Risk awareness during operation of analytical flow cytometers and implications throughout the COVID-19 pandemic.

The COVID-19 pandemic has brought biosafety to the forefront of many life sciences. The outbreak has compelled research institutions to re-evaluate biosafety practices and potential at-risk areas within research laboratories and more specifically within Shared Resource Laboratories (SRLs). In flow cytometry facilities, biological safety assessment encompasses known hazards based on the biological sample and associated risk group, as well as potential or unknown hazards, such as aerosol generation and instrument "failure modes." Cell sorting procedures undergo clearly defined biological safety assessments and adhere to well-established biosafety guidelines that help to protect SRL staff and users against aerosol exposure. Conversely, benchtop analyzers are considered low risk due to their low sample pressure and enclosed fluidic systems, although there is little empirical evidence to support this assumption of low risk. To investigate this, we evaluated several regions on analyzers using the Cyclex-d microsphere assay, a recently established method for cell sorter aerosol containment testing. We found that aerosol and/or droplet hazards were detected on all benchtop analyzers predominantly during operation in "failure modes." These results indicate that benchtop analytical cytometers present a more complicated set of risks than are commonly appreciated.

Trogocytosis by Entamoeba histolytica contributes to cell killing and tissue invasion.

Entamoeba histolytica is the causative agent of amoebiasis, a potentially fatal diarrhoeal disease in the developing world. The parasite was named "histolytica" for its ability to destroy host tissues, which is probably driven by direct killing of human cells. The mechanism of human cell killing has been unclear, although the accepted model was that the parasites use secreted toxic effectors to kill cells before ingestion. Here we report the discovery that amoebae kill by ingesting distinct pieces of living human cells, resulting in intracellular calcium elevation and eventual cell death. After cell killing, amoebae detach and cease ingestion. Ingestion of human cell fragments is required for cell killing, and also contributes to invasion of intestinal tissue. The internalization of fragments of living human cells is reminiscent of trogocytosis (from Greek trogo, nibble) observed between immune cells, but amoebic trogocytosis differs because it results in death. The ingestion of live cell material and the rejection of corpses illuminate a stark contrast to the established model of dead cell clearance in multicellular organisms. These findings change the model for tissue destruction in amoebiasis and suggest an ancient origin of trogocytosis as a form of intercellular exchange.

A Modified Injector and Sample Acquisition Protocol Can Improve Data Quality and Reduce Inter-Instrument Variability of the Helios Mass Cytometer.

Mass cytometry is a powerful tool for high-dimensional single cell characterization. Since the introduction of the first commercial CyTOF mass cytometer by DVS Sciences in 2009, mass cytometry technology has matured and become more widely utilized, with sequential platform upgrades designed to address specific limitations and to expand the capabilities of the platform. Fluidigm's third-generation Helios mass cytometer introduced a number of upgrades over the previous CyTOF2. One of these new features is a modified narrow bore sample injector that generates smaller ion clouds, which is expected to improve sensitivity and throughput. However, following rigorous testing, we find that the narrow-bore sample injector may have unintended negative consequences on data quality and result in lower median and higher coefficients of variation in many antibody-associated signal intensities. We describe an alternative Helios acquisition protocol using a wider bore injector, which largely mitigates these data quality issues. We directly compare these two protocols in a multisite study of 10 Helios instruments across 7 institutions and show that the modified protocol improves data quality and reduces interinstrument variability. These findings highlight and address an important source of technical variability in mass cytometry experiments that is of particular relevance in the setting of multicenter studies. © 2019 International Society for Advancement of Cytometry.

B cells secrete functional antigen-specific IgG antibodies on extracellular vesicles.

