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Unconjugated monoclonal antibodies (mAb) have revolutionized the treatment of B-cell malignancies. These targeted drugs can activate innate immune cytotoxicity for therapeutic benefit. mAb activation of the complement cascade results in complement-dependent cytotoxicity (CDC) and complement receptor-mediated antibody-dependent cellular phagocytosis (cADCP). Clinical and laboratory studies have showed that CDC is therapeutically important. In contrast, the biological role and clinical effects of cADCP are less well understood. This review summarizes the available data on the role of complement activation in the treatment of mature B-cell malignancies and proposes future research directions that could be useful in optimizing the efficacy of this important class of drugs.

Macrophage antibody (Ab)-dependent cellular phagocytosis (ADCP) is a major cytotoxic mechanism for both therapeutic unconjugated monoclonal Abs (mAbs) such as rituximab and Ab-induced hemolytic anemia and immune thrombocytopenia. Here, we studied the mechanisms controlling the rate and capacity of macrophages to carry out ADCP in settings of high target/effector cell ratios, such as those seen in patients with circulating tumor burden in leukemic phase disease. Using quantitative live-cell imaging of primary human and mouse macrophages, we found that, upon initial challenge with mAb-opsonized lymphocytes, macrophages underwent a brief burst (<1 hour) of rapid phagocytosis, which was then invariably followed by a sharp reduction in phagocytic activity that could persist for days. This previously unknown refractory period of ADCP, or hypophagia, was observed in all macrophage, mAb, and target cell conditions tested in vitro and was also seen in vivo in Kupffer cells from mice induced to undergo successive rounds of αCD20 mAb-dependent clearance of circulating B cells. Importantly, hypophagia had no effect on Ab-independent phagocytosis and did not alter macrophage viability. In mechanistic studies, we found that the rapid loss of activating Fc receptors from the surface and their subsequent proteolytic degradation were the primary mechanisms responsible for the loss of ADCP activity in hypophagia. These data suggest hypophagia is a critical limiting step in macrophage-mediated clearance of cells via ADCP, and understanding such limitations to innate immune system cytotoxic capacity will aid in the development of mAb regimens that could optimize ADCP and improve patient outcome., (© 2020 by The American Society of Hematology.)

Phagocytosis is a dynamic process central to immunity and tissue homeostasis. Current methods for quantification of phagocytosis largely rely on indirect or static measurements, such as target clearance or dye uptake, and thus provide limited information about engulfment rates or target processing. Improved kinetic measurements of phagocytosis could provide useful, basic insights in many areas. We present a live-cell, time-lapse and high-content microscopy imaging method based on the detection and quantification of fluorescent dye 'voids' within phagocytes that result from target internalization to quantify phagocytic events with high temporal resolution. Using this method, we measure target cell densities and antibody concentrations needed for optimal antibody-dependent cellular phagocytosis. We compare void formation and dye uptake methods for phagocytosis detection, and examine the connection between target cell engulfment and phagolysosomal processing. We demonstrate how this approach can be used to measure distinct forms of phagocytosis, and changes in macrophage morphology during phagocytosis related to both engulfment and target degradation. Our results provide a high-resolution method for quantifying phagocytosis that provides opportunities to better understand the cellular and molecular regulation of this fundamental biological process., Competing Interests: Competing interestsThe authors declare no conflict of interest. While we thank our funding sources for their support, none of these organizations, including Acerta Pharma LLC, contributed to the concept or design of this project, nor did they contribute to the data acquisition or data analysis presented here., (© 2020. Published by The Company of Biologists Ltd.)

Background: Neuroinflammation is thought to contribute to the pathogenesis of Alzheimer's disease (AD), yet numerous studies have demonstrated a beneficial role for neuroinflammation in amyloid plaque clearance. We have previously shown that sustained expression of IL-1β in the hippocampus of APP/PS1 mice decreases amyloid plaque burden independent of recruited CCR2 + myeloid cells, suggesting resident microglia as the main phagocytic effectors of IL-1β-induced plaque clearance. To date, however, the mechanisms of IL-1β-induced plaque clearance remain poorly understood., Methods: To determine whether microglia are involved in IL-1β-induced plaque clearance, APP/PS1 mice induced to express mature human IL-1β in the hippocampus via adenoviral transduction were treated with the Aβ fluorescent probe methoxy-X04 (MX04) and microglial internalization of fibrillar Aβ (fAβ) was analyzed by flow cytometry and immunohistochemistry. To assess microglial proliferation, APP/PS1 mice transduced with IL-1β or control were injected intraperitoneally with BrdU and hippocampal tissue was analyzed by flow cytometry. RNAseq analysis was conducted on microglia FACS sorted from the hippocampus of control or IL-1β-treated APP/PS1 mice. These microglia were also sorted based on MX04 labeling (MX04 + and MX04 - microglia)., Results: Resident microglia (CD45 lo CD11b + ) constituted > 70% of the MX04 + cells in both Phe- and IL-1β-treated conditions, and < 15% of MX04 + cells were recruited myeloid cells (CD45 hi CD11b + ). However, IL-1β treatment did not augment the percentage of MX04 + microglia nor the quantity of fAβ internalized by individual microglia. Instead, IL-1β increased the total number of MX04 + microglia in the hippocampus due to IL-1β-induced proliferation. In addition, transcriptomic analyses revealed that IL-1β treatment was associated with large-scale changes in the expression of genes related to immune responses, proliferation, and cytokine signaling., Conclusions: These studies show that IL-1β overexpression early in amyloid pathogenesis induces a change in the microglial gene expression profile and an expansion of microglial cells that facilitates Aβ plaque clearance.