weight-based clearance of mAbs in very young children, even if FcRn expression/activity was 90% lower in children than in adults.