B cells and the antibodies they produce are critical in host defense against pathogens and contribute to various immune-mediated diseases. B cells responding to activating signals in vitro release extracellular vesicles (EV) that carry surface antibodies, yet B cell production of EVs that express antibodies and their function in vivo is incompletely understood. Using transgenic mice expressing the Cre recombinase in B cells switching to IgG1 to induce expression of fusion proteins between emerald green fluorescent protein (emGFP) and the EV tetraspanin CD63 as a model, we identify emGFP expression in B cells responding to foreign antigen in vivo and characterize the emGFP+ EVs they release. Our data suggests that emGFP+ germinal center B cells undergoing immunoglobulin class switching to express IgG and their progeny memory B cells and plasma cells, also emGFP+, are sources of circulating antigen-specific IgG+ EVs. Furthermore, using a mouse model of influenza virus infection, we find that IgG+ EVs specific for the influenza hemagglutinin antigen protect against virus infection. In addition, crossing the B cell Cre driver EV reporter mice onto the Nba2 lupus-prone strain revealed increased circulating emGFP+ EVs that expressed surface IgG against nuclear antigens linked to autoimmunity. These data identify EVs loaded with antibodies as a novel route for antibody secretion in B cells that contribute to adaptive immune responses, with important implications for different functions of IgG+ EVs in infection and autoimmunity.

BMP-6 promotes type 2 immune response during enhancement of rat mandibular bone defect healing.

Introduction: Bone morphogenetic proteins (BMPs) are used as key therapeutic agents for the treatment of difficult fractures. While their effects on osteoprogenitors are known, little is known about their effects on the immune system. Methods: We used permutations of BMP-6 (B), vascular endothelial growth factor (V), and Hedgehog signaling pathway activator smoothened agonist (S), to treat a rat mandibular defect and investigated healing outcomes at week 8, in correlation with the cellular landscape of the immune cells in the fracture callus at week 2. Results: Maximum recruitment of immune cells to the fracture callus is known to occur at week 2. While the control, S, V, and VS groups remained as nonunions at week 8; all BMP-6 containing groups - B, BV, BS and BVS, showed near-complete to complete healing. This healing pattern was strongly associated with significantly higher ratios of CD4 T (CD45+CD3+CD4+) to putative CD8 T cells (CD45+CD3+CD4-), in groups treated with any permutation of BMP-6. Although, the numbers of putative M1 macrophages (CD45+CD3-CD11b/c+CD38high) were significantly lower in BMP-6 containing groups in comparison with S and VS groups, percentages of putative - Th1 cells or M1 macrophages (CD45+CD4+IFN-γ+) and putative - NK, NKT or cytotoxic CD8T cells (CD45+CD4-IFN-γ+) were similar in control and all treatment groups. Further interrogation revealed that the BMP-6 treatment promoted type 2 immune response by significantly increasing the numbers of CD45+CD3-CD11b/c+CD38low putative M2 macrophages, putative - Th2 cells or M2 macrophages (CD45+CD4+IL-4+) cells and putative - mast cells, eosinophils or basophils (CD45+CD4-IL-4+ cells). CD45- non-haematopoietic fractions of cells which encompass all known osteoprogenitor stem cells populations, were similar in control and treatment groups. Discussion: This study uncovers previously unidentified regulatory functions of BMP-6 and shows that BMP-6 enhances fracture healing by not only acting on osteoprogenitor stem cells but also by promoting type 2 immune response.

Flow cytometry imaging identifies rare T(H)2 cells expressing thymic stromal lymphopoietin receptor in a "proallergic" milieu.

