Role for Caspase-8 in the Release of IL-1ß and Active Caspase-1 from Viable Human Monocytes during Toxoplasma gondii Infection.

Monocytes are actively recruited to sites of infection and produce the potent proinflammatory cytokine IL-1ß. We previously showed that IL-1ß release during Toxoplasma gondii infection of primary human monocytes requires the NLRP3 inflammasome and caspase-1 but is independent of gasdermin D and pyroptosis. To investigate mechanisms of IL-1ß release, we generated caspase-1, -4, -5, or -8 knockout (KO) THP-1 monocytic cells. Genetic ablation of caspase-1 or -8, but not caspase-4 or -5, decreased IL-1ß release during T. gondii infection without affecting cell death. In contrast, TNF-α and IL-6 secretion were unperturbed in caspase-8 KO cells during T. gondii infection. Dual pharmacological inhibition of caspase-8 and RIPK1 in primary monocytes also decreased IL-1ß release without affecting cell viability or parasite infection. Caspase-8 was also required for the release of active caspase-1 from T. gondii-infected cells and for IL-1ß release during infection with the related apicomplexan parasite Neospora caninum. Surprisingly, caspase-8 deficiency did not impair synthesis or cleavage of pro-IL-1ß, but resulted in the retention of mature IL-1ß within cells. Generation of gasdermin E KO and ATG7 KO THP-1 cells revealed that the release of IL-1ß was not dependent on gasdermin E or ATG7. Collectively, our data indicate that during T. gondii Infection of human monocytes, caspase-8 functions in a novel gasdermin-independent mechanism controlling IL-1ß release from viable cells. This study expands on the molecular pathways that promote IL-1ß in human immune cells and provides evidence of a role for caspase-8 in the mechanism of IL-1ß release during infection.

Capers with caspases: Toxoplasma gondii tales of inflammation and survival.

Intracellular pathogens strike a delicate balance between maintaining their survival within infected cells, while also activating host defense mechanisms. Toxoplasma gondii is a protozoan parasite that initiates a variety of host signaling pathways as it invades host cells and establishes residence in a parasitophorous vacuole. Recent work has highlighted the interplay between T. gondii infection and innate immune pathways that lead to inflammation, several of which converge on caspases. This family of cysteine proteases function at the crossroads of inflammation and cell death and serve as a key target for parasite manipulation. This review focuses on the interaction of T. gondii with caspase-dependent inflammatory and cell death pathways and the role of parasite effector proteins in modulating these processes.

Internalization of HIV-1 by Phagocytes Is Increased When Virions Are Opsonized with Multimeric Antibody in the Presence of Complement.

The low abundance of envelope spikes and the inability of IgG to aggregate virions render HIV-1 an inadequate target for antibody-mediated clearance by phagocytes. In an attempt to improve the ability of antibody to mediate the internalization of HIV-1 virions, we generated multimers of the broadly neutralizing HIV-1-specific monoclonal antibody (MAb) VRC01 using site-directed mutagenesis of the Fc segment. We then measured virion internalization using primary human monocytes and neutrophils. We found that, in the absence of complement, immune complexes consisting of HIV-1 virions and VRC01 multimers were slightly more efficiently internalized than were complexes formed with monomeric VRC01. The presence of complement, however, greatly augmented internalization of immune complexes formed with the multimeric MAb but had little impact on monomeric MAb-mediated internalization. Multimerization and the presence of complement overcome the limited ability of monomeric antibody to mediate internalization of HIV-1 virions and may thus provide a therapeutic approach to clearing virus. IMPORTANCE Antibody-mediated internalization of HIV-1 by phagocytes, a potential mechanism for clearing virus, is very inefficient. In an effort to improve viral clearance, we produced a multimeric form of the broadly neutralizing monoclonal antibody VRC01. We found that VRC01 antibody multimers (primarily hexamers) were only slightly more efficient in mediating HIV-1 internalization than was monomeric VRC01. However, the addition of complement resulted in substantially greater internalization of multimer-opsonized virus. In contrast, complement had little if any impact on internalization of monomer-opsonized virus. Therefore, antibody multimerization in combination with complement may overcome the limited ability of monomeric antibody to mediate internalization of HIV-1 virions. Our findings may provide a therapeutic approach to clearing virus.