Fine Tuning Inflammation at the Front Door: Macrophage Complement Receptor 3-mediates Phagocytosis and Immune Suppression for Francisella tularensis

Abstract

Complement receptor 3 (CR3, CD11b/CD18) is a major macrophage phagocytic receptor. The biochemical pathways through which CR3 regulates immunologic responses have not been fully characterized. Francisella tularensis is a remarkably infectious, facultative intracellular pathogen of macrophages that causes tularemia. Early evasion of the host immune response contributes to the virulence of F. tularensis and CR3 is an important receptor for its phagocytosis. Here we confirm that efficient attachment and uptake of the highly virulent Type A F. tularensis spp. tularensis strain Schu S4 by human monocyte-derived macrophages (hMDMs) requires complement C3 opsonization and CR3. However, despite a>40-fold increase in uptake following C3 opsonization, Schu S4 induces limited pro-inflammatory cytokine production compared with non-opsonized Schu S4 and the low virulent F. novicida. This suggests that engagement of CR3 by opsonized Schu S4 contributes specifically to the immune suppression during and shortly following phagocytosis which we demonstrate by CD11b siRNA knockdown in hMDMs. This immune suppression is concomitant with early inhibition of ERK1/2, p38 MAPK and NF-κB activation. Furthermore, TLR2 siRNA knockdown shows that pro-inflammatory cytokine production and MAPK activation in response to non-opsonized Schu S4 depends on TLR2 signaling providing evidence that CR3-TLR2 crosstalk mediates immune suppression for opsonized Schu S4. Deletion of the CD11b cytoplasmic tail reverses the CR3-mediated decrease in ERK and p38 activation during opsonized Schu-S4 infection. The CR3-mediated signaling pathway involved in this immune suppression includes Lyn kinase and Akt activation, and increased MKP-1, which limits TLR2-mediated pro-inflammatory responses. These data indicate that while the highly virulent F. tularensis uses CR3 for efficient uptake, optimal engagement of this receptor down-regulates TLR2-dependent pro-inflammatory responses by inhibiting MAPK activation through outside-in signaling. CR3-linked immune suppression is an important mechanism involved in the pathogenesis of F. tularensis infection.

Complement Opsonization of HIV-1 Results in Decreased Antiviral and Inflammatory Responses in Immature Dendritic Cells via CR3

“…We examined the role of CR3 in the immune suppression induced by C-HIV, as this receptor has been proven to impair responses against other pathogens (34,35). Targeting CD11b, the a-integrin chain of CR3, with an activating Ab before adding F-HIV gave the same decreased immune response as seen for C-HIV, decreasing activation of both the inflammasome, as measured by IL-1b and TNF-a, and the antiviral responses, as measured by MxA (Fig.…”

Needle-Free Delivery of Acetalated Dextran-Encapsulated AR-12 Protects Mice from Francisella tularensis Lethal Challenge

“…Additionally, particles of this size are not optimal targets for dendritic cells and will not be trafficked away from the site of administration as readily, allowing for persistent passive targeting of macrophages (39). The potential for sustained passive treatment at the site of administration is important for an intracellular bacterium such as F. tularensis, which uses various strategies to avoid recognition by the host and allow for its replication and dissemination (14,15,19).…”

“…3 and 6). This inflammation, together with autophagy induced by AR-12/MPs, may overcome early immune suppression by F. tularensis in the lungs (15,44), enhancing control of the pathogen by the host. Indeed, the CFU data together with the lung histopathology data indicate that in- given for the duration of the experiment.…”

“…Bacterial cultures and analysis of bacterial growth in hMDMs. The F. tularensis SchuS4 strain (type A) used in this study was described previously (15). Bacteria were cultured on chocolate II agar plates (Becton Dickinson, Sparks, MD) for 48 h at 37°C prior to use for all experiments.…”

“…tularensis is difficult to treat because it is a facultative intracellular bacterium that targets macrophages and has several mech-anisms that enable it to evade immune clearance. Upon host cell entry, the bacterium affects signaling cascades to limit host protective inflammatory responses, block fusion of Francisella-containing phagosomes with lysosomes, and escape into the cytosol to proliferate, which results in host cell death (14)(15)(16)(17)(18)(19). Several hours after entry into the cytoplasm, subsets of wild-type Francisella organisms and bacterial mutants that are incapable of escaping from phagosomes have been observed in double-membrane vacuoles with markers of autophagosomes (20,21).…”