BACKGROUND: Thymic stromal lymphopoietin (TSLP) is expressed at sites of allergic inflammation, including eczematous skin. This cytokine has been reported to exert its T(H)2-inducing properties through dendritic cells. Expression of TSLP receptor on the surface of activated T(H)2 cells could amplify T(H)2 responses at inflamed sites through the direct actions of TSLP. OBJECTIVE: To test rigorously whether T(H)2 cells induced by "proallergic" factors express TSLP receptor and characterize these cells using an experimental platform that combines flow cytometry with microscopic capabilities. METHODS: CD4(+) T cells isolated from patients with atopic dermatitis or normal healthy controls were cocultured with autologous dendritic cells in the presence of T(H)2-promoting stimuli (TSLP ± allergen and staphylococcal enterotoxin B ± TSLP). Surface expression of TSLP receptor was analyzed by image-based flow cytometry, and responsiveness of purified T cells to TSLP was assessed by phosphorylation of signal transducer and activator of transcription-5 and cytokine secretion. RESULTS: T(H)2-promoting stimuli induced a robust population of activated T(H)2 cells (CD25(+)IL-4(+)). Regardless of the nature of the stimulus, flow cytometry imaging confirmed that T cells expressing TSLP receptor were rare, constituting a minor fraction of the IL-4(+) T cell pool; however, TSLP responsiveness was nonetheless detectable. Analysis of cell size and nuclear morphology revealed preferential expression of TSLP receptor on IL-4-expressing cells undergoing mitosis. Analysis of lesional skin in atopic dermatitis supported the view that rare IL-4(+) T cells expressing TSLP receptor are present at inflamed sites. CONCLUSION: In a "proallergic" milieu, TSLP receptor is preferentially expressed on rare actively dividing T(H)2 cells. The direct action of TSLP on T cells could amplify T(H)2 responses at sites of allergic inflammation.

Using Imaging Flow Cytometry to Quantify Neutrophil Phagocytosis.

Neutrophils are professional phagocytes that are important for innate host defenses against pathogens and resolution of inflammation. Traditionally, the phagocytic capacity of neutrophils was quantified by enumeration of cells containing either internalized or bound bacteria or other cargo from a series of microscopic images. Here we describe an imaging flow cytometry-based protocol and analysis method for quantifying the binding and uptake of Neisseria gonorrhoeae by primary adherent human neutrophils. Imaging flow cytometry combines the capacity for quantitative, high-throughput analysis of tens of thousands of cells per condition, with the imaging power of fluorescence microscopy. Here, all bacteria are labeled with Tag-it Violet™ and bound bacteria are differentially stained with a DyLight™ 650-conjugated antibody. Images are analyzed using spot count and other algorithms. Outputs include the percent of neutrophils associated with bacteria, the percent of neutrophils with internalized bacteria, and the percent of internalized bacteria. This basic protocol can be adapted to a variety of particle types and can be used for multiplex analysis in combination with staining for different neutrophil surface and intracellular markers.

IgG Antibodies against SARS-CoV-2 Correlate with Days from Symptom Onset, Viral Load and IL-10.

The emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has resulted in a pandemic of the respiratory disease coronavirus disease 2019 (COVID-19). Antibody testing is essential to identify persons exposed to the virus and potentially in predicting disease immunity. 183 COVID-19 patients (68 of whom required mechanical ventilation) and 41 controls were tested for plasma IgG, IgA and IgM against the SARS-CoV-2 S1, S2, receptor binding domain (RBD) and N proteins using the MILLIPLEX® SARS-CoV-2 Antigen Panel. Plasma cytokines were concurrently measured using the MILLIPLEX® MAP Human Cytokine/Chemokine/Growth Factor Panel A. As expected the 183 COVID-19 positive patients had high levels of IgG, IgA and IgM anti-SARS-CoV-2 antibodies against each of the viral proteins. Sensitivity of anti-S1 IgG increased from 60% to 93% one week after symptom onset. S1-IgG and S1-IgA had specificities of 98% compared to the 41 COVID-19 negative patients. The 68 ventilated COVID-19 positive patients had higher antibody levels than the 115 COVID-19 positive patients who were not ventilated. IgG antibody levels against S1 protein had the strongest positive correlation to days from symptom onset. There were no statistically significant differences in IgG, IgA and IgM antibodies against S1 based on age. We found that patients with the highest levels of anti-SARS-CoV-2 antibodies had the lowest viral load in the nasopharynx. Finally there was a correlation of high plasma IL-10 with low anti-SARS-CoV-2 antibodies. Anti-SARS-CoV-2 antibody levels, as measured by a novel antigen panel, increased within days after symptom onset, achieving > 90% sensitivity and specificity within one week, and were highest in patients who required mechanical ventilation. Antibody levels were inversely associated with viral load but did not differ as a function of age. The correlation of high IL-10 with low antibody response suggests a potentially suppressive role of this cytokine in the humoral immune response in COVID-19.

