Breast
Xavier L. Baldwin, MD
Resident
University of North Carolina Hospitals
Durham, North Carolina, United States
Disclosure: I do not have any relevant financial / non-financial relationships with any proprietary interests.
B-cells in triple negative breast cancer (TNBC) correlate positively with prognosis and their antibodies are critical for checkpoint blockade efficacy in animal models. Mechanistically, little is known of B-cells contribution to checkpoint blockade. We hypothesized that B-cell produced antibodies activate the classical complement pathway to contribute to tumor eradication during checkpoint blockade in TNBC.
Methods:
5x105 E0771 cells were implanted into abdominal mammary glands of C57/Bl6 mice. Mice were treated with control (PBS), anti-PD-1, anti-CTLA-4, and dual therapy (anti-PD-1+anti-CTLA-4) after tumors averaged 5 mm in greatest dimension. Flow cytometric analysis was used to characterize B-cells and plasma cells among tumor infiltrating lymphocytes and splenocytes. Expression of IgG, C3b, C4d, and the membrane attack complex (C5b-9) were measured by immunohistochemistry in tumors.
Results:
Mice treated with combination checkpoint inhibition showed tumor regression and increased survival compared to those with single checkpoint blockade and PBS. Plasma cells (the lineage of B-cells capable of producing antibody), defined by the marker CD138, were significantly increased in the tumor microenvironment compared to in the periphery (splenocytes) in untreated mice. Immunohistochemistry showed expression of IgG (indicative of antibody deposition) was significantly increased in mice treated with dual therapy compared with control treated mice (Fig 1a). There was increased expression of C3b, a marker of complement activation in mice treated with combination checkpoint blockade. C4d, a marker more specific for activation of the classical complement pathway, was also expressed in tumors in all four groups. C5b-9, which serves as the terminal endpoint, was significantly upregulated, as measured by IHC, in mice treated by dual compared to single-agent checkpoint blockade or control (Fig 1b).
Conclusions:
Increased expression of immunoglobulin and complement in TNBC tumors treated with checkpoint blockade suggest activation of the classical complement pathway via antibody production. Enhancing this pathway may augment tumor cytotoxicity with checkpoint blockade.