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Does the choice of anti-DENV antibody used to enhance infection impact Syk activation? Expanding the study of Syk activation by using additional enhancing antibodies will be important to gain more insight into stimulation of this pathway by ADE. Anti-DENV antibodies can target specific epitopes on prM, EDI/II, EDIII, or even epitopes that are only present on an intact virion (12). Some virions have no prM because of complete cleavage, while many virions maintain some or all of their prM expression (22). This alone indicates that immune complexes can be formed with a varying number of antibodies bound depending on the availability of each specific epitope. As recently identified, the signaling phenotype after FcγR cross-linking is dramatically impacted by the extent of cross-linking and receptor aggregation (146). Could enhancing antibodies that target epitopes with low availability contribute to intrinsic ADE? The lower number of antibodies bound could lead to less extensive receptor aggregation than

enhancing antibodies bound to more highly-expressed epitopes.

Additionally, a consideration should be given to the subclass of IgG used. IgG1 is the most abundant antibody in human serum (233), but humans can form anti-DENV antibodies of all 4 IgG subclasses (234). As each FcγR has varying affinity for the different IgG subclasses, could IgG subclass impact the strength of Syk activation induced by ADE? Low-affinity IgG- FcγR interactions by DENV immune complexes may induce differential Syk activation compared to high-affinity IgG-FcγR interactions. An interesting way to address this question

would be to make a chimeric antibody that only varied the Fc regions. Thus, ADE using the same epitope target could be assessed with IgG1, IgG2, IgG3, and IgG4 Fc regions.

Does antibody-mediated neutralization of DENV activate the same signaling pathway as ADE?

In theory, neutralization of DENV with antibody should activate Syk in a manner similar to ADE, as it is the cross-linking of the FcγRs that activates Syk. However, the situation is likely to be more complex. A virion bound for neutralization will be thoroughly coated with antibody, while a virion bound for enhancement has fewer antibodies attached. This could impact the degree of FcγR aggregation, thereby altering signaling.

It will be interesting to determine how cytokine expression changes when DENV is coated with neutralizing antibodies at a concentration that leads to neutralization. What could be the consequences of monocytes neutralizing virus but still producing cytokines in response? Is elevated inflammatory cytokine expression by monocytes only detrimental when they are also producing virus?

How does ADE impact inflammasome activation at later time points?

Coming into these studies, we needed to compare an interesting dichotomy. DENV reportedly activates the inflammasome in primary human monocytes 4 days after infection (98). However, it has been known for years that monocytes secrete IL-1β within 4 hours of infection (95). How could this be? Thus, we focused at early time points to assess both pro-IL-1β

synthesis and processing. In the early hours following infection, ADE was only able to induce elevated pro-IL-1β synthesis and not activate caspase-1. However, as described for monocytes

(169-173), a small portion of pro-IL-1β was processed without detectable increases in caspase-1 activation.

It will be interesting to extend caspase-1 assessments to later time points. Based on the reports, caspase-1 activation should occur at some point. At that time point, an important question will be to see how ADE modulates the caspase-1 activation. Will Syk activation increase inflammasome activation? Or will the proposed phenomena of intrinsic ADE and ITAMi suppress caspase-1 activation?

How does Syk activation impact intrinsic ADE?

There is increasing evidence that ADE suppresses the antiviral response to increase DENV replication (106,129,235). This includes dephosphorylation of STAT1 to impair IFN signaling. SHP-1, a phosphatase known to be activated by Syk, dephosphorylates STAT1 to inhibit its activation and is implicated in ITAMi signaling downstream of FcγR activation (142,143,146,149). If some of the phenotypes described in intrinsic ADE studies can be replicated in the system described here, Syk inhibition should be tested to see if intrinsic ADE can be reversed. There are Syk inhibitors currently in use in clinical trials for cancers and autoimmune conditions. Through a collaboration with a pharmaceutical company, we will be able to test Syk inhibition under ADE conditions using an inhibitor approved for clinical trials in humans.

An extension of this question is the need to reconcile the contrasting roles described for Syk in ADE. It has been reported that DENV immune complexes must inhibit Syk through co- ligation of the inhibitory receptor LILRB1 in order to induce ADE (204). However, much of the evidence implicating Syk in this pathway is based upon inhibition of Syk with piceatannol, an

inhibitor that is not very selective for Syk (236). Additionally, in the Chan, et al., study, all of the mechanistic findings were elucidated using THP-1 cells and not primary monocytes (204). Further investigation is clearly required, as inhibition of Syk in my studies with BAY 61-3606, a highly selective Syk inhibitor at the concentrations used, did not impact viral replication (188).

4.4 SUMMARY OF FINDINGS – VERO CELLS PRODUCE INFLAMMATORY