CD44 deficiency attenuates chronic murine ileitis.

BACKGROUND & AIMS: Lymphocyte recruitment to sites of inflammation requires the sequential engagement of adhesion molecules and chemokine receptors. In the current studies we analyzed the role of CD44 for the development of chronic small-intestinal inflammatory infiltrates in vivo. METHODS: By using a tumor necrosis factor (TNF)-driven model of chronic ileitis (ie, B6.129P-TNF(DeltaAU-rich element [ARE])) that recapitulates many features of Crohn's disease, we noticed dynamic changes in the expression and functional state of CD44 and its ligand hyaluronan via enzyme-linked immunosorbent assay, real-time reverse-transcription polymerase chain reaction, immunohistochemistry, and flow cytometry. In addition, we assessed the role of lymphocyte populations during induction of ileitis through adoptive transfer studies, and generated CD44-deficient TNFDeltaARE mice to assess the role of CD44 for development of ileitis. RESULTS: Soluble hyaluronan levels and expression of hyaluronan synthase-1 were increased in TNFDeltaARE mice. This coincided with increased expression of CD44 (including variant 7) and reactivity towards hyaluronan on CD4(+) T cells. CD44 was spatially colocalized with the gut-homing integrin alpha(4)beta(7), spatially linking lymphocyte rolling with arrest. These cells had an effector phenotype because they lacked L-selectin and a higher proportion in diseased mice produced TNF and interleukin-2 compared with wild-type littermates. Lastly, CD4(+) but not CD8(+) T cells conferred ileitis to RAG(-/-) recipients and deficiency of one or both alleles of the CD44 gene resulted in attenuation of the severity of ileitis in TNFDeltaARE mice. CONCLUSIONS: Our findings support an important role of CD44 expressed by CD4(+) and CD8(+) for development of ileitis mediated by TNF overproduction.

Analysis of CD25hiCD4+ "regulatory" T-cell subtypes in atopic dermatitis reveals a novel T(H)2-like population.

BACKGROUND: It is unresolved whether circulating CD25hiCD4+ T cells in patients with atopic dermatitis who have elevated IgE (IgE(high)) are regulatory or effector in nature. OBJECTIVE: To analyze the properties of CD25hi T-cell subtypes in IgE(high) atopic dermatitis. METHODS: The phenotype of circulating CD25hi T cells was analyzed by flow cytometry using PBMCs from patients with atopic dermatitis (total IgE > 250 IU/mL). Cytokines induced in CD25hi subtypes were analyzed after activation with anti-CD3 mAb (+/-IL-2) and in the presence of activated autologous effector T cells (CD25negCD4+). Reactivity to bacterial superantigen derived from the skin-colonizing organism Staphylococcus aureus was also evaluated. RESULTS: CD25(hi) T cells expressing regulatory T-cell markers (Foxp3, CCR4, cutaneous lymphocyte-associated antigen) were increased in atopic dermatitis compared with IgE(low) controls. This phenomenon was linked to disease severity. Two subtypes of CD25hi T cells were identified on the basis of differential expression of the chemokine receptor CCR6. Although the ratio of CCR6+ and CCR6neg subtypes within the CD25hi subset was unaltered in atopic dermatitis, each subtype proliferated spontaneously ex vivo, suggesting in vivo activation. Activated CCR6neg cells secreted T(H)2 cytokines, and coculture with effector T cells selectively enhanced IL-5 production. Moreover, induction of a T(H)2-dominated cytokine profile on activation with bacterial superantigen was restricted to the CCR6neg subtype. CONCLUSION: Despite a regulatory phenotype, activated CD25hi T cells that lack expression of CCR6 promote T(H)2 responses.

