The irreversible Bruton's tyrosine kinase (Btk) inhibitor ibrutinib has shown efficacy against B-cell tumors such as chronic lymphocytic leukemia and B-cell non-Hodgkin lymphoma. Fc␥ receptors (Fc␥R) on immune cells such as macrophages play an important role in tumor-specific antibody-mediated immune responses, but many such responses involve Btk. Here we tested the effects of ibrutinib on Fc␥R-mediated activities in monocytes. We found that ibrutinib did not affect monocyte Fc␥R-mediated phagocytosis, even at concentrations higher than those achieved physiologically, but suppressed Fc␥R-mediated cytokine production. We confirmed these findings in macrophages from Xid mice in which Btk signaling is defective. Because calcium flux is a major event downstream of Btk, we tested whether it was involved in phagocytosis. The results showed that blocking intracellular calcium flux decreased Fc␥R-mediated cytokine production but not phagocytosis. To verify this, we measured activation of the GTPase Rac, which is responsible for actin polymerization. Results showed that ibrutinib did not inhibit Rac activation, nor did the calcium chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N,N-tetraacetic acid tetrakis(acetoxymethyl ester). We next asked whether the effect of ibrutinib on monocyte Fc␥R-mediated cytokine production could be rescued by IFN␥ priming because NK cells produce IFN␥ in response to antibody therapy. Pretreatment of monocytes with IFN␥ abrogated the effects of ibrutinib on Fc␥R-mediated cytokine production, suggesting that IFN␥ priming could overcome this Btk inhibition. Furthermore, in monocyte-natural killer cell co-cultures, ibrutinib did not inhibit Fc␥R-mediated cytokine production despite doing so in single cultures. These results suggest that combining ibrutinib with monoclonal antibody therapy could enhance chronic lymphocytic leukemia cell killing without affecting macrophage effector function.
Background: Human monocyte inflammatory responses differ between virulent and attenuated Francisella infection.Results: A mixed infection model showed that the virulent F. tularensis Schu S4 can attenuate inflammatory cytokine responses to the less virulent F. novicida in human monocytes.Conclusion: F. tularensis dampens inflammatory response by an active process.Significance: This suppression may contribute to enhanced pathogenicity of F. tularensis. Francisella tularensis is a Gram-negative facultative bacterium that can cause the disease tularemia, even upon exposure to low numbers of bacteria. One critical characteristic of Francisella is its ability to dampen or subvert the host immune response. Previous work has shown that monocytes infected with highly virulent F. tularensis subsp. tularensis strain Schu S4 responded with a general pattern of quantitatively reduced pro-inflammatory signaling pathway genes and cytokine production in comparison to those infected with the less virulent related F. novicida. However, it has been unclear whether the virulent Schu S4 was merely evading or actively suppressing monocyte responses. By using mixed infection assays with F. tularensis and F. novicida, we show that F. tularensis actively suppresses monocyte pro-inflammatory responses. Additional experiments show that this suppression occurs in a dose-dependent manner and is dependent upon the viability of F. tularensis. Importantly, F. tularensis was able to suppress pro-inflammatory responses to earlier infections with F. novicida. These results lend support that F. tularensis actively dampens human monocyte responses and this likely contributes to its enhanced pathogenicity.
Fcγ receptors (FcγR) are critical mediators of monoclonal antibody cancer therapies, as they drive cytotoxic processes upon binding of effector cells to opsonized targets. Along with natural killer (NK) cells, monocytes are also known to destroy antibody-coated targets via antibody-dependent cellular cytotoxicity (ADCC). However, the precise mechanisms by which monocytes carry out this function have remained elusive. Here, we show that human monocytes produce the protease Granzyme B upon both FcγR and Toll-like Receptor (TLR) 8 activation. Treatment with TLR8 agonists elicited Granzyme B and also enhanced FcγR-mediated Granzyme B production in an additive fashion. Furthermore, monocyte-mediated ADCC against cetuximab-coated tumor targets was enhanced by TLR8 agonist treatment, and this enhancement of ADCC required Granzyme B. Hence, we have identified Granzyme B as an important mediator of FcγR function in human monocytes and have uncovered another mechanism by which TLR8 agonists may enhance FcγR-based therapies.
MiR-155 regulates numerous aspects of innate and adaptive immune function. This miR is induced in response to Toll-like receptor ligands, cytokines, and microbial infection. We have previously shown that miR-155 is induced in monocytes/macrophages infected with Francisella tularensis and suppresses expression of the inositol phosphatase SHIP to enhance activation of the PI3K/Akt pathway, which in turn promotes favorable responses for the host. Here we examined how miR-155 expression is regulated during infection. First, our data demonstrate that miR-155 can be induced through soluble factors of bacterial origin and not the host. Second, miR-155 induction is not a direct effect of infection and it requires NF-κB signaling to up-regulate fos/jun transcription factors. Finally, we demonstrate that the requirement for NF-κB-dependent de novo protein synthesis is globally shared by microbial ligands and live bacteria. This study provides new insight into the complex regulation of miR-155 during microbial infection.
Background: Toll-like receptors recognize bacterial components, leading to immune activation. Results: TLR2 ligands alter monocyte/macrophage Fc␥R and phosphatase expression to improve function both in vitro and in vivo. Conclusion: TLR2 ligands enhance monocyte/macrophage function. Significance: Fc␥R function is critical for antibody therapy. Deciphering the effect of TLR2 ligands provides a potential means to enhance therapy.
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