Complement receptor 3 (CD11b/CD18) mediates type I and type II phagocytosis during nonopsonic and opsonic phagocytosis, respectively

Toll-like receptors induce a phagocytic gene program through p38

Toll-like receptor (TLR) signaling and phagocytosis are hallmarks of macrophage-mediated innate immune responses to bacterial infection. However, the relationship between these two processes is not well established. Our data indicate that TLR ligands specifically promote bacterial phagocytosis, in both murine and human cells, through induction of a phagocytic gene program. Importantly, TLR-induced phagocytosis of bacteria was found to be reliant on myeloid differentiation factor 88–dependent signaling through interleukin-1 receptor–associated kinase-4 and p38 leading to the up-regulation of scavenger receptors. Interestingly, individual TLRs promote phagocytosis to varying degrees with TLR9 being the strongest and TLR3 being the weakest inducer of this process. We also demonstrate that TLR ligands not only amplify the percentage of phagocytes uptaking Escherichia coli, but also increase the number of bacteria phagocytosed by individual macrophages. Taken together, our data describe an evolutionarily conserved mechanism by which TLRs can specifically promote phagocytic clearance of bacteria during infection.

Release of arachidonic acid by stimulation of opsonic receptors in human monocytes: the FcgammaR and the complement receptor 3 pathways

The role of the opsonic receptors FcgammaR and CR3 on the release of arachidonic acid (AA) by human monocytes was studied using IgG-ovalbumin (OVA) equivalence immune complexes (IC), anti-OVA IgG bound to OVA-coupled latex beads, and C3bi-bound IC. Release of AA was produced by IC and latex-OVA beads bound to IgG, whereas binding of C3bi to IC inhibited the ability of IC to release AA. In contrast, coating of zymosan particles with C3bi enhanced AA release as compared with that produced by non-coated particles. Masking of C3bi on C3bi-bound IC by incubation with anti-C3 IgG resulted in the recovery of their ability to release AA, thereby suggesting that binding of C3b by IC reduces their flogogenic effects, whereas opsonization of microbial walls by complement may enhance their proinflammatory potential. The binding/uptake of opsonized zymosan particles was inhibited by anti-CR3 Ab and C3bi-bound IC, but not by beta-glucan, mannan, and anti-Toll-like receptor 2 Ab. These findings show that cooperative engagement of CR3 on both the lectin-like site involved in beta-glucan binding and the I-domain involved in C3bi binding, as it can be observed in the innate immune response, produces AA release, whereas the unique interaction of C3bi-bound IC with the I-domain of CR3, as it may occur in the adaptive immune response, diverts the IC lattice from a productive interaction with FcgammaR linked to AA release.

Macrophage--Mycobacterium tuberculosis interactions: role of complement receptor 3

Tuberculosis is the leading infectious disease in the world. Mycobacterium tuberculosis, the causal agent of this disease, invades macrophages and can replicate inside them. Because invasion of macrophages is a critical step for establishing a mycobacterial infection, there is much interest in understanding the mechanisms for M. tuberculosis entry into macrophages. Complement receptor 3 (CR3) is a heterodimeric surface receptor with multiple binding sites, which can mediate complement-opsonized as well as nonopsonic entrance of M. tuberculosis into macrophages. Here, we describe and discuss the role of CR3 in macrophage[bond]M. tuberculosis interactions. The actual information suggests that CR3 mediates a substantial amount of M. tuberculosis binding to macrophages, but CR3 is not related to the mechanisms that allow mycobacteria to survive and replicate intracellularly. Understanding the mechanisms of macrophage[bond]M. tuberculosis interaction will help developing more effective methods to prevent and treat tuberculosis in the future.

Effect of inhaled endotoxin on airway and circulating inflammatory cell phagocytosis and CD11b expression in atopic asthmatic subjects

BACKGROUND

In a cohort of 8 normal and 10 allergic asthmatic volunteers, we previously reported that inhalation of 5 microg of endotoxin (LPS) induced airway inflammation that correlated with CD14 expression that was, in turn, correlated with eosinophil numbers in the airway. Macrophage and neutrophil functions have been reported to be modified by endotoxin in vitro and in vivo, and response to endotoxin is mediated largely by airway phagocytes and related to allergic inflammation.

OBJECTIVE

We sought to examine functional and cell-surface phenotype changes in phagocytes recovered from atopic asthmatic subjects after endotoxin challenge.

METHODS

Sputum and peripheral blood from 10 allergic asthmatic subjects was recovered after saline and LPS challenge. Assessment of phagocytosis and cell-surface phenotype (CD11b, CD14, and CD64) was performed on phagocytes obtained from sputum (n = 7) and blood samples (n = 10).

RESULTS

Phagocytosis of blood and sputum phagocytes was blunted after LPS challenge in a fashion that correlated with the increase in airway neutrophils after LPS challenge. Cell-surface expression of CD14 (membrane-bound CD14) was increased in sputum cells, whereas CD11b was decreased in sputum and circulating phagocytes. Baseline expression of CD11b in blood correlated with the magnitude of the neutrophil response after LPS inhalation, as well as (inversely) with baseline airway eosinophil levels.

CONCLUSIONS

Inhalation of endotoxin at levels adequate to induce a neutrophil influx to the airways (but not systemic symptoms) results in decreased phagocytosis in both airway and circulating cells and modifies CD11b expression in a way that implicates its involvement in phagocyte responsiveness to inhaled LPS.

Pneumocystis carinii cell wall beta-glucans initiate macrophage inflammatory responses through NF-kappaB activation

beta-Glucans are major structural components of fungi. We have recently reported that the pathogenic fungus Pneumocystis carinii assembles a beta-glucan-rich cell wall that potently activates alveolar macrophages to release pro-inflammatory cytokines and chemokines. Purified P. carinii beta-glucans predictably induce both cytokine generation and associated neutrophilic lung inflammation. Herein, we demonstrate that P. carinii beta-glucan-induced macrophage stimulation results from activation of NF-kappaB. Although analogous to macrophage activation induced by bacterial lipopolysaccharide (LPS), P. carinii beta-glucan-induced macrophage NF-kappaB activation exhibits distinctly different kinetics, with slower induction and longer duration compared with LPS stimulation. Macrophage activation in response to P. carinii beta-glucan was also substantially inhibited with the NF-kappaB antagonist pyrrolidine dithiocarbamate. In addition to different kinetics of NF-kappaB activation, P. carinii beta-glucan and LPS also utilize different receptor systems to induce macrophage activation. Macrophages from Toll-like receptor 4-deficient and wild type mice produced equivalent amounts of tumor necrosis factor alpha when stimulated with P. carinii beta-glucan. However, Toll-like receptor 4-deficient macrophages were refractory to stimulation with LPS. In contrast, MyD88-deficient macrophages exhibited a significant (though partial) blunted response to P. carinii beta-glucan. These data demonstrate that P. carinii beta-glucan acts as potent inducer of macrophage activation through NF-kappaB utilizing cellular receptors and signaling pathways distinct from LPS.