High-Throughput Particle Uptake Analysis by Imaging Flow Cytometry.

Quantifying the efficiency of particle uptake by host cells is important in the fields of infectious diseases, autoimmunity, cancer, developmental biology, and drug delivery. Here we present a protocol for high-throughput analysis of particle uptake by imaging flow cytometry, using the bacterium Neisseria gonorrhoeae attached to and internalized by neutrophils as an example. Cells are exposed to fluorescently labeled bacteria, fixed, and stained with a bacteria-specific antibody of a different fluorophore. Thus, in the absence of a permeabilizing agent, extracellular bacteria are double-labeled with two fluorophores while intracellular bacteria remain single-labeled. A spot count algorithm is used to determine the number of single- and double-labeled bacteria in individual cells, to calculate the percent of cells associated with bacteria, percent of cells with internalized bacteria, and percent of cell-associated bacteria that are internalized. These analyses quantify bacterial association and internalization across thousands of cells and can be applied to diverse experimental systems. © 2017 by John Wiley & Sons, Inc.

An improved method for differentiating cell-bound from internalized particles by imaging flow cytometry.

Recognition, binding, internalization, and elimination of pathogens and cell debris are important functions of professional as well as non-professional phagocytes. However, high-throughput methods for quantifying cell-associated particles and discriminating bound from internalized particles have been lacking. Here we describe a protocol for using imaging flow cytometry to quantify the attached and phagocytosed particles that are associated with a population of cells. Cells were exposed to fluorescent particles, fixed, and exposed to an antibody of a different fluorophore that recognizes the particles. The antibody is added without cell permeabilization, such that the antibody only binds extracellular particles. Cells with and without associated particles were identified by imaging flow cytometry. For each cell with associated particles, a spot count algorithm was employed to quantify the number of extracellular (double fluorescent) and intracellular (single fluorescent) particles per cell, from which the percent particle internalization was determined. The spot count algorithm was empirically validated by examining the fluorescence and phase contrast images acquired by the flow cytometer. We used this protocol to measure binding and internalization of the bacterium Neisseria gonorrhoeae by primary human neutrophils, using different bacterial variants and under different cellular conditions. The results acquired using imaging flow cytometry agreed with findings that were previously obtained using conventional immunofluorescence microscopy. This protocol provides a rapid, powerful method for measuring the association and internalization of any particle by any cell type.

Systems analysis of the transcriptional response of human ileocecal epithelial cells to Clostridium difficile toxins and effects on cell cycle control.

BACKGROUND: Toxins A and B (TcdA and TcdB) are Clostridium difficile's principal virulence factors, yet the pathways by which they lead to inflammation and severe diarrhea remain unclear. Also, the relative role of either toxin during infection and the differences in their effects across cell lines is still poorly understood. To better understand their effects in a susceptible cell line, we analyzed the transciptome-wide gene expression response of human ileocecal epithelial cells (HCT-8) after 2, 6, and 24 hr of toxin exposure. RESULTS: We show that toxins elicit very similar changes in the gene expression of HCT-8 cells, with the TcdB response occurring sooner. The high similarity suggests differences between toxins are due to events beyond transcription of a single cell-type and that their relative potencies during infection may depend on differential effects across cell types within the intestine. We next performed an enrichment analysis to determine biological functions associated with changes in transcription. Differentially expressed genes were associated with response to external stimuli and apoptotic mechanisms and, at 24 hr, were predominately associated with cell-cycle control and DNA replication. To validate our systems approach, we subsequently verified a novel G1/S and known G2/M cell-cycle block and increased apoptosis as predicted from our enrichment analysis. CONCLUSIONS: This study shows a successful example of a workflow deriving novel biological insight from transcriptome-wide gene expression. Importantly, we do not find any significant difference between TcdA and TcdB besides potency or kinetics. The role of each toxin in the inhibition of cell growth and proliferation, an important function of cells in the intestinal epithelium, is characterized.

