PROJECT SUMMARY/ABSTRACT Fc-mediated antibody effector functions, primarily antibody dependent cell phagocytosis (ADCP) and antibody dependent cell cytotoxicity (ADCC), have been established to play a central role on the mechanism of action of therapeutic antibodies including anti-infective antibodies and immune checkpoint inhibitors. Effector functions are triggered by the crosslinking of Fc receptors (FcRs) expressed on cytotoxic leukocyte subsets following binding to target cells opsonized by multiple antibodies. Recent findings have highlighted the key role of myeloid- derived cells, on the clearance of opsonized pathogenic cells by ADCP/ADCC. Myeloid-derived effectors, express multiple Fc𝛾Rs, most relevant being the activating receptors Fc𝛾RI, Fc𝛾RIIa and Fc𝛾RIIIa and the inhibitory Fc𝛾RIIb. Because multivalent immune complexes (ICs) engage and activate all surface Fc𝛾Rs on myeloid-derived effectors to various degrees (depending on Fc𝛾R expression, Fc:FcR affinity, immune complex size and antigen density) the magnitude and kinetics of the ADCP and ADCC processes are determined by the integrated outcome of the activation of all surface FcRs to various degrees. The central hypothesis to be tested here is that the quantitative understanding of ADCP and ADCC by myeloid effectors triggered by the ligation of each individual FcR by taking advantage of bulk assays and high phenotypic content single-cell cytotoxicity assays together with phosphoproteomic data to map the specific signaling events on myeloid-derived effector cells, will be essential for providing a sound framework on how to engineer the Fc domain for optimal effector functions. This work will capitalize on our unique set of aglycosylated engineered Fc domains that bind with absolute selectivity and dialed-in affinity to each FcR type. In Specific Aim 1 we will exhaustively and quantitatively map the effector phenotypes (ADCC, ADCP, cytokine release, trogocytosis) performed by human macrophages and monocytes (as well as by neutrophils and by NK cells for thoroughness) triggered by ligation of each FcR as a function of affinity, IC target size and antigen density. In Sp. Aim 2 we will use novel high throughput single cell cytotoxicity assays and on-chip cytometry to determine the precise kinetics of immune synapse formation and cell killing in ADCC (or engulfment for ADCP) as a function of FcR expression levels on individual cells and to interrogate key relevant mechanistic aspects central to these processes. In Sp. Aim 3 we will use phosphoproteomics to: (a) identify and quantitate peptide phosphorylation events triggered by each FcR and (b) detect unique FcR ligation-induced phosphopeptide signatures that correlate with effector functions triggered by that receptor.