Abstract Macrophages hold tremendous promise as effectors of cancer immunotherapy, but the best strategies to provoke these cells to attack tumors remain unknown. Here, we evaluated the therapeutic potential of targeting two distinct macrophage immune checkpoints: CD47 and CD24. We found that antibodies targeting these antigens could elicit maximal levels of phagocytosis when combined together in vitro. However, to our surprise, via unbiased genome-wide CRISPR screens, we found that CD24 primarily acts as a target of opsonization rather than an immune checkpoint. In a series of in vitro and in vivo genetic validation studies, we found that CD24 was neither necessary nor sufficient to protect cancer cells from macrophage phagocytosis in most mouse and human tumor models. Instead, anti-CD24 antibodies exhibit robust Fc-dependent activity, and as a consequence, they cause significant on-target hematologic toxicity that was life-threatening in syngeneic mice. To overcome these challenges and leverage our findings for therapeutic purposes, we engineered a collection of 77 novel bispecific antibodies that bind to a tumor antigen with one arm and engage macrophages with the second arm. We discovered multiple novel bispecifics that maximally activate macrophage-mediated cytotoxicity and reduce binding to healthy blood cells, including bispecifics targeting macrophage immune checkpoint molecules in combination with EGFR, TROP2, and CD71. Overall, our findings indicate that CD47 predominates over CD24 as a macrophage immune checkpoint in cancer, and that the novel bispecifics we created may be optimal immunotherapies to direct myeloid cells to eradicate solid tumors. Citation Format: Anna Meglan, Juliet Allen, Kyle Vaccaro, José Velarde, Victor Chen, Juliano Ribeiro, Jasmine Blandin, Ranjan Mishra, Raymond Ho, Jennifer Love, Ferenc Reinhardt, George W Bell, Jin Chen, Robert Weinberg, Dian Yang, Jonathan Weissman, Kipp Weiskopf. CD47 predominates over CD24 as a macrophage immune checkpoint in cancer [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Tumor Immunology and Immunotherapy; 2024 Oct 18-21; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2024;12(10 Suppl):Abstract nr A055.

Mutations in the gene MCOLN1 coding for the TRP (transient receptor potential) family ion channel TRP-ML1 lead to the lipid storage disorder mucolipidosis type IV (MLIV). The function and role of TRP-ML1 are not well understood. We report here that TRP-ML1 is a lysosomal monovalent cation channel. Both native and recombinant TRP-ML1 are cleaved resulting in two products. Recombinant TRP-ML1 is detected as the full-length form and as short N- and C-terminal forms, whereas in native cells mainly the cleaved N and C termini are detected. The N- and C-terminal fragments of TRP-ML1 were co-immunoprecipitated from cell lysates and co-eluted from a Ni2+ column. TRP-ML1 undergoes proteolytic cleavage that is inhibited by inhibitors of cathepsin B (CatB) and is altered when TRP-ML1 is expressed in CatB-/- cells. N-terminal sequencing of purified C-terminal fragment of TRP-ML1 expressed in Sf9 cells indicates a cleavage site at Arg200 ↓ Pro201. Consequently, the conserved R200H mutation changed the cleavage pattern of TRP-ML1. The cleavage inhibited TRP-ML1 channel activity. This work provides the first example of inactivation by cleavage of a TRP channel. The significance of the cleavage to the function of TRP-ML1 is under investigation.

Ca2+-induced Ca2+ release (CICR) is a well characterized activity in skeletal and cardiac muscles mediated by the ryanodine receptors. The present study demonstrates CICR in the non-excitable parotid acinar cells, which resembles the mechanism described in cardiac myocytes. Partial depletion of internal Ca2+ stores leads to a minimal activation of Ca2+ influx. Ca2+ influx through this pathway results in an explosive mobilization of Ca2+ from the majority of the stores by CICR. Thus, stimulation of parotid acinar cells in Ca2+-free medium with 0.5 muM carbachol releases similar to5% of the Ca2+ mobilizable by 1 mM carbachol. Addition of external Ca2+ induced the same Ca2+ release observed in maximally stimulated cells. Similar results were obtained by a short treatment with 2.5-10 muM cyclopiazonic acid, an inhibitor of the sarco/endoplasmic reticulum Ca2+ ATPase pump. The Ca2+ release induced by the addition of external Ca2+ was largely independent of IP(3)Rs because it was reduced by only similar to30% by the inhibition of the inositol 1,4,5-trisphosphate receptors with caffeine or heparin. Measurements of Ca2+-activated outward current and [Ca2+](i) suggested that most CICR triggered by Ca2+ influx occurred away from the plasma membrane. Measurement of the response to several concentrations of cyclopiazonic acid revealed that Ca2+ influx that regulates CICR is associated with a selective portion of the internal Ca2+ pool. The minimal activation of Ca2+ influx by partial store depletion was confirmed by the measurement of Mn2+ influx. Inhibition of Ca2+ influx with SKF96365 or 2-aminoethoxydiphenyl borate prevented activation of CICR observed on addition of external Ca2+. These findings provide evidence for activation of CICR by Ca2+ influx in non-excitable cells, demonstrate a previously unrecognized role for Ca2+ influx in triggering CICR, and indicate that CICR in non-excitable cells resembles CICR in cardiac myocytes with the exception that in cardiac cells Ca2+ influx is mediated by voltage-regulated Ca2+ channels whereas in non-excitable cells Ca2+ influx is mediated by store-operated channels.

Receptor signaling at the plasma membrane often releases calcium from intracellular stores. For example, inositol triphosphate (IP3) produced by receptor-coupled phospholipase C activates an intracellular store calcium channel, the IP(3)R. Conversely, stores can induce extracellular calcium to enter the cell through plasma membrane channels, too. How this "reverse" coupling works was unclear, but store IP(3)Rs were proposed to bind and regulate plasma membrane TRP cation channels. Here, we demonstrate that the adaptor protein, termed Homer, facilitates a physical association between TRPC1 and the IP(3)R that is required for the TRP channel to respond to signals. The TRPC1-Homer-IP(3)R complex is dynamic and its disassembly parallels TRPC1 channel activation. Homer's action depends on its ability to crosslink and is blocked by the dominant-negative immediate early gene form, H1a. Since H1a is transcriptionally regulated by cellular activity, this mechanism can affect both short and long-term regulation of TRPC1 function.