ICAM-1 and LFA-1 play critical roles in LPS-induced neutrophil recruitment into the alveolar space.

Neutrophil recruitment into lung constitutes a major response to airborne endotoxins. In many tissues endothelial intercellular adhesion molecule-1 (ICAM-1) interacts with lymphocyte function associated antigen-1 (LFA-1) on neutrophils, and this interaction plays a critical role in neutrophil recruitment. There are conflicting reports about the role of ICAM-1 in neutrophil recruitment into lungs. We studied neutrophil recruitment into alveolar space in a murine model of aerosolized LPS-induced lung inflammation. LPS induces at least a 100-fold increase in neutrophil numbers in alveolar space, as determined by flow cytometry of bronchoalveolar lavage fluid. Neutrophil recruitment was reduced by 54% in ICAM-1 null mice and by 45% in LFA-1 null mice. In wild-type mice treated with anti-ICAM-1 and anti-LFA-1 antibodies, there was 51 and 58% reduction in the neutrophil recruitment, respectively. In chimeric mice, generated by the transplantation of mixtures of bone marrows from LFA-1 null and wild-type mice, the normalized recruitment of LFA-1 null neutrophils was reduced by 60% compared with wild-type neutrophils. Neither the treatment of ICAM-1 null mice with a function-blocking antibody to LFA-1 nor the treatment of LFA-1 null mice with anti-ICAM-1 antibody resulted in further reduction in the recruitment compared with untreated ICAM-1 null and LFA-1 null mice. We conclude that ICAM-1 and LFA-1 play critical roles in the recruitment of neutrophils into the alveolar space in aerosolized LPS-induced lung inflammation, and LFA-1 serves as a ligand of ICAM-1 in the lung.

L-selectin, alpha 4 beta 1, and alpha 4 beta 7 integrins participate in CD4+ T cell recruitment to chronically inflamed small intestine.

CD4+ T cells are essential for development and perpetuation of Crohn's disease, a chronic immune-mediated condition that affects primarily the small intestine. Using novel models of Crohn's disease-like ileitis (i.e., SAMP1/YitFc and CD4+ T cell transfer models), we have begun to understand the adhesive pathways that mediate lymphocyte trafficking to the chronically inflamed small bowel. Expansion of the CD4/beta7+ population and increased mucosal addressin cell adhesion molecule-1 (MAdCAM-1) expression were observed within the intestinal lamina propria with disease progression. However, Ab blockade of the beta7 integrin, the alpha4beta7 heterodimer, MAdCAM-1, or L-selectin did not attenuate inflammation. Blockade of two pathways (L-selectin and MAdCAM-1 or alpha4 integrins) was required to improve ileitis. Further analyses showed that 55 +/- 7% of the mesenteric lymph node alpha4beta7+CD4 expressed L-selectin. These L-selectin+ T cells were the main producers of TNF-alpha and the predominant ileitis-inducing subpopulation. Mechanistically, combined blockade of L-selectin and MAdCAM-1 depleted the intestinal lamina propria of CD4+ T cells that aberrantly coexpressed alpha4beta7 and alpha4beta1 integrins, markedly decreasing local production of TNF-alpha and IFN-gamma. Thus, pathogenic CD4+ T cells not only use the physiologic alpha4beta7/MAdCAM-1 pathway, but alternatively engage alpha4beta1 and L-selectin to recirculate to the chronically inflamed small intestine.

B cell complement receptor 2 transfer reaction.

