{
  "abstract": "Background PD-L1-directed immune-stimulating antibody conjugates (ISACs) have shown compelling efficacy in preclinical models, demonstrating the potential to treat cancers resistant to conventional immune checkpoint inhibitors (ICIs) and/or improve responses to ICIs when used in combination. PD-L1 is a unique target for ISACs and other antibody-drug conjugates as it can be expressed on both tumor and immune cells. Myeloid cells like macrophages and dendritic cells (DCs) can express high levels of PD-L1 and play a key role in regulating T cell responses through the PD-1/PD-L1 pathway. We have shown that PD-L1 expression by tumor cells is dispensable for efficacy with PD-L1 ISACs in syngeneic models, indicating a critical role for PD-L1-expressing immune cells in the mechanism of action. However, the key immune cell targets of PD-L1 ISACs and the properties affecting their immunostimulatory activity remain unclear. Here, we show that PD-L1 ISACs activate PD-L1-expressing macrophages and DCs in vitro and in vivo, with powerful immunostimulatory effects observed in both tumors and tumor-draining lymph nodes (TDLNs). Further, we demonstrate that our lead PD-L1 ISAC utilizing a dual TLR7/8 agonist payload and Fc-competent antibody has the potential for stronger myeloid cell activation and antitumor efficacy than alternative embodiments, while maintaining an encouraging safety profile.Methods ISACs were prepared using anti-PD-L1 antibodies and TLR7/8 agonist payloads. In vitro activity was assessed using monocyte-derived macrophages and DCs and tumor/DC cocultures. In vivo studies were performed in syngeneic mouse models.Results Our lead PD-L1 ISAC, an anti-PD-L1 human IgG1 conjugated to a TLR7/8 agonist, induced target-dependent activation of human macrophages and DCs polarized to express high levels of PD-L1, resulting in robust proinflammatory cytokine secretion. PD-L1 ISACs utilizing Fc-attenuated antibodies elicited weaker responses from human myeloid cells, both alone and in coculture with PD-L1-expressing tumor cells. In vivo, PD-L1 ISAC treatment led to the activation of PD-L1-expressing macrophages in tumors and DCs in TDLNs, with increases in markers associated with cytotoxic activity, antigen presentation, and T cell costimulation. Strong cytokine responses were also induced in tumors and TDLNs, with diminished effects in non-draining LNs. Consistent with their activity in vitro, Fc-competent PD-L1 ISACs demonstrated greater antitumor efficacy than Fc-attenuated variants in vivo.Conclusions PD-L1 ISACs drive powerful innate and adaptive immune responses through the targeted activation of PD-L1-expressing myeloid cells in tumors and TDLNs. A PD-L1 ISAC with an intact Fc and dual TLR7/8 agonist payload has the potential for enhanced immune activation and antitumor efficacy.",
  "authors": [
    {
      "affiliations": [
        "Bolt Biotherapeutics, Redwood City, CA, USA"
      ],
      "name": "Po Y Ho"
    },
    {
      "affiliations": [
        "Bolt Biotherapeutics, Redwood City, CA, USA"
      ],
      "name": "Rishali K Gadkari"
    },
    {
      "affiliations": [
        "Bolt Biotherapeutics, Redwood City, CA, USA"
      ],
      "name": "Shahid Khan"
    },
    {
      "affiliations": [
        "Bolt Biotherapeutics, Redwood City, CA, USA"
      ],
      "name": "Jennifer E Melrose"
    },
    {
      "affiliations": [
        "Bolt Biotherapeutics, Redwood City, CA, USA"
      ],
      "name": "David T Omstead"
    },
    {
      "affiliations": [
        "Bolt Biotherapeutics, Redwood City, CA, USA"
      ],
      "name": "Katelynn A McEachin"
    },
    {
      "affiliations": [
        "Bolt Biotherapeutics, Redwood City, CA, USA"
      ],
      "name": "Quyen Thach"
    },
    {
      "affiliations": [
        "Bolt Biotherapeutics, Redwood City, CA, USA"
      ],
      "name": "Karla A Henning"
    },
    {
      "affiliations": [
        "Bolt Biotherapeutics, Redwood City, CA, USA"
      ],
      "name": "William G Mallet"
    },
    {
      "affiliations": [
        "Bolt Biotherapeutics, Redwood City, CA, USA"
      ],
      "name": "Romas A Kudirka"
    },
    {
      "affiliations": [
        "Bolt Biotherapeutics, Redwood City, CA, USA"
      ],
      "name": "Michael N Alonso"
    },
    {
      "affiliations": [
        "Bolt Biotherapeutics, Redwood City, CA, USA"
      ],
      "name": "Justin A Kenkel"
    }
  ],
  "title": "994 Discovery of a PD-L1-directed ISAC optimized for the activation of PD-L1-expressing myeloid cells to drive potent antitumor immune responses",
  "uid": "62b69603-ecca-56cf-a239-cf12f0cfad58"
}