The B cell C receptor specific for C3dg (CR2) shares a number of features with the primate E C receptor (CR1). Previously, we have demonstrated, both in vitro and in animal models, that immune complexes (IC) bound to primate E CR1, either via C opsonization or by means of bispecific mAb complexes, can be transferred to acceptor macrophages in a process that also removes CR1 from the E. We have now extended this paradigm, the transfer reaction, to include B cell CR2. We used both flow cytometry and fluorescence microscopy to demonstrate that IC bound to Raji cell CR2, either via C opsonization or through the use of an anti-CR2 mAb, are transferred to acceptor THP-1 cells. This reaction, which appears to require Fc recognition of IgG bound to Raji cell CR2, also leads to transfer of CR2. Additional support for the B cell transfer reaction is provided in a prototype study in a monkey model in which IC bound to B cell CR2 are localized to the spleen. These findings may have important implications with respect to defining the role of C in IC handling during the normal immune response.

An anti-C3b(i) mAb enhances complement activation, C3b(i) deposition, and killing of CD20+ cells by rituximab.

We investigated deposition of the complement protein fragment C3b and its breakdown products (collectively designated as C3b(i)) on CD20-positive cells treated with rituximab (RTX) in the presence of normal human serum (NHS). Radioimmunoassay (RIA) demonstrates that about 500 000 C3b(i) molecules deposit per cell, and fluorescence microscopy reveals that C3b(i) colocalizes with bound RTX. Use of mAb 3E7, specific for C3b(i) bound to substrates, enhances C3b(i) deposition; > 1 million C3b(i) deposit when cells are incubated with NHS, RTX and mAb 3E7. Treatment of Raji cells in NHS plus RTX leads to robust cell killing (95%) after 24 to 48 hours, and mAb 3E7 significantly enhances RTX-mediated killing of Raji and DB cells. A cynomolgus monkey model based on intravenous infusion of RTX followed by mAb 3E7 demonstrated that RTX rapidly binds to B cells and promotes complement activation and C3b(i) deposition; fluorescence microscopy analyses revealed the same pattern of colocalization of C3b(i) on cell-bound RTX in vivo as observed in vitro. Preliminary in vitro studies with blood samples from patients with chronic lymphocytic leukemia lead to similar findings. These experiments suggest that complement plays a key role in the mechanism of action of RTX; moreover, the in vivo molecular form of RTX (and possibly other antitumor mAbs) in the circulation or in tissues may include C3b(i) molecules covalently bound to the therapeutic mAb, thus allowing it to interact with cells containing both Fc and complement receptors.

Rituximab infusion promotes rapid complement depletion and acute CD20 loss in chronic lymphocytic leukemia.

Complement plays an important role in the immunotherapeutic action of the anti-CD20 mAb rituximab, and therefore we investigated whether complement might be the limiting factor in rituximab therapy. Our in vitro studies indicate that at high cell densities, binding of rituximab to human CD20(+) cells leads to loss of complement activity and consumption of component C2. Infusion of rituximab in chronic lymphocytic leukemia patients also depletes complement; sera of treated patients have reduced capacity to C3b opsonize and kill CD20(+) cells unless supplemented with normal serum or component C2. Initiation of rituximab infusion in chronic lymphocytic leukemia patients leads to rapid clearance of CD20(+) cells. However, substantial numbers of B cells, with significantly reduced levels of CD20, return to the bloodstream immediately after rituximab infusion. In addition, a mAb specific for the Fc region of rituximab does not bind to these recirculating cells, suggesting that the rituximab-opsonized cells were temporarily sequestered by the mononuclear phagocytic system, and then released back into the circulation after the rituximab-CD20 complexes were removed by phagocytic cells. Western blots provide additional evidence for this escape mechanism that appears to occur as a consequence of CD20 loss. Treatment paradigms to prevent this escape, such as use of engineered or alternative anti-CD20 mAbs, may allow for more effective immunotherapy of chronic lymphocytic leukemia.